A short interruption can stop a contactor while the process and rotor continue moving. Reclosing without a sequence review can produce mechanical shock, high current or an unsafe unattended start.
| Dip outcome | Required decision | Evidence |
|---|---|---|
| Contactor holds | Ride-through remains acceptable | Coil and control-voltage test |
| Contactor drops | Restart allowed or inhibited | Cause-and-effect logic |
| Motor coasts | Residual speed/direction known | Coast-down data |
| Multiple motors return | Staggered priority required | Bus reacceleration study |
Define restart permission
Classify each motor by process consequence. Pumps may need valve state and minimum-flow checks; conveyors may contain material; compressors may need unloading and pressure equalisation. A VFD flying start must be commissioned for both speed and direction ranges actually permitted.
Retain event duration, minimum voltage, contactor states, speed, current and process permissives from witnessed tests. Do not increase protection delays merely to force a restart through an unexplained condition.
Related reading: VFD flying start and motor starting voltage drop.