Mechanical Latch for HV Motor Contactor

The latched contactor is essentially identical in construction to a magnetically-held contactor. However, instead of relying on a continuously energized main coil to keep the contactor closed, the latched contactor utilizes a mechanical latch to hold the contactor closed. A magnetically held contactor remains closed as long as the main coil is energised.

For motors mechanical latched contactors are generally not taken as you will not want the motor to start utomatically once the power supply is resumed. Generally mechanically latched contactors are used for loads like lighting transformers where you want the system to resume operation whenever power supply is resumed. Sometimes some motors may also have this requirements depending on what the driven equipment is. In this case the mechanical latched contactor is better than magnetically held contactor because it is not dependent on continuous availability of power to the main coil of the contactor.

I agree they are not a good idea in general, but it is sometimes done on large MV motors because the starting current causes a severe voltage drop and they do not want the contactor to drop out during start. This is only done when the voltage drop is acceptable, essentially making the supply transformer into a saturable reactor for starting the motor. Usually on such systems, there are no other loads nearby that will be negatively affected. The control circuit must then be designed in such a way that if power does completely fail, the "Unlatch" coil of the mecahnically held contactor is always pulsed immediately upon re-energization just to be sure.

Thanks JRaef for the insight. Great help.

Dear Current Maarey & JRaef, I thank you both a lot for explaining me on this subject. I now know the operation of a Mechanical Latch using for Contactor. But still I want to why do we need to use this for a HV Motor Starter Contactor which is powered by DC for it's Contactor Coil? Thanks in advance with Best Regards.