Two-speed motor won't change speed
A two-speed (e.g. Dahlander or two-winding) motor runs on one speed but won't switch to the other — the speed-change contactors or windings aren't doing their job.
Safety first
Two-speed schemes use multiple contactors with interlocks; an incorrect closure can short windings. Isolate and prove dead before working in the starter.
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
Speed-select contactor not operating
Most likelyThe contactor for the unavailable speed isn't energising — coil, control, or interlock fault.
- 2
Speed-change interlock holding it off
#2The interlock that prevents both speeds at once is stuck or mis-wired, blocking the change.
- 3
Winding connection fault for that speed
#3The winding configuration (star/delta or separate winding) for the missing speed has a fault.
- 4
Control selection fault
Least likelyThe speed-select switch/PLC output for that speed isn't being commanded.
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 ModeSelect the missing speed and check whether its speed contactor energises.
The selected speed's contactor pulls in.
Contactor operates — check the winding connection for that speed.
Contactor doesn't operate — check its control path and interlock.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Won't change speed
→ step 2 - 2decision
Does the selected speed's contactor energise?
Yes→ step 3No→ step 4 - 3decision
Are the winding connections for that speed correct?
Yes→ step 5No→ step 6 - 4decision
Is the control selection complete and interlock allowing it?
Yes→ step 7No→ step 8 - 5result
Re-verify the contactor contacts/motor.
- 6result
Winding connection fault — correct per the diagram.
- 7result
Command/interlock fine — see contactor coil fault-finding.
- 8result
Open selection or stuck interlock — repair (never bypass).
Common mistakes apprentices make
- Defeating the speed-change interlock and risking a winding short.
- Assuming a motor fault when a speed contactor isn't operating.
- Not checking the winding connection for the missing speed.
- Overlooking the speed-select control path.
When to stop & escalate
Two-speed winding configurations are easy to get wrong — work to the motor/starter diagram and never bypass the interlock. Winding faults go to a motor specialist.
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.
Was this guide helpful?
Related faults
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.
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.
Forward/reverse interlock locking out both directions
Neither direction will run — the interlock that stops both contactors closing together appears to be holding everything off, so no movement at all.