Three-Phase Motor Hums But Will Not Start Single-Phasing Decision Tree

Why this matters

A three-phase motor that hums but will not start, or that runs but trips overloads under load, is the classic presentation of single-phasing: one of the three supply phases is lost, so the motor sees only single-phase power and cannot develop starting torque. The hum is the remaining two phases pulsing without a rotating field. This is destructive in seconds to minutes: the windings on the energized phases draw locked-rotor or heavily unbalanced current and overheat, and a motor allowed to keep humming will burn its insulation. The diagnostic urgency is to de-energize fast, find which phase is missing, and trace upstream to the open - a blown fuse, an open contactor pole, a loose lug, or a lost utility phase. Misreading it as a bad motor when the supply lost a phase replaces a good motor and leaves the real fault.

Symptom presentation

The motor hums loudly and does not turn, or starts only with a manual spin, or runs noisy and hot and trips its overload. On startup it may draw heavy current and trip instantly. Ask what changed: a recent storm, a blown fuse, a contactor that was chattering, or a motor that was fine yesterday and dead today. A motor that hums without rotating is single-phasing until proven otherwise; do not let it sit energized while you think.

Quick checks before isolation

De-energize immediately if the motor is humming - every second of single-phasing cooks the windings. Then, with power restored briefly and the motor disconnected from load if possible, measure the three line-to-line voltages at the motor terminals: L1-L2, L2-L3, L3-L1. Balanced three-phase reads roughly equal on all three (within a few percent). A near-zero reading on one pair, or one phase reading low, identifies the lost or weak phase. Measure the same three voltages upstream at the disconnect and at the panel/feeder to localize where the phase disappears.

Isolation tree

Step 1 - Confirm single-phasing. Read all three line-to-line voltages at the motor. If one pair reads near zero or markedly low while the other two are normal, a phase is open - go to Step 2. If all three are balanced and present at the motor but it still hums and will not start, the supply is good and the fault is in the motor or its starter - go to Step 4.

Step 2 - Localize the open phase upstream. Measure the three phases at the motor disconnect, then at the starter/contactor line and load sides, then at the feeder. The point where a phase is present on the line side but missing on the load side is the open device. Common opens, in field-probability order: a single blown fuse in a fused disconnect (single-phasing a motor by one fuse is extremely common), an open or pitted contactor pole that fails to close, a loose or burned lug, an open overload heater element, and a lost utility phase.

Step 3 - Confirm the device. For a fused disconnect, ohm each fuse out of circuit - an open fuse confirms it; replace all three with the correct type and rating, not just the blown one, and find why it blew. For a contactor, inspect each pole's contacts for pitting and measure load-side voltage with the contactor energized; a pole not making is the open. For a lost utility phase, the open is upstream of the customer service - confirm at the service and hand off to the power company.

Step 4 - Motor or starter fault with good supply. With three balanced phases confirmed at the motor terminals and the motor still not starting, the fault is internal: an open motor winding (megger and measure winding resistance phase to phase, looking for one open or one much higher than the others), a failed start capacitor on a single-phase-converted setup, a seized bearing causing locked rotor, or a phase reversal preventing expected operation. A winding open or a grounded winding confirms the motor.

Confirming the diagnosis

A confirmed single-phasing fault shows one line-to-line voltage pair near zero or low at the motor, traced to a specific open device upstream (blown fuse, dead contactor pole, loose lug) that restores all three balanced phases when corrected, after which the motor starts normally. A confirmed motor fault shows balanced three-phase supply at the terminals but an open or unbalanced winding on a megger/resistance test, or a mechanical lock. Always confirm phase balance is within a few percent after repair, because chronic voltage unbalance also overheats motors (NEMA MG 1 derates a motor sharply above 1 percent unbalance).

Remediation

For a lost supply phase, replace all fuses in a fused disconnect with the correct class and rating and determine the cause, service or replace a contactor with pitted poles, re-terminate loose or burned lugs, and verify balanced three-phase at the motor. Install a phase-loss/phase-monitor relay on critical motors so the next single-phasing event drops the motor before it burns. For a faulted motor, repair (rewind) or replace per the winding test. Verify rotation direction and that all three phases are within the NEMA MG 1 unbalance limit before returning to service.

References

• NEMA MG 1 - Motors and Generators, voltage unbalance derating
• NEC 2023 Article 430 - Motors, Motor Circuits, and Controllers
• NEC 2023 Article 430.32 - Overload protection for continuous-duty motors
• NEC 2023 Article 430.52 - Motor branch-circuit short-circuit and ground-fault protection
• OSHA 29 CFR 1910.147 - Control of hazardous energy (lockout/tagout)