Guide · AC servo motor · Updated November 2026
How to check an AC servo motor before you send it for repair
When a CNC axis, robot joint or packing machine stops on a servo alarm, the first question is always the same: is it the motor, the drive or the cable? You can answer most of it on the shop floor in under an hour with a multimeter, an insulation tester and a known-good spare. This guide walks through the seven checks our repair bench uses first on every industrial AC servo motor (Fanuc, Yaskawa, Mitsubishi, Siemens, Panasonic, Delta, Allen-Bradley and other makes), what a good and bad reading looks like, and where field tests stop and a bench test is needed.
Star Automations has repaired servo motors since 2000. Multi-brand servo motor repair is our main work, done at our Puducherry, Chennai and Bengaluru sites.
- Step 1Look, smell, turn the shaft
- Step 2Winding resistance
- Step 3Insulation, 500 V
- Step 4Back-EMF
- Step 5Holding brake
- Step 6Encoder and cable
- Step 7Swap test
Before you start: safety
- Switch off and lock out the machine. Wait for the drive’s DC bus to discharge (most drives show a charge LED; wait until it is fully off, typically several minutes).
- On vertical or gravity-loaded axes, support the axis mechanically before you touch the brake or remove the motor. A released brake can drop the axis.
- Disconnect the motor power cable from the drive before any insulation test. A megger connected through the drive can destroy the drive’s output stage.
- Never put a megger on the encoder, its cable or the brake leads. They carry low-voltage electronics.
- Let a hot motor cool to room temperature before measuring. Readings on a hot or wet motor are misleading.
Step 1: Look, smell and turn the shaft
- Read the nameplate and note the model and serial number; you will need them for any spares or repair quote.
- Check the power and encoder connectors for burnt pins, green corrosion, loose crimps and coolant or oil inside. Coolant in the encoder connector is one of the most common causes of servo alarms on CNC machines.
- With the power leads disconnected from the drive and the brake released (or a motor without a brake), turn the shaft by hand. A healthy permanent-magnet servo motor turns with a light, even “cogging” feel. Gritty, rough or noisy rotation points to bearings. A shaft that will not turn at all with the brake released points to a seized bearing, a brake that is not releasing or a mechanical jam.
- Simple winding check by feel: short the three power leads U, V and W together and turn the shaft again. It should become clearly stiffer and even (the magnets generate current into the shorted winding). If it already feels stiff and uneven with the leads open, suspect an internal short between turns or phases.
Step 2: Winding resistance with a multimeter
Measure resistance between U-V, V-W and W-U at the motor connector (not at the drive end of a long cable).
- Good: all three readings are close to each other, within about 5 percent. Small motors may read a few ohms; large motors read a fraction of an ohm, so use the lowest range or a milliohm meter, and subtract the meter lead resistance.
- Bad: an open circuit on any pair, or one pair clearly lower or higher than the other two (shorted turns, a burnt joint or a loose crimp).
- Limit: a multimeter cannot see a few shorted turns or weak insulation between turns. That needs a surge (impulse) comparison test on a bench.
If the maker’s catalogue gives a phase-to-phase resistance, compare with it. Otherwise compare with an identical healthy motor.
Step 3: Insulation resistance with a 500 V megger
With the motor power cable disconnected from the drive, measure from each phase (U, V, W) to the motor frame using an insulation tester set to 500 V DC. Keep the test short.
FANUC’s maintenance manual (B-65425EN, βi-B AC servo motors) gives this judgement table for a 500 V DC test between winding and frame, and it is a sensible guide for most low-voltage servo motors:
| Insulation resistance | What it means |
|---|---|
| 100 MΩ or higher | Acceptable |
| 10 to 100 MΩ | Winding has begun to deteriorate; no performance problem yet, inspect periodically |
| 1 to 10 MΩ | Considerable deterioration; special care, inspect periodically |
| Below 1 MΩ | Unacceptable; motor must be repaired or replaced |
A reading that drops quickly over a few weeks, or a breaker that trips, usually means coolant, oil or moisture has entered the motor or cable. Record every reading against the motor’s serial number so you can see the trend, not just today’s value.
Step 4: Back-EMF (generator) check
A permanent-magnet servo motor works as a generator when you turn it.
- Disconnect the power leads from the drive.
- Set the multimeter to AC volts and connect across U-V.
- Turn the shaft at a steady speed (by hand, or carefully with a drill on a coupling).
- Repeat for V-W and W-U at the same speed.
- Good: all three voltages are equal and rise smoothly with speed.
- Bad: one pair clearly lower (shorted turns or a damaged phase), or all three lower than an identical healthy motor at the same speed (weakened magnets, often after overheating).
Step 5: Holding brake check
Most servo motor brakes are 24 V DC spring-applied brakes that release when powered. Check the nameplate for the brake voltage.
- Measure the brake coil resistance. Open circuit means a failed coil.
- Apply the rated brake voltage with correct polarity. You should hear a clear click, and the shaft should turn freely.
- Remove the voltage. The shaft should lock firmly.
A brake that does not release overheats the motor and causes overcurrent alarms. A brake that does not hold can drop a vertical axis. For Allen-Bradley motors see brake repair.
Step 6: Encoder and feedback checks
The encoder cannot be fully tested with a multimeter, but you can rule out the common causes:
- Inspect the encoder connector and cable for coolant, broken shields and crushed sections. Flex the cable along its length while watching the drive for an alarm.
- Check cable continuity pin to pin, and check that no signal wire is shorted to the shield or the motor frame.
- On absolute encoders, check the encoder backup battery. Battery-low and battery-alarm codes on Fanuc, Yaskawa and Mitsubishi systems often get mistaken for a failed encoder. See the servo alarm lookup.
Encoder replacement on most servo motors needs electrical alignment (commutation offset) to the rotor. Fitting a new encoder without alignment can cause overcurrent, noise or a runaway axis, so this is bench work. See encoder repair.
Step 7: Swap test to separate motor, drive and cable
If the motor passes Steps 2 to 6 but the alarm persists:
- Move the suspect motor to a known-good drive of the same model (or a known-good motor to the suspect drive), using a known-good cable.
- Alarm follows the motor: the motor (often the encoder) is faulty.
- Alarm stays with the drive: the drive is faulty.
- Alarm disappears with the new cable: the cable or a connector is the cause.
For drive alarm meanings see the Mitsubishi alarm list and the guides index.
Symptoms and what to check first
| Symptom | Likely cause | First check |
|---|---|---|
| Overcurrent or ground fault alarm at power-on | Winding to frame short, coolant in connector, damaged cable | Step 3 insulation test, then connectors |
| Encoder or feedback alarm | Coolant in encoder, cable fault, battery (absolute), failed encoder | Step 6 |
| Motor hot, overload alarm | Brake not releasing, bearing drag, mechanical load, shorted turns | Steps 1, 5 and 2 |
| Noise or vibration | Bearings, loose coupling, encoder misalignment, drive tuning | Step 1 |
| Makes noise but does not move | Brake not releasing, one phase open, mechanical jam | Steps 5 and 2 |
| Axis drops when power is removed | Brake not holding | Step 5 |
| Fault comes back after drying or cleaning | Insulation already weak, seal failure | Step 3 trend, plan a repair |
What field checks cannot tell you
Field checks find most hard faults. They cannot reliably find early shorted turns, encoder offset errors, bearing wear that only shows under load, or a motor that fails only when hot. On our bench every servo motor is tested before and after repair: insulation and surge comparison, winding balance, back-EMF, brake, encoder alignment, and a no-load and load run with a printed report. See servo motor testing with a no-load and load run and motor electrical testing. After a bearing change, read why a motor will not run after a bearing change.
For a shorter walk-through of the tests you can do without a drive, see how to test a servo motor without a drive.
Send it to us
If your checks point to the motor, WhatsApp a photo of the nameplate, the drive alarm code and your readings to +91 97866 22233. We repair Fanuc, Yaskawa, Mitsubishi, Siemens, Panasonic, Delta, Allen-Bradley and other servo motors at our Puducherry, Chennai and Bengaluru sites. See servo motor repair.
Questions
How do I check if a servo motor is working or not?
Disconnect it from the drive, then check winding resistance between the three phases (they should match), insulation resistance from each phase to the frame with a 500 V tester (100 MΩ or more is good), and back-EMF by turning the shaft (three equal voltages). Then check the brake and encoder cable. If all pass, swap the motor onto a known-good drive to confirm.
Can I check a servo motor with a multimeter?
Partly. A multimeter checks winding resistance balance, open circuits, brake coil resistance, encoder cable continuity and back-EMF. It cannot check insulation strength at voltage or find early shorted turns; those need an insulation tester and a surge test.
What insulation resistance is acceptable for a servo motor?
FANUC’s maintenance manual treats 100 MΩ or higher at 500 V DC as acceptable, 1 to 100 MΩ as deteriorating and below 1 MΩ as unacceptable. Always test with the motor disconnected from the drive.
Why does my servo motor get hot?
The most common causes are a brake that is not fully releasing, bearing drag, too much mechanical load or friction on the axis, and shorted turns in the winding. Check the brake first, then turn the shaft by hand, then check winding balance.
Is it the servo motor or the servo drive?
Do a swap test with a known-good motor, drive and cable of the same model. The fault follows the faulty part. Insulation and winding tests on the motor alone also tell you whether the motor is the cause.
Can a servo motor encoder be replaced on site?
Usually not. Most servo encoders must be aligned to the rotor magnets after fitting. Without alignment the motor can run rough, trip on overcurrent or run away, so encoder replacement is done on a bench with alignment equipment.
Which servo motor brands does Star Automations repair?
Multi-brand servo motor repair is our main work: Fanuc, Yaskawa, Mitsubishi, Siemens, Panasonic, Delta, Allen-Bradley, Rexroth, Lenze and other makes, at our Puducherry, Chennai and Bengaluru sites.
About the author
S. Suresh, Founder and CEO of Star Automations. More than 25 years in industrial electronics repair, with servo motor and drive repair as the main focus since 1998. Last updated: .
Send the nameplate and the alarm code.
WhatsApp a photo of the nameplate and the drive display to +91 97866 22233, or email info@starautomations.com.