Wireman · Motor won’t stop (welded contacts): troubleshooting for electricians · 2023 NEC ·

Motor won’t stop (welded contacts)

Make it safe first, then find out whether the contactor is stuck or the control circuit is still calling.

Before you start

How much electrical experience do you have?

This sets how much detail each step shows. You can change it any time.

How it works

Pressing stop opens the control circuit, the contactor coil loses voltage, and springs pull the power contacts apart. If the motor keeps running, one of two things is true: the coil is still getting voltage (the control circuit has a path around the stop button), or the coil is dead but the contacts are stuck shut, usually welded by heavy current or held by a jammed armature.

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

First make it safe: open the motor’s disconnect or breaker. Did the motor stop?
- Yes, it stoppedBefore you opened the disconnect, what did the contactor do when stop was pressed?
  - Stayed pulled inTest: Is the coil still getting voltage after stop?Expect: 0 V across the coil while stop is pressed.
    - 0 V, contactor still closedTest: Check the power contacts with power offExpect: OL (open) across every pole with the coil off.
      - One or more poles read near 0 ΩWelded contactor contacts
      - All open, armature moves freelySticking that cleared once power was off
      - Armature jammed or sluggishThe armature is mechanically stuck
    - Coil still has voltageWhat else can energize this coil?
      - HOA in Hand, or a device still callingThe control circuit is still calling for the motor
      - PLC output or solid-state relayPLC output stuck on, or leakage holding the coil
      - Just a stop/start stationThe stop button or its wiring has failed closed
    - Can’t test live (control power from the disconnect)Test: Check the power contacts with power offExpect: OL (open) across every pole with the coil off.
      - One or more poles read near 0 ΩWelded contactor contacts
      - All open, armature moves freelySticking that cleared once power was off
      - Armature jammed or sluggishThe armature is mechanically stuck
  - Dropped out, motor kept runningPower is going around the contactor
  - Not sureTest: Is the coil still getting voltage after stop?Expect: 0 V across the coil while stop is pressed.
    - 0 V, contactor still closedTest: Check the power contacts with power offExpect: OL (open) across every pole with the coil off.
      - One or more poles read near 0 ΩWelded contactor contacts
      - All open, armature moves freelySticking that cleared once power was off
      - Armature jammed or sluggishThe armature is mechanically stuck
    - Coil still has voltageWhat else can energize this coil?
      - HOA in Hand, or a device still callingThe control circuit is still calling for the motor
      - PLC output or solid-state relayPLC output stuck on, or leakage holding the coil
      - Just a stop/start stationThe stop button or its wiring has failed closed
    - Can’t test live (control power from the disconnect)Test: Check the power contacts with power offExpect: OL (open) across every pole with the coil off.
      - One or more poles read near 0 ΩWelded contactor contacts
      - All open, armature moves freelySticking that cleared once power was off
      - Armature jammed or sluggishThe armature is mechanically stuck
- No, it still runsThe motor is fed from somewhere other than that disconnect