
Inverter-Fed Induction Motor Drives 217
because the winding insulation will have been designed to
withstand not more than the rated voltage.)
Established practice is for the inverter to be capable of
maintaining the V/f ratio constant up to the base speed (50 Hz
or 60
Hz),
but to accept that at all higher frequencies the
voltage will be constant at its maximum value. This means
that the flux is maintained constant at speeds up to base
speed, but beyond that the flux reduces inversely with fre-
quency. Needless to say the performance above base speed is
adversely affected, as we shall see.
Users are sometimes alarmed to discover that both voltag ...