Why Does a 3-Phase Motor Stall After Losing One Phase?

A 3-phase motor loses speed, growls, or stalls while the panel shows zero current on one line and abnormal current on the other two. Start here: there is no universal current value after a feeding phase opens. Load torque, motor slip, winding connection, supply voltage, and the location of the open circuit determine what you measure.

Stop Trying the Wrong Fixes

• Do not calculate the answer from rated power alone. The balanced-power relationship no longer applies after one line opens.
• Do not hold power factor and efficiency constant. Both change when the motor becomes severely unbalanced and its slip rises.
• Do not assume each surviving line carries 150% current. That shortcut treats the motor as a balanced load. It is not.
• Do not raise the overload setting to keep the motor running. That masks the symptom and increases winding and rotor heating.
• Do not repeatedly reset a tripped overload. Find the open or high-resistance phase before another start attempt.
• Do not replace the motor first. A fuse, contactor pole, termination, cable, or upstream supply can produce the same symptom.

Current alone cannot identify the failed component. Measure all three line currents and all three line-to-line voltages at the same circuit location and under the same operating condition.

Understand What the Motor Is Doing

For a balanced running motor, input power is commonly expressed as P_in = sqrt(3) × V_LL × I_line × PF, with output power equal to P_in × efficiency. That equation assumes balanced three-phase operation. Once a line opens, one current becomes zero at the open point, the other currents are no longer a balanced three-phase set, and a single power-factor value cannot reconstruct the operating current.

The remaining supply creates positive- and negative-sequence magnetic fields rather than the original balanced rotating field. Motor torque falls, pulsating torque appears, and rotor slip rises as the motor tries to carry the mechanical load. Higher slip can drive the two energized line currents upward and produce rapid heating. With a light load, a running motor may continue turning; with a high load, it may slow or stall.

A fixed-impedance classroom model gives a useful warning, not a running-motor prediction. Assume three identical impedances Z that never change:

• For an ungrounded wye load with one line open, two impedances remain in series. Each surviving line carries V_LL / (2Z), and the open line carries zero.
• For an intact delta load with one supply line open, one winding remains directly across the two live lines while the other two form a series path. The winding currents are unequal even though the two incoming line currents have equal magnitude.
• In either idealized connection, the surviving line current is sqrt(3) / 2, about 0.866, of the original balanced line current for the same fixed Z.

That model predicts reduced current because it freezes impedance and ignores torque production. A real induction motor changes effective impedance as slip changes. Constant mechanical load, constant efficiency, and constant power factor therefore cannot all be imposed on the phase-loss calculation.

Start With Line-Current and Voltage Checks

Perform energized measurements only under the site electrical-safety procedure and with instruments rated for the circuit. Start at the motor controller output when accessible. This separates a supply-path problem from a motor or load problem quickly.

1. Record current in all three line conductors at one physical location.
2. Measure each line-to-line voltage pair at that location.
3. Repeat the voltage measurements on the input side of the controller.
4. Compare input and output readings while the fault is present. A normal input with an abnormal output localizes the fault inside or downstream of the controller.
5. Stop the motor if it slows, stalls, trips, smells hot, or shows sustained current imbalance. More runtime does not improve the diagnosis.

Trace the Open Phase

Move through the power path one boundary at a time. Avoid random continuity checks across the entire circuit; parallel paths and motor windings can hide the location of an open conductor.

1. Isolate the circuit according to the site procedure and prove it de-energized.
2. Inspect each fuse and fuse connection. Test the fuse electrically; appearance alone is insufficient.
3. Inspect all contactor poles for damaged contacts, incomplete closure, and heat discoloration.
4. Check terminations for looseness, conductor damage, contamination, and signs of overheating.
5. Test continuity from the controller output to each motor terminal, separating conductors where parallel paths could create a false reading.
6. Compare motor winding resistance using the same instrument, lead compensation, and motor temperature. A clear open circuit or a large unexplained difference requires motor-side investigation.
7. Check the driven machine for binding before reconnecting the motor. A mechanical overload can coexist with the electrical fault.

If the circuit is not completely open, measure voltage drop across each closed pole and connection under load. A high-resistance joint may pass a continuity test while producing damaging voltage imbalance at operating current.

Restore the Circuit in the Right Order

1. Repair or replace the identified open or high-resistance component using its specified rating and installation method.
2. Clean and remake heat-damaged terminations as required; tightening a damaged joint alone can leave excessive resistance.
3. Confirm conductor continuity and phase-to-phase isolation before energizing.
4. Verify that the overload setting matches the motor nameplate and the protection manufacturer's instructions. Return any temporary diagnostic changes to their documented values.
5. Uncouple or unload the motor for the first test when the machine design permits it.
6. Jog the motor briefly to confirm rotation and normal sound, then run it while recording all three line currents and line-to-line voltages.
7. Restore the mechanical load gradually and watch for renewed voltage or current imbalance.

Replacing only an opened fuse can waste time if a weak contactor pole, damaged termination, cable fault, or overloaded machine caused it to open. Find the initiating condition before returning the machine to production.

Verify the Repair Under Load

Verification must reproduce the operating condition that exposed the fault. A no-load run can look acceptable while a resistive connection fails again at higher current.

• Confirm that all three line currents are present and stable.
• Compare current imbalance against the motor and protection documentation rather than inventing a field limit.
• Confirm that all three line-to-line voltages remain stable at the controller input and output.
• Check that speed recovers under the normal mechanical load without abnormal noise or torque pulsation.
• Inspect repaired connections for renewed temperature rise using the site's approved method.
• Test the protective device according to its manufacturer procedure. Confirm that phase-loss or current-imbalance functions are active if the installed device provides them.

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