Applicable Systems
MELSEC FX3U
MELSEC FX5U
FR-A800
FR-A700
MR-J4
iQ-R Motion Controller (R32MTCPU/R16MTCPU)
Symptoms
The drive trips and the panel displays E.THM, indicating motor thermal overload. The electronic thermal relay function has detected that the motor current has exceeded the thermal protection threshold for a sustained period.
Possible Causes
- Actual motor overload due to excessive mechanical load
- Motor cooling fan or ventilation blocked causing thermal buildup
- Electronic thermal relay parameter (Pr.9) set too low for the motor rating
- Motor rated current parameter mismatch causing false overload detection
Troubleshooting Steps
- Step 1: Measure the actual motor current during operation
Current should be at or below the motor rated current; if above, reduce the load - Step 2: Inspect the motor cooling fan and ventilation paths
Fan should spin freely; air passages should be clear of dust and debris - Step 3: Verify parameter Pr.9 (electronic thermal relay) matches the motor rated current
Pr.9 should equal the motor nameplate rated current - Step 4: Check if the motor duty cycle is within the rated S1 (continuous) or S3 (intermittent) specification
If intermittent duty, ensure the on/off ratio is within the motor thermal capacity
How to Reset
To reset: 1) Press STOP/RESET. 2) Reduce the load or fix the cooling. 3) Verify Pr.9 setting. 4) Restart. Caution: Running a motor in an overloaded condition can cause insulation failure and permanent damage.
Frequently Asked Questions
- Q: How does the electronic thermal relay work without a physical sensor?
The drive calculates motor heating based on the measured current and the thermal time constant. It simulates the motor's thermal state using the current level and duration. For accurate protection, Pr.9 must match the motor rated current.
- Q: Can I disable the electronic thermal relay?
Yes, set Pr.9 to 0. However, this is strongly discouraged as it removes motor thermal protection. Use an external thermal sensor (PTC/PT100) instead if the built-in function is not suitable.
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Source: Mitsubishi FR-A800 Instruction Manual (Functions), 2024
Category: PLC & VFD
Published: 2026-10-31
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