FR-E700 Fault E.OC2 — Overcurrent at Constant Speed

Overview

The E.OC2 fault code on a Mitsubishi FR-E700 series variable frequency drive (VFD) indicates an Overcurrent at Constant Speed condition. This trip occurs when the drive’s internal current-sensing transformers detect an output current spike exceeding approximately 200% of the drive's rated current while the drive is operating at a stable, non-transitioning frequency (after acceleration is complete and before deceleration begins). Unlike startup overcurrent trips, E.OC2 signifies a sudden dynamic load change, an instantaneous electrical failure, or a controls-level breakdown that forces the drive's outputs beyond safe operating parameters during steady-state run time.

Symptoms

When an E.OC2 event occurs, the FR-E700 drive protects its internal components by immediately shutting off output to the motor. You will observe the following symptoms on the line and inside the control enclosure:

• Keypad Display: The integrated LED display flashes "E.OC2" continuously.
• Motor Coast-to-Stop: The driven motor immediately loses torque and coasts to a stop because the output transistors (IGBTs) are immediately gated off.
• Fault Relay State Change: The auxiliary fault output relays (terminals A, B, and C) switch states. This typically triggers an immediate emergency shutdown signal to the supervisory control system (such as a PLC or DCS).
• Audible Alerts: A physical mechanical thud or an audible whine from the motor winding can sometimes be heard immediately before the trip occurs.
• Status Indicators: Digital status monitoring parameters hold the fault history, including the output frequency, voltage, and current at the exact moment of the trip.

Possible Causes

Isolating an E.OC2 fault requires separating mechanical variables from electrical issues. The most common root causes include:

• Mechanical Load Fluctuations: A sudden physical jam, structural binding, or severe spike in mechanical resistance (such as a conveyor belt jamming, a pump experiencing cavitation, or a failed bearing on a fan shaft).
• Intermittent Motor Insulation Failure: The motor winding insulation may have deteriorated, causing a temporary, heat-activated phase-to-phase or phase-to-ground short circuit that only manifests at running speed and voltage.
• Output Cable Micro-Shorts: Moisture in wire conduits, vibration rubbing cable insulation against metal conduit paths, or a failing motor junction box terminal strip.
• Incorrect Torque Boost (Parameter 0): An excessively high manual torque boost configuration can inject too much current into the motor under nominal operating loads.
• VFD Hardware Degradation: Damaged output transistors (IGBTs), aged/leaking DC bus capacitors, or failing current detection sensors inside the FR-E700 unit.
• Sudden Line Voltage Instability: A severe incoming line voltage sag or phase drop can force the VFD to draw elevated output currents to maintain constant shaft torque.

Step-by-Step Troubleshooting

Follow these sequential diagnostics to locate and resolve the root cause of the E.OC2 fault on your FR-E700 drive.

Step 1: Record Operational Parameters at Trip

Before clearing the fault, use the digital PU dial/keypad to enter the drive's alarm history mode. Write down the operating parameters recorded at the moment of the E.OC2 trip:

• Output Frequency (Hz): Is it tripping exactly at the same speed every time?
• Output Current (A): How far beyond the motor's full-load amps (FLA) was the trip threshold?
• Output Voltage (V): Was the voltage depressed at the point of failure?

Step 2: Perform an IGBT Diode Check

To rule out internal hardware damage within the drive's output transistor stage, execute a static diode test:

1. Disconnect all incoming power to the VFD and lock out the source (LOTO).
2. Wait at least 10 minutes for the internal DC bus capacitors to fully discharge. Verify there is 0 VDC across terminals P/+ and N/- using a calibrated multimeter.
3. Disconnect the motor output cables from terminals U, V, and W.
4. Set your multimeter to Diode Mode.
5. Place the Red (+) probe on the N/- terminal and touch the Black (-) probe to U, V, and W in sequence. You should see a consistent forward bias reading of approximately 0.3V to 0.7V on all three phases.
6. Reverse the setup: Place the Black (-) probe on the P/+ terminal and touch the Red (+) probe to U, V, and W in sequence. Again, look for a balanced reading between 0.3V to 0.7V across all three phases.
7. If any reading is 0.0V (shorted) or OL/open, the internal IGBT module is damaged and the drive must be replaced.

Step 3: Test the VFD Offline (No-Load Test)

To isolate VFD electronics from external motor and cable issues:

1. Keep the motor cables disconnected from VFD terminals U, V, and W.
2. Restore input power to the VFD.
3. Change Parameter 79 (Operation Mode Selection) to 1 (PU operation mode).
4. Press RUN and ramp the drive up to your normal operating speed.
5. Monitor the drive for several minutes. If the E.OC2 fault trips without any motor connected, the VFD's internal current sensor or gate drive circuit is malfunctioning. Replace the drive.
6. If the drive runs cleanly without throwing a fault, turn off power, reconnect the motor cables, and proceed to Step 4.

Step 4: Inspect and Test Output Cabling and Motor

If the VFD behaves normally under no-load conditions, test the load components:

1. Use an insulation resistance tester (Megohmmeter) to test the motor and connecting cables. Do not apply the testing voltage while connected to the VFD, as this will destroy the drive's output semiconductors!
2. Disconnect the cables from the VFD and test insulation resistance from phase-to-phase (U-V, V-W, W-U) and phase-to-ground. Any reading below 10 Megohms indicates cable deterioration or moisture ingress in the motor terminal box.
3. Measure winding resistance using a micro-ohmmeter or high-quality digital multimeter. Ensure all winding resistances are balanced within a 2-3% tolerance range.

Step 5: Evaluate Mechanical Loads for Sags or Hard Jams

If electrical tests pass, evaluate physical mechanics:

1. Safely attempt to rotate the motor shaft and driven load by hand (with power disconnected). Switchgear, gearboxes, and pumps should rotate smoothly.
2. If the system has high load inertia, monitor the mechanical process during constant speed. Watch for sudden obstructions, valve changes, belt slippage, or conveyor mechanical misalignments that correlate with the E.OC2 trip pattern.

Step 6: Review Parameter Configurations

Optimizing parameters can mitigate nuisance overcurrent faults:

• Verify Parameter 0 (Torque Boost): The factory default is typically too aggressive for high-efficiency motors, generating high current. Reduce the manual torque value incrementally (e.g., from 6% down to 3% or 2%) and test operating stability.
• Verify Parameter 9 (Electronic Thermal O/L Relay): Confirm this matches the motor's actual full-load amperage (FLA) rating listed on the physical nameplate.
• Implement Auto-Tuning: If you are running in advanced magnetic flux vector control mode, ensure you run the motor auto-tuning procedure (Parameter 96) to properly match VFD output impedance to the motor's dynamic electrical characteristics.

Recommended Actions

To prevent recurrence of the E.OC2 fault, implement these field-proven preventative measures:

• Install an AC Line Reactor: If incoming line sag or transient surges are common in your facility, install an AC line reactor (such as the FR-HAL series) on the incoming supply side of the VFD. This smooths line current spikes and buffers the DC bus.
• Review Environmental Conditions: Ensure the cabinet housing the FR-E700 has operating cooling fans and clean ventilation filters. High cabinet temperatures lower the thermal limits of physical semiconductors, making current sensing circuits far more sensitive.
• Routine Cable Management: Ensure all shielding is correctly terminated at the ground plate. High electromagnetic interference (EMI) can corrupt current measurement feedback loops within the VFD's control board.

Recommended Replacement Parts

If diagnostics point to hardware failure, consider these exact mitigation paths and original equipment components:

Related Articles

• How to Migrate Your Legacy Mitsubishi FR-E700 to the New FR-E800 Series
• Sizing and Compatibility Guide for Mitsubishi FR-HAL Line Reactors
• Troubleshooting Mitsubishi VFD Faults: Resolving E.OC1 and E.OC3 Errors

FAQ

Q: What is the main difference between E.OC1, E.OC2, and E.OC3 faults?

A: These faults represent overcurrent conditions occurring at different operating states. E.OC1 indicates an overcurrent spike happened during acceleration. E.OC2 indicates it occurred while running at a constant, steady speed. E.OC3 denotes overcurrent occurring during deceleration or active braking.

Q: Can I bypass the E.OC2 fault by disabling the overcurrent protection parameters?

A: No. The overcurrent trip is a hardware-integrated self-preservation response designed to protect the IGBTs from vaporizing during critical over-amperage events. It cannot and should not be disabled. Failing to resolve the underlying fault will result in catastrophic VFD or motor winding destruction.

Q: Why does my E.OC2 fault only occur on hot summer afternoons?

A: Elevated ambient temperatures increase resistance within both the motor windings and the VFD's internal silicon pathways. Hotter components run less efficiently and draw more current. Furthermore, internal cooling fan degradation becomes critical under high ambient temperatures, leading to localized hot spots near the VFD current sensors that trigger erratic overcurrent trips.

Q: Does a mechanical brake sticking cause this fault?

A: Yes. If a mechanical motor brake fails to release properly, or releases late, it holds the shaft locked or highly resistant relative to the commanded V/F pattern frequency. Because the VFD is expecting the motor to run at a consistent speed but meets immense mechanical resistance, motor slip spikes dramatically, leading to an immediate E.OC2 overcurrent event.