Page 408 - Power Electronics Handbook

Electrical machine control 397
(2) Excitation field speed control
An alternative control method is to vary the speed of the stator field by
controlling the supply frequency, but now the supply voltage must be
changed in proportion to frequency in order to maintain motor flux
constant at its most economical value.
If supply voltage El is changed in proportion to the supply frequency fi
then from equation (14.7) Tm is constant. However, from equation (14.8)
Sm will vary inversely as fi so that as frequency is reduced the starting
torque approaches the peak torque.

I I 1 1 I I I I 1 1 1

s/sm

Figure 14.48 Effect of frequency change, or rotor resistance variation, on the normalised
curve

Fipre 14.49 Stator frequency control