Answer Posted / rajesh

In any process plant or factory , we need motor to move /
compress / pump/blow/ventilate etc . in motors we have dc ,
ac , synchronous motors . dc motor are not as efficient as
ac 3 phase motor , hence we use ac motor .

now a 3 ph ac motor has nonlinear speed /torque
characteristics u can see in any website . this means that
for any torque desired speed is not constant .to achieve
this we have to keep supply voltage to frequncy constant .
this is done by vfd . as such variation of suply voltage is
impossible hence we vary frequency .
3 types 1.pwm 2. direct control etc

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