Troubleshooting guide

Motor Keeps Tripping the Overload? Single-Phasing vs. a Real Overload

The overload relay is a messenger, not the culprit. Clamp the three phase currents before you reset it again — the way they go unbalanced is the entire diagnosis.

Last updated: August 2026 · written and reviewed by a working maintenance professional

A motor trips its overload, someone resets it, it runs for a bit, and it trips again. Reset number three is where good motors get cooked — because each reset on a motor that's single-phasing overheats the windings a little more.

Here's the thing the reset button hides: the overload relay reacts to current. It rarely creates the fault. So before you reset it a third time or reach for the adjustment screw, put a clamp meter on all three phases and read what it's actually reacting to. Three numbers tell you which of three problems you have.

Balanced is healthy. How it goes unbalanced is the answer

Two phases high, one at zero — single-phasing. A phase has been lost upstream.
All three high but equal — a genuine mechanical overload.
One phase low and jumping around — a loose, high-resistance connection.

Single-phasing: humming but won't pull

A motor that hums loudly, won't start turning, and trips within seconds is the classic. Clamp the phases and you'll see two legs spiking while the third reads zero — the motor is trying to run on two phases instead of three. The motor didn't fail; it lost a leg.

Find the lost phase upstream: a blown fuse, an open disconnect contact, a broken conductor, or a contactor pole that isn't closing. Replace or repair it — but find out why the fuse blew before you restart, or you'll blow the new one and be right back single-phasing. Whatever you do, don't rewind a perfectly good motor that was simply starved of a phase.

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A real overload: balanced, but high

If the motor runs a few minutes and then trips, and all three phases read equal but above the nameplate rating, the electrical side is healthy — the motor is genuinely overloaded by what it's driving. A jammed pump, a fouled impeller, a blocked discharge, a seized bearing downstream. Fix the load. Turning up the overload here just removes the protection on a motor that's working too hard, and you'll lose the windings.

A loose connection: low and wandering

Random, intermittent trips — fine for hours, then a trip — with one phase reading a little low and unsteady, point at a loose or corroded terminal. A high-resistance joint carries less current, heats up, and eventually drops out. Left alone, it arcs and burns until it opens completely and single-phases the motor. Clean, re-terminate, and torque all three connections to spec.

The one instant-trip exception

If the motor trips the instant it starts — not after minutes — with all three phases present, kill the power and try to turn the shaft by hand. If it won't budge, you've got a locked rotor: the motor is trying to start against a seized load and drawing enormous locked-rotor current. Free the mechanical seize; the trip was protecting the motor.

Two minutes with a clamp meter across all three phases turns "the motor keeps tripping" into a specific, fixable fault — and keeps you from cooking a motor with the reset button.