Unidrive AC servo/vector drives
Technical records for Control Techniques Unidrive SP and Unidrive M service cases, beginning with OI.AC overcurrent, Over Volts, PS.24V, O.ht heatsink temperature and encoder/feedback instability routes.
Scope of this technical record
Control Techniques / Nidec coverage is focused on Unidrive SP and Unidrive M service situations where the fault code only gives the starting point. The page connects OI.AC, Over Volts, PS.24V, O.ht and feedback instability to the field evidence needed before a drive, option module, encoder cable or power stack is blamed.
Use this page only for qualified drive-service work. Many Unidrive installations are closed-loop axes, winders, hoists, packaging machines or coordinated motion systems. Secure the load, verify brake state and isolate hazardous voltages before cabinet access or motor-cable testing.
Unidrive service routing map
Use the trip code only as the first marker. Timing, feedback and machine evidence decide the repair route.
Unidrive machine-evidence map
What this brand page should help decide
A Control Techniques search usually starts with a short trip label and little context: OI.AC, Over Volts, PS.24V, O.ht or encoder instability. The useful answer is not another fault-code list. It is a service route that separates motor current, regenerative braking, external 24 V loads, cooling, option modules and feedback wiring before the repair desk quotes a stack, control board or replacement drive.
Unidrive SP and Unidrive M are often part of the machine control loop rather than a simple pump inverter. A trip may be caused by a real power-stage event, but it may also be created by a held brake, wrong motor data, an encoder direction error, a noisy feedback cable, a failing option module, braking energy that has nowhere to go, or an external control load pulling down the 24 V rail.
High-value Unidrive fault families
The first split is the trip family. Current trips need a motor, load, brake and feedback boundary. DC-bus trips need a line, regeneration and brake-resistor boundary. Control-supply trips need an external load and option-module boundary. Thermal trips need airflow and heatsink evidence. Feedback instability needs encoder cable, shield, option card and tuning context.
This route prevents a common field mistake: replacing the drive because the panel shows a severe trip while leaving the machine condition unchanged. If a hoist brake is not releasing, if an encoder channel is reversed, or if a brake resistor is open, a replacement drive can trip the same way or fail again under the first loaded run.
Control Techniques / Unidrive quick index
| User search | Service boundary | Evidence to collect first |
|---|---|---|
| Unidrive OI.AC overcurrent | Output current, motor cable, mechanical load, brake, feedback loop | Trip timing, motor/cable isolation result, brake release state, control mode, encoder option status |
| Unidrive Over Volts | DC bus, deceleration ramp, braking resistor, regenerative load | When it trips, DC-bus trend, brake resistor value/condition, load inertia and stop command context |
| Unidrive PS.24V | Internal 24 V supply, external 24 V loads, option modules | What is connected to 24 V, whether the trip clears after load isolation, module photos and terminal evidence |
| Unidrive O.ht / O.ht1 | Fan, heatsink, ambient temperature, blocked airflow, thermal sensor path | Fan operation, heatsink contamination, cabinet temperature, load cycle and thermal history |
| Encoder feedback instability | Encoder cable, shield, supply, option card, direction and tuning | Encoder type, feedback option identity, cable route, fault timing and whether open-loop test is stable |
Before sending a repair request
Start with the exact trip label, then write down when it appears: immediately on enable, during acceleration, when the brake should release, during steady load, during deceleration, or only after heat soak. Timing is often more useful than the trip name because it tells you which circuit is being stressed at that moment.
Next separate the field side from the drive side. For output trips, prove the cable and motor only after disconnecting them correctly from the drive. For closed-loop work, record encoder and option-module evidence before changing power hardware. For DC-bus work, prove the braking path and ramp conditions before assuming an internal bus problem.
Minimum evidence for a useful Unidrive request
| Evidence area | Why it matters | Good field note |
|---|---|---|
| Nameplate and option modules | Service path depends on frame, supply, option cards and feedback hardware | Full front label, option module labels, control terminal photo |
| Trip timing | Separates enable, ramp, load, decel and heat-related causes | Trips at 3–5 Hz during lift command, not at idle |
| Control mode | Closed-loop trips need feedback evidence; open-loop trips route differently | Closed-loop vector with encoder option; open-loop test not yet performed |
| Motor / cable / brake | A braked or grounded motor can look like drive hardware failure | Motor disconnected test result, insulation result, brake contactor status |
| DC bus / braking | Overvoltage repeats if regeneration path is not corrected | Brake resistor measured, decel ramp value, load inertia noted |
| 24 V / thermal / airflow | External loads and blocked cooling are common non-drive causes | Fan runs, heatsink cleaned, external 24 V load isolated result |
Field record checklist
- Full Unidrive SP or Unidrive M type code and frame/rating
- Exact trip label and whether other trips appear before or after it
- Open-loop, closed-loop vector or servo/control mode
- Option-module and encoder/feedback photos
- Motor, cable, brake and mechanical-load evidence
- DC-bus, braking resistor, 24 V load and cooling evidence where relevant
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Internal service-map layer for Unidrive SP/M trip routing and evidence collection.