A three-phase motor winding consists of three sets: U, V and W. There are two internal connection types: Star (Y) and Delta (△).
Multimeter setting: Resistance mode (low resistance range)

I. Continuity and Integrity Test of Three-Phase Windings
(Conduct test with power disconnected; all external power cables must be detached)
1. Star Connection (neutral point connected together; three live leads plus an optional neutral wire)
Measure resistance between each pair: U-V, V-W, W-U
Normal condition: The resistance values of the three pairs are roughly equal with minimal deviation.
Measure resistance between each live wire and the motor housing (ground wire/casing)
Normal condition: Infinite resistance (intact insulation)
If measurable resistance exists between winding and housing: Winding leakage or ground fault; the motor cannot be put into service.
Fault Judgment Criteria:
Infinite resistance on any pair: Open circuit in the corresponding phase winding (internal wire breakage)
Significantly lower resistance on any pair: Inter-turn short circuit
Large discrepancy among the three resistance values: Partial winding burnout
2. Delta Connection (three windings connected end-to-end; only three outgoing leads)
Measure inter-phase resistance U-V, V-W, W-U as well.
Normal condition: Balanced resistance across three phases;
Open circuit on any phase indicates broken winding.
II. Distinguish Internal Winding Phase Loss vs. External Circuit Phase Loss
Many technicians fail to tell whether phase loss originates from the motor itself or faulty cables, contactors or circuit breakers.
Method 1: Winding resistance test with power off (to identify motor internal faults)
Remove all external wires from the motor terminal block
Test three-phase windings independently:
All three inter-phase paths conduct with balanced resistance → Motor windings are intact; phase loss comes from external wiring
One phase shows complete open circuit → Broken internal winding; phase loss is a motor inherent fault
Method 2: No-load voltage test with power on (strictly observe electrical safety during live test)
Switch multimeter to AC voltage mode and measure line-to-line voltage of the three incoming cables: AB, BC, CA
All three line voltages close to 380V → Stable power supply
One pair shows drastically low or zero voltage → Defective upstream circuit breaker, contactor contacts or broken cable (external phase loss)
III. Typical Symptoms of Motor Operation Under Phase Loss
Weak startup torque with constant buzzing; motor fails to rotate, and sustained power supply will burn windings rapidly
Barely rotates with reduced rated speed and abnormal overheating of the casing
Severe unbalance of three-phase current; one phase carries far higher current than the other two
Frequent overload tripping of thermal relays
IV. Step-by-Step Troubleshooting Procedure
Cut off power and disconnect all external cables at the motor terminals
Use multimeter resistance mode to test winding continuity and insulation to ground
✅ If windings test normal: Inspect upstream components including circuit breakers, fuses, contactor contacts and cable joints
❌ If one phase winding has an open circuit: Damaged motor coils; motor needs rewinding or full replacement
After repairing external wiring, verify balanced three-phase voltage under power supply before trial operation
V. Practical Tips
Three-phase motors must never run for extended periods with missing phase; full winding burnout can occur within minutes.
Install phase loss/phase sequence protectors for high-power motors to implement upstream preventive protection.
A megohmmeter (insulation resistance tester) delivers more accurate insulation measurement than a standard multimeter when available.