ADJUSTABLE-SPEED SYSTEMS (Electric Motor)

7.2
Many types of adjustable-speed systems are available. Some of the more popular types of adjustable-speed drives are the following: multispeed motors, adjustable-speed pulley systems, mechanical adjustable-speed systems, eddy current adjustable-speed drives, fluid drives, DC adjustable-speed systems, AC variable-frequency systems, and wound-rotor motors.
The selection of the most effective system for a specific application depends on a number of factors:
Life-cycle cost First cost
Duty cycle and horsepower range Energy consumption Control features required Size
Performance
Reliability
Maintenance
To assist in the selection of an adjustable-speed drive system, let us examine the characteristics of the more popular ones. The DC adjustable-speed systems have been specifically excluded from this section since their characteristics and application technology are well known to those who apply them.
7.2.1

Multispeed Motors

As discussed earlier, multispeed motors can be obtained with the following output characteristics:
Constant horsepower Constant torque Variable torque
However, in conventional multispeed motors, only a limited variety of speed combinations is available. One-winding, two-speed motors are available with 2-to-1 speed combinations such as
1750 rpm/850 rpm 1150 rpm/575 rpm
Two-winding, two-speed motors are available with speed combinations other than 2 to 1. Typical speed combinations are
1750 rpm/1150 rpm 1750 rpm/850 rpm
1750 rpm/575 rpm
1150 rpm/850 rpm
Thus, there are more combinations of speed ratios available in the two-winding, two-speed motors.
In addition, two-winding, four-speed motors are also available. A typical speed combinations is 1750 rpm/1150 rpm/850 rpm/575 rpm.
Since the power requirements for many fans and centrifugal pumps are a cube of the speed, the variable-torque multispeed motor can be used for two-step speed control, i.e., a high-speed and a low-speed operation. With a one-winding, two-speed motor, the output of the fan or pump on low speed will be 50% of the output on high speed, and the horsepower required will be 12.5% of high speed. Figure 1.7 shows a fan load curve superimposed on the speed-torque curves for a variable-torque multispeed motor. In the case of a two-winding, two-speed motor with a combination of 1750 rpm/1150 rpm, the output of the fan or pump on low speed will be 67% of the output on high speed, and the horsepower required will be 30% of high speed. This is illustrated by Fig. 1.8, which shows a fan load curve superimposed on the motor speed-torque curves. Given the speed limitations of this type of drive, it is an economical and reliable method to obtain incremental flow control.
Variable-speed pulley.
FIGURE 7.1 Variable-speed pulley. (Courtesy T. B. Wood’s Sons Company, Chambersburg, PA.)
7.2.2


Adjustable-Speed Pulley Systems

An adjustable-speed pulley system consists of the electric motor mounted on a special base, an adjustable-speed sheave on the motor shaft, and a fixed-diameter sheave on the load shaft connected by a V belt. Figure 7.1 shows the construction of one of the variable-speed sheaves, and Fig. 7.2 shows the special base required for the drive motor.
Figure 7.3 illustrates the cross-section construction of a double-side spring sheave, showing the maximum and minimum drive-belt locations, and Fig. 7.4 shows the cross-section construction of a single-side spring sheave.
The double-side spring sheave is usually recommended for the integral-horsepower V-belt drives. The V belt in the spring-loaded sheave changes its diametric position as the base is adjusted, resulting in a change in the ratio of the effective pitch diameters of the driven and driving sheaves and a change in output speed. This type of drive has a limited capacity up to approximately 125 hp and
MBA motor base.
FIGURE 7.2 MBA motor base. (Courtesy T. B. Wood’s Sons Company, Chambersburg, PA.)
Cross section of double-side spring sheave.
FIGURE 7.3 Cross section of double-side spring sheave. (Courtesy Lovejoy Inc., South Haven, MI.)
a limited speed range of approximately 2 or 3 to 1. In special cases, the speed range may be wider. The efficiency of these drives depends on the belt loading and minimum diameter of the sheaves. The belt efficiency as a function of load ranges from 92 to 99% and as a function of sheave diameter from 91 to 97%. When combined with a three-phase induction motor, the system efficiency ranges from 40 to 90%, depending on the type of load and speed reduction.
Figure 7.5 illustrates the system efficiency for an adjustable-speed pulley system with a 10-hp, 1750-rpm energy-efficient motor driving a constant-torque load over a 3-to-1 speed range.
Figure 7.6 illustrates the system efficiency for an adjustable-speed pulley system with a 10-hp, 1750-rpm energy-efficient motor driving
Cross section of single-side spring sheave.
FIGURE 7.4 Cross section of single-side spring sheave. (Courtesy Lovejoy Inc., South Haven, MI.)
a variable-torque load ovr a 3-to-1 speed range. Its major disadvantage is that the speed must be changed manually and does not lend itself to automatic or remote control. However, for applications that require only occasional adjustment in output, this system may be adequate and still provide energy savings at the lower speed settings, for example, an air-handling system that requires output adjustment only for summer and winter operation. Figure 7.7 shows a typical installation of this type of drive.
7.2.3

Mechanical Adjustable-Speed Systems

The broad group of mechanical ajustable-speed drives includes the more common stepless mechanical adjustable-speed drives that
Adjustable-speed pulley system with a 10-hp energy-efficient motor driving a constant-torque load.
FIGURE 7.5 Adjustable-speed pulley system with a 10-hp energy-efficient motor driving a constant-torque load.
Adjustable-speed pulley system with a 10-hp energy-efficient motor driving a variable-torque load.
FIGURE 7.6 Adjustable-speed pulley system with a 10-hp energy-efficient motor driving a variable-torque load.
Installation of variable-speed sheave.
FIGURE 7.7 Installation of variable-speed sheave. (Courtesy T. B. Wood’s Sons Company, Chambersburg, PA.)
provide an infinite number of speed ratios within a nominal speed range. These type of drives include packaged belt and chain drives, friction drives, and traction drives. These drive systems are usually driven by a constant-speed induction motor and convert this constant-speed input into a stepless variable-speed output.
Figure 7.8 is a cross section of the assembly of the U.S. Electric Motors varidrive system, showing the electric motor, driver, driven sheaves, and speed-adjusting mechanism.
A typical group of packaged adjustable-speed belt-drive systems is shown in Fig. 7.9. In the case of the belt-drive systems, the basis of rating is generally constant torque (variable horsepower) at speed ratios below 1 to 1 and constant horsepower (variable torque) at speed ratios above 1 to 1. Figure 7.10 illustrates this basis of rating.
Cross section of U.S. Electrical Motors Varispeed System.
FIGURE 7.8 Cross section of U.S. Electrical Motors Varispeed System. (Courtesy U.S. Electrical Motors, Division of Emerson Electric Co., St. Louis, MO.)
Electrical Motors varidrive units.
FIGURE 7.9 U.S. Electrical Motors varidrive units. (Courtesy U.S. Electrical Motors, division of Emerson Electric Co., St. Louis, MO.)
Horsepower output versus output speed of a mechanical adjustable-speed drive system.
FIGURE 7.10 Horsepower output versus output speed of a mechanical adjustable-speed drive system. (Courtesy U.S. Electrical Motors, division of Emerson Electric Co., St. Louis, MO.)
The efficiency of these systems at various loads and speeds is shown in Figs. 7.11 and 7.12 for representative ratings of the varidrive line of packaged mechanical belt drives.
Most of these types of drives have limited horsepower and speed ranges. Therefore, the selection of the drive systems should be based on the duty cycle of the load and the characteristics of the drive under consideration, including speed range, horsepower, torque characteristics, and efficiency over the duty cycle. When properly
tmp2614_thumb[2][2]Varidrive performance curves, typical data for a 15-hp, four-pole motor.
FIGURE 7.11 Varidrive performance curves, typical data for a 15-hp, four-pole motor. (Courtesy U.S. Electrical Motors, division of Emerson Electric Co., St. Louis, MO.)
applied, many of these drives have good efficiencies over their operating range. It is recommended that several types of systems be compared to determine the most suitable and effective life-cycle cost system. The requirements and type of remote control must also be a factor in the selection of the drive system.
Figures 7.13-7.15 illustrate the types of process controls available on packaged belt drives.
7.2.4

Eddy Current Adjustable-Speed Drives

The operating principle of the eddy current drive system involves a constant-speed AC induction motor that is magnetically coupled to
tmp2616_thumb[2][2]Varidrive performance curves, typical data for a 10-hp, six-pole motor.
FIGURE 7.12 Varidrive performance curves, typical data for a 10-hp, six-pole motor. (Courtesy U.S. Electrical Motors, division of Emerson Electric
Co., St. Louis, MO.)
an output shaft through an integral variable-speed eddy current coupling. The eddy current coupling consists of a constant-speed drum that is directly connected to the drive motor rotor and an inductor that is directly connected to the output shaft. As the drum rotates, eddy currents are induced and magnetic attraction occurs between the drum and the inductor, thus transmitting torque from the constant-speed drum to the output inductor. An excitation winding, which is usually stationary, is located in the magnetic circuit and is excited by a DC current to provide the magnetic field in the constant-speed drum and variable-speed inductor. The application of the field current creates a magnetic flux across the air
Output flow control of a mechanical adjustable-speed system.
FIGURE 7.13 Output flow control of a mechanical adjustable-speed system.
Line pressure control of a mechanical adjustable-speed system.
FIGURE 7.14 Line pressure control of a mechanical adjustable-speed system.
Speed control of a mechanical adjustable-speed system.
FIGURE 7.15 Speed control of a mechanical adjustable-speed system.
gap between the two members of the clutch, which induces eddy currents in the input drum. The net result is a torque available at the output shaft. The variation in the field current varies the degree of magnetic coupling between the motor-driven constant-speed drum and the variable-speed inductor connected to the output drive shaft. By adjustments in the field current, the output speed can be adjusted to match the output load requirements (speed and torque). Figure 7.16 shows the cross-section assembly of a self-contained eddy current drive system. Figure 7.17 is a general view of such a system. The power flow for this type of adjustable-speed drive is shown in Fig. 7.18.
The degree of coupling or slip between the two members is determined by the load and level of excitation. The slipping action (i.e., difference in speed) is the source of the major power loss and
Cross section of self-contained eddy current adjustable-speed drive system.
FIGURE 7.16 Cross section of self-contained eddy current adjustable-speed drive system. (Courtesy Magnetek, New Berlin, WI.)
inefficiency of the eddy current coupling. This slip loss is the product of the slip rpm, which is the difference in speed between the input and output members and the transmitted torque. This relationship may be expressed as follows:
tmp2622_thumb2[2][2]Self-contained eddy current adjustable-speed drive system.
FIGURE 7.17 Self-contained eddy current adjustable-speed drive system. (Courtesy Magnetek, New Berlin, WI.)
Power flow for eddy current drive system.
FIGURE 7.18 Power flow for eddy current drive system.
tmp2625_thumb[2][2]
where
rpm1 = coupling input speed (motor) rpm2 = coupling output speed (load) TL = load torque, ft-lb
The efficiency of an eddy current coupling can never be greater than the numerical percentage of the output speed. However, in addition to the slip losses, the friction and windage losses and excitation losses of the coupling must also be included in the efficiency determination. The friction and windage loss is about 1% of the rated input horsepower and can be considered constant over the speed range. The excitation loss is less than 0.5% of the input horsepower and decreases with reduction in speed. The maximum torque developed by the drive is limited to the maximum torque (breakdown torque) of the induction drive motor or the magnetic coupling of the eddy current clutch. With proper matching of the drive components, the full capacity of the drive motor can be utilized. Figure 7.19 illustrates the overload capacity of an eddy current drive system with an induction motor driver.
Since the eddy current coupling has no inherent speed regulation, it is necessary that the coupling include a tachometer generator that rotates at the coupling output speed. The tachometer-generator output signal is fed into a speed-control loop in the excitation system to provide close output speed regulation. The speed regulation is usually ±1% but with closed-loop control may be as close as ±0.1%. In addition to speed, the eddy current-control system can be used with any type of actuating device or transducer that can provide a mechanical translation or an electrical signal. Actuating devices include liquid-level control, pressure control, temperature control, and flow control. The performance of the eddy current adjustable-speed drive system driving a constant-torque load is illustrated in Fig. 7.20. The speed range for continuous operation is usually 16:1 but can be wider, depending
Eddy current adjustable-speed drive overload capacity.
FIGURE 7.19 Eddy current adjustable-speed drive overload capacity.
Eddy current adjustable-speed drive applied to a constant-torque load.
FIGURE 7.20 Eddy current adjustable-speed drive applied to a constant-torque load.
on the thermal dissipation capacity of the eddy current clutch. The performance of the eddy current adjustable-speed drive system driving a variable-torque load is illustrated in Fig. 7.21. The speed range shown in Fig. 7.21 is down to only 45% speed, which is within the range of most variable-speed applications; however, the eddy current drive can supply these types of loads to much lower speeds if necessary.
The advantages of this type of adjustable-speed drive are
Eddy current simplicity and high reliability
Stepless variable-speed control
Good speed regulation
High starting torque
High overload capacity
Controlled acceleration
Handle high-impact loads.
Figure 7.22 illustrates the application of an eddy current adjustable-speed drive to an extruder. Figure 7.23 illustrates the application of the eddy current adjustable-speed drive to a process pump.
Eddy current adjustable-speed drive applied to a variable-torque load.
FIGURE 7.21 Eddy current adjustable-speed drive applied to a variable-torque load.
The application of an eddy current adjustable-speed drive system to an extruder.
FIGURE 7.22 The application of an eddy current adjustable-speed drive system to an extruder. (Courtesy Magnetek, New Berlin, WI.)
7.2.5

Fluid Drives

Fluid drives can be described as any device utilizing a fluid to transmit power. The fluid generally used is a natural or synthetic oil. Fluid drives can be grouped into four categories: (1) hydrokinetic, (2) hydrodynamic, (3) hydroviscous, and (4) hydrostatic. The hydrokinetic, hydrodynamic, and hydroviscous drives are all slip-type devices.
The hydrokinetic fluid drive, commonly referred to as a fluid coupling, consists of a vaned impeller connected to the driver and a vaned runner connected to the load. The oil is accelerated in the impeller and then decelerated as it strikes the blades of the runner. Thus, there is no mechanical connection between the input and output shafts. Varying the amount of oil in the working circuit changes the speed. This provides infinite variable speed over the operating range of the drive. Figure 7.24 is a representation of such a drive.
The application of an eddy current adjustable-speed drive system to a process pump.
FIGURE 7.23 The application of an eddy current adjustable-speed drive system to a process pump. (Courtesy Magnetek, New Berlin, WI.)
The circulating pump, driven from the input shaft, pumps oil from the reservoir into the housing through an external heat exchanger and then back to the working elements. The working oil, while it is in the rotating elements, is thrown outward, where it takes the form of a toroid in the impeller and runner. Varying the quantity of oil in this toroid varies the output speed. A movable scoop tube controls the amount of oil in the toroid. The position of the scoop tube can be controlled either manually or with automatic control devices. The scoop-tube adjustment gives a fast response and smooth stepless speed control over a wide speed range, i.e., 4 to 1 with a constant-torque load and 5 to 1 with a variable-torque load. In addition to providing speed control, the fluid drive limits torque and permits no-load starting on high-inertia loads.
These units range in size from 2 to 40,000 hp, as illustrated in
Fig. 7.25.
Diahydrokinetic drivegram
FIGURE 7.24 Diahydrokinetic drivegram of a : (1) primary wheel;2 ()
secondary wheel; (3) shell; (4) scoop tube housing; (5) oil sump; (6) oil pump; (7) scoop tube. (Courtesy Voith Transmissions, Inc., York, PA.)

Efficiency. The fluid drives have two types of losses:

Circulation losses. These losses include friction and wind-age losses, the power to accelerate the oil within the rotor, and the power to drive any oil pumps that are part of the system. These losses are relatively constant and are approximately 1.5% of the unit rating.
Slip losses. As in the case of eddy current couplings, the torque at the input shaft is equal to the torque required at the output shaft:
tmp2632_thumb[2][2][2]Voith hydrokinetic fluid drive.
FIGURE 7.25 Voith hydrokinetic fluid drive. (Courtesy Voith Transmissions, Inc., York, PA.)
tmp2634_thumb2[2][2]
where
rpm1 = coupling input speed (motor)
rpm2 = coupling output speed (load) TL = load torque, ft-lb
The slip efficiency is then
tmp2635_thumb[2][2]
The maximum speed of the fluid drive at full load is about 98% of the driving motor speed, and with circulation losses of 1.5% the maximum efficiency is 96.5% at a maximum speed.
Figure 7.26 illustrates the typical performance of a fluid coupling driving a variable-torque load such as a fan or pump, where the torque varies as the speed squared and the horsepower varies as the speed cubed. Figure 7.27 illustrates the performance of a fluid coupling driving a constant-torque load such as a conveyor or piston pump, where the horsepower varies as the speed. Figure 7.28 illustrates a complete-package adjustable-speed fluid drive consisting of the drive motor, fluid coupling, and necessary accessories. Figure 7.29 shows the installation of a variable-speed fluid-drive system driving mud pumps at a mining installation. Figure 7.30 shows the installation of variable-speed fluid-drive systems driving blowers.
More complex units are available at ratings generally above 1000 hp. The Voith MSVD multistage variable-speed drives are an example of these drives, which consist of

Hydrodynamic variable-speed coupling Hydraulic-controlled lock-up clutch

Fluid-coupling variable-speed drive characteristics when driving a load that varies as the speed cubed.
FIGURE 7.26 Fluid-coupling variable-speed drive characteristics when driving a load that varies as the speed cubed.
Hydrodynamic torque converter Hydrodynamic brake Planetary gear, fixed Planetary gear, revolving
The operation of these units can be divided into two stages. In stage 1, the power is transmitted by the hydrodynamic variable-speed coupling directly through the planetary gear. The speed is controlled by changing the level of the oil in the hydrodynamic
Fluid-coupling variable-speed drive characteristics when driving a constant-torque load.
FIGURE 7.27 Fluid-coupling variable-speed drive characteristics when driving a constant-torque load.
Packaged fluid drive consisting of the drive motor, fluid coupling, and necessary accessories.
FIGURE 7.28 Packaged fluid drive consisting of the drive motor, fluid coupling, and necessary accessories. (Courtesy Voith Transmissions, Inc.,
York, PA.)
Variable-speed fluid drives driving mud pumps at mining installation
FIGURE 7.29 Variable-speed fluid drives driving mud pumps at mining installation. (Courtesy Voith Transmissions, Inc., York, PA.)
Variable-speed fluid drives driving blowers
FIGURE 7.30 Variable-speed fluid drives driving blowers. (Courtesy Voith Transmissions, Inc., York, PA.)
coupling. The operating range is approximately 0-80% speed. The torque converter has no function in this stage. The hydrodynamic brake generates the countertorque for the planetary gear. In stage 2, the impeller and turbine wheel on the hydrodynamic coupling are locked together by the hydraulic-controlled clutch bridging the input and output elements so that the drive motor is now coupled mechanically to the driven load. The operating speed range in this stage is 80-100% and is controlled by the hydrodynamic torque converter.

Hydrostatic Drives.

A hydrostatic variable-speed drive consists of a positive-displacement hydraulic pump driven by an induction motor, a positive-displacement hydraulic motor, and necessary hydraulic controls. The hydraulic pump and motor are usually separate units. This type of drive is also offered as a package consisting of the hydraulic pump, the piping, and the hydraulic motor mounted in a common housing.
When the hydraulic pump is driven by a constant-speed AC induction motor, the variable output is obtained by controlling the speed of the hydraulic motor. Commonly, the easiest system to design may be the most energy inefficient. Throttling any valve in the hydraulic system generates heat and consumes energy. The significance of this power loss is expressed as follows:
tmp2641_thumb1[2][2]
The most efficient hydraulic system is one that has no valves. However, such a system will also have very limited speed control. Many methods of control have been developed for hydraulic systems, and the method used depends on the types of pump and motor used and the characteristic of the load. Many of the systems are used on mobile equipment and machine tools, but they are not generally cost effective on industrial applications such as pumps and fans.
The Gibbs V/S drive shown in Fig. 7.31 is a packaged hydrostatic drive consisting of a constant-speed electric-drive motor, a constant-speed
Gibbs V/S hydrostatic drive package
FIGURE 7.31 Gibbs V/S hydrostatic drive package. (Courtesy Gibbs Machine Co. Inc., Greensboro, NC.)
hydraulic pump, and a variable-speed hydraulic motor. The hydraulic pump is a variable-volume positive-displacement pump, and the hydraulic motor is a fixed-volume positive-displacement motor. In the Gibbs package unit, the hydraulic pump is about 87% efficient over its working range, and the hydraulic motor has an efficiency of 92% over the working range. Figure 7.32 shows the hydraulic efficiency and the overall system efficiency for a 10-hp, 1800-rpm package unit operating over a 4:1 speed range, with a constant-torque load.
These types of adjustable-speed drives can operate from 0 to maximum speed at constant torque, with a recommended usable range of 27:1. The maximum output speed depends on the selection of the hydraulic motor in the package drive. These package drives are available up to 75 hp and can be provided with manual,
Efficiency of a hydrostatic package-drive unit driving a constant-torque load.
FIGURE 7.32 Efficiency of a hydrostatic package-drive unit driving a constant-torque load. (Courtesy Gibbs Machine Co., Greensboro,
NC.)
electronic, pneumatic, or hydraulic controls. This type of drive is basically a constant-torque drive and is not normally used on variable-torque loads.
Hydroviscous Drives. Another class of adjustable-speed fluid drives are the hydroviscous drive units. The basic components of the hydroviscous drive are (1) the torque-transmitting clutch plates, pressure plate, and flywheel assembly; (2) the oil pump for cooling and controlling oil; (3) the variable-orifice controller and control piston with a torque-limiting valve. Figure 7.33 is a cross section of one of these drives, manufactured by Great Lakes Hydraulic, Inc., showing the various components of the drive. Figure 7.34 shows a complete assembly for a horizontal unit.
Cross section of a hydroviscous clutch assembly.
FIGURE 7.33 Cross section of a hydroviscous clutch assembly. (Courtesy Great Lakes Hydraulics, Inc., Grand Rapids, MI.)
There is a continuous flow of fluid between the constant-speed and adjustable-speed elements. The torque is transmitted through this film of fluid according to the oil shear principle. The amount of torque transmitted is proportional to the amount of piston pressure applied. As the piston pressure increases, the slip between the plates decreases. At the maximum rated piston pressure, the plates are locked in, and the output shaft is then running at input motor speed. The orifice controller determines the pressure supplied to the piston area. Minimum pressure is supplied to the piston when the orifice is completely open, bypassing fluid to the sump. The piston pressure is increased as the orifice is closed, and the slip between the clutch plates decreases. The orifice controller, which controls the piston pressure, can be manual, pneumatic, hydraulic, or electronic, as required. With automatic control, the
Assembly of a hydroviscous drive package.
FIGURE 7.34 Assembly of a hydroviscous drive package. (Courtesy Great Lakes Hydraulics, Inc., Grand Rapids, MI.)
output speed can be regulated within ±2% of maximum speed. The torque transmitted by the drive is adjusted by changing the piston pressure.
This type of drive can be used for constant- and variable-torque applications and provides smooth operation at all speeds. The losses for these units include the slip loss that is common to all hydraulic drives and the fixed losses of the unit. Figure 7.35 illustrates the performance of a hydroviscous drive driving a variable-torque load
Performance of a hydroviscous drive system driving a variable-torque load.
FIGURE 7.35 Performance of a hydroviscous drive system driving a variable-torque load. (Courtesy Great Lakes Hydraulics, Inc., Grand
Rapids, MI.)
such as a centrifugal pump or fan; these data do not include the drive motor losses.
7.2.6

AC Variable-Frequency Drives

The squirrel-cage induction motor is normally considered a constant-speed device with an operating speed 2-3% below its synchronous speed. However, efficient operation can be obtained at other speeds if the frequency of the power supply can be changed. The synchronous speed of an induction motor can be expressed by
tmp2647_thumb2[2][2]
where
Ns = synchronous speed, rpm
f = power supply frequency, Hz
p = number of poles in motor stator winding
A four-pole induction motor that has a synchronous speed of 1800 rpm when operated on a 60-Hz power supply operates at the following synchronous speeds as the power supply frequency is changed:

Power Motor
frequency, synchronous
Hz speed, rpm
120 3600
90 2700
60 1800
30 SOO
in 450
7.5 225

Variable-Frequency Power Supplies. The utilization of power semiconductor technology has provided an economic means to generate a variable-frequency power supply from a fixed-frequency power source for industrial applications. Using the output of this variable-frequency semiconductor power system to supply three-phase power to a three-phase induction motor provides a means to vary the speed of the induction motor. Today, these systems are commonly identified as adjustable-frequency controllers or adjustable-frequency drives. These “controllers” consist of two basic power sections: the converter section, which converts the incoming AC power to DC power, and the inverter section, which inverts the DC power to an adjustable-frequency, adjustable-voltage AC power.
The size and types of power semiconductors used in the power sections of the controller depend on the voltage level, power level, and type of inverter.

CONVERTER POWER SECTION.

In the converter power section (AC to DC power), the power semiconductors are usually
1. Silicon rectifiers. These are commonly referred to as diodes. The silicon diode has the characteristic of permitting current flow in one direction and blocking current flow in the opposite direction. These rectifiers, along with the silicon control rectifiers (SCRs), are the workhorses of the semiconductors for power conversion. They range in current rating up to 4800 A and voltage rating up to 5000 V. These devices have no control characteristics and are either conducting or blocking power.
2. Silicon control rectifiers or thyristors. The silicon control rectifiers block current flow in one direction and permit current flow in the opposite direction, much as the silicon diode does. Unlike the diode, however, the start of current flow can be controlled in the SCR. The SCR switches on and conducts current from the anode to the cathode when a proper voltage pulse is applied to the gate terminal. Current continues to flow until the device switches itself off. The SCRs have large power handling capability. They range in current rating up to 4000 A and in voltage ratings up to 4500 V. The rating of the device depends on the case temperature and duty cycle of the application.
3. Gate turn-off thyristors. The gate turn-off thyristor (GTO) is a semiconductor device that can be turned on like the thyristor (SCR) with a single pulse of gate current, but it can also be turned off by the injection of a negative gate current pulse. The GTO power losses are higher during switching, but elimination of forced commutation circuits improves the overall efficiency of the converter. In addition, GTOs are suitable for higher switching frequencies than SCRs. They are available with turn-off current ratings up to 3000 A as well as a blocking voltage capability up to
4500 V.

INVERTER POWER SECTION.

In the inverter power section (DC power to AC power), the power semiconductors used depend on the type of inverter, voltage, and power ratings and may be any of the following:
1. Silicon control rectifiers. See above comments on SCRs. Because of their limited switching frequency, these devices are generally not used in pulse width modulation inverters.
2. Gate turn-off thyristors. See above comments on GTOs. Again, the frequency of operation is limited but is higher than the switching frequency of the SCR. GTOs have been used in pulse width inverters.
3. Bipolar transistors. These devices can be switched at higher frequencies than SCRs. However, the current ratings are limited; they may be on the order of 400 A rating, with VCEO ratings of 600 V, and 120 A rating, with 1000-V VCEO. Significant drive power is required for these devices.
4. Bipolar Darlingtons. These devices are generally two-or three-stage devices with built-in emitter-base resistances, speed-up diodes, and freewheeling diodes. The frequency of operation is typically in the 5- to 8-kHz range, but the devices can operate at higher frequencies, and the gain is considerably higher than for the bipolar transistor. Bipolar Darlingtons are available in the range of 140 A at 1400 V and 600 A at 1200 V. The units can be operated in parallel, and this is common practice in many inverters, with as many as four devices in parallel.
5. Insulated gate bipolar transistors (IGBTs). The IGBT combines on a single chip the high-impedance, voltage-controlled turn-on and turn-off capabilities of power MOSFETS and the low on-state conduction losses of the bipolar transistors. These devices can be switched at higher frequencies than the Darlington units and can be connected in parallel. They also have lower base-power requirements than the Darlington units. The ratings range up to current
rating Ic of 600 A and voltage VCES of 1200 V. IGBTs are finding increased use in pulse width modulation inverters. The devices can be operated in parallel.
Figure 7.36 illustrates the relative rating of some of these power semi-conductor devices. The Darlington transistors and the IGBTs can be switched at frequencies above the range of human hearing. In addition, they can be operated in parallel. The types of power semi-conductor devices used in a particular type of inverter can change as the quality, capacity, and cost of existing devices and new devices improve.
Types of AC Inverters. The AC three-phase induction motor can be used for adjustable-speed applications when the power to the motor is supplied by a variable-frequency power supply (inverter).
 Relative rating range of various power semiconductor devices.
FIGURE 7.36 Relative rating range of various power semiconductor devices. (Courtesy Powerex, Inc., Youngwood, PA.)
The input voltage to the motor is varied proportionally to the frequency, i.e., at constant volts/hertz. At low frequencies, however, the voltage may be increased above its proportional level to obtain adequate torque. The torque developed by the induction motor is proportional to the magnetic flux in the motor air gap and to the rotor slip. As the frequency is decreased, the reactance of the motor decreases so that the applied voltage must be decreased proportionally to the frequency decrease to maintain constant air gap flux. If the applied voltage is not decreased, the motor magnetic circuit becomes saturated and there are excessive motor losses. At normal frequencies, the stator winding resistance drop is only a small percentage of the stator voltage drop so that the difference between the applied voltage and the net air gap voltage is relatively small. However, since the stator resistance is constant as the frequency is decreased and the reactance decreases proportionally to the frequency, the stator resistance drop voltage becomes a high percentage of the applied voltage. This results in a
Typical voltage boost compared  to constant volts/hertz moto voltage
FIGURE 7.37 Typical voltage boost compared to constant volts/hertz moto voltage.
decrease in the net air gap voltage and air gap flux. Therefore, at lower frequencies (about 10 Hz and lower), to compensate for this increased stator resistance voltage drop and maintain the flux in the air gap, the applied voltage must be increased above the constant volts/hertz level. Figure 7.37 shows the typical voltage boost compared to constant volts/hertz at the lower frequencies. The amount of voltage boost should be limited so that the current drawn by the motor does not exceed 150% of the current rating of the adjustable-frequency power supply. If a higher motor current is needed to achieve the necessary starting torque, a higher current-rated adjustable-frequency power supply will be required. This high-voltage boost should be maintained only during the starting of the motor to protect both the drive motor and the inverter from damage. A number of inverter types are used in adjustable-frequency power supplies, but the most common types are
Voltage-source inverters Current-source inverters Pulse width modulation inverters Vector control inverter systems
VOLTAGE-SOURCE INVERTER. Figure 7.38 illustrates the basic power circuit for a variable-voltage-source, six-step inverter. In this system, the 60-Hz input voltage is converted to a DC adjustable voltage by means of a three-phase semibridge converter. Then, by
Voltage-source inverter.
FIGURE 7.38 Voltage-source inverter.
means of a DC-to-AC transistor inverter, each of the three-phase output lines is switched from positive to negative for 180° of each 360° cycle. The phases are sequentially switched at 120° intervals, thus creating the six-step line-to-neutral voltage, or square wave line-to-line voltage, as shown in Fig. 7.39. The DC power supply is normally controlled by SCRs in the bridge rectifier, and an LC filter is used to establish a stiff DC voltage source. The output frequency is controlled by a reference signal that sets the control logic to achieve the correct gate or base-drive signals for the semiconductors in the inverter section. The semiconductors in the inverter section can be SCRs, GTOs, transistors, or Darlington transistors.
Speed control beyond the 10:1 range becomes a problem with the six-step inverter because at low voltage and frequency the harmonic currents become excessive, causing motor heating, torque pulsations, and cogging.

Advantage of the voltage-source inverter include

• Inverter section can use SCRs, GTOs, or transistors.
• Low switching frequency devices can be used.
• It is the simplest regulator.
• Standard or energy-efficient motors can be used with proper derating.
Voltage-source inverter wave shapes.
FIGURE 7.39 Voltage-source inverter wave shapes.
• It has multimotor capability.
• Voltage stress on motor insulation system is low.
Disadvantages of the voltage-source inverter include
• Poor input power factor that decreases with decreasing output frequency
• Harmonics fed into the 60-Hz AC supply system
• Limited speed control beyond the 10:1 range
• Torque pulsations and cogging
• High-harmonic currents, causing excessive motor heating

CURRENT-SOURCE INVERTER. In contrast to a stiff voltage

source as in a voltage-source inverter, the current-source inverter has a stiff DC current source at the input. This is generally accomplished by connecting a strong inductive DC filter reactor in series with the DC source and controlling the voltage within a current loop. Figure 7.40 illustrates the power circuit for the current-source inverter. A three-phase bridge consisting of six SCRs converts the AC input to DC, and a three-phase bridge autosequential-commutated inverter inverts the DC to the AC output voltages. With a stiff current source, the output current waves are not affected by the load. The power semiconductors in the current-source inverter
Current-source inverter.
FIGURE 7.40 Current-source inverter.
have to withstand reverse voltages; therefore, devices such as transistors and power MOSs are not suitable. Figure 7.41 illustrates the waveforms for the current-source inverter. Note the high spikes in the line-to-neutral voltage.
Advantages of the current-source inverter include
• Simples SCR-type circuit
• Low-frequency inverter switches
• Inherent short-circuit capability
• Inherent regeneration capability
• Rugged construction
Disadvantages of the current-source inverter include
• Motor and control must be matched.
• Not suitable for multimotor operation.
• Poor input power factor.
• Low-speed torque pulsations and cogging.
• It can cause high-voltage spikes at the motor.
Current-source inverters have been developed using GTO devices and pulse width modulation to overcome some of the disadvantages of the current-source inverter.
Current-source inverter wave shapes.
FIGURE 7.41 Current-source inverter wave shapes.
PULSE WIDTH MODULATION INVERTERS. In the pulse width modulation (PWM) system, the input AC power is rectified to a constant potential DC voltage. The DC voltage is then applied to the motor in a series of pulses. A number of methods have been devised to control the pulse width and to vary the frequency of the pulses as the motor speed is changed. In some cases, at the higher speed, the system becomes a six-step inverter.
Figure 7.42 shows the power circuit for a PWM inverter with a diode bridge to convert the AC voltage to DC voltage and a transistor DC to AC inverter to generate the AC output voltage.
The technology of pulse width modulation is not new. However, the use of microprocessors to provide improved modulation techniques and higher-speed switching power semiconductor devices such as transistors and IGBTs are making the PWM inverter the standard inverter in the 1- to 500-hp range.
A number of pulse width modulation procedures are used in today’s PWM inverters. Some of these are
• Sinusoidal with a sine wave signal and a triangular carrier wave
• Harmonic elimination, particularly the fifth, seventh, eleventh, and thirteenth harmonics
Pulse width modulation inverter.
FIGURE 7.42 Pulse width modulation inverter.
• Distortion minimization with five switching angles/quarter cycle
• Minimum ripple current
• Uniform sampling
The ideal PWM system balances the switching losses in the inverter with the current and torque ripples and the heating losses in the drive motor for the best overall performance (Figure 7.43).
By selecting the width and spacing of the pulses, lower-order harmonics, such as the fifth, seventh, and eleventh, can be eliminated in the waveform. If the pulse rate is high enough, the motor inductance presents a high impedance so that the pulse-rate-frequency current is insignificant. From the motor viewpoint, it is desirable to have a high-frequency pulse rate. From the inverter viewpoint, since most of the losses occur during switching, it is best to have a low
Pulse width modulation inverter wave shapes.
FIGURE 7.43 Pulse width modulation inverter wave shapes.
pulse rate. However, the number of pulses per cycle must be maintained high enough to avoid troublesome harmonics that may be resonant with the motor components and cause noise and vibration in the drive. The switching and recovery time of SCRs limits their use on PWM systems. IGBTs, power transistors, Darlington transistors, and GTOs have faster switching times with lower losses, and so they are used at the higher pulse rates required for smooth operation. While the PWM inverters improve the waveforms by eliminating the low-order harmonics, they impose a series of high-voltage impulses on the motor winding. Although the winding inductance smooths the current waveform, the rapid voltage changes produce insulation stresses on the first few turns of each of the motor windings. Full-voltage PWM systems produce the most severe stresses, particularly at low speeds, where the motor back-
EMF is low.
Advantages of PWM inverters include
• Wide speed range.
• Smooth low-speed operation.
• Multimotor operation.
• Standard or energy-efficient motors can be used with proper derating.
• Minimum problems matching motor and inverter.
• High-input power factor.
Disadvantages of PWM inverters include
• Complex control
• Requires high-frequency power semiconductors in the inverter
• Higher motor heating and noise (depends on the modulation system used)
• Not regenerative
• Imposes high-voltage gradients on the motor insulation system
There are numerous variations of these three types of adjustable-frequency inverters, but the principle of operation is essentially the same. As with any product, changes and improvements are being
accomplished every day. The improvements come primarily from the increasing use of integrated circuits as well as microprocessors, which have greatly reduced the number of control logic components. Also the use of power IGBTs, power transistors, and GTOs has reduced the cost of power elements. These factors, plus improved designs and techniques, have reduced and will continue to reduce the size and cost of the inverters. At the same time, performance and reliability continue to improve.

VECTOR CONTROL INVERTER SYSTEMS.

One important variation or addition to the previously discussed inverter systems is the vector control inverter system. Vector control considers the analogy between AC and DC electrical machines. The ultimate object of the vector control system is to control the AC induction motor as a separately excited DC motor is controlled, i.e., to control the field excitation and torque-generating currents separately and independently. To control the induction motor in this manner, the air gap flux (net air gap voltage) and rotor current must be separately controlled. The vector control drives available are based mostly on the indirect flux control method. The magnitude, frequency, and phase of the stator current components are controlled as a function of the rotor position, slip frequency, and torque command. Figure 7.44 is a block diagram of a vector
Block diagram of vector control logic.
FIGURE 7.44 Block diagram of vector control logic.
control logic system, with the input signals received from the motor and the output control signal from the vector control system supplied to the inverter section of a PWM inverter. The excitation current component and the torque component of the current are calculated from the motor terminal voltage and current and the motor speed. The performance of this method of control depends on how closely the algorithm of the vector system matches the induction motor characteristics. The precision of the system also depends on the precision of the tachometer or rotor speed sensor since the slip control is based on this signal. Without a tachometer or rotor speed sensor, the precise speed range is 20:1, with the speed
Family of PWM adjustable-frequency power supplie
FIGURE 7.45 Family of PWM adjustable-frequency power supplie; (Courtesy Magnetek, New Berlin, WI.)

STANDARD FEATURES

■ Frequency resolution 0.1 Hz with digital reference; 0.06 Hi with analog reference
• Frequency regulations 0.01% with digital command: 0.1% with analog (15 to 36* C)
■ Stanctara frequency range IS to 800 HZ
• Volts/Hertz ratio, 15 preset patterns, one fully adjustable pattern
• Independent accel/decel 0.16000 sec.
• DC injection braking amplitude & duration, current limited
• Signal follower externa bias & gain
• Critical frequency rejection, 3 selectable
• Torque limit, 30-150%
• Jog speed, adjustable zero to 100%
• Multi-speed setting. 9 possible
• Forward/reverse operation
■ Speed range 40:1
. NEMA 1 enclpsure [NEMA 12 osl onal)
■ 24 VDC Logic
■ Run/fault contacts 1 amp, 250 VAC or 30 VDC
- Remote speed reference capability 0-10 VDC (2DK ohms) or 4-20 mA (250 ohm)
PROTECTION and MONITORING
• Overload capability to 150% rated, 60 sec.
■ Instantaneous overcuirent trip and Indication
• Overvoltoge trip and indication
• Undervottoge trip and Indication
■ Overtemperature Irip and indication
■ External fault trip and Indication
- Blown "use -Nd ard ndicaticn
• Control circuit error trip and indication
■ DC bus charge indication
• EC bus fuse
• Ground fault protection
■ Stall prevention
■ Electronic motor overload o'otect.on
■ Momentary power failure ride-through [2 sec. 5 HP and above); [0.2 sec. below 5 HP with option of 2 sec.)

I :Nvl RON MENTAL CONDITIONS

■ Altitude to 3300 feet above seo level
■ Ope'afng ar^hsnt temperature -10 to WC
• Storage temperature -20 to 60° C
' Nc^conde^src reniivr; humidity to 90%
■ Vibration 1G max under 20 Hz; Q2G at 20-50 Hz

INPUT POWER REQUIREMENTS

■ 230V model for 200. 208. 220 or 230 VAC, ±10%
• 460 V model for 380. 400. 415, 440, or 460 VAC. +10%
» 3-phose, 3-wire. phase sequence insensitive
■ Frequency 50 or 60 Hz, +5%
FIGURE 7.46 Typical features of PWM adjustable-frequency power supplies. (Courtesy Magnetek, New Berlin, WI.)
the types of volts/hertz patterns that can be selected. In addition, some units automatically select the optimum voltage for a given frequency and load condition. • Automatic carrier frequency. As the motor load increases, or the operating frequency decreases, the carrier frequency will automatically increase; this increase in the carrier switching frequency reduces the output current harmonics and, as a result, provides more motor torque per ampere.

7.2.7 AC Variable-Frequency Drive Application Guide

Unfortunately, the selection and application of an AC variable-frequency induction motor drive system are more complex than the
Improvement in the efficiency of a 10-hp,
FIGURE 7.48 Improvement in the efficiency of a 10-hp, energy-efficient, four-pole induction motor by reducing the volts/hertz when supplying a variable-torque load.
Adjustable-frequency power supply efficiency as a function of load and output frequency.
FIGURE 7.49 Adjustable-frequency power supply efficiency as a function of load and output frequency.
with the load and the operating frequency, and there are additional losses in the motor as a result of the harmonics in the motor supply frequency. Figure 7.49 shows the efficiency of an adjustable-frequency power supply as a function of load and operating frequency. Figure 5.19 compares the efficiency of a 100-hp induction motor with a sinusoidal power supply and a nonsinusoidal power supply (such as an adjustable-frequency power source), reflecting the decrease in motor efficiency as a result of the harmonics in the supply voltage. The overall efficiency of the adjustable-frequency induction motor system is the product of the component efficiencies and is illustrated in Fig. 7.50. This figure also compares the induction motor efficiency when the motor is operating on a sine-wave power source to the overall efficiency when it is operating with an adjustable-frequency power system.
Efficiency of a 100-hp,
FIGURE 7.50 Efficiency of a 100-hp, energy-efficient induction motor with a constant-torque load on a sine-wave power supply versus the overall efficiency on an adjustable-frequency power supply.
The following is a summary of the types of loads suitable for application of adjustable-frequency induction motor systems:
• Variable-torque loads
Centrifugal fans Centrifugal pumps Agitators
Axial centrifugal compressors Centrifugal blowers
• Constant-torque loads
Calenders
Positive-displacement blowers
Conveyers
Centrifuges
Reciprocating and rotary compressors
Positive-displacement pumps
Slurry pumps
Cranes
Elevators
Mixers
Printing presses Washers
• Constant-horsepower loads
Drill presses
Grinders
Lathes
Milling machines Tension drives Winders Recoilers
• Impact loads. The following types of impact loads may be suitable for application of adjustable-frequency induction motor systems but require special consideration of the adjustable-frequency power supply in order to provide the peak induction motor output torques required and stay within
the current limitations of the adjustable-frequency power supply.
Lathes
Milling machines Rolling mills Punch presses Shakers Shears Crushers
7.2.8

Wound-Rotor Motor Drives with Slip Loss Recovery (Static Kramer Drives)

The wound-rotor motor has normally been used for short-time duty applications such as cranes and hoists where torque control is of prime importance. When it has been used on continuous-duty installation, the major purpose has been to obtain controlled starting and acceleration. The reason for this limited use has been that high slip losses occur at speeds below normal operating speed. With the development of power electronics and solid-state inverters, systems have been developed to recover these slip losses.
These drives are commonly referred to as static Kramer drives. The original Kramer drives used a rotary converter instead of power semi-conductors and fed the power back to the line from a DC motor coupled to the induction motor. With the recovery of the rotor slip losses, the efficiency of the wound-rotor feedback system is comparable to the efficiency of an adjustable-frequency induction motor drive. It has an advantage in that the inverter has only to be large enough to handle the rotor slip losses. The system has the disadvantages, however, of the unavailability and high cost of the wound-rotor motor. Today, these systems are generally custom-designed for specific applications with a limited speed range, such as large pumps and compressors.
Figure 7.51 is a power circuit diagram for the static Kramer drive. As shown, the output of the wound rotor is connected to a three-phase rectifier bridge. The output of the bridge is connected to a
Static Kramer drive.
FIGURE 7.51 Static Kramer drive.
fixed-frequency inverter, the output of which is connected to the primary power supply that supplies the motor stator. The connection from the inverter output to the primary power supply is generally through a matching transformer. The effective rotor resistance, and hence the motor speed, is controlled by controlling the firing angle of the power SCRs in the inverter section.
The speed range that can be obtained is determined by the motor secondary (rotor) voltage; for instance, for a 100% speed range system,
480-V power supply: The rotor voltage must be 380 V or less.
600-V power supply: The rotor voltage must be 480 V or less.
and for a 50% speed range system, 480-V power supply: The rotor voltage must be between 600
and 760 V.
600-V power supply: The rotor voltage must be between 750
and 960 V.
For power supply voltages above 600 V, such as 2300 and 4160 V, the motor primary can utilize the line voltage. However, a matching transformer is required at the output of the rotor inverter, and it need only be large enough to handle the rotor losses, not the total motor input.
The efficiency of the controller is approximately 98.5% and is constant over the speed range; thus, the system is very efficient in recovering the slip losses and raising the system efficiency.
Consider a 200-hp wound-rotor motor on a pumping installation where the motor horsepower load is a cubic function of the speed. Without the slip recovery controller, at full speed (1764 rpm):
Horsepower output, 200 hp Motor efficiency, 94%
At one-half speed (882 rpm):
Horsepower output, 25 hp Motor efficiency, 46%
With the slip recovery controller, at full speed (1764 rpm):
Horsepower output, 200 hp Motor efficiency, 94% Overall system efficiency, 94%
At one-half speed (882 rpm):
Horsepower output, 25 hp Motor efficiency, 46% Overall system efficiency, 84%
Note that recovery of the slip losses at one-half speed increased the efficiency from 46 to 84%.
The wound-rotor motor with a slip recovery system used on a pump or fan application can usually be operated over a 50% speed range with self-ventilation. For applications requiring continuous operation below 50% speed, forced ventilation may be required for the motor.
This type of drive system offers an energy-efficient system comparable to adjustable-frequency systems and superior to slip-loss systems.

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