Yaskawa Drive OV Fault Troubleshooting on Uncoilers: GPD 515/G5

Problem Overview: OV Fault on Yaskawa GPD 515/G5 Driving an Uncoiler

An OV (Overvoltage) trip on a Yaskawa GPD 515/G5 series drive used on an uncoiler / payoff reel is one of the most common field faults. The drive is typically rated for a 575/600 V class supply and feeds a three-phase induction motor coupled directly (no encoder) to the coil. When the operator commands full speed reference from the speed pot, the drive accelerates the motor, reaches the commanded frequency, and then trips with OV on the keypad. Reducing the speed-pot reference allows the drive to run indefinitely.

The fault is almost never a defective drive. It is a symptom of one or more of the following physical conditions:

• Regenerative energy from the load being pumped back into the DC bus faster than the bus capacitors and braking resistor can absorb it.
• A dynamic braking resistor that is undersized, open, or incorrectly wired to terminals B1 and B2.
• Input (line) voltage that is too high, reducing the available headroom between nominal bus voltage and the OV trip threshold.
• Deceleration ramp (C1-02) set too aggressive for the inertia of the coil, or b1-03 set to Ramp to Stop while the load naturally wants to coast in less time than the ramp.
• An incorrect voltage-class jumper (460 V vs 575/600 V) on the drive's input rating selection.

Because uncoilers carry very high stored rotational energy (steel coil plus reel), the load is always trying to drive the motor, not the other way around, once the strip tension is established. Any disturbance that momentarily unloads the motor, or any commanded decel that is faster than the load's natural coast time, results in the motor acting as a generator. That regenerated current flows back through the drive's rectifier in reverse, charges the DC bus capacitors, and pushes the bus voltage up toward the OV trip level.

How the Yaskawa G5 Detects Overvoltage

The GPD 515/G5 samples the DC bus voltage directly and compares it against a fixed hardware threshold that is not user-adjustable. Understanding these thresholds is essential when sizing braking or interpreting trace data:

On a 575 V uncoiler installation the bus sits around 1000 VDC during normal motoring at full speed. Any regeneration event of more than about 30 V of bus rise will trip the drive. This is why a 460 V drive mistakenly installed on a 575 V supply, or a 460 V voltage-class jumper left in place on a 575 V drive, will trip almost immediately on any regen event.

Why the Fault Disappears at Reduced Speed

Speed-pot reduction lowers the commanded output frequency. Below the speed at which the strip tension is fully developed, the motor is not yet back-driving the load, so the net energy flow is from the drive to the motor. As speed reference is raised, the uncoiler's dancer / tension loop demands more holding torque. If the strip is being pulled by a downstream process, the uncoiler motor is effectively motoring in reverse rotation relative to its commanded direction (the motor is acting as a brake). The faster the line, the more regenerative current is produced.

The trip threshold is hit when this regenerative current multiplied by the deceleration ramp (or by strip-tension-induced negative slip) exceeds the drive's ability to dissipate it. Below the critical speed, the energy is small enough to be absorbed by the DC bus capacitors and the modest internal braking transistor's on-resistance path.

Diagnostic Procedure: Confirming the Root Cause

Step 1 - Inspect the Voltage-Class Jumper

Locate the input voltage selection jumper on the GPD 515/G5 control board. Verify it is set to the 575/600 V position. A 460 V jumper on a 575 V drive will cause the drive to under-rate its OV threshold and trip early.

Step 2 - Measure the DC Bus on the Keypad

The G5 keypad does not display DC bus voltage directly on the main screen by default, but it is available in monitor parameter U1-07 (DC Bus Voltage). Navigate to the Monitor menu (U1 group) on the digital operator and scroll to U1-07. The displayed value is in VDC. With the drive in Run at no load, record the steady-state value. Then accelerate to full speed and record the value again. Any reading above the trip threshold listed in the table above confirms an OV event.

If a digital operator with LED display is in use and the value cannot be found, an external DMM connected between terminals +1 (positive) and - (negative) on the power board will read the same DC bus voltage directly. Use a 1000 V (CAT III 600 V or better) rated meter with isolated probes.

Step 3 - Review the Fault Trace

The GPD 515/G5 retains the last four faults plus a fault trace (output frequency, output current, bus voltage, and run status at the moment of trip) in the U2 and U3 parameter groups. Read U2-01 through U2-04 for the fault history, and U3-01 through U3-10 for the trace snapshot. A U3-07 value above the OV trip threshold at the moment of the trip is definitive proof of a DC bus overvoltage.

Refer to the Yaskawa GPD 506/P5 and GPD 515/G5 Unit Troubleshooting Manual for the complete fault-trace decoding tables.

Step 4 - Inspect the Braking Resistor

If a dynamic braking resistor is fitted, verify the following:

1. The resistor is wired between terminals B1 and B2 on the drive. Polarity is not observed for a resistive element, but the wiring must land on the correct terminals and not on the DC bus studs.
2. The resistor's ohm value matches the drive's minimum allowable resistance. The G5 internal braking transistor will fault on its own overcurrent if the resistor is below the minimum.
3. Measure the resistor cold with the drive de-energized and locked out. An open resistor reads infinite ohms and is the single most common cause of persistent OV faults after a long service interval.
4. Verify the wiring is not daisy-chained through a contactor or interlock that drops out during run.

Step 5 - Verify Supply Voltage at the Drive Input

Use a true-RMS meter at the drive's input terminals L1, L2, L3. Compare to the drive's nameplate. A 575 V supply that is actually running at 600 - 615 V (common on lightly loaded industrial feeders) reduces the available headroom for braking. The drive cannot absorb regen energy above the OV threshold regardless of resistor sizing if the bus is already sitting at 1020 VDC before any braking occurs.

Parameter Settings That Influence OV Behavior on the GPD 515/G5

The following parameters from the GPD 515/G5 menu are most often involved in OV trips on uncoiler duty. Refer to Yaskawa GPD 515/G5 Troubleshooting Manual Section Three for the full list.

Recommended Solution Sequence

1. Confirm root cause. Use the U1-07 monitor or an external meter to observe the DC bus at steady state and at the moment of trip. Capture the U3 trace after every trip.
2. Correct the input-voltage setting. Set E1-01 to the actual measured line-to-line voltage (575 or 600). Set the hardware jumper to match.
3. Verify and repair the braking resistor. Confirm continuity, confirm ohm value is above the drive's minimum, confirm wiring is on B1/B2.
4. Enable OV suppression. Set b3-11 = 1. This is the safest first software fix because it limits the decel rate automatically and prevents nuisance trips without changing the operator's stop behavior.
5. Lengthen deceleration ramp. Increase C1-02 in 2 - 5 second increments until the OV fault clears. Typical uncoiler values run 15 - 30 s for a full speed-to-zero ramp.
6. Re-tune the motor. Run a rotational autotune with the motor uncoupled if possible (or static autotune if coupled). Verify E2-01, E2-02, and E2-03 are written correctly afterward.
7. Re-size the braking resistor if steps 1 - 6 do not hold. Calculate the peak regen power from the load inertia and decel time, and select a resistor rated for at least 125 % of that peak for the full decel duration.

Dynamic Braking Resistor Sizing for Uncoiler Duty

The peak regenerative power the resistor must absorb is approximated by:

P_regen (W) = 0.5 x J_total x (ω_final^2 - ω_initial^2) / t_decel

Where:

• J_total = total inertia at the motor shaft in kg·m² (motor rotor + gearbox + reel + coil), all reflected to motor rpm.
• ω = angular velocity in rad/s at start and end of decel.
• t_decel = deceleration time in seconds.

For a typical uncoiler with motor rotor inertia of 0.5 kg·m², reel + coil combined of 8 kg·m² (reflected), full speed at 1750 rpm (183 rad/s), and a target decel of 15 s, the peak regen power is:

P_regen = 0.5 x 8.5 x (183^2 - 0) / 15 ≈ 9,490 W

At a 575 V class bus of approximately 1000 VDC, the resistor ohm value is then approximately R = V^2 / P = 1000^2 / 9,490 ≈ 105 Ω. Always consult the Yaskawa G5 braking resistor selection table for the drive's specific horsepower before substituting values, because the drive's internal braking transistor has a minimum allowable resistance it will switch.

Related Fault Codes to Rule Out

While OV is the reported fault, two other Yaskawa G5 fault codes can appear in similar circumstances and should be ruled out during diagnostics:

• LF (Output Open Phase) — Indicates an open phase on the inverter output side; loose terminal screws at T1, T2, or T3, or programmed parameters set incorrectly. Reference: Yaskawa LF Fault FAQ. On uncoilers, a momentary phase loss from a loose pull-on terminal can drop one phase, causing the other two to over-flux and the bus to spike.
• PF (Input Phase Loss) — Indicates a missing input phase or severe input imbalance. Reference: Yaskawa PF Fault FAQ. A lost input phase causes the DC bus to ripple heavily and can fool the OV detector or trip the PF protection first.

Always clear and read the actual fault code displayed (not assumed). The G5 stores the most recent four faults; if the OV entry is preceded by an LF or PF in the trace, treat those as the primary fault and the OV as a downstream consequence.

Verification After Repair

1. Reset the fault (Reset key on the digital operator, terminal S5 closed if programmed, or cycle power). Confirm the drive returns to Ready with no remaining fault in U2.
2. Run the drive at 25 %, 50 %, 75 %, and 100 % of speed reference with no coil loaded. Monitor U1-07 (DC bus voltage) and U1-03 (output current) at each step. Bus voltage should remain within ±5 % of nominal.
3. Load a coil. Run from zero to full speed and back to zero using the operator's standard command sequence. Confirm the bus voltage does not approach the OV threshold.
4. Force a worst-case stop (full speed to zero in the operator's standard stop mode). Watch the bus voltage on the meter. If the bus peaks above 95 % of the trip threshold, the resistor is still undersized — repeat step 7 of the solution sequence.
5. Document final parameter set: b1-03, b3-11, C1-01, C1-02, E1-01, E2-01, and braking resistor part number. Save the trace to a maintenance record.

Preventive Maintenance Recommendations

• Measure DC bus voltage monthly under load and trend it. A creeping upward trend indicates a degrading bus capacitor or a slowly failing resistor.
• Measure braking resistor resistance cold every six months. Resistor values drift upward as they age; an open resistor means OV trips return.
• Torque-check power terminals (L1, L2, L3, T1, T2, T3, B1, B2, +1, -) annually. Loose terminals at T1/T2/T3 on uncoilers are a documented cause of LF and downstream OV events.
• Verify the input voltage-class jumper during every commissioning or after any board replacement. This is a single jumper, easily missed after service.
• Keep a printed copy of the G5 fault code list at the cabinet — fault code OV versus OV1 (overvoltage during accel) versus OV2 (overvoltage during decel) versus OV3 (overvoltage during constant speed) all have different root causes.

FAQ