WiremanWireman · Soft starter faults: troubleshooting for electricians · 2023 NEC ·
Soft starter faults
Generic fault categories: supply, SCRs, bypass, start settings and load.
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How it works
A soft starter ramps a motor up by using SCRs to chop the voltage, then once the motor is up to speed, many models close a bypass contactor so the SCRs stop carrying current. It controls voltage, not frequency, so it reduces starting current and torque but can’t make the motor run at a different speed. Faults usually fall into a few groups: supply problems (phase loss, wrong rotation), power device problems (a shorted SCR), a start that takes too long, and overload.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Which kind of fault does the display or manual describe?
- Phase loss or phase sequenceTest: Measure the incoming phasesExpect: Three equal readings within about 1–2% of each other and within the starter’s rated voltage range.
- One phase missing or lowLost or weak phase on the supply
- Balanced, but sequence is reversedPhase sequence reversed
- Balanced and correctLook at the motor and its leads
- Shorted SCR or power deviceTest: Check each pole for a shorted SCRExpect: A high reading on every pole, with all three similar. A few ohms or less means a shorted device.
- One pole reads very lowShorted SCR
- All three high and similarCheck for capacitors and the bypass contactor
- Start too long, stall or current limitHas the load or the start setting changed?
- Heavier load or a tight machineLoad too heavy for the start profile
- Settings were changed, or never tunedSettings don’t match the motor
- Overload or overcurrent while runningTest: Measure running currentExpect: At or below FLA and within about 10% of each other, with the starter’s setting matching the nameplate.
- Current high on all linesLoad too heavy for the start profile
- UnbalancedLook at the motor and its leads
- Normal, setting is lower than FLASettings don’t match the motor
- Phase loss or phase sequenceTest: Measure the incoming phasesExpect: Three equal readings within about 1–2% of each other and within the starter’s rated voltage range.