Servo Alarm: Is It the Amplifier or the Motor? How to Tell Before You Spend

An axis faults with an overcurrent-class alarm — FANUC alarm 8/12-series, Mitsubishi AL.32, Yaskawa A.10 — and you're staring at the two most expensive suspects on the machine: the servo amplifier and the servo motor. Guess wrong and you either replace a healthy amplifier, or worse, install a good amplifier onto a shorted motor and watch it die in seconds.

Here's the field-level isolation that costs nothing but a megohmmeter and twenty minutes.

First: what the alarm is actually saying

An overcurrent/IPM alarm means the amplifier's power stage saw current beyond its limit and shut down to protect itself (or failed trying). Three things cause that:

1. The amplifier's own power stage failed — a shorted IGBT or intelligent power module (IPM). The most common servo amplifier failure, period. Often announced by the alarm appearing the instant the drive enables, or the machine breaker tripping at power-on.
2. The motor or its cable is shorted — winding-to-winding or winding-to-ground. The amplifier is (so far) healthy and doing its job by faulting.
3. A mechanical jam demanded more current than the axis limit — less common, and usually obvious from what the machine was doing.

The 20-minute field isolation

Step 1 — Read the timing. Alarm at enable/power-up with the axis doing nothing → lean amplifier. Alarm only under load, during rapids, or at a particular spot in travel → lean motor/cable/mechanics.

Step 2 — Disconnect the motor leads at the amplifier (machine locked out, drive powered down, DC bus verified discharged — these buses hold lethal charge after power-off).

Step 3 — Megger the motor + cable. With the leads off the amp, test insulation resistance from each motor phase to ground at 500V. Healthy: hundreds of megohms. A reading in the low megohms or kilohms = shorted motor or cable — do not connect a good amplifier to it. Also check phase-to-phase resistance with a plain multimeter: the three readings should match closely; a significantly low pair suggests a winding short.

Step 4 — Power the amplifier with the motor disconnected (where the platform allows). Many amplifiers will sit enabled with no motor and no alarm if their power stage is healthy; an amp that faults instantly with nothing connected has convicted itself.

Step 5 — Check the amplifier's power stage passively. With everything isolated, a multimeter in diode mode across the amplifier's output terminals to the DC bus terminals will reveal a dead-shorted IGBT leg on most units — a reading near zero ohms where its neighbors read a normal diode drop.

What each verdict costs

• Shorted motor/cable: motor rewind or replacement territory — and be glad you didn't feed it a fresh amplifier.
• Failed amplifier: a repair, not a replacement. IGBT/IPM replacement with new components, gate-drive verification, DC-bus recap, and powered testing typically runs $450–$950 on Mitsubishi MR-J2S-class amps and $650–$1,500 on FANUC alpha-series — against exchange units at $2,500+ and new units that may not exist anymore.

If you'd rather not open anything

Fair. Ship us the amplifier with the exact alarm number and what the machine was doing — the evaluation is free. We test the amplifier independently on the bench: if it's healthy, you pay nothing, and you've just cleared the expensive suspect and can chase the motor side with confidence. If it's failed, you get a firm quote before any work happens.

Either way: never "test" a spare amplifier on an un-meggered motor. That experiment has a 100% documented cost and a 0% information yield.