A Step-by-Step Guide to Testing Servo Motors

Servo motor problems can show up as position errors, overload alarms, rough movement, or a dead axis. Those symptoms don’t always point straight to the motor because cables, feedback devices, brakes, and amplifiers can cause similar faults.

If you’re troubleshooting motion problems, here’s a quick step-by-step guide to testing servo motors that can help you narrow down where the issue starts.

1. Start With the Alarm and Machine History

Start by reviewing the alarm history to identify when the problem occurs. Does it happen during startup, rapid movement, cutting, deceleration, or at a specific point in the axis travel?

Timing provides valuable clues. A motor that faults after a prolonged cycle might have a heat-related issue. An alarm appearing at the same axis position could indicate a damaged cable or connector.

Record the motor model, serial number, axis, drive type, and connector layout. These details will assist in locating the correct winding resistance, brake voltage, feedback type, and pinout before taking any measurements.

2. Shut Down the Machine Safely

Follow the manufacturer’s lockout procedure before opening the cabinet or disconnecting the motor. Ensure power is shut off, stored energy is released, and voltage has dropped to zero. Exercise extra caution with vertical axes or suspended loads.

Support the load before releasing the motor brake, as the axis may move once the brake is disengaged. Keep insulation testers away from feedback wiring, encoder electronics, and connected servo amplifiers, since the test voltage can cause damage.

Always disconnect the motor power leads and feedback connector before performing electrical checks.

3. Give the Motor and Cables a Close Look

Performing a quick visual inspection can save time before removing a meter. Check the motor body, shaft, connectors, cable jackets, and mounting points for signs of heat, coolant, oil, corrosion, loose pins, or physical damage.

Special care should be given to cable flex points, as repeated bending can cause internal conductor breaks that might only trigger an alarm at specific axis positions.

Watch for signs such as:

• Burnt or discolored connector pins

• Coolant inside a connector

• Cracked or crushed cable jackets

• Loose mounting bolts

• Shaft noise or excess movement

A clean-looking motor can still have an internal problem, though visible damage can help guide the next checks.

4. Check Phase-to-Phase Winding Resistance

Use a calibrated low-resistance meter or a digital multimeter capable of reading the motor’s expected values. Measure between each pair of power leads—U-to-V, V-to-W, and W-to-U. The three readings should be similar; a significantly different reading may indicate a damaged winding, weak connection, or cable fault.

An open circuit reading could mean a broken conductor, while a very low reading might suggest a short. Keep in mind that low-ohm measurements can include resistance from the meter leads themselves.

To account for this, touch the leads together first and subtract that resistance if your meter lacks a zero or relative setting. Always compare each reading to the values specified in the motor manual, considering that winding design, motor size, and temperature can influence expected readings.

5. Test Insulation Resistance to Ground

This check detects current leakage between motor windings and the frame. Disconnect the amplifier's power leads and isolate the feedback electronics. Use an insulation tester between each phase lead and the motor frame, following the manufacturer’s recommended voltage and procedure rather than default settings. Low insulation resistance can indicate moisture, contamination, damaged cables, or winding failure. Environmental factors like temperature and humidity may affect the readings, making it helpful to compare with previous measurements.

After testing, allow the winding circuit to discharge properly through the tester’s approved method before handling the leads.

6. Check the Brake Circuit

Many servo motors feature a spring-applied, electrically released holding brake. The motor documentation should specify the correct brake voltage, coil resistance, connector pins, and release method.

First, measure the brake coil resistance. An open circuit indicates a coil or conductor failure. A resistance outside the specified range could suggest heat damage, coil issues, or cable problems. When possible, apply the designated brake voltage using a controlled test source.

Listen for a smooth release and verify that the shaft moves freely. Brake drag can cause excessive current, heat buildup, positioning inaccuracies, and amplifier alarms. These symptoms can resemble winding problems.

7. Inspect the Feedback Device

The feedback device indicates the shaft's position and movement to the control. Depending on the motor, this device could be an encoder, resolver, or another type of feedback system.

Examine the connector carefully for bent pins, contamination, loose contacts, or damaged shielding. Check cable continuity only in accordance with the wiring diagram and ensure the electronics are disconnected during the process.

Many encoders require specialized test equipment or drive diagnostics. A basic multimeter can be used to check power supply and continuity but will not verify signal quality, alignment, or communication reliability.

When working with a Haas servo system, keep the motor, amplifier, encoder, and cables in the same diagnostic picture. A feedback alarm can start with any one of those parts.

8. Check the Bearings and Mechanical Load

With the power turned off and the brake safely released, manually turn the shaft if the motor design permits. The movement should be smooth, free of grinding, binding, or uneven resistance.

Inspect for any unusual side-to-side or in-and-out movement. Worn bearings may produce noise, heat, vibration, or poor surface finish, along with the following errors. A damaged coupling or misaligned load might cause similar symptoms.

When the service instructions permit, separate the motor from the driven mechanism. This check can help determine whether the resistance originates from the motor or the machine.

Get Help With a Servo System Fault

Servo motor testing involves high voltage, stored energy, suspended loads, and sensitive electronics. If a check exceeds your training, tools, or machine documentation, seek help from someone with the right expertise.

Following a clear step-by-step process for testing servo motors can help you narrow down the fault and gather useful information before deciding on repair or replacement.

At CNC Service Company, we assist maintenance teams with troubleshooting Haas servo amplifiers, drives, motors, and related components. Contact us for technical support, repair guidance, or a second opinion if your test results raise questions.