VSD powered up but won't start the motor
The drive is energised and the display is alive, but it won't run the motor. No fault may be shown — it just sits in 'ready' or 'stopped' and ignores the start command.
Safety first
A VSD's DC bus holds a dangerous charge after power-down. Wait for the documented discharge time and prove dead before touching internal terminals. The motor can start the instant the missing condition is satisfied.
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
Missing run/enable signal or wrong control source
Most likelyThe drive is set to take its start command from a source you're not using (terminals vs keypad vs network). The command never arrives, so it stays stopped.
- 2
Safe-torque-off / enable input not satisfied
#2A hardware enable or safe-torque-off input is open, so the drive will accept a run command but won't actually output to the motor.
- 3
Zero or missing speed reference
#3The drive has a valid run command but the speed reference is at zero (pot at 0, missing analogue/network reference), so it 'runs' at 0 Hz and the motor doesn't turn.
- 4
An interlock or permissive in the control wiring is open
#4An external interlock (guard, e-stop chain, upstream permissive) wired into the drive's control terminals is holding it from running.
- 5
Latched fault or warning not cleared
Least likelyA previous trip is still latched, or a warning condition is blocking the run, even if the main display looks normal.
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.
Check the drive's status/monitor screen for its commanded run state, the active control source, and the live speed reference value.
Run command shown as received, control source matches how you're starting it, and a non-zero speed reference.
Drive believes it should run — look at the enable/STO hardware and output side.
The drive isn't seeing a run command, is on the wrong control source, or has zero reference — fix that first.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
VSD powered but won't start
→ step 2 - 2decision
Does the drive show a run command, correct control source, and non-zero reference?
Yes→ step 3No→ step 4 - 3decision
Are the enable / safe-torque-off inputs satisfied?
Yes→ step 5No→ step 6 - 4result
Fix the control source, run command, or zero speed reference.
- 5decision
Are all external interlocks/permissives closed and no fault latched?
Yes→ step 7No→ step 8 - 6result
Open enable/STO input is blocking output. Trace the safety chain/wiring (do not bypass).
- 7result
Command path is healthy — if it still won't run, treat as a drive fault and check the specific code.
- 8result
Open permissive or latched fault. Clear/repair, then re-test.
Common mistakes apprentices make
- Assuming the drive is broken when it's simply set to take its command from a different control source.
- Overlooking a zero speed reference — the drive is 'running' but at 0 Hz.
- Not realising an open safe-torque-off input will allow 'run' but block motor output.
- Clearing a fault repeatedly without finding why it keeps coming back.
When to stop & escalate
If an enable/safe-torque-off input is open because of the machine's safety system, do not bypass it — confirm with the safety design and the responsible person. Persistent or repeating drive fault codes that aren't explained by wiring should go to the drive manufacturer's documentation or support.
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
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.
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Encoder
Gives precise position or speed feedback by producing pulses (or a coded value) as a shaft turns.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.