Drive cooling fan
Common heat-related VFD and servo-drive failure point.
Mitsubishi FR Series Fault E6 — What It Means
E6 (or E.6) on a Mitsubishi FR series VFD (FR-A700, FR-A800, FR-E700, FR-E800) indicates a brake transistor fault. The drive detected an anomaly in the built-in brake circuit — either the brake transistor has failed, the braking resistor is open or short-circuited, or the brake circuit detected excessive on-time indicating a thermal protection condition. The drive stops to prevent damage to the brake transistor and resistor.
Common Causes
- Failed brake transistor (IGBT) — The internal brake transistor has shorted or opened. A shorted transistor causes continuous resistor current; an open transistor means no braking capability and bus voltage rise.
- Open or shorted braking resistor — The external braking resistor has burned out (open circuit) or shorted. An open resistor prevents energy dissipation; a shorted resistor draws excessive continuous current.
- Braking resistor overload (thermal protection) — The resistor has been used beyond its duty cycle rating, and the drive’s thermal model tripped E6 to protect the resistor from burning out.
- Wiring fault at the PR/P+ terminals — A loose or incorrect connection between the drive’s brake terminals and the external resistor causes abnormal brake circuit behavior.
Step-by-Step Fix
- Power down the drive and wait for DC bus discharge — Turn off the input breaker and wait at least 5 minutes for the DC bus capacitors to discharge to a safe level before touching internal components.
- Measure the braking resistor — Disconnect the braking resistor from the PR and P+ terminals. Measure resistance with a multimeter. The resistance should match the resistor’s rated value (e.g., 20Ω, 40Ω). An open (OL) reading means the resistor has burned out; a very low reading (near 0Ω) means it has shorted.
- Inspect the braking resistor for visible damage — Look for burned windings, melted insulation, or carbonized resistor wire. Any visible damage confirms resistor failure.
- Check the PR/P+ wiring — Verify the wiring between the drive’s brake terminals and the resistor is correct, tight, and properly sized for the resistor’s current rating.
- Test the brake transistor — If the resistor tests good, the brake transistor (IGBT) may have failed. This requires a technician with power electronics test equipment or a replacement drive for comparison testing.
- Reduce braking duty cycle — If thermal overload is the cause, review the deceleration rate (Pr.8) and the braking duty cycle. Extend the decel time or upgrade to a higher-duty braking resistor.
- Reset the fault and test — After replacing the failed component, restore power, clear E6 via the parameter unit or reset terminal, and test through a full deceleration cycle.
Parts Often Needed
| Part | Notes |
|---|---|
| Braking resistor | Match to Mitsubishi FR drive model; verify Ω rating and wattage |
| Brake transistor / drive power board | Replace if transistor is confirmed failed; typically requires whole board |
| Braking resistor wiring | Use wire rated for resistor peak current; keep leads short |
Technician Tips
When diagnosing VFD faults, always check the drive’s fault history before resetting. Modern drives store the last 5-10 faults with the drive state at the time (running, accelerating, decelerating, stopped). An overcurrent fault that occurs only during acceleration points to incorrect motor data or mechanical binding. The same fault during deceleration suggests regeneration issues. A ground fault that clears after disconnecting the motor and returns when reconnected means the motor or cable insulation has failed.
Prevention: Schedule annual infrared scanning of drive cabinets. Loose power connections and failing capacitors produce heat long before they cause faults. A 5-minute scan can prevent hours of unplanned downtime.
Need Replacement Parts?
Before ordering, match the drive part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
Brake transistor testing and power board replacement require power electronics expertise. A Mitsubishi Electric-authorized drive service technician can test the brake circuit under load and replace the internal brake components correctly.