Star-delta starter trips the overload at the star-to-delta changeover
The motor starts fine in star, but at the moment it switches to delta the overload (or breaker) trips. Stays running if it survives the transition; trips reliably on the changeover.
Safety first
Changeover faults involve heavy currents and fast switching between live contactors. Work to the star-delta control drawings, confirm isolation, and don't defeat the transition timing or interlocks to 'get it going'.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Transition timer too long (motor slows in open transition)
Most likelyIf the open-transition gap is too long, the motor slows and the inrush when delta closes is large enough to trip the overload/breaker.
- 2
Changing to delta under too much load
#2If the driven load is too heavy or the timer switches before the motor is near speed, the delta current surge on changeover exceeds the protection.
- 3
Incorrect delta wiring / winding connection
#3A wiring error in the delta connection (or a swapped winding end) causes a large current imbalance the instant delta is made.
- 4
Overload in the wrong position or set wrong
Least likelyIn star-delta the overload position and setting matter (it senses line or phase current depending on design). A wrong setting trips on normal changeover current.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeObserve the start sequence and time the star period; confirm the motor is near full speed before transition.
Motor reaches near full speed in star before switching to delta.
Timing looks right — check the delta wiring and overload setting.
Switching too early (or too late with a long open gap) causes the surge — adjust the transition timing.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Trips at star-to-delta changeover
→ step 2 - 2decision
Is the motor near full speed before transition?
Yes→ step 3No→ step 4 - 3decision
Is the delta connection wired correctly?
Yes→ step 5No→ step 6 - 4result
Transition timing wrong — adjust so delta closes near full speed.
- 5decision
Is the overload positioned and set correctly?
Yes→ step 7No→ step 8 - 6result
Delta miswired — correct against drawing and motor terminal markings.
- 7result
Load may be too heavy for star start — review the starting method (escalate).
- 8result
Overload wrong position/setting — correct it.
Common mistakes apprentices make
- Lengthening the transition timer the wrong way and making the surge worse.
- Assuming the overload is faulty when the delta is miswired.
- Not confirming the motor is near speed before delta closes.
- Setting the overload as if it sees line current when it sees phase current (or vice versa).
When to stop & escalate
If the load genuinely can't be started in star, escalate to review the starting method (e.g. whether soft-start/VSD suits the load) rather than forcing the changeover. Don't bypass the transition logic to mask the trip.
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.
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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.
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.
Motor overload trips after running for a while
The motor starts and runs fine, then the thermal overload trips minutes or hours later. Reset it and it runs again before tripping once more — a heat/loading pattern, not a start fault.