Mechanical Latch with a Vacuum Contactor

When using a vacuum contactor, it is essentially a given that the motor is going to be very large because the need for that technology and extra expense is difficult to justify for small motors. So given thatcondition, there is an inherent danger in having a large rotating mass such as a big induction motor come off-line and then immediately back on-line, as might be the case if there is a momentary voltage drop below the hold-in threshold of the contactor coil. What can happen is that the motor starting CAUSES the voltage drop, which dips below the hold-in of the coil, the coil lets go, the contactor opens, the cause of the voltage drop goes away and immediately the coil voltage goes back up, the contactor then recloses. But the motor field was still colapsing, so when re-close, the motor is still acting as a generator for a few seconds. The resulting voltage spike of that regenerating motor being re-closed into the line can cause massive damage.

So a latching coil is used in the vacuum contactor so that the contactor will NOT open during a severe voltage drop caused by the motor itself.