Answer / raghavendran.r

An LCI drive using SCRs does not need extra forced turn-off and does not use PWM to vary the frequency. It uses SCRs on the front-end to covert the incoming AC to variable voltage DC, then uses multiple SCRs on the back end to fire into the motor at a varying frequency, but relies upon the motor's rotation itself to make the current passing through the SCRs to cross through zero and turn off (hence the term "Load Commutated"). This is done through very complex firing schemes that require a relatively high powered computer to accomplish. That's why you will not see LCI drives on small motors; the cost is prohibitive and LCI drives cannot compete with PWM where PWM is effective and reliable. But on very large synchronous motors the transistor technology becomes a quantum leap more expensive and highly susceptible to damage, so LCI drives start to make sense again.

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