Single-phasing draws roughly 173% current on the remaining legs and cooks windings fast.
1
Blown fuse on one leg
criticalCheckWith the machine safely isolated for the connection, measure voltage L1-L2, L2-L3, L1-L3 at the load side of the fuses. One pair reading low or zero indicates an open leg.
FixReplace all three fuses as a set; find why the one leg failed before re-running.
ToolsMultimeter (CAT III 600V+)
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2
Burnt or pitted contactor pole
highCheckDe-energise and inspect all three contact sets; measure voltage drop across each pole under load (should be near zero).
FixReplace the contactor — do not file contacts.
ToolsMultimeter · Clamp meter
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3
Loose lug or corroded termination
highCheckThermal image the panel under load; check torque on every power lug against the OEM value.
FixClean, re-terminate and torque properly. Replace heat-damaged lugs and conductors.
ToolsThermal camera · Torque screwdriver
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4
Utility phase loss or severe voltage imbalance
highCheckMeasure all three legs at the service; calculate imbalance (max deviation from average ÷ average). Above 2% is unacceptable.
FixReport to the utility; fit a phase-loss / phase-imbalance monitor relay to protect the motor.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.