Page 211 - Electric Drives and Electromechanical Systems

206 Electric Drives and Electromechanical Systems

Work by Sunter and Clare (1996) has demonstrated that the matrix controller can be
used to provide servo-grade performance, in particular high-speed reversal, when used
as a power controller within a vector controller.
As noted in Wheeler et al. (2002), the matrix converter can be used to provide the high
power quality required for vector controllers. The problems with over currents and
voltage spikes resulting from the arrangements of the power switches can be minimised
by the use of soft commutation techniques.

7.5 Summary

The use of vector-controlled induction motors represents an alternative to the other
forms of brushless motors for servo drives. In the selection of a vector-controlled
induction motor, the following points need to be considered, particularly when they
are being compared with permanent-magnet sine-wave-wound motors:
Induction motors are inherently more difficult to control.
Induction-motor drives are typically larger than permanent-magnet sine-wave
wound motor-drives, for identical output powers; this is because of the rotor power
loss in induction motors. Therefore, a provision may have to be made for forced
cooling.
For the same output torque, the efficiency (which directly dictates the frame size)
is lower for induction motors. Permanent-magnet sine-wave wound machines have
of higher efficiencies because of the lack of any rotor losses.
Induction motors can be designed for higher flux densities than those of
permanent-magnet sine-wave-wound motors, which are limited by the design of
the rotor and its permanent magnets.
Induction motors cost less than the equivalent permanent-magnet sine-wave-
wound motors, due to their simplicity and lack of permanent magnets.
In induction motors, field weakening is easily achieved over a wide speed range;
this is not possible in permanent-magnet sine-wave-wound motors.
The vector control of induction motors requires a considerable amount of
computing power, and while microprocessors are an advantage they are not a
necessity. In safety-critical applications, the use of motors incorporating sophisti-
cated microprocessor-based controllers may constitute an undue safety risk.

This chapter has reviewed vector control applied to squirrel-cage induction motors;
the resultant characteristics are suitable for servo applications. This development
represents significant alternative for design engineers. Vector-controlled induction
motors are rapidly becoming accepted as the preferred choice for high-power servo
applications. Their undoubted advantages and disadvantages need to be critically
compared (because of their complexity and hence their cost) when they displace
conventional servo drives from any application. It should be recognised that induction