0857

Common Causes

• Faulty motor winding insulation causing a phase-to-phase or phase-to-ground short
• Damaged or chafed motor power cable shorting to ground or between phases
• Inverter unit power stage failure
• Mechanical overload or seized axis driving current beyond the limit

Troubleshooting Steps

1. 1Read the code number shown with the alarm: code XXXX = U-phase current at the moment of the alarm, signed hexadecimal (tolerance limit 3FFF).; code YYYY = V-phase current at the moment of the alarm, signed hexadecimal (tolerance limit 3FFF).
2. 2Note the code: XXXX is the U-phase current and YYYY the V-phase current (signed hex, max 3FFF) at the trip — large values confirm a genuine overcurrent.
3. 3Lock out the machine and let the DC bus discharge before touching any motor or inverter wiring.
4. 4Disconnect the motor power cable at the motor and megger the motor windings phase-to-phase and phase-to-ground; low insulation resistance means the motor insulation has failed.
5. 5Inspect the motor cable along its full run (especially at cable track bends and connectors) for chafing, crushed sections, or coolant-soaked insulation; megger the cable separately from the motor.
6. 6Check the axis moves freely by hand with power off — a seized ballscrew, bearing, or brake can drive sustained overcurrent.
7. 7If motor, cable, and mechanics are all good, the inverter unit itself is the remaining suspect — record the alarm code and contact Okuma service.