FR-A800 Fault E.THM — Motor Thermal Overload

Overview

The E.THM fault code on the Mitsubishi FR-A800 variable frequency drive (VFD) indicates a Motor Thermal Overload trip. This protective function activates when the drive's internal electronic thermal relay calculates that the connected motor is running dangerously hot. Rather than relying on physical temperature sensors in some setups, the VFD uses a mathematical model based on the output current, operating frequency, and elapsed run-time to estimate the heating state of the motor and cuts power to protect the motor windings from permanent insulation damage.

Symptoms

When an E.THM fault occurs, you can expect the following symptoms in your automated system:

• Unexpected Machinery Shutdown: The VFD immediately cuts output voltage, causing the connected motor to coast to a stop.
• Keypad Fault Display: The FR-A800 parameter unit (FR-DU08 or FR-PU07) prominently displays E.THM.
• Alarm Signal Activation: The drive's physical alarm output relay (typically terminals A-B-C) switches state, alerting the PLC or SCADA system.
• Physical Signs of Heat: The motor itself may feel exceptionally hot to the touch, emit a distinct burning smell, or show signs of paint blistering if it has been operating near its thermal limit for extended periods.
• Warm-Up Phase Failures: The trip occurs either progressively over a shifts as load increases, or rapidly within a few minutes of starting high-torque cycles.

Possible Causes

To effectively troubleshoot the E.THM fault, you must understand the contributing factors. The root cause usually falls into mechanical overload, bad parameters, or environmental/cooling limitations:

• Incorrect Parameter Configuration (Pr. 9): Parameter 9 (Electronic thermal O/L relay) is either left at the factory default setting or programmed with a current limit lower than the motor's actual nameplate Full Load Amps (FLA).
• Mechanical Overload or Jamming: The driven machine (pump, conveyor, fan, gearbox, etc.) has bearing damage, blockages, or high friction that forces the motor to draw current above its rated capacity.
• Low-Speed, High-Torque Operation: Operating a standard self-cooled motor (with a shaft-mounted fan) at low frequencies (below 30 Hz) under a heavy load reduces airflow over the motor casing, leading to rapid heat buildup monitored by the drive's mathematical algorithm.
• Incorrect Motor Type Selection (Pr. 71): Parameter 71 (Applied motor configuration) is improperly set, meaning the thermal model does not match the cooling characteristics of the actual motor connected (e.g., standard motor vs. inverter-duty constant torque motor).
• High Duty-Cycle Rates: Excessive start/stop duty cycles or aggressive deceleration and acceleration ramp profiles without dynamic braking options cause heat spikes in the motor windings.
• Unbalanced Output Phases: A high-resistance connection at the VFD output terminals, a loose contactor pole, or a damaged motor cable can restrict phase current balance, causing localized phase winding overload.

Step-by-Step Troubleshooting

Follow these systematic diagnostic steps to pinpoint and resolve the E.THM fault safely.

Step 1: Verify and Match VFD Parameters

Never trust pre-programmed VFD settings on a replacement or commissioning drive.

1. Cross-reference the motor nameplate Rated Current (Amps) with Parameter 9 (Electronic thermal O/L relay) of the FR-A800.
2. Ensure the value in Pr.9 matches the motor's nominal nameplate current exactly. If the motor is running at a 1.15 service factor, consult engineering values before adjusting beyond nominal limits.
3. Check Parameter 71 (Applied motor selection). For standard induction motors, verify that Pr. 71 is set to 0 or 3 (for constant-torque motors). Under certain conditions, choosing the incorrect motor profile alters the dynamic thermal limit assumptions of the E.THM algorithm.

Step 2: Measure Current and Check for Imbalance

Determine if the overload is an actual physical overcurrent condition or an instrumentation/calculation error:

1. Hook up an industrial clamp-on True-RMS multimeter around the motor output phases (U, V, W) at the drive's output terminal block.
2. Navigate to the monitor menu of the FR-A800 and select the output current monitor option.
3. Run the application (if safe to reset the motor temporarily) and compare the real physical measurement on the clamp meter with the drive’s digital panel reading.
4. If the readings match and both exceed the motor's rated current, the motor is drawing too much actual current. If the real clamp-meter readings are normal but the drive shows an artificially high rating, check your current detection CTs within the VFD or inspect for severe electromagnetic interference on the control lines.

Step 3: Audit Mechanical Load and Machinery

If the motor current is genuinely high under load:

1. Safely lock out and tag out (LOTO) power connections to the FR-A800.
2. Decouple the motor shaft from the driven load (gearbox, drive belt, or fan shaft).
3. Rotate the driven equipment manually to check for binding, stiff bearings, lack of lubrication, or mechanical barriers.
4. Spin the motor shaft independently to check for bearing wear within the motor frame. If the shaft does not rotate smoothly, overhaul or replace the motor bearings.

Step 4: Evaluate Motor Operating Speeds & Cooling Method

Low speed is a common killer of self-cooled (TEFC) induction motors:

1. Assess the operational speed profile of the machine. Is the VFD constantly driving the motor below 25-30 Hz?
2. Standard motors use fan blades attached directly to the rotor shaft; running slowly seriously degrades their velocity and heat dissipation capabilities.
3. If your application demands continuous running at low speeds under high-torque conditions, install an independent, constant-velocity electric fan (blower kit) powered by an external source to cool the motor body, and adjust parameter settings to reflect an inverter-duty motor configuration.

Step 5: Run Diagnostics on Motor Insulation and Winding State

An electrical fault in the motor windings can manifest directly as a thermal overload:

1. Disconnect the motor leads (U, V, W) physically from the VFD output terminal block. Caution: Never perform an insulation test or Megger test directly on VFD output terminals, as this will destroy the drive's IGBT power transistors.
2. Utilize a Megohmmeter (500V or 1000V DC setting) to check isolation resistance between each winding phase and the motor's ground connection.
3. Use a high-quality micro-ohmmeter to measure Phase-to-Phase resistances (U to V, V to W, W to U). Significant imbalance (greater than 1-2%) indicates internal winding degradation or short-circuited turns.

Recommended Actions

• Correct Configuration: Program Pr. 9 to match the exact nameplate rating of your motor.
• Keep Motor Heat in Check: If low-speed operation is required, install an external motor blower unit. Switch to a motor characterized for constant torque performance (Inverter Duty).
• Adjust Acceleration and Deceleration Ramps: If the thermal trip triggers primarily during dynamic velocity shifts, increase acceleration time (Pr. 7) and deceleration time (Pr. 8) to round out current peaks.
• Enclosure Ventilation: Ensure that the cabinet cooling fans and filters of the VFD enclosure are functioning correctly to minimize drive ambient temperatures, which can slightly influence thermal calculations or lead to parallel E.THT limits.

Recommended Replacement Parts

If manual inspection reveals component degradation, utilize these diagnostic replacements:

Related Articles

• Mitsubishi FR-A800 Family Migration Guide
• Sizing External Braking Resistors for High Dynamic Loads
• Troubleshooting VFD-Related Motor Insulation Failures

FAQ

Q: Why does the E.THM fault trigger immediately upon starting up the motor?

A: If the E.THM fault trips instantaneously as soon as you output voltage, this usually points to a severe parameter error (such as Pr. 9 being set to an inappropriately low decimal value), a massive short-circuit in the motor winding, or thermal accumulation memory from a previous run. If the motor has just tripped repeatedly, the drive uses an internal decay timer to mock the cooldown of the motor. Leaving the drive powered up allows the thermal value to scale down to 0%.

Q: Can I disable the E.THM protective trip altogether on my FR-A800 drive?

A: You can disable the electronic thermal relay function by setting Parameter 9 (Pr. 9) to 0. However, doing so disables all built-in thermal protection for the connected motor. This should only be programmed if you have installed an external, physical thermal overload relay or separate thermistors (like PTC/NTC) directly tied to a safety trip circuit. Doing so without auxiliary protection presents an immediate fire and motor-burnout hazard.

Q: What is the difference between E.THM and E.THT faults on the FR-A800 VFD?

A: While both are thermal overload faults, E.THM refers specifically to the Motor Thermal Overload, representing calculated overheating in the motor rotor/stator windings. E.THT represents Inverter Thermal Overload, which indicates that the transistors (IGBTs) or internal components of the FR-A800 drive itself have reached critical temperature thresholds due to high carrier frequencies or excessive ambient cabinet heat.

Q: How does the drive know the motor is hot without internal thermal RTD/PTC sensors?

A: The FR-A800 drive relies on a software-based electronic thermal algorithm. It dynamically monitors output current over time alongside current operating frequency. Because heat generation in a standard electric motor scales with the square of the current ($I^2t$) and dissipation decreases at lower shaft speeds, the software calculates current accumulation profiles and projects winding temperatures accurately without requiring external sensor wiring.