Faults Are Information

A Mitsubishi Electric IPM reports its faults through a fault output, but that one signal can mean a short circuit, an undervoltage condition or an over-temperature event. The temptation is to treat the fault as a nuisance and restart the drive, yet the fault is telling you something specific and useful. This article presents a systematic method for diagnosing the common Mitsubishi Electric IPM faults in servo, inverter and compressor drives, with a practical fix for each.

Step 1: Identify the Fault Type

The first step is to determine which protection tripped. Many controllers log only that a fault occurred, which is not enough. Record the fault type and note the load and operating condition at the moment of the trip. A trip during acceleration points to short circuit or overcurrent; a trip at high ambient temperature points to over-temperature; a trip with no obvious load fault points to noise or a supply problem. Matching the symptom to the protection saves a great deal of time.

Reading the Fault Signal

Where the controller does not decode the fault type directly, the timing of the fault can still help. Measure when and how the fault asserts relative to the switching and the load, and correlate it with the motor current. That correlation usually identifies the cause.

Short-Circuit Trips

A short circuit is the most serious fault. It has three typical causes: a genuine load fault such as a short or a locked rotor, a trip level that is set too low by the sense resistor, or a control-interface problem that produces a spurious trip. Start by checking the sense-resistor value against the intended trip current. Then measure the phase current with a clamped probe during the trip to see whether the current is genuinely high or the trip is premature. Inspect the motor and wiring for a short, and check the DC-link capacitance and power-loop layout for the ringing that can produce a spurious trip.

Locked Rotor and Overload

A locked rotor or a mechanical overload draws a large current that the module correctly detects. An IPM treats the overload as a fault and shuts down, so the fix is at the mechanical or control level, not the power stage. Confirm that the fault clears once the load is free and the drive re-enabled.

Undervoltage Faults

An undervoltage fault means the control supply fell below the protection threshold, which weakens the gate drive and risks linear-mode operation of the switches. Check the control supply for ripple and sag, especially during switching, and verify that its decoupling is close to the module. A supply that measures its nominal voltage with a multimeter can still collapse for microseconds during a switching edge, so inspect it with an oscilloscope near the module pins.

Over-Temperature Faults

An over-temperature fault means the module temperature exceeded its threshold. This is usually a thermal-design problem: an insufficient heatsink, a thick or uneven thermal interface, an incorrectly mounted module or an ambient temperature higher than assumed. Inspect the mounting for flatness and torque, measure case temperature at rated load, and confirm the heatsink against the loss estimate. Repeated over-temperature events stress the module and shorten its life, so treat them as a design defect rather than a transient.

Thermal Interface and Mounting

The thermal interface is a frequent culprit. A thick, uneven or contaminated interface raises the thermal resistance enough to cause an over-temperature fault at rated load. Use a thin, uniform layer and the specified mounting torque, and re-measure case temperature after any rework.

False Faults from Noise

Sometimes the fault is real to the module but not to the load: electrical noise couples into the control supply or the sense path and trips the protection. Symptoms include faults at specific switching transitions and faults that disappear when the load is disconnected. Fixes include better control-supply decoupling, a shorter sense loop, shielding and a tighter power loop.

Clear the Fault Correctly

Once the cause is identified and fixed, clear the fault and re-enable the drive only when the load is safe. Do not rely on fast auto-restart, because a persistent fault will trip again and may damage the hardware. Logging the fault type and the operating condition at every trip turns the fault output from an annoyance into a diagnostic tool.

Conclusion

A Mitsubishi Electric IPM fault is a precise, useful signal once you decode it. Identify the protection that tripped, match the symptom to the cause, fix the design or the load, and clear the fault properly. With that discipline, IPM faults become a guide to better design rather than a recurring interruption.