Unidrive SP braking resistor
Use only when OV trips are tied to rapid deceleration or high-inertia loads after checking decel parameters.
Control Techniques Unidrive SP Fault Codes — Quick Reference
Control Techniques (now part of Nidec) manufactures the Unidrive SP and Unidrive M series variable-speed drives, widely used in industrial applications. The drives display fault codes as text “trip” codes on the keypad display. The trip history is accessible via parameter Pr 10.20–10.29 (last 10 trips).
A trip code alone rarely tells the whole story. The same OI.AC trip means something different if it happens the instant you press start versus after the drive has run steadily for an hour, so always read the trip history and the operating conditions at the moment of the fault before reaching for a replacement part. Work through the diagnostic steps for the specific trip family below in order, since drive faults are almost always wiring, parameter, cooling, or load issues rather than a failed drive.
| Trip Code | Meaning | Common Cause | Quick Fix |
|---|---|---|---|
| OI.AC | Overcurrent — AC output | Short circuit; too fast acceleration | Check motor wiring; increase accel time |
| OI.DC | Overcurrent — DC bus | Braking energy; voltage spike | Add brake resistor; check decel |
| OV | DC Bus Overvoltage | Fast deceleration; high supply voltage | Increase decel time; add brake resistor |
| UV | DC Bus Undervoltage | Low input voltage; power interruption | Check supply voltage; check fuses |
| OH | Drive heatsink overtemperature | Blocked fan; high ambient temp | Clean heatsink; improve ventilation |
| OH2 | Control board overtemperature | Ambient too hot | Improve enclosure cooling |
| t.dEF | Thermistor fault — motor PTC | PTC resistance out of range | Check PTC wiring; check motor temp |
| Ot | Motor overtemperature — model-based | Motor overloaded | Reduce load; check motor cooling |
| SCL | Serial comms loss | Fieldbus or serial link interrupted | Check comms wiring; check master device |
| EEF | EEPROM fault — parameter error | Parameter corruption | Restore factory defaults and reprogram |
| PSF | Power supply fault — internal | Internal power supply failed | Replace drive |
| O.SPd | Overspeed | Speed feedback exceeded limit | Check speed reference; check encoder |
| EnC | Encoder fault | Encoder signal lost or invalid | Check encoder wiring; check speed |
| Ph | Input phase loss | Missing L1, L2, or L3 | Check input wiring; check fuses |
| GF | Ground fault | Motor or cable insulation fault | Megger motor; check cable |
Tools You Will Need
- Digital multimeter with AC/DC voltage, resistance, and continuity ranges
- A megohmmeter (insulation tester) rated for motor and cable insulation testing
- A clamp meter for measuring motor running current without breaking the circuit
- Small flat and Phillips screwdrivers for terminal and fieldbus module connections
- The Unidrive SP parameter list for your firmware version, since parameter numbers can shift slightly between software releases
- CT SyPTPro or Connect software and a laptop, if available, for parameter backup and trip log export
Safety Before You Start
Lock out and tag out the drive’s input disconnect before opening the drive enclosure, and always wait for the DC bus capacitors to discharge per the manufacturer’s specified time, typically several minutes, before touching any internal terminal. Confirm zero voltage at the DC bus terminals with a meter rated for the drive’s voltage class rather than assuming the capacitors have discharged just because power has been off for a short time. These drives switch three-phase power at line voltage and can retain a lethal charge on the bus capacitors well after the input power is removed. Never perform a megohmmeter insulation test on a motor or cable while it is still connected to the drive output terminals, since the test voltage can damage the drive’s output stage. Disconnect the motor leads at the drive first.
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Need Replacement Parts?
Before ordering, match the drive part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Most Common Faults
OI.AC — AC Output Overcurrent
The output current exceeded the instantaneous trip threshold (typically 200–220% of rated current). This is the most common trip on the Unidrive SP in industrial pump and fan applications.
Diagnosis:
- Check if the trip occurs at startup or while running — startup OI.AC often indicates too-short acceleration time or a high-inertia load
- If running, check for a mechanical jam in the driven load
- Disconnect the motor and check output wiring for short circuits between phases or phase-to-ground
Fix:
- Increase acceleration time (Pr 2.11) by 50% and retry
- If the motor was recently replaced, verify the motor nameplate amps and update the drive’s motor rated current parameter (Pr 5.07)
OV — DC Bus Overvoltage
Occurs when regenerated energy from rapid deceleration raises the DC bus voltage above the trip threshold (typically 830VDC on 400VAC drives, 415VDC on 200VAC drives).
Fix:
- Increase deceleration time (Pr 2.21)
- Enable the drive’s built-in voltage-dependent deceleration (Pr 2.32 = 1) — this automatically extends decel time to keep DC bus below trip level
- Add an external braking resistor connected to the drive’s braking terminals — essential for applications with high inertia (large fans, centrifuges)
OH — Heatsink Overtemperature
The Unidrive SP uses forced-air cooling via an internal fan. The heatsink trip occurs at approximately 90°C.
Fix:
- Clean the heatsink fins thoroughly — the fins on the back of the drive pack with dust rapidly in industrial environments
- Verify the internal cooling fan is running — it should be audible when the drive is powered
- Check ambient temperature in the panel: maximum for Unidrive SP is 40°C
- Verify minimum clearances: 100mm above, 100mm below, and 30mm on sides (or per installation guide)
SCL — Serial Communications Loss
The drive lost contact with its Fieldbus master (PROFIBUS, EtherNet/IP, Modbus RTU, etc.) or serial link. Check the communications cable, the module connector on the drive, and the scan rate settings. Verify the master device is polling the drive.
EEF — EEPROM Fault
Parameter memory has corrupted. This can occur after a power interruption during a parameter save. Reset to factory defaults (Pr 00.09 = 1000 for most models), then reprogram all required parameters. Save all parameters to a computer before this reset if possible using CT’s SyPTPro or Connect software.
UV — DC Bus Undervoltage
An undervoltage trip means the DC bus voltage dropped below the minimum threshold the drive needs to operate safely, most often from low or interrupted incoming supply voltage. Check the incoming line voltage at the drive’s input terminals under load, not just at no-load, since voltage can sag when the drive draws current during acceleration. Inspect input fuses and any upstream disconnect or contactor for a poor connection, and confirm the supply is not simply undersized for the drive and motor combination.
EnC — Encoder Fault and O.SPd — Overspeed
An encoder fault means the drive lost or received an invalid feedback signal from the motor’s speed or position encoder, which matters most on closed-loop vector or servo applications. Check the encoder cable for damage, confirm the connector is fully seated at both the encoder and the drive, and verify the encoder power supply voltage is within spec at the encoder itself, not just at the drive terminal. An overspeed trip is often a downstream symptom of a marginal encoder signal or a speed reference that briefly commands more speed than the load or motor can safely achieve, so check both the encoder wiring and the speed reference source together.
GF — Ground Fault
A ground fault trip means the drive detected current leaking to ground rather than returning through the expected motor phase conductors. Disconnect the motor leads at the drive output terminals and use a megohmmeter to test insulation resistance between each motor phase and ground, then test the motor cable separately from the motor itself if practical. A motor or cable reading near zero ohms to ground confirms an insulation failure that requires motor or cable repair, not drive replacement. If the motor and cable both test healthy in isolation, suspect moisture intrusion in a junction box or a nicked cable jacket along the run.
Step-by-Step Diagnostic Sequence
- Read the exact trip code on the keypad and check the trip history in Pr 10.20 through Pr 10.29 to see the operating conditions logged at the moment of the fault.
- Note when the trip occurs relative to the drive’s operating state: at power-up, during acceleration, during steady running, or during deceleration, since this narrows the cause significantly before you open a single panel.
- Match the trip code to its section above and follow that specific diagnostic path rather than resetting and hoping the fault does not return.
- Check incoming supply and motor wiring with a multimeter for any trip that could plausibly involve voltage or current, since supply-side problems are common and easy to rule out early.
- Back up parameters before making changes, using CT software if available, so a parameter reset or adjustment can be undone if it does not resolve the fault.
- Reset the drive only after correcting the identified cause, then run the load through a full cycle while monitoring current, DC bus voltage, or the relevant parameter for that trip type.
- Escalate to a drive specialist if the same trip returns after supply, wiring, motor insulation, cooling, and parameters have all been confirmed correct, since a drive that continues tripping under those conditions likely has an internal component failure.
Unidrive SP Parameter Menu Access
- Hold the M button for 2 seconds to access the full parameter list
- Trip history: navigate to Menu 10 → Pr 10.20 through 10.29
- Trip reset: press the STOP/RESET key after correcting the fault condition
When to Call a Pro
PSF (internal power supply), GF (ground fault with motor failure), and Ph (input phase loss) on a persistent basis require an industrial drive specialist. Control Techniques service centers offer drive repair and board-level service.