Yaskawa V1000 and A1000 Faults oC, GF, ov and Uv1: Drive or Wiring?

On a Yaskawa V1000 or A1000, oC is output overcurrent, GF is a ground fault on the output, ov is DC bus overvoltage and Uv1 is DC bus undervoltage. Of those four, only oC and GF regularly mean the drive has failed — and one test separates them: disconnect the motor leads at the drive and give it a run command. If the fault appears with nothing connected, the drive's output stage is damaged. If it runs clean, the problem is in your motor, cable or load.

That test takes fifteen minutes and it is the most valuable quarter-hour in VFD troubleshooting. This guide walks the four codes people search most, the two that sit behind them (SC and Uv3), what each one is actually measuring, and the point at which the drive should come off the panel and go in a box.

What the drive is measuring

A VFD rectifies incoming AC into a DC bus, stores that energy in a capacitor bank, and then switches it through six IGBTs to synthesise a variable-frequency output. Every fault on this page is the drive reporting a measurement at one of three places: the output current sensors, the DC bus voltage, or the power devices themselves. Knowing which measurement tripped tells you where to look.

The design and test requirements for drives of this type are set out in the IEC 61800 series (iec.ch), which is why the same logic applies to the PowerFlex and ABB drives covered in our PowerFlex F004, F005 and F012 and ABB ACS fault code guides. The codes differ; the physics does not.

The fault table

As of October 2026 this is how the common codes read on the V1000 and A1000. Trip levels are the nominal values from Yaskawa's technical manuals; confirm them for your drive and your parameter settings, because the undervoltage level in particular is adjustable.

Code	Meaning	What tripped	Most likely cause	Usually the drive?
oC	Overcurrent	Output current exceeded the overcurrent level	Shorted motor or cable, too-short accel, failed output stage	Often
GF	Ground fault	Current to ground exceeded roughly 50% of drive rated output	Motor or cable insulation, long leads, failed current sensor	Sometimes
SC	IGBT short circuit	Power device protection operated	Shorted IGBT, or a dead short on the output	Usually
ov	DC bus overvoltage	Bus above about 410 V (200 V class) or 820 V (400 V class)	Decel too fast for the load, high line, no braking path	Rarely
Uv1	DC bus undervoltage	Bus below about 190 V (200 V class) or 380 V (400 V class)	Lost phase, line sag, loose terminals, worn bus capacitors	Sometimes
Uv3	Soft-charge circuit fault	Precharge bypass did not confirm	Failed precharge relay or contactor in the drive	Yes
oH / oH1	Heat sink overheat	Heat sink temperature over its limit	Dead fan, blocked fins, hot cabinet	Fan only
PF / LF	Input / output phase loss	Bus ripple, or missing output phase	Blown line fuse, open motor lead	Rarely

oC and SC: the output stage question

An oC fault means the drive's current sensors saw more output current than its overcurrent level. That is a measurement, not a verdict. The causes, in the order they actually turn up:

1. A shorted motor winding or motor cable. Insulation that has finally broken down, a cable crushed in a tray, a flooded conduit, a peckerhead full of coolant.
2. Acceleration or deceleration time too short for the load. Especially after someone "tuned" the ramp to hit a cycle time.
3. A mechanical jam or a load that has changed. A seized bearing, a conveyor with a new and heavier product.
4. A contactor opened or closed on the drive output while it was running. Drives do not like that, and it damages them over time.
5. A failed IGBT or gate-drive circuit in the drive. The output stage itself.
6. A failed current sensor in the drive reporting current that is not there.

SC is the blunter version: the drive's power-device protection operated directly. It is far more likely than oC to mean a damaged output stage, but a dead short at the motor terminals will produce it too.

The isolation test. Lock out, wait the discharge time on the drive's label, prove the bus dead with a meter, and disconnect the three motor leads at the drive's output terminals. Tape them clear. Restore power and, in V/f control, give the drive a run command at low speed with nothing connected.

• Fault appears with no motor connected: the drive's output stage or current detection is damaged. It needs a bench.
• Drive runs up normally with no motor: the drive is healthy enough to exonerate. Megger the motor and cable — leads still off the drive, because insulation-test voltage across the output terminals will damage it — and inspect the load.

When the fault is oC on acceleration only and the no-motor test passes, lengthen the accel time before anything else. It is the cheapest fix in the manual.

"I have lost count of the drives I have seen swapped three times on the same motor. Each new one lasts a week, because nobody put a megger on the cable. The drive is the fuse in that story, and it is an expensive fuse."

— Industrial electrician and controls technician, 26 years, food and beverage packaging; name withheld by request

GF: ground fault, and the cable-length trap

GF trips when the drive detects current returning through ground at roughly half its rated output. Three very different things cause it.

Real insulation failure in the motor or cable is the first and most common. It is frequently intermittent — it appears when the motor is hot, or when washdown water finds a conduit — which is why a megger reading taken on a cold, dry Monday morning can look fine. Test warm if you can.

Leakage through long motor leads is the second, and it is not a fault in anything. The fast voltage edges of a PWM output charge the capacitance between the motor conductors and ground on every switching cycle, and on a long cable run or with several motors on one drive that leakage adds up. If GF arrived after a motor was relocated further from the cabinet, lowering the carrier frequency or adding an output reactor is the fix, and the drive is innocent.

A failed current sensor inside the drive is the third. If GF appears with the motor leads disconnected, that is your answer.

ov and Uv1: the bus voltage codes

These two are the drive reporting its DC bus, and most of the time it is reporting something true about the world outside the drive.

ov, overvoltage. A decelerating motor is a generator. The energy it returns charges the DC bus, and if it arrives faster than the drive can dispose of it the bus rises to the trip level — about 410 V on a 200 V class drive and 820 V on a 400 V class drive. The usual causes are a decel time too short for the inertia, an overhauling load, a missing or failed braking resistor on an application that needs one, or incoming line voltage that is already high. Power-factor capacitor switching elsewhere in the plant can also push a surge onto the bus. If ov appears only during deceleration, lengthen the ramp or look at the braking circuit. If it appears at rest or at constant speed, measure the incoming line.

Uv1, undervoltage. The bus fell below the undervoltage level while the drive was running — nominally around 190 V on a 200 V class drive and 380 V on a 400 V class drive. Work from the outside in: all three line fuses, the disconnect, the line contactor, and every power terminal on the drive for tightness. A single lost phase will often let a lightly loaded drive keep running on the remaining two, then trip Uv1 the moment load is applied.

Two causes of Uv1 are genuinely inside the drive. Bus capacitors that have lost capacitance with age can no longer hold the bus up between line peaks under load. And a precharge circuit whose bypass relay has failed leaves the charging resistor in circuit, which starves the bus as soon as current is drawn — that often shows as Uv3 on these drives, and it is a drive repair.

Supply quality deserves a real measurement rather than a glance. The U.S. Department of Energy's motor-systems guidance (energy.gov) notes that current unbalance typically runs 6 to 10 times the percentage voltage unbalance on a three-phase motor circuit, and that motor-driven systems account for roughly two-thirds of industrial electricity use — which is to say most of what a plant's power system exists to feed is exactly this kind of load. Recommended harmonic and power-quality practice at the point a drive connects is described in IEEE 519 (ieee.org).

Let the drive tell you what happened

Both families keep a record, and it is better evidence than anyone's memory of the event.

The fault trace monitors in the U2 group capture the conditions at the moment of the last fault — output frequency, output current and DC bus voltage among them. The fault history in the U3 group lists the most recent faults in order. Read them before you clear anything. A bus voltage reading that was normal when oC occurred points at the output side; a bus voltage that was collapsing when Uv1 occurred points at the supply.

The maintenance monitors in the U4 group are worth a look on any drive more than a few years old. They estimate remaining life for the cooling fan, the bus capacitors, the soft-charge relay and the IGBTs as percentages. They are estimates, not measurements — but a capacitor monitor near its limit on a drive that has started throwing Uv1 under load is not a coincidence.

The rule of thumb for electrolytic capacitors is that every 10 °C of sustained temperature rise roughly halves service life. A drive in a hot enclosure with a tired fan is ageing its bus capacitors on a fast clock, which is the mechanism behind the "VFD tripping" pattern described on our symptoms pages.

Safety is not optional on this one

Every test above happens next to a capacitor bank. The product safety standard IEC 61800-5-1 requires a warning label wherever stored charge takes longer than 5 seconds to decay below 60 V, and the wait time is printed on the drive. Observe it, then measure the bus terminals before touching anything.

OSHA estimates that compliance with its control-of-hazardous-energy standard (osha.gov) prevents around 120 fatalities and 50,000 injuries each year. Any measurement you take live at the line terminals is energized work under NFPA 70E (nfpa.org), whose arc-flash boundary is defined at an incident energy of 1.2 cal/cm². Drives are built to the power-conversion safety requirements now published as UL 61800-5-1 (ul.com); none of that protects a hand inside an open cabinet.

When to ship it, and what it costs

Send the drive when:

• oC, SC or GF appears with the motor leads disconnected.
• Uv3 appears, or Uv1 persists with verified three-phase input at the drive terminals under load.
• The drive is dead, or the keypad is dark with input power present.
• A heat-sink fault returns after a new fan and a cleaned heat sink.

Keep the drive and fix the installation when the no-motor test passes, when ov tracks deceleration, or when the megger finds the motor.

Yaskawa V1000, A1000 and GA500 drives up to 25 HP are covered by our multi-brand VFD repair service. It is quote-track: the bench evaluation is free, typical repairs run $300–$900 — V1000-size frames typically $300–$800 — and no work starts without your approval of a firm quote. Repairs cover the IGBT and rectifier stages, bus capacitor replacement, precharge and control circuits, and a powered test with a motor load appropriate to the frame, with a 24-month warranty. Standard turnaround is 3–5 business days in lab and rush service is $149.

We state the 25 HP ceiling on purpose. Proving a repair on a larger drive needs three-phase load capacity we do not claim, and a drive returned under-tested is worse than a drive declined.

A note on naming, because it sends boxes to the wrong queue: a Yaskawa Servopack is a servo amplifier, not a VFD, and its alarm A.10 is a different fault on different hardware — see Yaskawa Servopack alarm A.10. For the full price picture across drive brands, read VFD repair cost. When you are ready, request a quote with the fault code, horsepower and voltage, and pack the drive per the mail-in instructions.

Frequently asked questions

What does oC mean on a Yaskawa V1000 or A1000?

oC is an overcurrent fault: the drive's output current exceeded its overcurrent detection level. The cause is either on the load side — a shorted motor or cable, a ramp that is too short, a jammed load — or a failed output stage in the drive. Disconnect the motor leads and run the drive; if oC still appears, the drive is at fault.

How do I know if a Yaskawa GF fault is the motor or the drive?

Disconnect the motor leads at the drive and give it a run command. A GF that appears with nothing connected is a failed current sensor or output stage in the drive. A GF that disappears is on the load side: megger the motor and cable with the leads off the drive, and check whether the cable run is long enough for leakage current to be the cause.

Why does my Yaskawa drive trip ov when it stops?

Because the decelerating motor is returning energy to the DC bus faster than the drive can absorb it. Lengthening the deceleration time is the first fix. If the application needs a fast stop, it needs a braking resistor that is connected, sized correctly and not open-circuit. An ov at rest or at steady speed points to high incoming line voltage instead.

Is Uv1 a bad drive or bad power?

More often bad power. Uv1 means the DC bus dropped below the undervoltage level, and the common causes are a lost input phase, a line sag, or loose power terminals. It becomes a drive problem when input voltage is verified good at the drive terminals under load, which points to worn bus capacitors or a failed precharge circuit.

What is the difference between oC and SC on a Yaskawa drive?

oC is the drive measuring too much output current through its current sensors; SC is the power-device protection operating directly. SC is the more serious of the two and more often means a damaged IGBT, though a dead short at the motor terminals can cause either. Both get the same first test: run the drive with the motor disconnected.

How much does Yaskawa VFD repair cost?

Typical repairs on Yaskawa V1000, A1000 and GA500 drives up to 25 HP run $300–$900 after a free bench evaluation, with V1000-size frames typically $300–$800. You approve a firm quote before any work, standard turnaround is 3–5 business days in lab, and the repair carries a 24-month warranty.

Do I lose my parameters when the drive is repaired?

Usually not, but back them up before you ship. Unlike a servo amplifier whose settings live in the CNC, a VFD stores its parameters in the drive itself. Save them before you ship — to the keypad's copy function or to the manufacturer's PC tool — so that reinstalling is a restore rather than a recommissioning.