E.THT: Inverter Overload Trip

Different from an instantaneous short circuit

The official manual describes E.THT as inverter overload protection by electronic thermal relay function. Where current reaches at least 150% of rated output without reaching instantaneous overcurrent trip, the thermal model protects the output transistors; the manual states a 150% for 60 seconds inverse-time characteristic.

This distinction is important. A drive that runs before E.THT is not exhibiting the same evidence as an E.OC1 event at startup. The initial questions are sustained load, duty cycle, acceleration/deceleration, torque boost, load pattern and cooling—not an immediate assumption of a shorted output module.

Triage table

Where board-level evidence matters

A purely workload-driven E.THT may be corrected without internal board work. However, recurring overheating, lost fan command, temperature-signal irregularity or an earlier IGBT event can justify inspection of the supply/driver/fan interface represented in the reviewed drawing family. The correct order preserves the distinction between a thermal overload condition and a destructive switching failure.

Completion criteria

A technically complete resolution documents operating current/duty, verifies cooling performance, restores correct settings and confirms the trip does not recur under intended service. Where a damaged output stage or control/supply evidence appears, the unit requires a repair assessment rather than further operation.

Overload evidence must include time and thermal conditions

E.THT is separated from E.OC1 by its time behaviour: the official protection model concerns output current sustained above permitted overload capability, rather than an abrupt short-circuit-class event. A field inquiry should therefore capture current trend, motor load, acceleration/deceleration profile, fan and heatsink condition, ambient temperature and whether the fault occurs after a reproducible operating interval.

The board-level branch becomes more credible when cooling conditions are proven acceptable yet the drive reports persistent overload/thermal behaviour without corresponding external current demand, or when previous power-stage damage has compromised current measurement or protection behaviour. This keeps a thermal complaint from being misdiagnosed as an immediate short circuit.

Practical branching record

A competent workflow records observations before making a replacement decision. First capture the displayed trip and operating instant. Next identify whether the fault can occur with the motor disconnected under manufacturer-approved conditions, whether deceleration/braking conditions are relevant, and whether prior module or board work was performed. Only after these branches are documented should the investigation move to board-level evidence.

The value of this sequence is that identical-looking trips can have entirely different root causes. An acceleration overcurrent may come from an output short or mechanical demand; a regenerative overvoltage may be caused by deceleration energy and braking configuration; a repeated immediate trip after output-stage repair may justify analysis of gate-drive or feedback circuitry.

Stop conditions for a field technician

Stop further attempts when there is visible power-stage damage, a repeat protective trip with external causes excluded, unstable control supplies, evidence of carbonisation or flashover, or uncertainty about DC-bus discharge and safe isolation. Repeated reset-and-run attempts can convert a repairable board problem into a destroyed power module and collateral control-board damage.

A useful escalation request contains the exact model, trip history, whether the trip occurs before or after output command, motor/cable test status, brake-unit details where relevant, and clear photographs of affected board and module markings. This turns a vague fault inquiry into a diagnostic case that a specialist can evaluate.