Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Safety first
Repeated start attempts heat the motor and stress the supply. Limit attempts. The motor may start unexpectedly when protection is 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
Protection too sensitive for the inrush
Most likelyA breaker/overload with the wrong trip characteristic or setting trips on normal starting current.
- 2
High load inertia / hard start
#2A high-inertia or loaded start draws heavy current for longer, tripping time-limited protection.
- 3
Star-delta / starter not reducing starting current
#3A reduced-voltage starter that isn't working (e.g. stuck in delta or failed star) lets full inrush through.
- 4
Weak supply sagging on start
Least likelyA weak supply that dips on inrush can cause protection or drive faults at start.
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.
Identify which device trips and its type/setting; compare to the motor's starting requirements.
Protection rated/charactered to ride through normal starting.
Protection suits the motor — look at load inertia and the starter.
Wrong characteristic/setting — select protection suited to the start.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Trips on start only
→ step 2 - 2decision
Is the protection's characteristic/setting suited to starting?
Yes→ step 3No→ step 4 - 3decision
Is the reduced-voltage starter actually limiting inrush?
Yes→ step 5No→ step 6 - 4result
Wrong characteristic/setting — select suitable protection.
- 5decision
Are starting current/time and supply within limits?
Yes→ step 7No→ step 8 - 6result
Starter not reducing inrush — repair it.
- 7result
Within limits — investigate intermittent protection behaviour.
- 8result
Excess inrush or supply sag — address load/supply.
Common mistakes apprentices make
- Upsizing protection to stop the trips instead of fixing the start.
- Not realising a reduced-voltage starter has stopped working.
- Ignoring a supply that sags badly on inrush.
- Repeatedly slamming start and overheating the motor.
When to stop & escalate
If the start genuinely needs more than the supply/protection can give, the starting method or supply should be reviewed at design level rather than defeating protection.
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
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
Soft starter faulting or not ramping the motor
A soft starter won't ramp the motor up smoothly — it faults out, the motor jerks or doesn't reach full speed, or the bypass doesn't engage.
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.
Learn the theory
How the gear and circuits behind this fault actually work.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Inrush current
The brief, high current many loads draw at switch-on — and why it trips protection if not allowed for.
Motor slip & torque-speed
Why an induction motor must run slightly slower than its field, and how its torque changes from start to full speed.