A drive declares an overcurrent fault when the current on its output rises faster or higher than its protection allows. The cause is either the motor circuit downstream, the load the motor is fighting, or a drive setting that does not match either of them.
A variable frequency drive measures its own output current continuously, and it protects its output transistors far more aggressively than a fuse or overload ever could. There are usually two families of current fault. A fast, hard overcurrent trip — often displayed with a short code and an instant stop — means current spiked, and the drive shut off in a fraction of a cycle to save itself. A slower overload or thermal fault means current stayed above rating long enough to accumulate heat.
The distinction matters because they point in different directions. An instant trip on the first attempt to run usually means a short or ground on the output, a shorted output transistor, or an acceleration ramp far too short for the load. A fault that only appears under load, after the machine has been running, usually means the mechanical load, a motor problem, or a drive sized or programmed too tight for the job.
Record the exact fault code and the state the drive was in when it faulted. Two drives from the same manufacturer can use different codes for a ground fault and a hard overcurrent, and a photo of the display saves an hour of guessing.
This is the most frequent cause on newly commissioned or newly reprogrammed equipment. A drive told to reach full speed in two seconds on a loaded conveyor, a large fan wheel or a pump with a long column of liquid has to supply enormous torque to do it, and torque is current. The same applies in reverse: a deceleration ramp too short on a high-inertia load drives current back into the drive and produces either an overcurrent or a bus overvoltage fault. Lengthening the ramp is often the whole fix, but it should be done knowing the process, not by trial and error.
Everything that trips a conventional overload will trip a drive too: a dry bearing, a plugged auger or screw, a conveyor packed with material, a gearbox starved of oil, a pump running against a closed valve or drawing thicker product than it was set up for, or a belt over-tensioned during maintenance. The drive gives you a useful clue here that a starter does not — a display showing output current and frequency in real time. If current climbs at a specific point in the process, the problem is at that point.
A phase-to-phase short, a phase-to-ground fault, or moisture in a motor connection box downstream of the drive produces an instant trip the moment the drive tries to modulate. Rodent damage, a conduit that filled with water, a cable chafed on a machine frame, or a motor lead crushed in a junction box are the usual origins. So is a motor with failed winding insulation. On a long cable run, the reflected wave from fast switching can stress the motor insulation over time, which is why cable length limits and output filtering matter on 600 V installations.
Motor nameplate data entered wrong is a standing cause. If the drive is told the wrong full-load current, the wrong base frequency, the wrong voltage or the wrong number of poles, its internal model of the motor is wrong and its current limit lands in the wrong place. Vector or sensorless-vector modes that were never auto-tuned to the motor behave badly under load. A boost or V/Hz curve set too aggressively pushes excess current at low speed.
A fan that is windmilling, a pump coasting down, or a load still turning when the drive restarts will draw a large current spike if the drive starts at zero frequency into a rotating rotor. Most drives have a flying-start or speed-search function for exactly this, and it is frequently left disabled.
Anything that opens or arcs on the drive output is hard on the drive. A loose terminal, a contactor switching between the drive and the motor while the drive is running, or a disconnect operated with the drive energized can produce an instant overcurrent. Output-side switching needs to be interlocked so the drive is stopped first.
Voltage unbalance, a lost input phase, a weak supply on a long feeder or a nearby large load starting can all upset the drive. Some drives report input phase loss distinctly; others show it as a bus or current fault. If other equipment on the same supply also misbehaves, look upstream. The related article on phase loss on a motor covers where that usually originates.
A drive sized on horsepower alone can be wrong for a high-torque or high-inertia application, or for one that needs sustained overload capacity. Constant-torque and variable-torque ratings are not interchangeable. Drives also derate with ambient temperature, altitude and switching frequency, and a drive in a hot, dusty panel will run out of headroom before its nameplate says it should.
A shorted output transistor or a failed current sensor produces a fault that will not clear regardless of what is done downstream. The tell is a drive that faults instantly with the motor leads disconnected. That test is a qualified-person job and it settles the question quickly.
Safety: anything that involves opening enclosures or taking readings on live equipment is work for a qualified electrician. The checks below are the ones that do not. A drive holds a dangerous charge in its DC bus after power is removed, so opening one is never a casual check.
Stop cycling power on a drive that faults instantly every time. Each attempt drives the fault current again into whatever is shorted, and a downstream short can take the drive output with it. Stop immediately if there is any smell of hot plastic or burnt insulation, any visible scorching inside the panel, or a snap or bang at the moment of the fault. Do not attempt to clear a fault by disabling protection or raising a current limit — those settings are what stand between a nuisance trip and a destroyed drive.
Manufacturer experience includes drives such as Schneider Altivar, TECO and Lenze. Grasse Electric is an independent electrical service contractor, not an authorized distributor or factory service provider.
Repeat drive faults get sorted with parameters, readings and a look at the mechanical side together, which is what motor, control and VFD service and electrical troubleshooting cover. Send photos of the drive display and both nameplates through the request form and the call starts with the right information in hand.
No. The current limit protects the drive output and the motor. Raising it does not remove the load that caused the current, it just moves the failure from a fault display to hardware damage.
Most often a short or ground fault on the output wiring or in the motor, or a failed output section in the drive. It can also be an acceleration ramp set absurdly short. It is not a load problem, because the load has not had time to matter yet.
Electrically it usually reduces current, but it changes how the machine behaves. On a process with timing, interlocks or material flow, a longer ramp can cause its own problems. Change it knowing what the machine has to do.
Never. An insulation test applied to drive output terminals can destroy the drive. The motor leads must be disconnected from the drive first, which is qualified-person work.
Cold thickens lubricant and product, which raises breakaway torque and starting current. Condensation in an unheated building can also lower insulation resistance in motor cable and connection boxes.
Only if the drive is the fault. A drive swapped onto a shorted cable or a seized machine fails the same way. Confirm what is downstream before spending on hardware. See why a motor keeps tripping the overload for the mechanical side of the same question.
Fault code, nameplate, and what the machine was doing gets us started.
