AdvancedHigh risk

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.

Start fault-finding

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.

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

Likely causes

Ranked from most to least likely.

1
Speed-select contactor not operating
Most likely
The contactor for the unavailable speed isn't energising — coil, control, or interlock fault.
2
Speed-change interlock holding it off
#2
The interlock that prevents both speeds at once is stuck or mis-wired, blocking the change.
3
Winding connection fault for that speed
#3
The winding configuration (star/delta or separate winding) for the missing speed has a fault.
4
Control selection fault
Least likely
The speed-select switch/PLC output for that speed isn't being commanded.

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

Premium fault tree

The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Testing sequence

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

Test 1 of 3

Select the missing speed and check whether its speed contactor energises.

Expected reading

The selected speed's contactor pulls in.

Your actual reading

If it passes

Contactor operates — check the winding connection for that speed.

If it fails

Contactor doesn't operate — check its control path and interlock.

Often leads toMotor goes one way but won't go the other (e.g. down but not up)Star-delta starter not transitioning to delta

View all expected readings at once

1. Select the missing speed and check whether its speed contactor energises.

The selected speed's contactor pulls in.

2. If it doesn't operate, trace the control selection and the speed-change interlock.

Speed command reaches the coil and the interlock allows it.

3. If the contactor operates, isolate and check the winding connections for that speed against the diagram.

Correct, sound winding connections for the selected speed.

Full test sequence

The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Fault-finding flowchart

The same logic as a decision tree.

1
start
Won't change speed
→ step 2
2
decision
Does the selected speed's contactor energise?
Yes→ step 3No→ step 4
3
decision
Are the winding connections for that speed correct?
Yes→ step 5No→ step 6
4
decision
Is the control selection complete and interlock allowing it?
Yes→ step 7No→ step 8
5
result
Re-verify the contactor contacts/motor.
6
result
Winding connection fault — correct per the diagram.
7
result
Command/interlock fine — see contactor coil fault-finding.
8
result
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.

Related faults

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

QualifiedHigh risk

Contactors & relays Premium

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.

4 causes3 test steps

AdvancedHigh risk

Forward / reverse circuits

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.

4 causes3 test steps

QualifiedMedium risk