How to Test a Servo Motor (and 5 Signs One Is Failing)

When a servo-driven axis starts faulting, positioning erratically, or refusing to move, the motor is the obvious suspect—but it isn't always the guilty one. Before you pull a servo and send it out (or worse, replace a healthy motor), a handful of straightforward tests will tell you whether the motor is actually the problem or whether the fault lives in the drive, the cabling, or the feedback path.

Here's how to test a servo motor properly, what each result is telling you, and the warning signs that one is on its way out.

5 Signs a Servo Motor Is Failing

Servo motors usually warn you before they quit. Watch for:

1. Overheating. A motor running hotter than normal—too hot to keep a hand on—points to winding, bearing, or load problems.
2. Unusual noise or vibration. Grinding, whining, or rumbling almost always means worn bearings. Vibration accelerates every other kind of wear.
3. Erratic motion or positioning errors. Jitter, drift, overshoot, or an axis that can't hold position often traces back to a failing encoder or feedback signal.
4. Repeated drive faults or alarms. Overcurrent, following-error, or feedback alarms on the drive are frequently the motor talking through the drive.
5. A burning smell or visible discoloration. Scorched insulation or a burnt odor means winding damage—stop and test before powering it again.

Any one of these is a reason to test. Several together usually means the motor is the cause.

Safety First

Servo motors demand respect before you touch them:

• Lock out and tag out the drive and disconnect power before doing anything.
• Allow time to discharge. Drives store energy in their capacitors after power-down.
• Remember servos generate voltage. A servo motor spun by hand or back-driven by a load acts as a generator—it can produce real voltage at its terminals. Treat the leads as live.

The Tools You Need

You can complete a solid servo test with three instruments:

• A digital multimeter — for winding resistance and continuity (your basic servo motor tester).
• An insulation resistance tester (megohmmeter/"megger") — for checking insulation to ground, which a standard multimeter can't measure.
• An oscilloscope (optional) — for verifying encoder or resolver feedback signals if you suspect the feedback device.

You'll also want the motor's nameplate data and, ideally, a servo motor windings diagram or datasheet so you know the expected phase configuration and resistance values for your specific model.

How to Test a Servo Motor, Step by Step

Most industrial servos are three-phase AC (brushless) motors with windings labeled U, V, and W. The tests below apply to that common type.

1. Visual and Mechanical Inspection

Start simple. Inspect the cables, connectors, and motor body for damage, moisture, or burnt spots. Then—with power removed—rotate the shaft by hand. It should turn smoothly with light, even resistance. Roughness, grinding, or play indicates worn bearings.

2. Winding Resistance Test

Set your multimeter to the low-ohms range and measure phase-to-phase resistance across each pair of windings: U–V, V–W, and W–U. On a healthy motor, all three readings should be very close to each other and match the manufacturer's spec.

• Balanced, in-spec readings: windings are good.
• One reading much higher or infinite: an open winding.
• One reading much lower: a shorted winding.

An imbalance between the three phases is one of the clearest indicators of internal winding damage—this is exactly why the windings diagram matters, so you're comparing against the right expected values.

3. Insulation Resistance Test

Using the megohmmeter (typically at 500V for these motors), measure from each motor phase to the motor frame/ground. You're checking whether the winding insulation is still isolating the windings from the chassis.

• High reading (in the megohms): insulation is intact.
• Low reading: the insulation is breaking down, often from moisture, contamination, or overheating—the motor needs attention even if it still runs.

4. Back-EMF Test

With the motor disconnected from the drive, set your multimeter to AC volts and spin the shaft by hand at a steady rate (or with a drill, carefully). Measure the voltage generated across each phase pair. A healthy motor produces balanced AC voltage across all three; a large imbalance points again to a winding fault.

5. Feedback Device Check

If the windings test clean but the axis still misbehaves, the encoder or resolver is the prime suspect. Verify its wiring and connector first, then—if you have an oscilloscope and the signal specs—confirm it's producing clean, correctly-phased feedback. Many "motor" faults are really feedback faults.

Interpreting the Results

Put the picture together:

• Windings balanced + insulation high + feedback good → the motor is likely healthy. Turn your attention to the drive, the motor cable, or the tuning.
• Winding imbalance, open, or short → internal winding failure. The motor needs rewinding or replacement.
• Low insulation resistance → insulation breakdown. Sometimes recoverable with professional cleaning and drying; often a sign the motor is near end of life.
• Bad feedback signal → encoder/resolver repair or replacement, not necessarily a new motor.

When to Repair vs. Replace—and When to Call a Pro

A motor with worn bearings or a failed encoder is very often worth repairing rather than replacing, especially for higher-end or discontinued servos where a new unit carries a long lead time and a steep price. Winding faults and insulation breakdown are more involved but still frequently repairable by a shop with the right equipment.

The catch is that a proper diagnosis—and a genuine repair-vs-replace answer—usually needs load testing and calibration equipment beyond a field multimeter. If your tests point to a motor fault, or if they come back clean but the axis still won't behave, a specialist can bench-test the motor under load and confirm the root cause before you spend money on the wrong part.

IVS Incorporated repairs, rebuilds, and load-tests servo motors across all major brands, including obsolete models, with full functional testing before return. If you've got a servo that's failing your tests—or one you can't quite pin down—see our motor repair services, and if the fault turns out to sit in the drive, we handle drive repair as well.