Page 119 - Servo Motors and Industrial Control Theory -

6.5 Conclusion 113

a
ω o := τ + 2 e (6.9)
s1

The time constant ( τ) is usually given by the manufacturer of frequency converter.
The time constant may vary from a few milliseconds to 10–30 s depending on the
rated power of the motor and frequency converter. The gain (a1) changes with fre-
quency because of the nonlinear characteristics of output voltage and the error sig-
nal ( e) which is more pronounced at low frequency.
The Eqs. (6.8) and (6.9) describe the dynamic behavior of frequency converter.
However, to model the complete system, that is, the converter, motor, and load, the
voltage equation of (6.8) and the operating point of the constant, must be used; and
Eq. (6.7) must be used to model the dynamic behavior of the model. The frequency
changes are inherent in the system and will be accounted for implicitly. The dy-
namic behavior of other part of the system is similar to DC servo motors.

6.5 Conclusion

In this chapter, the properties of AC induction motors and various frequency con-
verters were studied. The main object was to describe the principle behavior when
AC induction motors are used as servo motors. They are very reliable because there
is no brush. The three phase motors are considered because their speed changes with
frequency and this is the best method of changing speed.
There are various research centers that are studying how to improve the dynamic
behavior of AC servo motors especially at low frequency. The reader is encouraged
to obtain a catalogue of a typical AC motor and a catalogue of a matching frequency
converter and to model the complete system. This is a good exercise to study the dy-
namic behavior of AC servo motors in speed or position control. It should be noted
that AC motors are available at various speed ratings and the reader should study
the benefits of using gearboxes to reduce the speed. The procedure is similar to that
described for DC servo motors.