AC Drives (Motors And Drives)


The term AC drives covers a wide range of drive types. When talking mostly industrial terms, an AC drive could also be considered a variable frequency drive (VFD), adjustable-speed drive (ASD), variable-speed drive (VSD), and “inverter.” If a technician was discussing ASDs and the factory contained mostly DC equipment, then ASD or VSD would refer to a DC drive. The term inverter is actually the final or power section of the drive—and is considered an acceptable term for the entire unit.
There are actually many different types of AC drives, but all of them have one concept in common—they convert fixed voltage and frequency into a variable voltage and frequency output.
Though they do not meet the strict definition of an AC drive, reduced voltage starters (e.g., soft starts) and wound rotor slip recovery units fall into the variable-speed category. Soft starts immediately deliver line frequency to the motor but at a reduced voltage value for a specified period of time. The result is reduced motor torque, as the motor is accelerating.
Load commutated inverters (LCIs) and cycloconverters are also part of the variable-frequency drive category. Cycloconverters actually use SCRs in very large horsepower amounts that require regeneration. Though the output of a cycloconverter may not be considered variable, the unit does alter the frequency output, thereby reducing the speed. This unit would actually step down the frequency to 1/2 or 1/3 of the line frequency. This frequency would then be applied to motors of 30- or 20-Hz design. The cycloconverter got its start in the 1930s but is not much in demand today because of its complexity and cost of circuitry.
The primary focus of this section will be on three common types of VFDs: the variable voltage input or inverter (VVI, sometimes referred to as the six-step drive), the current source inverter (CSI), and the pulse width modulated (PWM). The VVI and PWM would be considered a voltage source inverter, while the CSI would be considered a current source inverter.
Several other types of drives fall under the category of voltage source inverters. Vector (or flux vector) drives and sensorless vector drives will be considered later in this topic.
If not already done, it would be helpful to review the section on AC induction motors in topic 3. That section will provide a foundation upon which to build the basic concepts of variable-frequency drives.
Basic Theory of Major Drive Types
The easiest way to understand drives is to take a brief look at what a drive application looks like. Figure 4-22 shows a simple application with a fixed-speed fan using a motor starter. The three-phase motor starter can be replaced by a VFD, allowing the fan to be operated at variable speed.
Simple VFD/fan application
Figure 4-22. Simple VFD/fan application
If the fan can operate at virtually any speed required (below maximum motor speed), the air outlet damper can be “fixed” open. Using the fixed-speed motor starter method, the only means of varying the air out was by adjusting the air outlet damper.
Now the question is: How does this induction motor work with a drive? As mentioned earlier, a standard three-phase induction motor can be controlled by a VFD. There are two main elements that are controlled by the drive, speed, and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motor characteristics.
Figure 4-23 shows the formula for determining the shaft speed of an induction motor.
As you can see, this formula includes a characteristic called slip. The slip speed in a motor is actually termed base speed. As indicated earlier, a VFD controls two main elements of a motor—speed and torque.
The speed of a motor is conveniently adjusted by changing the frequency applied to the motor. The VFD adjusts the output frequency, thereby adjusting the speed of the motor. The torque of a motor is controlled by a
Motor speed formula (including slip)
Figure 4-23. Motor speed formula (including slip)
basic characteristic of every motor—the volts per hertz ratio (V/Hz). A review of this ratio is shown in Figure 4-24.
AC motor linear volts per hertz ratio
Figure 4-24. AC motor linear volts per hertz ratio
If an induction motor is connected to a 460-V power source at 60 Hz, the ratio is 7.67 V/Hz. As long as this ratio is kept in proportion, the motor will develop rated torque.
Figure 4-25 indicates a typical speed/torque curve for a motor. This curve represents a motor operated at a fixed voltage and frequency source.
A VFD provides many different frequency outputs, as shown in Figure 426. At any given frequency output of the drive, another torque curve is established. A typical operating point is where the curve intersects the 100% level, indicating rated torque.
Motor speed/torque curve
Figure 4-25. Motor speed/torque curve
Drive frequency output vs. motor torque
Figure 4-26. Drive frequency output vs. motor torque
The VFD provides the frequency and voltage output necessary to change the speed of a motor. A block diagram of a basic PWM drive (pulse-width modulated) is shown in Figure 4-27. All PWM drives contain these main parts, with subtle differences in hardware and software components. A PWM drive will be used as an example here, then the characteristics of a VVI and CSI drive will be compared with the PWM.
Although some drives accept single-phase input power, a three-phase unit will be considered for illustration purposes. (To simplify illustrations, the waveforms in the following drive figures only show one phase of input and output.)
PWM drive (VFD) block diagram
Figure 4-27. PWM drive (VFD) block diagram
Three-phase power is applied to the input section of the drive, called the converter. This section contains six diodes, arranged in an electrical bridge. These diodes convert AC power to DC power. Because diodes are used in the converting process, the next section sees a fixed DC voltage.
The DC bus section accepts the now converted, AC-to-DC voltage. The role of this section is to filter and smooth out the waveform. The “L” and “C” indicate inductors and capacitors. (Note: Many drive manufacturers install only one DC bus inductor, along with the filter capacitor(s). Some manufacturers install line reactors, “L” ahead of the drive converter. Later in this topic, innovations and technology improvements will be reviewed in detail.)
As shown, the diodes actually reconstruct the negative halves of the waveform onto the positive half. An average DC voltage of ~650-680 V is seen, if the drive is a 460-VAC unit. (Line voltage x 1.414 = DC bus voltage.) The inductor and the capacitor(s) work together to filter out any AC component on the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.
Once filtered, the DC bus voltage is delivered to the final section of the drive, called the inverter section. As the name implies, this section actually inverts the DC voltage back to AC—but in a variable voltage and frequency output. Devices called insulated gate bipolar transistors (IGBTs) act as power switches to turn on and off the DC bus voltage at specific intervals. In doing so, the inverter actually creates a variable AC voltage and frequency output. Control circuits, called gate drivers, cause the control part of the IGBT (gate) to turn “on” and “off” as needed.
As seen in Figure 4-28, the output of the drive does not provide an exact replica of the AC input sine waveform. It actually provides voltage pulses that are at a constant magnitude in height. The IGBTs switch the DC bus voltage on and off at designated intervals.
PWM output waveform (voltage and current)
Figure 4-28. PWM output waveform (voltage and current)
The control board in the drive signals the gate driver circuits to turn on the waveform positive half or negative half IGBT. This alternating of positive and negative switches recreates the three-phase output.
The longer the IGBT remains on, the higher the output voltage; the less time the IGBT is on, the lower the output voltage (shown in Figure 4-29). Conversely, the longer the IGBT is off, the lower the output frequency.
Frequency and voltage creation from PWM
Figure 4-29. Frequency and voltage creation from PWM
The speed at which IGBTs are switched on and off is called the carrier frequency or switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3000 x 4000 times per second (3-4 kHz), but some manufacturers use a carrier as high as 16 Hz.
As you can imagine, the higher the switch frequency, the smoother the output waveform (the higher the resolution). However, there is a disadvantage. Higher switch frequencies cause decreased efficiency of the drive. The faster the switching rate, the faster the IGBTs turn on and off. This causes increased heat in the IGBTs.
Now that the standard PWM VFD has been reviewed, the next step will be to review the characteristics of VVI and CSI VFDs. This will be followed by an analysis of how PWM compares with VVI and CSI drives.

VVI (Variable Voltage Inverter) — Input

This design takes the supply voltage (e.g., 230 or 460 V), rectifies it, and sends the variable voltage to the DC bus and then to the inverter section. The inverter section then inverts (changes DC to AC) the variable voltage DC to a variable-voltage and variable-frequency AC. The inverter section contains power semiconductors such as transistors or thyristors (SCRs).
To deliver variable voltage to the inverter, the input rectifier section or front-end consists of a controllable rectifier—SCRs. The control logic fires the SCRs at the appropriate time during the sine wave, thereby providing the variable voltage to the DC bus. Figure 4-30 shows a block diagram of a
VVI drive.
VVI drive block diagram
Figure 4-30. VVI drive block diagram
Some of the advantages of a VVI drive include good speed range, ability to connect multiple motors to the drive (within drive current limitations), and a fairly simple control regulator. However, there are some limitations.
One of the major disadvantages, in terms of AC drives, is the input power factor. It decreases as the speed of the drive/motor decreases. This is due to the “controllable” rectifier front end being constructed of SCRs. This issue is identical to that of DC drives.
Another disadvantage is the inability of the drive to “ride through” a low-input voltage situation. The term power loss ride-through is defined as the ability of a drive to ride through a low- or zero-voltage input and still remain in operation. This may be 2 to 3 cycles of the AC Sine wave, or more. (Note: Each AC cycle lasts about 16 ms). At low operating speeds (low hertz output), the SCR rectifier is not constantly keeping the DC bus charged at full potential. This means that the motor has voltage to draw from, in the event of a low line input.
Other disadvantages include the requirement of an input isolation transformer or line reactor. This is needed because of the SCR control technology (line spike generation). In addition, the generation of additional output harmonics is a possible result of the technology being applied to the AC motor.
The VVI drive has one additional disadvantage—the characteristic of low-speed motor cogging (shaft pulsations/jerky motion). Though not an issue at mid to high speeds, cogging at low speeds can cause equipment problems. This limitation is best illustrated in Figure 4-31.
 VVI voltage and current waveforms
Figure 4-31. VVI voltage and current waveforms
As seen in Figure 4-31, the voltage waveform approximates a series of steps. Because of this characteristic, this drive is sometimes called a six-step drive. During low-speed operation (>15-20 Hz), the rotor actually searches for the next available magnetic field in the stator. The result is a jerky rotation of the motor shaft. Because of this, gears or gear reducers connected to the motor shaft will suffer additional friction and wear. At high speeds, the inertia of the motor will provide continuous movement of the motor shaft. Therefore, cogging is not a problem.
As shown in the current waveform, there are several spikes that occur at regular intervals. These spikes, or transients, are caused by the SCRs gating on, or triggering. The DC bus filter circuit (shown by an L and C) does reduce the effects of these spikes, but they are not eliminated. These spikes translate into additional motor heating and inefficiency.
The VVI drive was one of the first AC drives to gain acceptance into the industrial drives market. It may be considered one of the most economical drives in the 25- to 150-HP range if a 6:1 speed range is acceptable (operation from 10-60 Hz). This type of drive is also widely used in high-speed drive applications—400 to 3000 Hz.

CSI (Current Source Inverter)

This type of AC drive (sometimes referred to as current source input) has basically the same components as a VVI drive. The major difference is that it is more of a current-sensitive drive as opposed to a VVI, which is more of a voltage-sensitive drive.
This design also takes the supply voltage (e.g., 230 or 460 V), rectifies it, and sends the variable voltage to the DC bus and then to the inverter section. As with the VVI, the CSI drive inverter section inverts (changes DC to AC) the variable-voltage DC to a variable-voltage and variable-frequency AC. The inverter section is made up of power semiconductors such as transistors or thyristors (SCRs).
To deliver variable voltage to the inverter, the input-rectifier section also consists of a controllable rectifier—SCRs. The control logic fires the SCRs at the appropriate time during the sine wave, thereby providing the variable voltage to the DC bus. Figure 4-32 shows a block diagram of a CSI drive.
CSI block diagram
Figure 4-32. CSI block diagram
Some of the advantages of a CSI drive include high efficiency, inherent short-circuit protection (due to the current regulator within the drive), inherent regenerative capability back to the AC line during overhauling load situations, and the capability of synchronous transfer (bringing other motors on-line during full-voltage output).
However, there are also some limitations. As with a VVI drive, the input power factor decreases as the speed of the drive/motor decreases. Also, this drive has a limited speed range due to low-speed motor cogging (shaft pulsations/jerky motion). This drive is also unable to “ride through” a low-input voltage situation. This drive also has the requirement of an input isolation transformer, due to the SCR control technology (line spike generation).
Unlike the VVI drive, the CSI drive cannot operate more than one motor at a time. The motor is an integral part of the drive system and its characteristics must be matched to the drive. (Usually the motor and drive are sold as a complete package.) Multiple motors would cause malfunctions in the drive-control system. In addition, the motor normally requires a feedback device (e.g., tachometer) to provide information to the drive current regulator.
Also related to motors is the requirement for the motor to always be connected to the drive. This means that the drive cannot be tested without the motor connected. In some cases, one additional disadvantage is the drive size. Typically, it is physically larger than other drive types because of internal power components.
As mentioned earlier, the VVI and CSI drives produce low-speed cogging. This is illustrated in Figure 4-33.
CSI voltage and current waveforms
Figure 4-33. CSI voltage and current waveforms
As seen in Figure 4-33, line notching or spikes developed from the gating of SCRs in the drive front end. Compared with a VVI drive, the voltage waveform is somewhat closer to the sine wave voltage required by the motor. The current waveform appears to simulate a trapezoid. In addition, there are times when no current flows. These gaps in current cause the rotor to search for the next available magnetic field in the stator. This characteristic, like that of the VVI, results in jerky rotation of the motor shaft at low speeds (<15-20 Hz).
As with the VVI drive, the DC bus filter circuit (shown by an L) does reduce the effects of these spikes, but they are not eliminated. Here again, these spikes translate into additional motor heating and inefficiency.
CSI drives are the latest addition to the line-up of AC variable-frequency drives. They are usually used in applications requiring 50 HP or larger. These VFDs are well suited for powering pumps and fans because of the inherent synchronous transfer capability. The cost of a CSI drive may be less than either a VVI or PWM in powering pumps, fans, or similar applications. However, the efficiency of the CSI drive matches that of the DC drive and may not provide a total energy-saving package compared with the PWM drive.

PWM (Pulse-Width Modulated)

As seen earlier, the power conversion principle of this drive is different from that of VVI and CSI. One of the major differences is that of a fixed diode front end, not a controllable SCR front end. This fixed diode bridge
provides a constant DC bus voltage. The DC bus voltage is then filtered and sent to the inverter section. Another difference between PWM and the other types is the operation of the inverter section. The inverter in the PWM drive has a dual purpose—it changes fixed-voltage DC to variable-voltage AC and changes fixed frequency to variable frequency. In the other types, the inverter’s primary purpose is to change the fixed-frequency to a variable-frequency output.
PWM drives use several types of power transistors; IGBTs, and GTOs (gate turn-off—SCRs) are examples. These semiconductors offer the advantages of PWM technology without the expense of commutation circuits. (Commutation circuits are required to turn off the SCRs once they start conducting. They are found in early VVI or CSI units.)
Another major difference is the actual voltage output of the inverter itself. The DC bus voltage is fixed and approximately equal to the RMS value of the drive input voltage (e.g., 460 V x 1.414 = 650 V). By chopping or modulating the DC bus voltage, the average voltage (output voltage) is increased or decreased. The output voltage value is controlled by the length of time the power semiconductors actually conduct. As seen earlier, the longer the on time for the semiconductors, the higher the output voltage. The longer the off times occur in the process, the lower the frequency output. Thus the inverter accomplishes both variable voltage and frequency. Figure 4-34 shows a block diagram of a PWM drive.
PWM block diagram
Figure 4-34. PWM block diagram
Some of the advantages of a PWM drive include high efficiency, the capability of optional common bus regeneration (operating several inverter sections off of one DC bus), and a wide controllable speed range (in some cases up to 200:1, with no low speed cogging under 20 Hz operation).
The PWM drive offers other advantages, such as power loss ride-through capability, open circuit protection, and constant input power factor. This is due to the fixed diode front end and DC bus inductor. Constant power factor is not seen by CSI, VVI, or DC drives.
Like the VVI drive, the PWM also allows multi-motor operation (within the current capability of the drive). However, there are a few limitations.
Extra hardware is required for line regenerative capability (discussed later in this topic). Also, the regulator is more complex than a VVI. However, microprocessor control has nearly eliminated significant economic differences between the two drives.
As mentioned, low-speed cogging is not an issue with PWM drives. This is illustrated in Figure 4-35.
PWM voltage and current waveforms
Figure 4-35. PWM voltage and current waveforms
Figure 4-35 has been seen before. Of particular interest is the fact that there is no line notching or spikes developed, thanks to the diode front end. The voltage waveform, which could be superimposed on the modulations, very closely approximates the sine wave voltage required by the motor. If the carrier frequency is high (8-16 kHz), the quality of low-speed operation is improved. The higher the carrier frequency, the smoother the motor operation. (Remember—carrier frequency is the speed at which the power semiconductors are switched on and off.)
Another benefit of high carrier frequencies is that of reduced audible noise. The higher the frequency, the less motor noise is generated. Audible motor noise can be an issue with low switching rates (e.g., 1-3 kHz). The current waveform, though it contains some ripple, is the smoothest of the three types of drives. It closely approximates the AC sine wave. The efficiency is therefore very high with little motor heating.
Continued improvements in drive technology have enabled PWM drives to deliver a response almost equal to that of DC servos. High response applications such as machine tools and robots require very precise control of motor speed and torque. PWM flux vector drives provide this type of capability and are covered later in this topic.

AC Drives — Braking Methods

Braking methods of DC motors has already been reviewed earlier in this topic. In this section, attention will be given to AC drive braking methods, which, for the most part, are similar to DC drive braking methods,
with a few exceptions. Figure 4-36 is a review of the stopping methods of an AC motor, with a minor variation.
Braking methods for AC drives
Figure 4-36. Braking methods for AC drives
As indicated, the easiest way of bringing an AC motor to a stop is the simple method of coast-to-stop. This is followed by the next fastest means, called ramp-to-stop.
During this method, the drive actually forces the motor down to a stop by systematically reducing the frequency and voltage. This is done in a deceleration ramp format, which is set through a parameter in the drive. It should be noted that the motor will contain energy or inertia that must be dissipated—in this case voltage. The DC bus circuit will have to absorb the back fed voltage. When this happens, the DC bus voltage rises—possibly to a point of a voltage trip (called over-voltage fault or DC bus fault). A typical drive will automatically protect itself by shutting down at ~135% of nominal DC bus value. (For example, a 460-VAC drive will carry ~650 VDC on the bus. The trip point would be ~878 VDC.)
The DC bus of a typical AC drive will take on as much voltage as possible without tripping. If an over-voltage trip occurs, the operator has three choices—increase the deceleration time, add DC injection braking, or add an external dynamic braking package. If the deceleration time is extended, the DC bus has more time to dissipate the energy and stay below the trip point. This may be a trial-and-error approach (keep setting the deceleration time until the drive does not trip). A few of the recent drives offered on the market automatically extend the deceleration time, without an operator having to do so. If 30 s is a deceleration time and the drive stops the motor in 45 s, the motor cannot be stopped in 30 s without DC injection braking or external hardware (e.g., dynamic braking). If a 30-second stop is required by the application, DC injection braking is a possibility.

DC Injection Braking

As the name implies, during this braking process, DC voltage is “injected” into the stator windings for a preset period of time. In doing so, a definite north and south pole is set up in the stator, causing the same type of magnetic field in the rotor. Braking torque (counter torque) is the action that results, bringing the motor to a quicker stop, compared with ramp. The rotor and stator dissipate the energy within itself through heat. This method of braking is usually used in lightly loaded applications, where braking is not often used. Repetitive operation of injection braking can cause excessive heat buildup, especially in high-inertia applications, such as flywheels or centrifuges. Excessive heat can cause permanent damage to the stator windings and rotor core.

Dynamic Braking

If DC injection braking cannot bring the motor to a stop in the required time, then dynamic braking will need to be added. Figure 4-37 indicates a typical dynamic braking system for an AC drive.
 AC drive dynamic braking
Figure 4-37. AC drive dynamic braking
This form of stopping uses a fixed, high-wattage resistor (or bank of resistors) to transform the rotating energy into heat. When the motor is going faster than commanded speed, the energy is fed back to the DC bus. Once the bus level increases to a predetermined point, the chopper module activates and the excess voltage is transferred to the DB resistor. The chopper is basically a sensor and is constructed of a transistor or IGBT switch device. The DB resistor is not mounted within the drive box or inside a drive cabinet. It is always mounted in an area where the heat developed cannot interfere with the heat dissipation requirements of the drive.
As previous indicated, the main stopping power of a DB system occurs when the resistor is cold, during the first few seconds of the process. Once the resistor heats up, the amount of braking torque diminishes. The num-
ber of times per minute DB is engaged will also determine the effectiveness of braking torque. Duty cycle, as it is called, is the number of times per minute the DB resistor is used. Many DB circuits consider a maximum of 10% duty cycle (6 s on, 54 s off-time to cool).

Regenerative Braking (Four Quadrant)

The process of regenerative braking has already been discussed. However, this type of braking uses different components in the AC drive, compared with DC. The end result is still the same—generation of voltage back to the AC line in synchronization with utility power.
To accomplish this, a second set of reverse-connected power semiconductors is required. Some AC drives use two sets of fully controlled SCRs in the input converter section. The latest AC regenerative drives use two sets of IGBTs in the converter section (some manufacturers term this an active front end).
The reverse set of power components allows the drive to conduct current in the opposite direction (taking the motor’s energy, and generating it back to the line). Figure 4-38 indicates a block diagram of a regenerative braking (four quadrant) system.
Regenerative AC drive (two IGBT bridges)
Figure 4-38. Regenerative AC drive (two IGBT bridges)
As expected with a four-quadrant system, this unit allows driving the motor in the forward and reverse directions, as well as regeneration in both the forward and reverse directions. The control board contains the microprocessor that controls the status of the forward and reverse IGBT bridges. When the speed of the motor is faster than commanded, the motor’s energy is fed back into the DC bus. The regeneration circuit senses the increase in reverse voltage and turns on the reverse IGBT bridge circuit.
In this method, the reverse IGBTs need to be able to conduct in the reverse direction. Therefore, if power is removed from the drive, the microprocessor and the reverse IGBTs would not operate. Therefore, this method would not be used for emergency stop situations. However, one method of working around this issue is to include brake resistor and chopper across the DC bus. This provides the best of both worlds, regeneration and e-stop capability.

Drives (AC)—Torque Control

Up until now, standard PWM voltage-controlled drives have been discussed. In this type of drive, the voltage and frequency applied are the controlling variable, when talking about motor torque produced. Torque produced is actually a product of the amount of slip in the motor. The motor has to have a certain amount of slip present to produce torque. As the motor load increases, slip increases and so does torque. This type of AC drive is termed a volts per hertz drive, primarily because of the two controlling elements—volts and hertz. It is also given the label of a scalar drive.
The drive technology of today has moved beyond a “motor turner” philosophy. Drive systems of today need to accurately control the torque of an AC induction motor. Controlled torque is required by automation systems such as wind-unwind stands, process-control equipment, coating lines, printing, packaging lines, hoists and elevators, extruders, and any place where standard motor slip cannot be tolerated (typically 3-5%). Enter the realm of controlled-slip drives—called flux vector or simply, vector drives.

Flux Vector Drives

One of the basic principles of a flux vector drive is to simulate the torque produced by a DC motor. As indicated in the DC drive section, one of the major advantages of DC Drives, and now Flux Vector Drives – is full torque at zero speed. Up until the advent of flux vector drives, slip had to occur for motor torque to be developed. Depending on motor design, 30-50 rpm of slip might be needed for torque to be developed. An output frequency of 3-7 Hz may be needed from the drive before the motor actually starts turning. With flux vector control, the drive forces the motor to generate torque at zero speed.
A flux vector drive features field-oriented control—similar to that of a DC drive where the shunt field windings continuously have flux, even at zero speed. The motor’s electrical characteristics are simulated in the drive controller circuitry called a motor model. The motor model takes a mental impression of the motor’s flux, voltage, and current requirements for every degree of shaft rotation. Due to the way the drive gathers information for the motor model, it would be termed a closed loop drive. Torque is indirectly controlled by the creation of frequency and voltage on the basis of values determined by a feedback device. Figure 4-39 indicates a block diagram of a closed loop, flux vector-controlled AC drive.
To emulate the magnetic operating conditions of a DC motor, that is, to perform the field orientation process, the flux vector drive needs to know the spatial angular position of the rotor flux inside the AC induction motor. With flux vector PWM drives, field orientation is achieved by electronic means rather than the mechanical commutator and brush assembly of the DC motor.
During field orientation, information about the rotor status is obtained by feeding back rotor speed and angular position. This feedback is relative to
Closed loop flux vector AC drive (block diagram)
Figure 4-39. Closed loop flux vector AC drive (block diagram)
the stator field and accomplished by means of a pulse encoder. A drive that uses speed encoders is referred to as a closed-loop drive. In addition, the motor’s electrical characteristics are mathematically modeled with microprocessors, processing the data. The electronic controller of a flux vector drive creates electrical quantities such as voltage, current, and frequency. These quantities are the controlling variables, which are fed through the modulator and then to the AC induction motor. Torque, therefore, is controlled indirectly.
The advantages of this type of drive include good torque response (<10 ms). Some manufacturers consider this response as the limiting response of standard AC induction motors because of the inherent inertia of the machine. Other advantages include full torque at zero speed (at ~0.5 Hz output).
Note: Special caution must be taken when an “off-the-shelf” motor is used to provide full torque at zero speed. A specialized cooling system may be needed, in addition to the internally mounted fan. This is due to drastically reduced airflow. The motor is developing full torque and increased heat buildup.
Accurate speed control is possible because of the pulse tachometer feedback. This speed control approaches the performance of a DC drive. Accurate speed control would be stated as ±5% of rated torque.
Depending on point of view, there may be several drawbacks to this type of control. They may be considered drawbacks when compared with the next version of vector control discussed—sensorless flux vector control).

Sensorless Vector Drives

To achieve a high level of torque response and speed accuracy, a feedback device is normally required. This can be costly and adds complexity to the traditionally simple AC induction motor. Also, a modulator is used, which is a device that simulates the AC sine wave for output to the motor. A modulator, slows down communication between the incoming voltage and frequency signals and the ability of the drive to quickly respond to signal changes.. Although the motor is mechanically simple, the drive is electrically complex. Figure 4-40 indicates a simple sensorless flux vector control scheme, which is achieving increased recognition in recent years.
Sensorless flux vector control block diagram
Figure 4-40. Sensorless flux vector control block diagram
Sensorless flux vector control is similar to a DC drive’s EMF control. In a DC drive, the armature voltage is sensed, and the field voltage is kept at constant strength. With sensorless flux vector control, a modulator is used to vary the strength of the field, which is in reality, the stator.
The role of sensorless flux vector fits generally in between the standard PWM open loop control method and a full flux vector, closed loop control method. As previously indicated, the role of sensorless flux vector control is to achieve “DC-like” performance, without the use of a shaft position feedback device. This method provides higher starting and running torque, as well as smoother shaft rotation at low speed, compared with standard V/Hz PWM drives. Additional DC-like performance comes from benefits such as a wide operating speed range and better motor-speed control during load variations.
As indicated, there are many advantages of sensorless flux vector over standard PWM control, a main advantage being higher starting torque on demand. However, standard sensorless flux vector drives may not accomplish torque regulation or full continuous torque at zero speed, without more complex circuitry. Several drive manufacturers use a software design that estimates rotor and stator flux. The result of the flux calculations (estimations) is current that produces a type of regulated motor torque.
If more accurate torque control is required by the application, even more sophisticated control technology is needed. Though more complex in design, high-speed digital signal processors and advanced micro circuits make the electronics design easier to manage. These newer designs also do not require a feedback device and provide the smooth control of torque, as well as full torque at zero speed.

The Direct Torque Control Method

The idea of vector control without feedback (i.e., open loop control) has been researched for many years. A German doctor, Blaschke, and his colleague Depenbrock published documents in 1971 and 1985 on the theory of field-oriented control in induction machines. The publications also dealt with the theory of direct self control. One manufacturer in particular, ABB Inc., has taken the theory and converted it into a refined hardware and software platform for drive control. The result is similar to an AC sensor-less vector drive, which uses a direct torque control scheme. The theory was documented and tested in lab experiments for more than 30 years. How-
ever, a practical drive ready for manufacture was not possible until the development of application-specific circuitry.
Specific circuits such as the DSP (digital signal processor) and ASIC (application specific integrated circuit) are imbedded in IC (integrated circuit) chips. These chips perform a certain function in the overall production of direct torque control. The controlling variables are motor magnetizing flux and motor torque.
With this type of technology, field orientation is achieved without feedback using advanced motor theory to calculate the motor torque directly and without using modulation.
There is no modulator used in direct torque control and no need for a tachometer or position encoder for speed or position feedback of the motor shaft. Direct torque control uses the fastest digital signal processing hardware available and a more advanced mathematical understanding of how a motor works.
The result is a drive with a torque response that is as much as 10 times faster than any AC or DC drive. The dynamic speed accuracy of these drives are many times better than any open-loop AC drive. It is also comparable with a DC drive that uses feedback. One drives manufacturer indicates this drive is the first universal drive with the capability of performance like either an AC or DC drive. It is basically the first technology to control the induction motor variables of torque and flux.
Figure 4-41 shows a block diagram of direct torque control. It includes the basic building blocks upon which the drive does its calculations, based on a motor model.
The direct torque control (DTC™) method
Figure 4-41. The direct torque control (DTC™) method
The two fundamental sections of direct torque control are the torque control loop and the speed control loop. During drive operation, two output-phase
current values and the DC bus voltage value are monitored, along with the IGBT switch positions. This information is fed to the adaptive motor model. The motor model calculates the motor data on the basis of information it receives during a self-tuning process (motor identification).
During this automatic tuning process, the drive’s motor model gathers information such as stator resistance, mutual inductance, and saturation coefficients, as well as the motor inertia. In many cases, the motor is operated by the drive automatically for a short period of time, to gather the information required.
The output of this motor model is the representation of actual motor torque and stator flux for every calculation of shaft speed. The values of actual torque and actual flux are fed to their respective comparators, where comparisons are performed every 25 |is.
The optimum pulse selector is a fast digital signal processor (DSP) that operates at a 40-MHz speed. Every 25 | s, the inverter IGBTs are sent information for an optimum pulse for obtaining accurate motor torque. The correct IGBT switch combination is determined during every control cycle. Unlike standard PWM control, in this control scheme there is no “predetermined” IGBT switching pattern. The main motor control parameters are updated as much as 40,000 times per second. This high-speed processing brings with it static speed control accuracy of ±0.5% without an encoder. It also means that the drive will respond to changes in motor torque requirements every 2 ms.
The speed controller block consists of a PID controller and a circuit that deals with dynamics of acceleration. The external speed reference signal is compared with the actual speed signal given by the motor model. The resulting error signal is fed to the PID section of the speed controller. The flux reference controller contains circuitry that allows the drive to produce several dynamic motor features. Flux optimization is performing just-in-time IGBT switching. This IGBT switching method is controlled by a hysteresis block, which controls the switching action—when to switch, for how long to switch, and which IGBT switches are to be used. This reduces the resulting audible noise emitted from the motor and reduces energy consumption. In addition, flux braking is also possible, which is a more efficient form of injection braking.
Field-oriented control is a term commonly used by one manufacturer when describing continuous torque control. Similar to the direct torque control method, an advanced motor reference model acquires motor parameters during actual operation. An auto-tuning procedure determines the motor values to be used in the motor reference model. These voltage values are fed back to an adaptive software control block, which controls output current, thereby controlling torque. The proper amount of slip is provided, thereby maintaining field orientation (precise stator flux control).

Drives (AC)—Technical Concerns

SCR and GTO control of AC drive power structures have been around since the 1960s. Forced commutated SCR PWM drives gained increased acceptance in the mid-1970s. This was followed by GTO and bipolar transistor-based PWM drives in the mid-1980s. In the late-1980s, IGBT PWM drives were emerging as the drive to take the variable-speed industry into the 21st century. By the early 1990s, several manufacturers were promoting a full-line of IGBT based AC drives for the industrial, as well as HVAC, marketplace. With these AC drive offerings came several advantages and some challenges. Figure 4-42 illustrates to one of the advantages and challenges.
Voltage reflection/standing waves
Figure 4-42. Voltage reflection/standing waves
As seen in Figure 4-42, as the technology era of power semiconductor devices changed, so did the number of circuit boards needed to support that technology. In the 1960s and 1970s, SCRs and GTOs needed more than a dozen circuit boards to support the gating of the power device. Given the fact that each board had a retail value of $500-$1000, it is easy to understand the high cost of AC drives in that era. Separate gate driver boards were needed for each SCR or GTO device to turn off the device and control timing circuitry. In relative terms, the device turn-on time was rather slow, compared with the other emerging technologies. With slower turn-on or switch times, the drives caused audible noise in the motor of 500-1000 Hz, quite a noticeable level. The laminations in the stator winding vibrated at the switch frequency, producing the noise much like an audio speaker.
With the advent of bipolar transistors came the requirement for fewer boards. Less sophisticated control circuitry was required since there was no need for separate gate driver boards. Fewer circuit boards meant less overall cost for the drive. In addition, the relative size of the drive was
reduced, compared with SCR-based products. On the positive side, the bipolar transistor switched 3-6 times faster than SCRs or GTOs (in the 1-to 3-kHz range). This meant that the audible noise was also reduced to a more tolerable level.
When IGBT technology emerged in the early 1990s, it was considered the power technology of the future. The device switched over 10 times faster than bipolar transistors (3-12 kHz), which meant a drastic reduction in audible noise. The circuit board count was reduced to two. The control board contained all the circuits for timing and signal processing. The motor control board contained all the circuits to turn on and off the device. With only two circuit boards needed, the drives industry realized the lowest cost drive possible. There was also a reduction in size to about 1/3 that of bipolar transistor drives. With this technology advancement came an acute challenge for the device connected to the drive—the motor.
With the extremely fast switching times, came the rise of a phenomenon called voltage reflection. Voltage reflection is caused by the fast-rising voltage waveform versus unit of time. In essence, the IGBT turns on immediately compared with 30 times longer with other devices. When a drive switches at this high rate, a reflected wave back from the motor adds to the voltage leaving the output of the drive. The result is a voltage at the motor terminals greater than the original voltage output from the drive. This is illustrated in Figure 4-43.
Voltage reflection characteristics
Figure 4-43. Voltage reflection characteristics
As seen in Figure 4-43, this situation is more of an issue when an impedance mismatch exists between the drive output/motor cables and the motor terminals. The phenomena is similar to the standing wave ratio (SWR) that exists in citizens band (CB) radio antenna setups. The coil of the CB antenna must be installed and tuned correctly, so that there are no waves reflected back to the transmitter, which could cause damage. If tuned properly, the antenna absorbs all the energy the transmitter can deliver.
The amount of increased voltage at the motor terminals is a function of the drive output voltage, length of motor cable, and the amount of mismatch. This situation is a possibility more often in smaller motors, which
have a higher impedance compared with motors in the 150-HP range or more. In some cases, it is not uncommon to see more than twice the drive output voltage at the motor terminals. Many 460-V motor insulation systems are not designed to handle that type of spike voltage. The motor voltage spike issue can be seen in Figure 4-44.
Motor terminal voltage
Figure 4-44. Motor terminal voltage
The spike voltages created in this particular case are close to 1500 V (460 V drive). Because AC, IGBT drives are installed at significant distances away from the motor, the impedance mismatch can be present for various types and brand names of motors. For example, it has been determined that a typical IGBT drive would cause twice the output voltage at the motor terminals. This would be true if the motor is installed greater than 75 feet away from the drive.
There are several possibilities in protecting motors against damage or coping with the issue. On new drive installations, verify that motors installed a significant distance away from the drive meet NEMA MG1, part standards. These motors are designed with insulation systems that are able to handle the over-voltage stress. Figure 4-45 indicates the construction of a random wound versus a form wound motor. The concentric wound or form wound motor is designed to handle spike voltages generated.
Random wound vs. concentric wound motors
Figure 4-45. Random wound vs. concentric wound motors
Motors that meet the MG1 standard are concentric wound and are termed inverter duty motors. These motors contain stator windings that are carefully formed around the stator slots, so that the first winding turn is not next to the last winding turn. Voltage spikes poke minute holes in the insulation. When that occurs in a random wound motor, the likelihood is that the first and last turn are next to each other. A voltage spike hole would therefore short out the winding and make the motor useless until rewound. Inverter duty motors also have extra slot paper insulation separating the windings of different phases. In addition, these motors are typically dipped in lacquer insulation after the windings are complete to add to the insulation strength and cover insulation holes that may have occurred. Some inverter duty motors are actually dipped a second time to improve the dielectric (insulation) strength.
Another means of protecting the motor against possible damage is to install output reactors, (similar to line reactors) at the output of the drive. The drive manufacturer can make recommendations. Usually 1.5-3% impedance will protect existing motors to about 500 feet. If distances greater than 500 feet are encountered, dv/dt filters can be installed at the output of the drive. These filters are usually effective up to distances of about 2000 feet. (Note: dv/dt means change of voltage vs. change in time.) This is a special resistor-inductor-capacitor filter designed to drastically reduce the over-voltage spikes at the drive output.
Additional precautions include installing a sine filter at the output of the drive, which is not limited to motor distance. In addition, a snubber circuit installed at the motor will have over-voltage reduction similar to dv/dt filters. Snubber circuits do not usually have any distance limitation. Figure 4-46 shows a reduction in spike voltage generation with the installation of an output dv/dt filter.
Effects of a dv/dt filter on voltage reflection
Figure 4-46. Effects of a dv/dt filter on voltage reflection
At long motor cable lengths (e.g., 250 to 300 ft or more), another phenomenon can occur – that of capacitive coupling. Conductors separated by an insulator make up a capacitor. With additional capacitance at the VFD output, higher current is calculated by the VFD. To the VFD, the motor will appear to consume increased amounts of current, which can cause nuisance “overcurrent” trips. This condition can be exaggerated by higher IGBT switch frequencies and the lack of output snubber circuits.
In most cases, the motor is consuming appropriate current, but the motor cabling causes inappropriate readings by the VFD. Typically, an output reactor, as previously indicated, can serve a dual purpose—protect motor insulation from damage, and improve the VFD current calculations. This ultimately results in more stable VFD performance with less nuisance faults. Some manufacturers include hidden parameters to assist in tuning up the VFD current sensing circuit.

Harmonics Generation

Harmonics are basically a distortion of the original waveform. In the case of AC drives, harmonics are a distortion of the three-phase waveform, with the harmonic components fed back onto the AC power line. Harmonics are caused by the fact that AC drives draw current from the supply line in “bursts” rather than in a “linear” fashion. Because of this characteristic, AC drives are considered nonlinear loads. As a matter of fact, any electronic device that draws nonlinear currents causes a certain amount of harmonics.
VCRs, big screen TVs, stereos, and laptop and desktop computers all fall into the nonlinear load category. They include a switch mode power supply that changes AC to DC—a rectifier. Because rectifiers draw current in bursts, they create harmonic currents, which are fed back to the power source.
A six diode bridge AC drive (called a 6 pulse drive) produces the 5th, 7th, 11th, and 13th harmonic. The values of these harmonics is enough to distort the AC supply waveform. An example of distortion created by the 5th and 7th harmonic is illustrated in Figure 4-47.
Note: The fundamental frequency is 60 Hz. Harmonic frequencies are multiples of the 60-Hz waveform (e.g., 5th = 300 Hz, 7th = 420 Hz, and so on).
Users of AC drives need to be concerned about harmonics, which are generated back to the line supply. Harmonic currents do not provide any useful work. Harmonic current distortion generates additional heating in transformers and cables, reducing the available capacity of the equipment. Current distortion can also create resonance conditions between the line supply reactance and power factor correction capacitors (if used). In addition, the high frequencies of harmonics can cause electronic interference with telephone and telecommunications equipment.
Distortion caused by the 5th and 7th harmonic
Figure 4-47. Distortion caused by the 5th and 7th harmonic
Harmonic voltage distortion causes increased heating in motors. Voltage distortion can also cause malfunctions in sensitive communications and computer equipment. In many areas, local electrical codes or drive-installation specifications require compliance with IEEE 519-1992. The locations where voltage and current harmonics can be an issue are termed the point of common coupling or simply, PCC.
The local power utility deals with the PCC, where the customer’s building connects directly with utility power. This is the current harmonic distortion concern and is termed the total demand distortion or simply, TDD. The utility customer is faced with the voltage harmonic distortion concern, where nonlinear loads meet other loads, such as linear loads (inductors, line operated motors). Figure 4-48 indicates the locations of concern for both harmonic current and voltage distortion.
Overall, harmonics are a system issue. Harmonics that are produced by an individual drive are only important when they represent a significant portion of the total system. For example, if the drive load on a transformer represents over 1/4th of the kVA load, then harmonics could be an issue and requires further investigation. If the total drive load is 5 HP on a 1000-kVA transformer, harmonics would not be an issue. It is worth noting that the addition of linear loads, such as line-operated motors, tend to reduce the overall system harmonic levels.
IEEE 519-1992 indicates limits of THD (voltage distortion) as 5% for general systems (e.g., factories and general office buildings, not including hospitals, airports, and power systems dedicated to drive loads). Current distortion limits (TDD) are based on a ratio. The ratio is the short-circuit current available at the PCC divided by the maximum fundamental load current. Therefore the limits will vary on the basis of the amount of the
PCC and harmonic distortion
Figure 4-48. PCC and harmonic distortion
electrical current tank available. If the current capacity ratio is high, the allowable TDD will be high, compared with a low ratio. (Example: If a 10-lb rock is dropped in a bathtub full of water, the waves created represent the current harmonics generated—quite a significant amount. If that same rock is dropped off the Golden Gate bridge in San Francisco, the amount of waves hitting the shoreline would be almost non-existent—no significant current harmonics generated.)
It is important to note that an 80% THD nonlinear load will result in only a 8% TDD if the non-linear load is 10% of the total system load. With that in mind, there are several ways of reducing (mitigating) harmonics.
Using the above 80% THD example, the following comparisons could be drawn. Line reactors could be added to the input of the drive. A 5% line reactor (or equivalent DC bus inductor) could drop the THD from 80% down to 28%. Adding a 5th harmonic trap filter to the line reactor could drop the THD down to 13%. The cost of a line reactor may be 15-25% the cost of the drive (depending on drive horsepower). A harmonic trap filter could add 25-50% the cost of the drive (depending on drive horsepower).
Beyond these techniques, more serious mitigation could be realized, including higher associated costs. A 12-pulse drive input rectifier could be installed, along with a 5% impedance transformer. (Note: A 12-pulse drive is two six-diode bridge rectifiers, with a special delta-delta-wye input transformer, which could be 1/2 the cost of the drive unit itself. A 12-pulse drive effectively reduces the 5th and 7th harmonic. The total cost of the drive and transformer could be about double that of a six-pulse drive, depending on horsepower.) With this configuration, the THD could be reduced to 8%. With a 12-pulse drive, a 5% impedance transformer and
an 11th harmonic trap filter, a reduction of THD down to 4% could be realized. Installing an active harmonic filter would reduce the THD down to
An active harmonic filter is essentially a regenerative drive. As stated above, this type of drive would yield the highest amount of harmonic mitigation of all the techniques. It could also be over twice the cost of a standard 6-pulse drive.
When dealing with harmonics, it is helpful to work with the drive’s manufacturer or a company specializing in harmonic mitigation techniques. Some drive manufacturers offer a “no-cost” analysis service—submitting a harmonics report on the basis of the installation of their drive in a specific system. Harmonics will be even more of an issue in the future, with VFD’s being applied in applications traditionally deemed fixed speed. The positive side to harmonics is that cost-effective techniques are available for a wide variety of installations.

Power Factor

When discussing electronic power conversion equipment, there are two ways to identify power factor: displacement and total or true power factor. Displacement PF is the power factor of the fundamental components of the input line voltage and current. Total PF indicates the effects of harmonic distortion in the current waveform. No matter how PF is viewed, the power utility imposes penalties for customers that use equipment with a poor PF.
Displacement PF for an AC drive is relatively constant. It is approximately 0.96-0.97. This value is primarily independent of the speed of the motor and its output power. The harmonic current distortion is determined by the total values of inductance, capacitance, and resistance, from power source to the load. The power distribution system has impedance, which also enters into the calculations for total PF. Figure 4-49 gives a general indication of displacement and total PF for AC as well as DC drives.
The curves shown in Figure 4-49 are for drives at full load and constant torque operation. The shaded area indicates a variation of total PF for typical drive installations. For an AC drive, the total (true) PF varies from roughly 0.94 at rated load, down to below 0.75 when under a light load.
As you may recall, a lagging PF is seen for an AC induction motor used in a power system. The AC drive does an effective job in isolating the input power source from the lagging PF at which the motor operates. In a certain sense, AC drives could be considered PF correctors by means of its isolation from the AC line. Because of this fact, PF correction is not normally applied to AC, PWM drives. If existing PF capacitors are connected of the motor, they must be removed when an AC drive is installed in place of a full-voltage starter. PF capacitors between the drive output and the motor can cause physical damage to the drive output IGBTs.
Total and displacement PF (DC and AC drives)
Figure 4-49. Total and displacement PF (DC and AC drives)

Shielding and Grounding

Nearly all of the procedures related to DC drives shielding and grounding apply to AC drives. However, there are some additional guidelines that must be followed regarding AC installations.
AC, PWM drives tend to expose AC induction motors to high levels of common mode voltages. Common mode voltages are created at the neutral zero point of the output of a three-phase AC drive. This is due to the creation of three-phase AC, generated from a DC bus. Also at issue is the fact that a high level of dv/dt is also possible because of the IGBT power structure. ( Note: dv/dt is defined as delta voltage per delta time, that is, change of voltage per change of time.) Common mode voltages can become apparent in several ways.
Damaging high-frequency-bearing currents can occur, causing damage inside the bearing inner race. The phenomena of bearing currents have been around for years. The incidence of motor damage, however, has increased during recent years, after the introduction of IGBTs. This is due to the rising voltage pulses and high switching frequencies that are created by the IGBT power structure. These voltage pulses and frequencies can cause repeated discharging through the bearings and result in a gradual erosion of the bearing inner race. Figure 4-50 indicates this discharge path through the bearings.
High-frequency current pulses are generated through the motor bearings. If the energy of the pulses is high enough, metal transfers from the ball bearing and the races to the lubricant. This process is known as electrical discharge machining (EDM). Because of the high-frequency pulses, thousands of these machining pits are created, which translate to metal erosion that can accumulate quickly. Figure 4-51 indicates the inner race damage caused by bearing currents.
Bearing currents discharge path
Figure 4-50. Bearing currents discharge path
Figure 4-51. Bearing “fluting” caused by bearing currents
Bearing "fluting" caused by bearing currents
In addition to bearing erosion, high-frequency ground currents can lead to malfunctions in sensitive sensor and instrumentation equipment.
To avoid these damaging bearing currents, a high-frequency, low-impedance path to ground must be provided between the drive and the motor. This is accomplished by installing continuous corrugated aluminum armored cable, shielded power cable, and a properly installed conduit system. Figure 4-52 indicates cabling, shielding, and grounding that provides
low impedance paths to avoid high frequency and bearing current damage.
Motor cabling, shielding, and grounding
Figure 4-52. Motor cabling, shielding, and grounding
Recommended motor cable construction is described in Figure 4-53. It consists of aluminum armor that provides a low-impedance, high-frequency return path to ground.
Recommended motor cable construction
Figure 4-53. Recommended motor cable construction
Once the motor cable is installed in the proper location, the termination of the cable is vitally important. Figure 4-54 indicates the recommended termination method for AC drives.
Recommended motor cable termination method
Figure 4-54. Recommended motor cable termination method
The proper termination method includes 360° contact with the corrugated armor and grounding bushings for the connection of safety grounds. Metal-to-metal contact with the mounting surface is extremely important in the installation.
Following these guidelines will reduce the effects of frequency generation in typical AC drive installations.


AC variable frequency drives generate a certain amount of electromagnetic interference (EMI). The same installation procedures for DC drives apply to AC, with a few additional guidelines. The following would be considered general wiring practices related to AC drives. Many of these items have been presented before, but deserve a review.
Never install motor cables (e.g., 460 VAC) and control wiring (e.g., 4-20 mA or 0-10 VDC) in the same conduit or cable tray. It is also recommended that control wiring be shielded cable (for signals less than 24 V). Even though shielded control cable is used, the PWM output can have adverse effects on the low-power control signals. Unstable operating conditions can develop. It is further recommended that the control wiring be installed in its own individual conduit. In essence, the best installation procedure would be three separate, grounded, metallic conduits: one conduit for input power to the drive, one conduit for output power to the motor, and one conduit for control wiring. If EMI could be a serious issue with the installation, the use of ferrous metallic conduit rather than aluminum can help. The traditional steel conduit contains a certain amount of iron that can provide additional shielding against stray EMI signals.
If control wiring and power wiring are not in separate conduits, then the control wiring must be kept a minimum of 12 inches away from power wiring. The crossing of control and power wiring at 90° angles will reduce the EMI effects if the sets of cables must be close to each other. The previously indicated grounding techniques are also required to reduce EMI. Process control sensors and equipment must also be connected using shielded cable. The shield should be grounded only at the drive end, with the shield at the signal end cut back and taped to avoid contact with ground. This will avoid the possibility of ground loops that could cause


On any installation that is to conform to EMC compliance, the manufacturer’s recommendations must be strictly followed. Their documentation indicates that the required research was conducted. If their instructions are followed, the complete installation will be EMC-compliant. This includes the use of shielded cable and CE-rated equipment, as well as specific cable-termination techniques. Any deviations from their guidelines would void the compliance.
Radio frequency interference (RFI ) can be an irritating problem, just like EMI. RFI could be considered electrical noise and would take the form of conducted or radiated.
Conducted noise is noise that is conducted or reflected back onto the line supply. Since the AC VFD generates a carrier frequency, it could be considered a radio station, with the power input cable being the transmitter antenna. The simple procedure of installing power input and output cable in separate conduits will reduce the possibility of RFI. RFI can have serious effects on other control equipment connected to the same line input, especially if the equipment is frequency controlled (e.g., carrier current lighting, theft control, or security screening systems).
If RFI is not below a federally mandated level, the FCC has the authority to shut down the installation until compliance is proven. In cases where the interference is not reduced by separate input and output power conduit, the installation of RFI filters would be required. RFI filters are designed to reduce a specific frequency that is causing the disturbance to other equipment. The manufacturer of the drive can assist in identifying the proper filter, if required.
Radiated electrical noise is just as implied—the radiation of radio frequencies, much like the conducted noise that eventually radiates from the power input cables. Radiated noise is typically described as the radio frequencies emitting directly from the drive itself, without external connections. Any drive manufacturer that complies with CE ratings will have already designed the drive with radiated noise reduction in mind. The CE compliance standards contain stricter guidelines compared with those of the United States, related to AC, VFD design. The point to keep in mind is that if the drive is designed to meet certain RFI standards, it must be installed, grounded, and shielded per the manufacturer’s requirements. Otherwise, it would be in violation of the standards and would be subject to penalties or shut down until remedies are installed.

Drives (AC) — Innovations and Technology Improvements

AC drives have been sold in increasing numbers during recent years. Up until the 1950s and early 1960s, the reliability of some AC drives was questionable, at best. The failure rate of some AC drives, out of the box, was 20% or more. Many drive service technicians always came with a package of spare parts. It was assumed that something would go wrong during startup. Fortunately, those days are behind us as industrial and HVAC drive consumers. The reliability and intelligence of AC drives from all major manufacturers has increased dramatically over the past 30 years. AC drives are fast becoming more and more a commodity item for simple applications like pumps, fans, and conveyors. The more complex AC drive applications are now accomplished with AC drives, but with modifications in drive software—which some manufacturers call “firmware.”
The AC drive, with its high-speed microprocessor and PLC compatibility, is viewed as a critical piece of the automation system. The ability to connect to a variety of industrial networks makes the AC drive a viable variable-speed choice for years to come. The sizes and shapes of AC drives have been reduced over recent years. On the other hand, the power density (per square inch of chassis space) has increased dramatically. IGBT technology and high-speed application chips and processors have made the AC drive a true competitor to that of the traditional DC drive system. In this age of efficiency and network control, AC drives have emerged as a prominent choice. The emphasis is now on less motor maintenance and increased energy savings, with the ability to communicate to a variety of network systems. With these benefits in mind, the AC drive will continue to gain acceptance in applications that have been traditionally fixed speed or DC variable speed.
To make intelligent applications choices, it is necessary to review the improvements made in AC drive technology. The following sections will outline, in general and specific terms, improvements in AC drive design and control. In certain cases, AC drive improvements have paralleled that
of DC drives. This section is meant to generate ideas as to where AC drives and motors can be applied, with little additional equipment required.

Compact Package Design

The AC drive systems of today include all of the needed components to operate, troubleshoot, and maintain the system. Because of the use of microprocessors and IGBTs, a 1-HP drive of today is about 1/3 the size of a 1-HP drive 10 years ago. This size reduction is also attributed to the surface mount technology used to assemble components to circuit boards. Twice as many components are possible on one board because of the placement of components on both sides of the board. The entire unit can be operational with three wires in (power input) and three wires out (power output). An additional set of wires would also be needed if optional external control is used.
AC drive designs of today are more compact with little documentation required to troubleshoot (many diagnostic features are visible in software), and have less parts that require replacement. In most cases, packaged AC drives of approximately 50 HP or less use only two circuit boards—control board and motor control board. In short, AC drives of today are more reliable than their ancestors of 30 years ago. Figure 4-55 illustrates this type of package design.
Standard AC drive package
Figure 4-55. Standard AC drive package

Digital I/O (Inputs/Outputs)

AC digital drives allow for simple programming and a high degree of application flexibility. The idea is to connect all control and power wiring and set up the drive for the application, through software programming. If the application is altered, the drive functions can quickly be reprogrammed through software, instead of rewiring the drive. The programming is typically done with a removable keypad or remote operator panel. Both AC and DC drives of today share in this technology improvement.
In earlier versions of AC drives, the drive had to be shut down and the control terminal block rewired for the new application. Downtime is costly. The less time a system is shut down, the more productivity is obtained.
Typically, the control wiring section of an AC drive will contain analog inputs (Al’s for speed reference), digital inputs (Dl’s for controls such as start/stop, reverse, etc.), AO’s (analog outputs) (connection for an auxiliary meter), and relay outputs (RO’s for devices such as fault relays, at-speed relays, etc.). The digital inputs and outputs would typically operate on ±10 or 12 VDC or ±24 VDC logic. A software function would operate if the assigned terminal voltage is high (meaning 8 V or higher on a 10-VDC control). Any voltage less than that would indicate a digital logic “low” and the function would not operate.
In addition to the standard start/stop speed reference inputs, the drive would also include I/O for diagnostics such as auxiliary fault, motor overload, and communications status. Many of the drive manufacturers include a section in software called I/O status. This section of software is dedicated to the monitoring and viewing of drive inputs and outputs. When a DI is “high,” it would register as an “I” on the LCD display. If “low,” it would register as a “0.”
Note: High and low are relative terms in digital technology. A “high” would mean that a high control voltage is applied to a circuit. A “low” would indicate that a low control voltage is applied to a circuit. For example, in a 24-V control system, a “high” value might mean 20-24 V. On the other hand, a “low” value might mean 5 V or less.
The same type of display would be seen for analog signals, with a true readout appearing. By viewing the I/O status section of software, it can quickly be determined if the drive has a problem or some interconnection device in the system, outside of the drive.

IGBT Technology

IGBT technology has been successfully applied to AC drives since the late 1980s. Before IGBTs, bipolar transistors and SCRs were the standard output power conversion devices. IGBTs can be turned on and off with a small milliamp signal. Smaller control driver circuits are required because of the smaller control signals needed compared with SCRs. Smaller control circuitry also means a smaller sized control circuit boards, which translates to less cost.

Multi-Language Programming Panel

Many of the programming panels (touch keypads) are removable and may or may not include a panel extension cord. Up until the last decade and a half, programming panels required continuous attachment to the drive control board. Storage of parameter values was a function of the control board and associated memory circuits.
With the latest advancements in E2PROM™s and flash PROMs, the programming panel can be removed from power. Values can now be stored for an indefinite period of time with no batteries required for backup. This type of capability allows for a backup plan in case any or all parameter values are lost because of a drive malfunction or electrical damage due to lightning.
Drive panels are in many cases back-lit, meaning that the LCD digits have an illuminated background that can increase or decrease in intensity. This is helpful when the drive is installed in a brightly or dimly lit room. Many programming panels allow for individual display of several different languages. This is very helpful when the drive is mounted onto a machine and shipped to another country. The programming setup can be accomplished in English, for example, and then the language changed to Spanish before shipment to Mexico. Figure 4-56 indicates a typical programming keypad.
AC drive programming keypad
Figure 4-56. AC drive programming keypad
Additional functions of the modern-day drive panels include “soft keys” similar to that of a cell phone. The function of these keys change depending on the mode of the keypad (e.g., operating, programming, local/ remote, menu, etc.).
The ability of the keypad to guide the user through a multitude of situations is also the trend of current drives. Inherent programs such as a “Start-up Assistant,” guide the user through the required steps to start up a drive for the first time. The “Diagnostic Assistant” aids the user in providing suggestions as to where to correct a fault situation. The “Maintenance
Assistant” can be programmed to alert the user when routine maintenance is suggested (e.g., checking or replacing the heatsink fan).

Programming Macros

Because of the digital design, many of the AC drive parameters are preprogrammed in software before shipment from the factory. During drive startup, the operator need only load motor data values and values to customize the drive to the application. In most cases, an operator can install parameters and start up a drive in a matter of minutes, compared with an hour or two for analog AC drives.
Many AC drive manufacturers use pre-assigned values to each of the parameters, in what would be known as default values. The values would not exactly match the motor and application, but would allow the drive to operate a motor. In addition to defaults, several manufacturers include preprogrammed sets of values, known as macros. Individual macros would allow the operator to match the drive parameters and diagnostics to the motor and application that it is connected to. In many cases, all of the parameters can be set in a matter of seconds, rather than individually set parameters, which could take more than an hour. Macros such as hand-auto, three-wire control, torque control, and PID are available for easy configuration and set up of the drive.
Several drive manufacturers offer a macro or default setup for proportional integral derivative (PID) control. Proportional integral derivative is essentially the automatic control of drive speed by receiving a controlling input such as temperature, pressure, humidity, or tank level. Because of the microprocessor power in today’s AC drives, much of the mathematical functions are now standard features of the drive’s control board. A water treatment application would be a prime candidate for PID control and is illustrated in Figure 4-57.
A simple principle of PID is to keep the error (difference between set point and feedback) at zero. AI1 would be the set point or desired value—in this case 120 ft, which would be changed to voltage or current signal. The drive takes that signal and matches it with the transducer feedback signal (AI2) from inside the tank. This actual level would also be converted to a feedback signal current (milliamp). The PID controller takes the resulting error signal and increases the speed of the well pump to match the demand. When the level in the water reservoir goes down, the drive speeds up.
In the past, PID controllers were separate units, which added $400-$500 to the installation costs. With the latest AC drives, the PID function resides in the software.
Note: Some companies refer to software as “firmware” indicating that it is a software program firmly imbedded, into the drive memory – normally not user changeable.
PID Control in a pumping application
Figure 4-57. PID Control in a pumping application
Several manufacturers include the software logic to engage fixed-speed lag pumps. Alarm circuits wired into the drive would warn an operator or automated system control when the level reached the danger low-level of 90 ft. If that would occur, another fixed-speed lag pump could be engaged to keep up with demand, not handled by the regulated drive pump.
Duplex and tri-plex pumping systems can easily be accomplished with software logic inherent to the drive.

Enhanced Programmability

Even though “pre-programmed” macros are a tremendous time-saver when setting up a drive to match an application, there are instances where a “customized” macro is required. Many drive manufacturers provide a “firmware customization” service for their customers. The innovative manufacturers provide the customer with a means to construct their own specialized macro. The capability of “function block programming” allows the user to reassign many I/O points of the software blocks that create the original firmware. Software blocks, such as the “AND” or “TIMER” block, can be found in PLC programs and high-performance DC drives. Now they have been brought into the programmable functions of the VFD. Figure 4-58 indicates an example of a function block program.

Sensorless Vector as a Standard Industrial Drive

The operation of V/Hz drives has been discussed, as well as the benefits of sensorless vector drives. The use of the sensorless vector drive has increased in industrial applications due to ease of set-up and the require-
Function block programming
Figure 4-58. Function block programming
ment to handle high-starting torques. V/Hz drives require the motor to “slip” in order for torque to be developed. With sensorless vector drives, full-rated motor torque is available at zero speed, with no slip required in the process.

Self-Tuning Speed and Torque Loops

A high-performance AC drive system normally requires tuning, once the standard software parameters are set in the drive. This would especially hold true for vector, flux vector, and DTC™ drives. The fine adjustments allow the drive to match the feedback loop with the speed and torque reference circuits. Through the tuning process, the operator is able to obtain efficient response times when accelerating, decelerating, and changing directions.
Some older AC drives require a manual tuning procedure. The operator must accelerate and decelerate the load, observe the behavior of the system, and make manual adjustments. In some drives, however, this tuning process is done automatically by the drive control circuitry. The response gains and recovery times are preloaded at the factory. When the drive sees dynamic adjustments occurring during commissioning, it adjusts the tuning parameters to match the outcome of a pre-assigned value set. Drive response times as low as 1 to 5 ms are possible with some systems.
Several manufacturers have reduced the guesswork during the process of dynamic speed and torque loop tuning. After the motor data is entered and when prompted by the drive, the operator conducts an identification
run (ID). During this process, the motor is disconnected from the application. This is required so the drive can obtain a complete mental image of the motor characteristics (magnetic properties, hysteresis, thermal time constants, etc.). The drive conducts a 30-60 second program of fast accelerations and decelerations and full energizing of the stator windings to develop the complete mental image. Once the ID is done, the operator can install any standard parameter values required by the application (accel/ decel times, digital and analog inputs and relay outputs).

Serial and Fiber-Optic Communications

Access to digital communications is a must in today’s automated facilities. Drives used before the digital age required hard wiring to the control terminal block. This allowed only remote operation from a distance where control voltage or current loss could be kept to a minimum (maybe 25 feet or less). With today’s serial and parallel mode of information transfer, the AC drive can accept control and speed commands from process equipment several thousand feet away. This allows the AC drive to be integrated into the factory automation environment, where process control equipment is located in a clean, dry control room.
Serial links (three wires plus a shield conductor) are more of the norm today, compared with a decade ago. Control and diagnostic data can be transferred to the upper level control system, at a rate of 100 ms. With only three wires for control connections, the drive “health” and operating statistics can be available at the touch of computer button. The communication speed of a serial link makes it ideal for simple process lines and general coordination of conveyors, where high-speed accuracy is not required.
Fiber-optic communications use long plastic or silica (glass fiber) and an intense light source to transmit data. With optical fiber, thousands of bits of information can be transmitted at a rate of 4 megabaud (4 million bits per second). An entire factory can be wired with high-speed fiber optics with very little, if any, electrical interference. This is due to the high frequency of light waves, as opposed to the lower frequency of a wire conductor serial link. With fiber-optic communications, steel processing, coating lines, and high-speed cut-to-length applications are possible. With small error signals fed back to the speed controller, the drive can immediately respond with a correction. This keeps the quality of the product very high, and the deviation in size very low.
Several drive manufacturers offer serial and fiber-optic software that installs directly to a laptop or desktop computer. With this software installed, all drive parameters are accessible from the stand-alone computer. This makes parameter changes simple and fast. Parameters can be changed in the computer, downloaded to the drive for verification, and saved in the computer as a file or macro. The file can then be easily transferred to other computers or a network or e-mailed to another factory with the same company. Hundreds, even thousands, of macros and file
sets can be saved. The ultimate results are the ability to quickly respond to required changes in drive and application setup.
Serial and fiber-optic communications will be discussed in more detail later in topic 6. Figure 4-59 shows an operator interface scheme with fiber optic, serial, and hardwired connections.
Operator interface scheme (communications)
Figure 4-59. Operator interface scheme (communications)

Field Bus Communications (PLCs)

Some types of communication systems are almost always specified with AC drives sold today. Data links to PLCs (programmable logic controllers) are common in many high-speed systems that process control and feedback information. PLCs provide the mathematical calculations, timing circuits, and software “and/or” logic signals required to process drive, sensor, and switch status.
Several manufacturers of PLCs offer a direct connection to many drive products. Because each PLC uses a specific programming language (usually ladder-logic programming), drive manufacturers are required to build an adapter box. This adapter (sometimes called a field bus module) is used to translate one language to another (called a protocol). Refer to Figure 4-59 for an example. The drive manufacturer installs one internal protocol, and the PLC installs another. Field bus modules allow for a smooth transfer of data to the PLC, and vice versa, with little loss of communication speed.

Drive Configurations

Many drive manufacturers offer out-of-the-box configurations. Several manufacturers offer sensorless vector drives that include the automatic tuning described above. In addition, some manufacturers offer a vector-ready configuration. The circuitry for a vector drive is included in the package, but the customer must add a feedback option to make the performance a reality. Because these products are packaged products, the drive vendor can offer very competitive pricing. Little to no additional optional devices are required to be installed at the job site.
Several manufacturers offer a variation of the standard six-pulse drive. An AC drive that is termed 12-pulse ready offers the optional feature of converting a standard six-pulse drive to a 12-pulse drive. As previously discussed, the 12-pulse drive does an impressive job of reducing the 5th and 7th harmonic content back to the power line. This type of drive includes 12 diodes in the converter section as standard. A delta-delta-wye transformer must be connected to the drive input to make the required phase shift possible. Depending on horsepower, this approach may require a slightly smaller initial investment compared with packaged 12-pulse drives. Active front-end drives that include two sets of IGBTs also make it possible for a packaged approach to harmonic mitigation.
The fact is harmonic reduction is a requirement in more applications today. Twelve-pulse or active front-end drives will increase in their importance, as EMI, RFI, and harmonics issues become more acute in the future.
Many AC drives are seen in centrifugal fan and pump applications. The energy savings realized is of major importance in this age of energy conservation. One of the features of AC drive technology is the ability to bypass the drive, if the drive stopped operating for any reason. This configuration, known as bypass, is used in many applications where the fan or pump must continue operating, even though it is at fixed speed. Figure 4-60 indicates a block diagram of the bypass drive.
Figure 4-60. AC drive bypass unit
 AC drive bypass unit
As seen in Figure 4-60, many bypass AC drives include all the features required for operation in drive or bypass mode. Many manufacturers include a service switch to allow for drive troubleshooting while the bypass contactor is closed—running the motor at full speed. The line reactor is also standard on many packaged units as is line input fusing.
One manufacturer in particular offers electronic bypass circuitry. An electronic circuit board operates all the diagnostics and logic required for an automatic transfer to bypass. Because of the electronic means of bypass control, information can be fed back to the drive and to a building automation system control. This would not be possible if the overload element was just a mechanical device. Light-emitting diodes are also a part of the bypass control board, allowing instant and clear indication of drive operation, bypass operation, and the run status. Building safety indications and run enable signals are also indicated with this type of bypass system.

Features/Software Enhancements

Additional features and innovations include a wide voltage input tolerance. Many manufacturers specify their drive as a 460-V drive, ±10% input voltage. However, several manufacturers offer a low and high value as part of the range of operation. The range of a 460-V drive may have input parameter values of 440, 460, 480, and 500 V, ±10%. This means that the drive input voltage could drop as low as 396 V or increase as high as 550 V. With this wide range of operation, the drive will continue to run and not trip offline because of a slight power dip or short-duration brownout condition.
Along with this circuit, many manufacturers offer the capability of “power loss ride through.” This circuit is standard from almost all drive manufacturers, but efficient handling of the power loss is not. One manufacturer, in particular, uses the regenerated voltage from the motor inertia to backfeed the DC bus voltage. Figure 4-61 indicates the effects of one such design.
Power loss ride-through
Figure 4-61. Power loss ride-through
The DC link (bus) voltage will drop slightly in response to the loss of supply voltage. When supply voltage is cut off, the drive control automatically reduces the speed reference command. The motor acts as a generator since, for a short period of time, it is rotating faster than the speed reference. The following example may illustrate this type of circuit.
A drive is set for a 60-Hz speed reference. The motor spins at the 60-Hz commanded speed. The building suffers a power outage. The drive immediately reduces the speed reference to 59 Hz. For a short period, the motor continues to spin at 60 Hz, which now causes regenerative voltage (the motor acts as a generator). The excess energy is fed back to the drive DC bus, and the microprocessor continues to operate. Eventually, the motor will coast down to 59 Hz.
Power has not returned to the building. The drive then responds with a speed reference of 58 Hz. The motor is spinning at 59 Hz, which causes regenerative voltage to be pumped back to the DC bus. The motor eventually coasts down to 58 Hz. If input power doesn’t return, this same scenario is repeated until the drive DC bus drops below a minimum level (typically 65% of nominal value). At that point, the drive would shut down due to lack of DC bus voltage.
If the amount of motor inertia is high (as with a fan or flywheel), the “ride-through” time may be several seconds to over several minutes. This circuit has an advantage where short-duration power outages are common (and the application can tolerate automatic speed reduction while the drive stays operational).
Critical frequency or skip frequency is another circuit offered by many manufacturers. A critical frequency is a frequency that can cause severe mechanical vibration if the drive operates the application continuously at that speed. HVAC system cooling towers, some pumping applications, and certain machines have critical frequencies. Figure 4-62 shows this type of circuit.
Critical frequency circuit
Figure 4-62. Critical frequency circuit
As an example, a critical frequency may appear at 32-34 Hz. When programmed, the drive would continue to output 32 Hz until there was enough speed reference to cause the drive to output 34 Hz. The drive would pass through that range, but the operator could not unknowingly set the drive to continuously operate at 33 Hz. When the operating speed was above 34 Hz, the critical frequency circuit would operate the same, only in reverse. The drive output would stay at 34 Hz until the speed reference was decreased below 32 Hz. This circuit causes less stress for mechanical equipment and is easily programmed in drives on the market today.

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