Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
What it is
An overload relay protects a motor against sustained over-current — the kind caused by a jammed load, single-phasing, or a motor working too hard. It's set to the motor's full-load current.
How it works
A thermal overload passes the motor current through bimetallic strips. Excess current heats them; they bend, and after a time delay they operate a contact that breaks the contactor's coil circuit — dropping the motor out.
The time delay is deliberate: it rides through the brief high current of starting but trips on a sustained overload. Electronic overloads do the same job by measuring current directly.
Where it's used
Fitted to almost every motor starter, between the contactor and the motor. It's set (in amps) to the motor's nameplate full-load current.
If it keeps tripping, it's usually protecting against something real — don't just wind it up. Causes include a mechanical overload, single-phasing, a winding fault, or a setting that's too low.
Safety first
A tripping overload is protecting the motor — find why before resetting. Resetting can restart machinery.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
Overload relay won't reset
After an overload trip, the reset won't take — the starter won't re-arm, or it resets and trips straight back. The control circuit stays broken at the overload's contact.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.