
192 Electric Motors and Drives
condition, the rotor and stator currents will be at their rated
values.
Now suppose that the voltage falls to 90 per cent. The
load torque is assumed to be constant so the new operating
point will be at Y. Since the air-gap flux is now only 0.9 of its
rated value, the rotor current will have to be about 1.1 times
rated value to develop the same torque, so the rotor
e.m.f.
is
required to increase by 10 per cent. But the flux has fallen by
10 per cent, so an increase in slip of 20 per cent is called for.
The new slip is therefore 6 per cent.
The drop in speed from 95 per cent of synchronous to 94
per cent may wel ...