IndustrialDriveDataDrive Board & Diagnostic Database
Control Techniques / Nidec fault record

O.ht / O.ht1: Heatsink or Thermal Overload Trip

Unidrive reports O.ht or O.ht1 after load operation, high ambient temperature or poor cabinet cooling.

Deep thermal route9 min read

Scope of this technical record

Routes Unidrive O.ht thermal trips through cold/hot timing, fan, heatsink, ambient, load duty and temperature-feedback evidence.

Safety boundary

Fans, heatsinks and braking hardware can be hot or live. Isolate and verify before access.

Unidrive thermal route

1Cold / hot timing
→
2Fan
→
3Heatsink
→
4Load duty
→
5Thermal feedback

Cold and hot thermal trips have different repair boundaries.

Unidrive O.ht thermal route image

Unidrive O ht thermal route cold hot timing fan heatsink load duty and thermal feedback
The image separates cold sensor feedback from real hot-load cooling faults.

Cold versus hot thermal trips

A thermal trip that appears cold is a different case from one that appears after sustained load. Cold trips point to sensor/feedback wiring or control electronics; hot trips point to airflow, fan, heatsink contamination, ambient temperature, load duty or braking heat.

Thermal matrix

Split temperature evidence by timing.

O.ht evidence

TimingLikely boundaryEvidence
Cold at power-upTemperature feedbackSensor/board evidence
After long runCooling/load dutyFan, heatsink, ambient
After decel/brakingBrake/thermal couplingResistor and cabinet heat
Intermittent with vibrationFan wiring or sensor connectionConnector evidence
After repairFan installed wrong or airflow blockedAir path photos

Repair boundary

The repair record should include fan condition, heatsink contamination, ambient temperature and whether the trip is cold or load-related before a board is suspected.

Field record checklist

  • Cold/hot timing
  • Fan evidence
  • Heatsink condition
  • Load duty
  • Thermal feedback

Technical basis and reference documents

This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.

Unidrive SP trip-code referenceDrives and Automation

Trip-code context for Unidrive thermal trips.

Diagnostic workflow

Workflow
Unidrive PS.24V / Thermal / Feedback Evidence Workflow

Unidrive reports PS.24V, thermal feedback symptoms, O.ht/O.ht1 or feedback instability with option modules and external control wiring.

Open →
Evidence intake

Turn this record into a qualified service request

A repair decision is much more reliable when the request includes the exact identity of the drive, the first fault evidence and the machine condition when the symptom appeared.

  • Complete drive type code / MLFB or nameplate model
  • Fault code, fault value and first event before reset
  • When the event appears: power-up, enable, ramp, run, decel or stop
  • Motor/cable connected or isolated during the symptom
  • Visible board, option-card, module and connector identifiers
  • Previous repair history, replacement parts and repeat-failure pattern
Prepare request →