This questions pertains to a variable speed operations of an

This questions pertains to a variable speed operations of an induction machine

a. How can the speed of an induction machine be controlled by a variable speed drive over a wide range of speeds from close to zero to rated speed?

b. what relationship between voltage and frequency must be established to avoid saturation of the core?

Solution

Ans)

a) The speed of induction motor is controlled by changing the  frequency of the input supply and magnitude of voltage applied to it,In a variable speed drive or variable frequency drive VFD ,both the voltage and frequency of the input is controlled by power electronic circuits like inverter with PWM(Pulse Width Modulation),to get desired speed,With this method you can vary speed from 0 to rated speed

-----------------------------------------

b)The voltage to frequency ration must be kept constant i.e V/F=constant to avoid saturation of the core because the flux in the core is directly proportional to V/F ratio,When we kept it constant the flux in the core is constant .So speed can be varied by changing the voltage and frequency simultaneously so that V?F=constant