Motor draws fluctuating current / unstable running
The motor runs but its current swings up and down, speed surges, or it runs roughly — pointing at load variation, supply, or control instability rather than a hard fault.
Safety first
Surging machinery can be unpredictable — keep clear of moving parts. Isolate before investigating the mechanical drive.
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
Varying mechanical load
Most likelyA genuinely fluctuating load (pump cavitation, intermittent jam, fluctuating process) shows up as swinging current.
- 2
Control instability (drive tuning / hunting)
#2On a drive, poor tuning or a mismatched control mode causes the motor to 'hunt' around setpoint.
- 3
Supply voltage fluctuation
#3A fluctuating or sagging supply makes current vary as the motor compensates.
- 4
Loose connection or intermittent winding fault
Least likelyAn intermittent connection can cause current to jump around.
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.
Watch current and speed together — does the swing track a process/load change?
Current variation correlates with a known load/process change.
It tracks the load — the variation is mechanical/process-driven.
Variation with steady load points to control, supply, or a connection fault.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Unstable / fluctuating current
→ step 2 - 2decision
Does the current swing track a load/process change?
Yes→ step 3No→ step 4 - 3result
Mechanical/process-driven variation — refer to process/mechanical.
- 4decision
On a drive, does tuning/mode cause hunting?
Yes→ step 5No→ step 6 - 5result
Hunting from tuning/mode — re-tune the drive.
- 6decision
Is supply steady and are connections sound?
Yes→ step 7No→ step 8 - 7result
Re-examine the load/process more closely.
- 8result
Fluctuating supply or loose connection — address it.
Common mistakes apprentices make
- Chasing the motor when the process/load is genuinely varying.
- Not considering drive tuning as a source of hunting.
- Overlooking a loose connection causing intermittent current jumps.
- Ignoring supply fluctuation feeding the instability.
When to stop & escalate
Process/load variation is for the mechanical/process team. Drive tuning that won't stabilise should go to the drive documentation/support. Supply fluctuation traced upstream is a distribution issue.
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.
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
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.