Why does my VFD trip on overcurrent as soon as I start it?

An overcurrent trip the instant you start, before the motor turns, usually points to a short or ground fault in the motor cable or windings, a motor brake that isn't releasing, or a contactor between the drive and motor switching while the drive is running. A trip partway through acceleration points instead to an acceleration time that's too short for the load, too much torque boost, a mechanical bind, or motor data that doesn't match the nameplate.

Can I megger a motor with the VFD connected?

No. Disconnect the motor leads from the drive output first. Megger test voltage applied to the drive's output terminals can damage the drive's power electronics. Isolate the drive, lock out, and test the cable and motor on their own.

Why does a fan VFD trip on overcurrent at startup?

Fans are often already spinning when the drive starts, either windmilling backward from airflow or coasting down from the last run. Starting from zero into a spinning load causes a current surge. Most drives have a flying start or catch-a-spinning-load function that matches the motor's speed first; enabling it usually stops the trip.

Troubleshooting guide

VFD Overcurrent Fault on Startup? When It Trips Tells You Why

Same fault code, very different causes. A drive that trips the instant you hit start is telling you something different from one that trips halfway up the ramp. Get the timing right and you've already cut the suspect list in half.

Last updated: October 2026 · written and reviewed by a working maintenance professional

An overcurrent fault on startup is one of the most common drive calls there is, and one of the most common reasons good drives get swapped. The drive isn't usually the problem. It's reacting, fast, to a current spike it didn't expect, and protecting itself and the motor the only way it can.

The first thing to work out isn't why. It's when. Pull up the drive's fault history before anything else — many drives log the output frequency and current at the moment of the trip — and answer one question: did it trip instantly, or during acceleration?

Instant trip: before the motor even turns

If the drive faults the moment you command run — output frequency at or near zero, motor never moves — the current surge is happening before the motor has a chance to do anything. Think short, ground, or something mechanical holding the motor:

A motor brake that isn't releasing. If the motor has a brake and it doesn't lift on start (bad brake coil, failed rectifier, wiring), the drive is trying to turn a locked shaft. Watch or listen for the brake release. Five seconds, and it's one of the most commonly missed causes.
A contactor or disconnect between the drive and the motor. If something on the drive's output side opens or closes while the drive is running, you get a huge current spike. The output contactor should be closed before the drive starts, and only opened after it stops. Check the sequence.
A short or ground fault in the cable or motor. A nicked cable, water in a junction box, or a winding breaking down to the frame. This one needs testing, below — but test it properly.

Trip during acceleration: partway up the ramp

If the motor starts to turn and the drive faults a second or two in, the electrical path is probably fine. The drive is being asked to deliver more current than it can while it accelerates the load. Think settings and load:

Acceleration time too short. A heavy flywheel, a loaded conveyor, or a big fan can't come up to speed in two seconds. If someone shortened the ramp to "speed up the cycle," this is your trip. Lengthen it and see.
Too much torque boost (or V/Hz boost). Boost adds voltage at low speed to help starting. Too much of it pushes current way up at the bottom of the ramp.
Motor data that doesn't match the nameplate. In vector or sensorless-vector mode the drive models the motor. Wrong full-load amps, wrong poles, or a skipped autotune after a motor swap and the drive's model is off, along with its current control. Compare the parameters to the nameplate.
A mechanical bind. A dragging bearing or a jammed load shows up as extra current while accelerating.
Starting into a spinning load. Fans are the classic. Airflow windmills a fan backward, or it's still coasting from the last run, and the drive starts it from zero. Turn on the drive's flying start (catch-a-spinning-load) function and the trip usually disappears.

Test the cable and motor — with the drive disconnected

When you suspect a short or ground, test it. Lock out, verify dead, and disconnect the motor leads from the drive's output terminals before you megger anything. Megger voltage on the drive's output can damage its power electronics. Then:

Megger each phase to ground through the cable and motor. A low reading means insulation breakdown somewhere along that path. Split it by disconnecting at the motor to tell cable from motor.
Check winding resistance phase to phase. The three readings should be close. A low or unbalanced reading points to a winding fault.
Turn the load by hand while you're locked out. If it's hard to turn, the load is the problem, not the electrics.

Only then suspect the drive

If the cable and motor test clean, the brake releases, the output contactor sequence is right, the load turns freely, and the parameters match the nameplate, then it's time to look at the drive itself. A drive that trips on overcurrent with the motor disconnected (if your drive allows running with no motor for testing) is pointing at its own output section. That's the expensive answer, though, so make it the last one you reach for.

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Timing first, then work outward

Start with the fault log and settle when it trips. An instant trip sends you to the brake, the output contactor, and a short or ground. A trip during the ramp sends you to accel time, boost, motor data, the load, and flying start. Either way, the drive is the last suspect, not the first. That one question about timing saves more drives than any spare-parts shelf.

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