When a CNC milling machine or automated robotic cell trips on a servo drive Over-Current (OC) or Over-Voltage (OV) alarm code (such as Fanuc Alarm 401/414 or Siemens Sinumerik Fault 25201), immediate systematic troubleshooting is required to pinpoint whether the fault originates inside the servo amplifier or externally in the motor cabling.
Step 1: Isolate the Motor Cabling & Megger Test
Before replacing internal drive modules, disconnect U, V, W power phase wires from the servo drive output terminals. Measure insulation resistance using a 500V Megohmmeter between each motor phase and ground chassis. A reading below 10 Megaohms indicates stator winding breakdown or liquid ingress into motor junction boxes.
Step 2: Diode Test the Internal IGBT Power Module
Using a digital multimeter in Diode Test mode, measure forward and reverse voltage drops between DC Bus terminals (+ / -) and motor output terminals (U, V, W). A healthy IGBT freewheeling diode typically reads between 0.3V and 0.6V. A 0.00V reading signifies a shorted IGBT transistor phase.
Step 3: Inspect Optocoupler Gate Driver Circuits
If an IGBT short circuit occurred, high voltage often back-feeds through gate resistors into Optocoupler isolation driver ICs. Replacing only the IGBT without replacing blown gate driver resistors will instantly destroy the new IGBT module upon powering on!
Encountering PCB or Servo Drive Issues?
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