WiremanWireman · Reversing starter problems: troubleshooting for electricians · 2023 NEC ·
Reversing starter problems
One direction dead, fuses or breakers on reversal, or wrong rotation both ways.
Before you start
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How it works
A reversing starter is two contactors, forward and reverse, feeding the same motor. The reverse contactor swaps two of the three phases (typically L1 and L3), which reverses rotation. If both closed at once they would short two phases together, so they are interlocked mechanically (a bar that blocks one while the other is in) and electrically (a normally closed contact from each contactor in series with the other’s coil).
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What’s wrong?
- Works one direction onlyTest: Measure across the interlock contactExpect: Near 0 V across the button and the interlock contact (both closed), and full control voltage across the coil.
- Full voltage across the interlock contactThe interlock contact on the opposite contactor is open
- Full voltage across the buttonBad push button or wiring for that direction
- Coil has voltage but won’t pull inCoil or contactor on that side
- Trips or blows fuses when reversingDoes it trip when you go straight from one direction to the other while the motor is still turning?
- Yes, only on a quick reversalPlugging: reversing before the motor stops
- It trips even from a stopTest: Check that both contactors can’t close togetherExpect: Only one contactor can close at a time, mechanically and electrically.
- Both can close togetherThe interlocks don’t prevent both contactors closing
- Interlocks are fineCheck the motor and its leads
- Runs backward in both selectionsPhase order is reversed at the supply or motor
- Hums or single-phases in one directionTest: Check voltage across each pole in the bad directionExpect: Near 0 V across each closed pole. More than a volt or two means a burned contact.
- One pole reads highBurned contact on one contactor
- All poles near 0 VCheck the motor and its leads
- Works one direction onlyTest: Measure across the interlock contactExpect: Near 0 V across the button and the interlock contact (both closed), and full control voltage across the coil.