Mitsubishi Inverter Error E.OC1 (Overcurrent During Acceleration)

E.OC1 (Overcurrent During Acceleration) in a Mitsubishi Inverter indicates that the output current exceeded the inverter's overcurrent protection level while the motor was accelerating. To protect the IGBT power module and motor, the inverter immediately trips and displays Error E.OC1.

This fault generally indicates a problem with the motor load, acceleration settings, motor wiring, or inverter output circuit, rather than the control system.

Possible Causes

• Acceleration time is too short.
• Sudden increase in mechanical load during acceleration.
• Motor is overloaded.
• Motor shaft is locked or jammed.
• Short circuit in the motor cable.
• Ground fault in the motor or motor cable.
• Incorrect motor wiring.
• Incorrect motor parameter settings.
• Motor capacity is too large for the inverter.
• Output phase-to-phase short circuit.
• Defective motor.
• Failed IGBT power module.
• Incorrect V/F or vector control settings.
• Excessive torque boost setting.

Solutions

1. Increase the acceleration time.
2. Check for sudden load changes during startup.
3. Inspect the motor and driven machine for mechanical binding or seizure.
4. Check the motor load and reduce it if necessary.
5. Inspect the motor cable for short circuits or damage.
6. Check the motor wiring for correct phase connections.
7. Measure the motor insulation resistance according to the motor manufacturer's recommendations.
8. Verify that the motor parameters match the motor nameplate.
9. Ensure the inverter is correctly sized for the motor.
10. If the fault persists with the motor disconnected, inspect the inverter output stage (IGBT module).

Additional Information

Mitsubishi inverters continuously monitor the output current during all operating modes.

Error E.OC1 is generated specifically when an overcurrent occurs during the acceleration period. The inverter immediately disables its output to prevent damage to the power semiconductor devices.

The most common causes are:

• Acceleration time set too short.
• Excessive starting load.
• Mechanical jamming.
• Motor or cable short circuit.

Unlike E.OC2 (Overcurrent During Constant Speed) or E.OC3 (Overcurrent During Deceleration), E.OC1 occurs only while the inverter is accelerating the motor.

Common Symptoms

• Inverter displays E.OC1.
• Motor trips immediately after the RUN command.
• Fault occurs before reaching the commanded speed.
• Motor jerks or hesitates during startup.
• High output current is observed during acceleration.
• Drive repeatedly trips during startup.
• Motor fails to reach operating speed.
• Machine cannot complete the acceleration cycle.
• Fault resets but returns on the next start attempt.
• In severe cases, inverter output remains disabled until the fault is reset.

Recommended Troubleshooting Sequence

1. Verify that the fault is E.OC1 (Overcurrent During Acceleration).
2. Disconnect power and follow lockout/tagout procedures.
3. Check the driven machine for mechanical binding.
4. Inspect the motor load.
5. Increase the acceleration time.
6. Check motor wiring and cable condition.
7. Measure motor insulation resistance according to the motor manufacturer's recommendations.
8. Verify motor parameters against the motor nameplate.
9. Ensure the inverter capacity matches the motor.
10. Disconnect the motor and test whether the inverter still trips.
11. Inspect the inverter output stage if the fault persists.
12. Repair or replace defective components.
13. Restore power and perform a functional test.
14. Monitor output current during acceleration.

E.OC1 Check Table

Check Point	Action
Mechanical load	Check for binding or seizure
Acceleration time	Increase if too short
Motor load	Reduce excessive load
Motor wiring	Verify correct connections
Motor cable	Inspect for damage or short circuits
Motor insulation	Test according to manufacturer recommendations
Motor parameters	Verify against the motor nameplate
Inverter sizing	Confirm correct capacity
IGBT output stage	Inspect if the fault persists
Drive operation	Monitor output current during acceleration

Preventive Maintenance Tips

• Set appropriate acceleration times for the application.
• Avoid starting the motor under excessive mechanical load.
• Inspect motor cables regularly for insulation damage.
• Periodically test motor insulation resistance.
• Keep motor terminals clean and tight.
• Verify motor parameters after commissioning or replacement.
• Ensure the inverter is properly sized for the motor.
• Maintain the driven equipment to prevent mechanical jamming.
• Record recurring E.OC1 faults to identify developing electrical or mechanical issues.
• Follow Mitsubishi's recommended maintenance procedures for the inverter and motor.

Proper inspection of the acceleration settings, motor load, motor wiring, motor cable, motor parameters, and inverter output stage will help prevent recurring E.OC1 (Overcurrent During Acceleration) faults in Mitsubishi inverters, ensuring reliable motor starting, effective inverter protection, and long-term system performance.