Reacceleration Schemes LV Motors

Whenever an AC motor is disconnected from it's source, the magnetic fields in the core take a bit of time to collapse because of residual magnetism in the steel components. If the motor is re-connected before that happens, the motor is essentially a generator that is out of phase with the line source and a large and dangerous transient of voltage and current can cause protective devices to trip off line, not to mention potential damage to nearby equipment. A minor power loss, such as a utility grid switch, automatic transfer switch changeover etc., can cause the motor controller to turn off and if an automatic control system immediately reconnects it, the damage is done. So a re-acceleration scheme prevents this by forcing a delay of sufficient time in between re-starts to allow the motor residual magnetism to dissipate.

We tend to only worry about it on large motors because on small motors, the effect is somewhat negligible due to having less steel and it's difficult to justify spending budget money to prevent. But it can happen on any motor and in some critical systems, I have seen it done on as small as 5HP motors, especially if connected to high inertia loads such as centrifuges.