Why Does a Pump VFD Show Undervoltage or Phase Loss?

A pump VFD shows undervoltage or phase loss when the input supply is unstable, one phase is missing, voltage drops during acceleration, wiring is loose or the power source cannot support the pump load. The fault should be checked under running conditions, not only at no load.

Recommended Configuration

What Must Be Confirmed?

• Actual input voltage at the VFD terminals.
• Whether voltage drops during pump acceleration.
• Three-phase balance if using three-phase input.
• Breaker, contactor and terminal tightness.
• Generator or weak grid condition.
• Other loads sharing the same supply.

Selection Logic

Measure voltage when the pump is accelerating and running. A supply may look normal when the VFD is stopped but drop when the pump demands torque. Long cables, undersized feeders and weak transformers can all create low-voltage faults.

Phase loss can be caused by a missing supply phase, a bad breaker pole, loose terminal, damaged contactor or upstream power issue. The VFD is often reporting a real electrical problem rather than creating it.

If the pump is powered by a generator, voltage and frequency stability should be checked carefully. A longer acceleration time may reduce sudden demand, but it cannot replace adequate generator capacity.

Common Mistakes

• Measuring voltage only when the pump is stopped.
• Ignoring loose terminals or aging contactors.
• Increasing ramp speed on a weak supply.
• Blaming the VFD before checking upstream power.
• Running other heavy loads on the same feeder.
• Using cables that are too small for the input current.

AUSENIST Solution

AUSENIST can help interpret pump VFD fault behavior after receiving the model, fault code, motor data and input voltage measurements. Site electrical inspection should be performed by qualified personnel.

FAQ

Why does undervoltage happen only during startup?

The pump draws more power during acceleration, and weak supply voltage may dip under load.

Can a loose terminal cause phase loss?

Yes. Loose wiring, bad contactors or breaker faults can create intermittent phase-loss alarms.

Should I make acceleration slower?

It may help reduce input demand, but the supply and wiring still need to be correct.

Can generator power cause undervoltage?

Yes. Generator load response may be too weak for the pump acceleration or running load.

Is phase loss possible with single-phase input?

Fault behavior depends on the VFD model and input design. Confirm the selected drive is suitable for the supply type.

What should I measure?

Measure input voltage at the VFD terminals before start, during acceleration and under normal running load.

What information should I send?

Send the fault code, supply voltage, phase type, motor nameplate, cable size, generator data if used and when the fault occurs.

Related AUSENIST Pages

Send us your fault code, input voltage measurements and pump motor nameplate. AUSENIST can help review the likely cause of the VFD power fault.