How to Diagnose VFD Overcurrent at Start
A drive that trips on overcurrent the instant it tries to start is one of the most common commissioning headaches, and it has a short list of usual suspects. The current spikes past the drive's limit before the motor gets moving, the drive protects itself, and the load never turns. This page is for the technician staring at a fresh overcurrent fault at startup. It works the causes in order of likelihood, each with a quick test and the fix, so you can rule out the cheap and common problems before pulling megger leads or suspecting a shorted output.
VFD Overcurrent at Start in one line: A VFD overcurrent at start usually means the drive is commanding more current than the motor can accept while moving, so work the likely causes in order: an accel time too short for the load's inertia, excessive torque boost or a bad motor model, a stalled or jammed load the motor cannot break away, and then genuine faults like a shorted or grounded motor lead. Lengthen the accel ramp and reduce boost first because they are free and common, confirm the load turns freely by hand, and only then test the motor and cabling for a real fault.
First Checks: Ramp, Boost, and Direction
Start with the parameters, because the cheapest fixes live there. An accel time set too short forces the drive to pour current into the motor to meet an aggressive ramp, and on a high-inertia load that current runs straight into the overcurrent limit. Lengthen the accel time and try again; if the trip moves later in the ramp or disappears, inertia was the problem, and the method for setting a ramp the load can follow is in the guide on setting VFD accel and decel times.
Check torque boost and the motor model next. Too much manual boost, or an IR-compensation setting cranked up, over-fluxes the motor at low speed and drives current high before rotation even begins. On a vector drive, a poor motor model from a bad or skipped autotune does the same thing by mis-estimating the current needed. Back the boost off to a modest value, and if the drive is vector, re-run the tune per the guide on running a VFD motor autotune.
Confirm the trip really is at start and not a symptom of the motor fighting itself. Give a brief start command and watch whether the shaft so much as twitches. If it lurches and trips, the drive is producing torque but hitting a limit; if nothing moves at all and current pins immediately, suspect a stalled load or a genuine electrical fault, which the next sections separate. Noting exactly when in the sequence the current spikes tells you which branch to follow.
Rule Out a Stalled or Jammed Load
A mechanical jam looks exactly like an electrical problem from the keypad, so prove the load can move before you chase the electrics. With the drive safely locked out, try to turn the shaft or coupling by hand or with a bar. A pump seized by a locked impeller, a conveyor jammed against debris, or a gearbox bound up will refuse to turn, and no drive setting can start a load that is mechanically stuck. If it will not budge, the fault is mechanical and the drive was doing its job.
Consider breakaway torque even when the load does eventually turn. A positive-displacement pump against pressure, a compressor with residual head, or a cold, stiff gearbox can demand far more torque to break away than to keep running, and a drive short on low-speed torque trips trying to supply it. This is often really a control-mode question, since scalar control struggles to produce breakaway torque where vector control can, a trade covered in the comparison of sensorless vector control against simpler modes.
Separate a true overload from a transient inrush. If the load turns freely by hand yet the motor still overcurrents the moment it is asked to move it, the motor may simply be undersized for the breakaway demand, or the process may be presenting more load than expected, such as a valve lined up wrong or a system starting against a full head. Confirming the process is staged for a light start, then trying again, tells you whether the drive settings or the plant lineup is the real culprit.
Test the Motor and Cabling for a Real Fault
If the parameters are sane and the load turns freely, look for a genuine electrical fault. A shorted turn in the motor, a phase-to-phase short in the cable, or a lead touching ground presents the drive with a near-dead-short at start and pins current instantly. With the drive isolated, disconnect the motor leads and check phase-to-phase and phase-to-ground resistance; a winding or cable short shows up as a very low or zero reading where you expect balanced, sensible values. The full method is in the guide on insulation-resistance testing a motor circuit.
Rule the output stage itself in or out by testing with the motor disconnected. If the drive still faults on overcurrent with nothing connected to its output, the fault is inside the drive, most likely a failed output transistor. If it runs clean with no motor but faults with the motor connected, the problem is downstream in the cable or motor, and you have narrowed it to the field wiring and machine rather than the drive electronics.
Do not overlook cable length as a contributor on longer runs. A long motor cable adds capacitance the drive must charge at each switching edge, and on a marginal installation that charging current stacks onto the motor current and nudges the drive into overcurrent, especially at higher carrier frequencies. Verifying the run length and whether an output filter is warranted is covered in the guide on checking VFD motor cable length, and it is worth ruling out before condemning the motor.
Frequently Asked Questions
Will lengthening the accel time fix a startup overcurrent?
Often, yes, when the load is high-inertia. A short accel time forces the drive to command heavy current to meet an aggressive ramp, and on a flywheel, fan, or loaded conveyor that current hits the overcurrent limit. Lengthening the accel time lets the load speed up more gently, keeping current within bounds. If the trip disappears or moves later in the ramp once you extend the time, inertia was the cause; if it still trips immediately, look at boost, the load, or a fault.
How do I tell a stalled load from an electrical fault?
Lock out the drive and try to turn the shaft or coupling by hand. If it refuses to move, the load is mechanically jammed and no drive setting will start it. If it turns freely yet the drive still overcurrents at start, the fault is electrical or the settings are wrong, so test the motor and cable for shorts and check accel time and torque boost. Separating the mechanical from the electrical this early saves chasing the wrong system.
Can too much torque boost cause an overcurrent trip?
Yes. Excess manual torque boost or IR compensation over-fluxes the motor at low speed, driving current high before the shaft even turns, which can trip the drive on overcurrent right at start. Back the boost down to a modest value and retry. On a vector drive, a bad or skipped autotune has the same effect by mis-estimating the current the motor needs, so re-running the tune with correct nameplate data often clears the fault.
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