How to Troubleshoot Mitsubishi Inverter Error E.OC1 (Overcurrent During Acceleration)?

Getting the Mitsubishi VFD E.OC1 error? This overcurrent alarm happens during acceleration. Learn how to check motor load, ramp times, and output cables to get your drive running smoothly.

 

The Mitsubishi Inverter is designed to protect itself from excessive current. If your keypad displays E.OC1, the drive has detected an Overcurrent condition during the acceleration phase.

In simple terms, the drive is trying to speed up the motor, but the motor is demanding more current than the drive's output transistors (IGBTs) can safely supply. If the drive did not stop, these internal components would likely burn out. Here is a systematic approach to identifying and fixing the cause of this trip.

Common Causes of E.OC1

• Ramp-Up Time Too Short: The drive is trying to force a heavy load (high inertia) to reach full speed too quickly.
• Mechanical Stalling: The driven load (conveyor, pump, or fan) is physically jammed or experiencing high starting torque.
• Short Circuit: A partial short in the motor windings or motor cables.
• Incorrect Motor Parameters: The drive has been programmed with the wrong motor nameplate data, causing it to miscalculate the required current.

Step-by-Step Troubleshooting Guide

Safety Warning: Always disconnect power and wait at least 10 minutes for the inverter's internal capacitors to discharge before opening the unit or touching motor connections.

1. Check Acceleration Time

This is the most common fix. If your application has a heavy flywheel or high inertia, the default acceleration time (often 5.0 seconds) may be too aggressive.

• Locate the Acceleration Time parameter.
• Increase the time value.
• If the drive starts successfully with a longer ramp time, your issue was simply excessive inertia.

2. Inspect the Motor and Cabling

If the ramp time adjustments don't help, look for physical faults:

• Disconnect the Motor: Detach the motor cables (U, V, W) from the inverter.
• Test the Drive: Attempt to run the drive without the motor connected. If the drive trips with E.OC1 *without* a motor attached, the internal power module of the inverter has failed.
• Insulation Test: Use a Megger (insulation tester) to check for a short circuit between the motor phases and the ground. A damaged cable or a burnt motor winding will easily trigger an overcurrent fault.

3. Mechanical Check

Look at the driven equipment:

• Ensure there is no mechanical binding, seized bearings, or debris inside the fan or pump housing.
• If the motor is struggling to turn the load by hand, it will definitely draw excessive current when the inverter tries to start it.

4. Verify Motor Data

Ensure the inverter "knows" what motor it is running. Verify that parameters like Rated Motor Current and Rated Voltage match the motor nameplate exactly. If the drive thinks the motor is smaller than it actually is, it will trigger an overcurrent fault prematurely.

How to Reset the Error

Once you have addressed the mechanical or electrical cause:

• Press the [STOP/RESET] button on the keypad.
• Alternatively, you can trigger a reset by cycling the main power or activating the reset terminal (if configured).

Summary Advice

If your Mitsubishi drive trips on E.OC1 every single time it starts, your motor or cabling is likely damaged. If it trips occasionally during acceleration, your acceleration time is likely just slightly too aggressive for your current load. Minor increases in ramp time can often solve the problem without hardware replacement.