QualifiedHigh risk

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.

Start fault-finding

Safety first

A repeatedly tripping overload is protecting something. Don't just wind it up or strap it out. Power terminals are live; rotating machinery can start unexpectedly when reset.

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.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

  1. 1

    Genuine mechanical overload on the driven load

    Most likely

    Seized bearings, a jammed conveyor, a blocked pump, or an over-tight belt makes the motor draw more current than its rating.

  2. 2

    Overload set too low / wrong rating

    #2

    The overload dial is set below the motor's full-load current, or an undersized overload was fitted, so normal current trips it.

  3. 3

    Supply problem (single-phasing or voltage imbalance)

    #3

    A lost phase or unbalanced supply makes the remaining phases draw excessive current, tripping the overload — often shortly after running, not instantly.

  4. 4

    Motor winding fault

    #4

    A partial short or degraded insulation raises current draw and unbalances the phases.

  5. 5

    High ambient or poor cooling

    Least likely

    A blocked fan, clogged cooling fins, or hot environment makes a thermal overload trip earlier than the motor's actual current would suggest.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 4
1

Note when it trips: instantly on start, or after running a while? Check the overload setting against the motor nameplate FLC.

Expected reading

Overload set at or just above the nameplate full-load current.

If it passes

Setting is sensible — measure the actual running current next.

If it fails

Overload set too low or mis-rated — correct the setting/rating before going further.

Often leads toThree-phase equipment single-phasing (lost a phase)Motor goes one way but won't go the other (e.g. down but not up)
View all expected readings at once
1. Note when it trips: instantly on start, or after running a while? Check the overload setting against the motor nameplate FLC.
Overload set at or just above the nameplate full-load current.
2. Clamp each phase current while running (or up to the trip) and compare the three readings to each other and to nameplate FLC.
Three roughly balanced currents, at or below nameplate full-load current.
3. Isolate, lock off, uncouple the load if practical, and turn the shaft by hand. Then check supply voltage balance across the three phases.
Shaft turns freely; the three supply voltages are close to balanced.
4. With the motor isolated and disconnected, carry out insulation resistance and winding balance checks per your test procedure.
Balanced winding resistances and insulation resistance well above the minimum your procedure allows.

Fault-finding flowchart

The same logic as a decision tree.

  1. 1
    start

    Overload keeps tripping

    → step 2
  2. 2
    decision

    Is the overload set at/above nameplate FLC?

    Yes→ step 3No→ step 4
  3. 3
    decision

    Are the three phase currents balanced and within rating?

    Yes→ step 5No→ step 6
  4. 4
    result

    Correct the overload setting/rating to match the motor, then re-test.

  5. 5
    result

    Suspect cooling/ambient or a marginal overload. Check fan, fins, environment.

  6. 6
    decision

    Shaft free and supply voltages balanced?

    Yes→ step 7No→ step 8
  7. 7
    result

    Test motor windings/insulation. Out of spec → motor repair/replace.

  8. 8
    result

    Mechanical overload (seized) or supply fault (single-phasing/imbalance). Chase that first.

Common mistakes apprentices make

  • Winding the overload dial up to stop the tripping instead of finding why current is high — defeating the protection.
  • Reading current on one phase only and missing an imbalance.
  • Forgetting that a trip 'a few minutes in' often points to single-phasing or cooling, not a dead short.
  • Not uncoupling the load to separate a motor fault from a driven-machine fault.

When to stop & escalate

If currents are balanced and within rating but it still trips, or if insulation/winding tests are out of spec, escalate to a motor inspection/rewind. A genuine mechanical jam on the driven equipment should go to the mechanical team.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

General testing

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.

5 causes3 test steps
QualifiedHigh risk
Forward / reverse circuits

Motor goes one way but won't go the other (e.g. down but not up)

A reversing drive works in one direction only. One command (say, down) runs fine; the other (up) does nothing, or just hums/trips. Common on hoists, doors, and conveyors.

5 causes4 test steps
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VSDs Premium

VSD trips on overcurrent / overload

The drive trips with an overcurrent or overload code — on start, on acceleration, or under running load. It may restart and trip again on the same point in the cycle.

5 causes4 test steps
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Learn the theory

How the gear and circuits behind this fault actually work.

Device

Induction motor

The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.

Device

Overload relay

Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.

Principle

Start/stop circuit (seal-in)

A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.

Principle

Single-phasing

What happens when a three-phase load loses one phase — and why it's so damaging to motors.

Principle

No-volt release

After a power cut, equipment stays off until deliberately restarted — preventing dangerous auto-restart.