WiremanWireman · Contactor won’t pull in or chatters: troubleshooting for electricians · 2023 NEC ·
Contactor won’t pull in or chatters
Trace the control circuit, find the open device, check the coil.
Before you start
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How it works
A contactor is a big relay. When its coil gets control voltage, it pulls in and closes the power contacts. The control circuit is a series path: stop button, start button, overload contact, safeties, then the coil. If any device in the series is open, the coil gets nothing.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Test: Measure the coil voltageExpect: The coil’s rated voltage, within about 10%.
- Full rated voltage, contactor doesn’t moveTest: Check the coilExpect: A steady reading, typically a few ohms to a few hundred ohms depending on coil voltage. Not 0 and not OL.
- OL (open)Open coil
- Near 0 ΩShorted coil
- Normal readingMechanical binding
- 0 VTest: Find the open deviceExpect: Voltage present at every point up to the coil.
- No control power at allNo control power
- Voltage stops at the overload contactThe overload relay has tripped
- Voltage stops at a button or safetyAn open control device
- Low voltage, it chatters or buzzesLow control voltage
- Full rated voltage, contactor doesn’t moveTest: Check the coilExpect: A steady reading, typically a few ohms to a few hundred ohms depending on coil voltage. Not 0 and not OL.