VSD trips on undervoltage / loses supply
The drive trips on undervoltage — the DC bus drops too low, often on start, under load, or with supply dips, and the drive shuts down to protect itself.
Safety first
Even with an undervoltage trip the bus can hold charge. Prove dead before touching internal terminals. Loose supply connections can arc and overheat.
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
Loss of an input phase
Most likelyA missing input phase (blown fuse, open connection) collapses the bus and trips undervoltage.
- 2
Loose / high-resistance supply connection
#2A loose input terminal drops voltage under load and trips the drive.
- 3
Genuine supply sag/brownout
#3A weak supply or large load elsewhere causes dips that trip the drive.
- 4
Undersized supply for the drive's demand
#4A supply too small for the drive sags when the motor loads up.
- 5
Drive input rectifier/precharge fault
Least likelyAn internal input-stage fault reduces or destabilises the bus.
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.
Measure all three input phases at the drive terminals (under load if it trips under load).
Three balanced input phases within rating, steady under load.
Input looks healthy — check connections and the drive input stage.
Missing/low/unbalanced phase — chase that supply fault.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
VSD undervoltage trip
→ step 2 - 2decision
Are all three input phases healthy and steady under load?
Yes→ step 3No→ step 4 - 3decision
Are the input connections tight and cool?
Yes→ step 5No→ step 6 - 4result
Missing/low phase — chase the supply fault.
- 5decision
Does the supply hold under the drive's load?
Yes→ step 7No→ step 8 - 6result
Loose/hot connection — repair it.
- 7result
Suspect the drive input stage — refer to documentation.
- 8result
Supply sag/undersized — address the supply.
Common mistakes apprentices make
- Not checking all three input phases (missing one looks like undervoltage).
- Overlooking a loose, heating input terminal.
- Blaming the drive when the supply genuinely sags.
- Ignoring a supply that's simply too small for the drive.
When to stop & escalate
A lost input phase or a sagging supply is upstream — raise it appropriately. Suspected internal input-stage faults go to the drive documentation/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 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.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
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.