Hard starts, contactor chatter, or breaker trip during S→D transition.
1
Open transition timing wrong
CheckTime S→D should be 0.5–2.5 sec (per starter). Motor must reach ~75% speed before transition.
FixAdjust timer per OEM specs.
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2
Wye contactor not dropping out
criticalCheckAux contact wiring; mechanical interlock between S and D contactors.
FixReplace contactor / repair interlock — risk of phase-to-phase short.
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3
Delta (run) contactor not pulling in
CheckCoil voltage after timer transition.
FixRepair control circuit / replace contactor.
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4
Motor leads wired wrong (T1–T6)
CheckVerify per nameplate diagram: T4 to T1+R, T5 to T2+S, T6 to T3+T after delta close.
FixRe-terminate per diagram.
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5
OL relay sized wrong for delta connection
CheckHeater = motor FLA × 0.58 (line current in delta winding when OL is inside the delta).
FixResize per OL location in the circuit.
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6
Closed-transition resistor bank failed
CheckVisual; resistance check on transition resistors.
FixReplace resistor bank; verify contactor sequencing.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.