Load Commutated Inverter - How does it work

Simply put, an LCI drive is a way to perform speed control (or low current soft starting) on very large motors by using low cost but very rugged SCR technology instead of transistors (or GTO thyristors). The problem with SCRs is that they cannot be gated off (called "commutation"); you can remove the gate power, but they only stop conducting when the current going though them crosses zero.

First a comparison to understand what an LCI is NOT. A PWM drive functions by creating high speed pulses of DC in a pseudo AC format using transistors. A GTO (Gate Turn Off) thyristor is similar to an SCR but requires a separate gate control signal to turn off and was used for a time on CSI (Current Source Inverters) but required impedance matching with the motor in order to function. CSI drives were the first technology to fall by the wayside with the advent of cheap PWM (although they still have their place)

An LCI drive using SCRs does not need that 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.

Hope that helped. GE offers a 3 day course to teach you what I just said, so obviously this is boiled down quite a bit!