WiremanWireman · Two-speed motor runs at the wrong speed: troubleshooting for electricians · 2023 NEC ·
Two-speed motor runs at the wrong speed
Check the speed contactor, its interlocks and the lead connections against the nameplate.
Before you start
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How it works
Two-speed three-phase motors come in two common types. A consequent-pole motor has one winding that the starter reconnects to double the number of poles, so low speed is half of high speed (for example 1800/900 rpm). A two-winding motor has a separate winding for each speed, so the ratio can be anything (for example 1800/1200 rpm). The starter has a contactor for each speed, interlocked so both can’t close at once, and the leads must land exactly as the nameplate diagram shows.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What is it doing?
- Runs on one speed onlyTest: Check the dead speed’s contactor coilExpect: Rated coil voltage, and the contactor pulls in.
- No voltage at the coilMeasure across each device in that speed’s control circuit. Which one is open?
- Other contactor’s interlock contactInterlock blocking the speed
- Selector switch, push button or timerSelector, push button or timer
- Voltage, but the contactor doesn’t pull inBad contactor coil or contactor
- Contactor pulls in, motor doesn’t run that speedTest: Check the lead connections and windingsExpect: Connections match the diagram, and each set reads balanced within about 5%.
- Leads landed wrongLeads connected wrong
- A winding open or unbalancedWinding fault
- All correctBurned contactor contacts
- No voltage at the coilMeasure across each device in that speed’s control circuit. Which one is open?
- Same speed on both settingsTest: Check the lead connections and windingsExpect: Connections match the diagram, and each set reads balanced within about 5%.
- Leads landed wrongLeads connected wrong
- A winding open or unbalancedWinding fault
- All correctBurned contactor contacts
- Runs backward on one speedOne speed’s phase order is reversed
- Trips when switching high to lowSwitching high to low too fast
- Runs on one speed onlyTest: Check the dead speed’s contactor coilExpect: Rated coil voltage, and the contactor pulls in.