VFD stall means the actual motor speed fails to follow the command speed during acceleration, deceleration or constant-speed operation. It may trigger overcurrent, overvoltage alarms or rough motor running. Essentially, it is an energy balance issue: insufficient motor output current or excessive regenerated energy feedback. The core solution is to match the VFD precisely with load characteristics.

1. Stall during acceleration: High current

Symptoms: Frequency stalls during startup or speed-up, or over current (OC) alarm pops up.

Cause: Large load inertia. The VFD tries to raise motor speed in a short time, creating instant high current and activating current stall protection. 

Solutions (priority order):

· Increase acceleration time: The most direct fix. Adjust acceleration time parameter (e.g. P.7, F07). For high-inertia loads, start with 5–10 seconds for testing.

· Modify start mode: If the motor is still coasting after stop, enable speed tracking start instead of 0Hz start to avoid current surge.

· Raise V/F torque boost: If the motor lacks torque at low frequency with high current, moderately increase torque boost parameter (F09).

2. Stall during deceleration: High DC bus voltage

Symptoms: Long deceleration time, or over voltage (OV) alarm immediately when decelerating.

Cause: The load drives the motor, turning it into a generator. Regenerative energy flows back to the VFD and spikes DC bus voltage, triggering over voltage stall protection. 

Solutions:

· Extend deceleration time to release regenerative energy gradually.

· Install braking resistor: If fast stop is required and longer deceleration time is not acceptable, fit braking unit and resistor between terminals P/+ and PR to dissipate surplus energy as heat.

· Enable over voltage stall protection: The VFD will pause deceleration automatically when voltage rises, and resume after voltage drops.

3. Stall in constant operation: Speed fluctuation or sudden speed drop

Symptoms: Frequency fluctuates randomly or speed drops unexpectedly.

Cause: Abrupt load increase (mechanical jamming) causes over current; sudden load reduction (fan loss of load) leads to over speed.

Solutions:

· For sudden heavy load: Turn on current stall prevention. If it triggers frequently, check mechanical jamming or raise stall action current (P.22).

· For sudden light load: Enable over voltage stall prevention. For fans and compressors, activate frequency reduction upon momentary power dip to stabilize bus voltage.

· Eliminate interference: For closed-loop control with encoder, inspect PG card wiring. Noise will cause false speed feedback and incorrect speed reduction.

4. Basic hardware inspection (if parameter tuning fails)

· Verify matching of VFD and motor; confirm correct motor wiring (e.g. star connection for 380V motors).

· Check mechanical overload: Monitor VFD output current. If the current continuously exceeds motor rated current, the load is overloaded. Parameter adjustment alone will not work; upgrade equipment or repair machinery.