Cooling fan
Common VFD, drive, UPS, and cabinet overheating failure point.
Mitsubishi FR-E800 Fault E6 — What It Means
Fault E6 on the Mitsubishi FR-E800 inverter indicates a brake transistor fault — the internal braking transistor (used to dissipate regenerative energy during deceleration) has failed or is operating outside normal parameters. The FR-E800 monitors the braking transistor duty cycle and temperature; if the transistor shorts or the braking resistor circuit is open, E6 trips to prevent a DC bus overvoltage condition.
Common Causes
- Failed braking transistor — The internal brake IGBT can fail short-circuit, triggering E6 immediately on power-up.
- Braking resistor open or disconnected — If an external braking resistor is specified and becomes open-circuit or disconnected, the transistor has no path to dissipate energy and the fault trips.
- Braking resistor overheating/overload — Excessive regenerative energy from a large inertia load overheats the braking resistor. Most resistors have a thermal switch that opens on overheat.
- Deceleration time too short — An aggressive decel ramp on a high-inertia load puts more energy into the braking circuit than it can handle.
Step-by-Step Fix
- Check for a shorted braking transistor — With power off, measure resistance between the P/+ terminal and PR terminal. Near-zero resistance in both directions indicates a shorted transistor.
- Inspect the braking resistor — Measure resistance across the braking resistor terminals (if external resistor installed). Open circuit = burned-out resistor. Check thermal fuse/switch on the resistor.
- Extend deceleration time — Increase Pr.8 (Deceleration Time) to reduce regenerative current per unit time. This is the first parameter fix to try before replacing hardware.
- Check wiring to braking resistor — Verify the resistor is connected between P/+ and PR terminals with correct wire gauge. Loose connections cause intermittent E6.
- Reset and test — After repair, cycle power and run a ramp-down cycle. Confirm E6 does not return.
Parts Often Needed
| Part | Notes |
|---|---|
| Braking resistor | Match to FR-E800 frame size and duty cycle requirement |
| FR-E800 inverter unit | If internal brake transistor has failed |
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
Internal braking transistor replacement requires drive disassembly and component-level repair. Mitsubishi authorized service handles transistor-level repairs for FR-E800 units.