WiremanWireman · Motor starts, then stops: troubleshooting for electricians · 2023 NEC ·
Motor starts, then stops
Find out what opens: the seal-in, an overload, a protector, a control switch or the supply.
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How it works
A motor that starts proves the power circuit and the motor can work. Something then takes it away: the control circuit (a missing seal-in, a pressure or float switch, a protective relay), a protective device (the overload relay or an internal thermal protector), or the supply dropping out under load. How long it runs and what has opened when it stops point to which one.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- How long does it run?
- Only while the start button is heldNo seal-in (holding) circuit
- Seconds to minutes, then stopsWhen it stops, what has opened?
- The overload relay trippedOverload relay is tripping
- Motor’s internal protector (restarts after cooling)Test: Measure running currentExpect: At or below nameplate FLA.
- Above nameplateMotor is drawing too much current
- At or below nameplateMotor overheating at normal current
- Contactor dropped out, nothing trippedTest: Watch the coil voltage as it drops outExpect: Steady rated coil voltage the whole time it runs.
- Goes to 0 V when it stopsMeasure across each device in the control circuit while it’s stopped. Which one has voltage across it (is open)?
- Pressure, float, flow or temperature switchProcess switch is stopping it
- Phase monitor or protective relayProtective relay trips
- Timer, PLC output or interlockTimer, PLC or interlock
- Sags low before it dropsControl voltage sags
- Stays at full voltage, contactor drops anywayContactor fault
- Goes to 0 V when it stopsMeasure across each device in the control circuit while it’s stopped. Which one has voltage across it (is open)?
- The breaker or fusesBreaker or fuses opening
- Starts and stops over and overIs it controlled by a pressure, float or temperature switch?
- YesShort cycling on the control switch
- NoWhen it stops, what has opened?
- The overload relay trippedOverload relay is tripping
- Motor’s internal protector (restarts after cooling)Test: Measure running currentExpect: At or below nameplate FLA.
- Above nameplateMotor is drawing too much current
- At or below nameplateMotor overheating at normal current
- Contactor dropped out, nothing trippedTest: Watch the coil voltage as it drops outExpect: Steady rated coil voltage the whole time it runs.
- Goes to 0 V when it stopsMeasure across each device in the control circuit while it’s stopped. Which one has voltage across it (is open)?
- Pressure, float, flow or temperature switchProcess switch is stopping it
- Phase monitor or protective relayProtective relay trips
- Timer, PLC output or interlockTimer, PLC or interlock
- Sags low before it dropsControl voltage sags
- Stays at full voltage, contactor drops anywayContactor fault
- Goes to 0 V when it stopsMeasure across each device in the control circuit while it’s stopped. Which one has voltage across it (is open)?
- The breaker or fusesBreaker or fuses opening