Page 410 - Power Electronics Handbook

Electrical machine control 399

Due to the high efficiency at all speeds, variable-frequency control is
suitable for motors of all sizes requiring infinite speed variation coupled
with high performance. Frequency control can be accomplished statically
by inverters or cycloconverters, as described in Chapters 10 and 13.
In Chapter 13 various forms of inverter voltage-control systems were
discussed, and Figure 14.50 shows the motor voltage and current for
quasi-square wave control. The stalled current corresponds to that
discussed in Chapter 13, but when the motor is running, the back e.m.f.
distorts this current waveform so that it is now difficult to determine
equations for the ratings of the inverter design.
Figure 14.51 shows the output voltage and current waveforms for a
pulse-width modulated inverter output voltage-control system, in which
the current is seen to be relatively sinusoidal. Since the motor inductance is
low a higher chopping frequency should be used to reduce further the
high-frequency component in the current. This is evident from Figure
14.52, which also shows the current distortion caused by motor back e.m.f.

Figure 14.51 Motor-stalled waveforms for a bridge inverter using pulse-width modulation with
a sine wave. fT = 555 Hz, fs = 20 Hz; (i) voltage; (ii) current

(3) Induced current control
In the previous sections the value of S,,, was controlled by varyingfi. From
equation (14.8) it is seen that the same effect can be accomplished by
changing the rotor resistance R2. Note, however, that S, varies directly as
R2 but inversely as fi so that Figure 14.48 also represents the characteristics
for variable rotor resistance control. There is one important difference
between this method of speed control and that using variable frequency,
since in this instance the stator field speed is constant so that at low speeds
the slip energy is dissipated in the rotor resistance and the efficiency is low.
It is necessary to consider again the principle involved in the transfer of
energy between stator and rotor. If the rotating field gives power P to the