E.OC1: Overcurrent Trip During Acceleration

Official meaning and why timing matters

Mitsubishi defines E.OC1 as an overcurrent trip during acceleration. In the FR-A700 manual, the protective circuit stops inverter output when output current reaches or exceeds approximately 220% of rated current during acceleration. Timing is therefore diagnostic evidence: an event at the first start impulse is different from an event only after a mechanically coupled load gains torque.

The manual points to sudden acceleration, output short circuit, base-frequency error, stall prevention/current-limit settings, regeneration effects and certain vector-control wiring conditions. A board-level record adds a second layer: if external causes are eliminated and the trip persists immediately, the output bridge and six-channel gate-drive/support path must be treated as suspect until proven otherwise.

Branch the investigation before replacing hardware

A motor or cable defect must be addressed before the drive is accused. The correct field branch begins with evidence capture and controlled isolation. A mechanic jam or aggressive acceleration ramp can create real current demand without hardware failure; a persistent instant trip with the output separated raises the probability of internal damage.

Internal evidence from the reviewed circuit family

The FR-A740 circuit record shows a multi-rail auxiliary supply and a six-channel isolated gate-drive relationship. After a destructive output event, a collapsed isolated rail, damaged optocoupler/driver channel, altered gate resistance or protection-interface defect can cause a fresh module to be driven incorrectly. Comparative measurement across corresponding U/V/W upper and lower channels matters more than an isolated check of the visibly damaged phase.

Repair-completion criteria

A repaired drive is not complete merely because E.OC1 clears once. The technician needs a defensible cause, restored protection, stable auxiliary/driver evidence, balanced output under a controlled test and confirmation that the motor/load does not recreate the fault. Where these criteria cannot be obtained safely in the field, route the unit to a qualified industrial drive repair bench.

Acceleration-time symptom matrix

E.OC1 has unusually high diagnostic value because the operating instant is known: the event occurs during acceleration. Record whether the motor begins to move, whether the event follows a recent parameter or motor change, whether a heavily inertial or jammed load is present, and whether the fault remains after the motor/cable branch is evaluated. A trip that only occurs under loaded acceleration and a trip that occurs instantly after a repaired output stage are not equivalent cases.

For a board-level assessment, an instantaneous repeat event after verified external isolation raises the relevance of the six-channel driver path, gate support rails and output module. A fault that disappears when the application is separated requires the opposite service decision: avoid unnecessary internal board replacement and pursue motor/cable/load causes first.

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.