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 {#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 | Amazon | Match to Mitsubishi FR drive model; verify Ω rating and wattage |
| Brake transistor / drive power board | Amazon | Replace if transistor is confirmed failed; typically requires whole board |
| Braking resistor wiring | Amazon | Use wire rated for resistor peak current; keep leads short |
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.