COMMERCIAL ADJUSTABLE SPEED DRIVES (Induction Motor)

10.5
A topic on control of induction motors would be incomplete without a short overview of commercial ASDs available on the market. The majority of these drives are sensorless. If available, the speed or position sensors are usually offered as an option. Most of the drives are used in industry, but domestic and vehicular applications of induction motor ASDs are on the rise.
The growth in controlled induction motor drives over the last decade has been greater than that in the previous three decades. It has been mostly a response to demands for: (a) increased efficiency and reliability of electromechanical power conversion, (b) process automation, and (c) flexibility of the power train. The rapidly decreasing cost of information processing (dedicated DSP controllers for motion control can be purchased for less than $5 apiece) and the continuing improvement in operating characteristics of semiconductor power switches are additional growth-supporting factors. In 1998, worldwide sales of ac ASDs were close to five billion dollars. The highest saturation of industry by induction motor drives occurs in Japan, followed by Europe and the United States. As time progresses, the drives become more compact and less expensive.
Voltage source inverters tend to phase out the current source inverters. The inverters operate mostly in the PWM mode, but in the field-weakening region, where the highest stator voltage is needed, the square-wave operation is employed. IGBTs are the most common semiconductor power switches. Modern IGBTs have rise times of 5 to 10 kV/|xs, voltage ratings up to 3.3 kV, and current ratings up to 2.4 kA. Consequently, they are able to replace GTOs in many high-power applications.
Apart from the control capabilities described in this topic, commercial ASDs have several other operation-enhancing features. In most drives, the dc-link voltage is sensed and the drive is shut down when the voltage drops below a predetermined level. However, in so-called critical drives, measures are applied to maintain the drive operational when, as often happens, the supply power is lost for only a second or so. These include a flywheel, an extra large dc-link capacitance, and an override of the automatic shutdown. When the power reappears, the motor is automatically accelerated to the set speed.

Electric braking is used in most ASDs supplied from the power line via a diode rectifier,

incapable of transferring power from the drive to the line. Three basic approaches are: (a) dynamic braking, using an external resistor connected in parallel with the dc-link capacitor by an extra switch (see Figure 4.20); (b) flux braking, used below the rated frequency and consisting in increasing the flux to convert portion of kinetic energy of the drive into iron losses in the motor; and (c) dc-current braking, by injection of a dc current into stator windings.
With the increase in switching frequencies, the dead time (see Section 4.3) in the inverter is no longer negligible, and it can compromise the voltage and current control. When the polarity of output currents is known, the dead time can be compensated by appropriate adjustment of the switching times. Another method consists in comparing the output voltage integral (virtual flux) with a reference integral. This feedback arrangement allows continuous correction of the voltage error.

As already mentioned, drive manufacturers classify induction motor ASDs into three categories:

(a) Constant Volts per Hertz (CVH) drives, (b) Sensorless Vector (SV) drives, and (c) Field-Oriented (FO) drives. The CVH ASDs are based on the open-loop voltage and frequency control (see Section 5.2). The current is sensed for protection purposes only, usually in the dc link. Dynamics at low speeds is poor. However, typical applications of CVH drives, such as pumps, blowers, compressors, and fans, do not really need precise speed or torque control.
Many general purpose ASDs have the user-selectable Sensorless Vector option. SV drives, although somewhat inferior to systems with the
speed or position feedback, are a good choice for applications requiring decent dynamic performance at low speeds. Specifically, low static errors and overshoots, fast reaction to command changes, and wide ranges of torque and speed control are called for. The SV ASDs are employed in printing lines (1:100 speed range), paper machines (1% control error tolerance), steel mills (1:50 speed range), and coating machinery (smooth speed changes). Typical parameters of commercial sensorless drives are:
• speed range from 1:5 to 1:120
• static speed error at the minimum/rated speeds: —3.0% to — 0.1% / 1.6% to 0.0%
• dynamic speed error (at 30-Hz frequency): 2.5%/s to 0.25%/s
• speed control bandwidth (at 30 Hz frequency): 10 rad/s to 20 rad/s
• starting torque from 100% to 150% of the rated torque

For sensorless drives,

commissioning procedures are necessary to determine parameters of the motor used in control algorithms and to set controller gains. Performance requirements dictate accuracy of the parameters, extent of the commissioning tests, and feedback resolution. Many manufacturers of ASDs provide complete drive systems, including motors, which facilitates the commissioning. Certain self-commissioning and auto-tuning functions are incorporated in advanced drives. Fine tuning is also available through user interface devices, including Windows-based software for interfacing the drive with a personal computer.

The highest level of performance is obtained in Field-Oriented ASDs,

also called Flux Vector Control or Full Vector drives, equipped with encoders or resolvers. Speed ranges of FO drives are much wider than these of other ASDs with induction motors, approaching the 1:20,000 value. Bandwidths of the torque and speed control are an order of magnitude greater than those of the sensorless drives; for instance, 1000 rad/s for the torque loop and 100 rad/s for the speed loop. Speed control errors can be as low as 0.01% of the rated speed. Motors produced especially for FO drives, apart from an independent cooling fan, often have an optional integrated encoder. These motors are more expensive than the standard motors, but they can operate in similarly harsh environments. Certain manufacturers of ASDs also equip their motors with a spring-loaded friction brake for rapid stopping of the drive system. High-quality gear boxes of various ratios and configurations are offered with the motors.
The progress in current control techniques and fast semiconductor power switches has made PWM voltage-source inverters to evolve into high-bandwidth power amplifiers. Electric power conversion efficiency
at rated power approaches 98%. Selectable switching frequencies are common. Typical range of output frequencies is 0 to 240 Hz, but frequencies as high as 800 Hz are available in certain drives. For multimotor drive systems, single dc sources, usually with the reverse power flow capability, are employed to supply several inverters. Certain small drives have the power electronic and control circuits integrated with the motor. Typical protection systems include:
• phase-loss protection
• overvoltage protection
• undervoltage tripping (with ride-through options)
• line reactors for protection from the supply voltage transients
• overcurrent protection (plus input fuses)
• short-circuit and ground fault protection
• overtemperature protection
• motor overload (stall) protection
Data recorders and displays allow storing and displaying vital information about normal and faulty operating conditions of the drive. Critical ASDs may be equipped with the Essential Service Override (ESO): In the case of an emergency, the drive is required to run as long as it can, even under the threat of complete destruction.