Checklist: Verifying Torque Ripple on Servo Motors

Why Torque Ripple Matters for Precision Systems

Torque ripple is the fluctuation in motor torque that occurs as the rotor turns. In high-precision applications, this ripple translates into vibration, noise, and position error. A motor that looks fine on a datasheet can still fail in a real system if the ripple exceeds the tolerance of the spindle, encoder, or mechanical frame.

This checklist helps you verify servo torque ripple before purchase. It assumes you have access to a bench or test rig. If you are buying directly without testing, focus on the specification and documentation sections first.

Check the Datasheet Specifications

Start with the manufacturer’s datasheet. Look for a section titled torque ripple, cogging torque, or torque fluctuation. The unit is usually percent of rated torque or millinewton-meters.

Some datasheets list ripple only at steady-state speed. Others provide data at multiple speeds or under load. Check for both. A low ripple rating at no load can mask high ripple under load.

Red flags:

• No torque ripple data provided, or data only at a single speed.
• Ripple values listed as “typical” without a minimum or maximum range.
• Missing information on how the data was measured.

What to look for:

• A specific torque ripple percentage at rated speed.
• Data under rated load or at maximum torque.
• Notes on measurement method, such as a dynamometer or torque sensor.

Review the Motor Type and Construction

Servo motors come in different forms. Permanent magnet synchronous motors are common in high-precision applications. Induction servos are used where high torque at low speed is needed. The construction affects ripple.

Permanent magnet motors can have cogging torque, which is a mechanical ripple caused by the interaction between the stator teeth and the rotor magnets. A well-designed motor uses magnet shaping, skewing, or a slotless stator to reduce this.

Red flags:

• Standard cylindrical rotor with unskewed magnets in a high-precision application.
• No mention of cogging torque mitigation in the design description.
• A motor with a high slot count but no information on magnet shape.

What to look for:

• Magnet skewing or shaped magnets.
• A slotless or distributed winding design, which often reduces cogging.
• Clear documentation of the torque ripple mitigation strategy.

Verify the Drive and Cabling

Torque ripple is not only a motor issue. The servo drive can add ripple through current control, switching frequency, and communication errors. Cabling can introduce noise that the drive interprets as torque fluctuation.

Check the drive’s current control resolution and switching frequency. Higher switching frequencies generally reduce current ripple. Poorly shielded cables can pick up electromagnetic interference, causing the drive to overcorrect and introduce torque fluctuations.

Red flags:

• Drive with a low switching frequency, such as under 8 kHz, for a precision application.
• Cables routed near inverter outputs or power lines without proper shielding.
• Drive firmware that has not been updated to the latest revision.

What to look for:

• A drive with adjustable switching frequency and current loop tuning.
• Twisted pair or shielded cables for motor and encoder connections.
• A ground plane or star-grounding scheme for the test setup.

Run a No-Load Test

Connect the motor to the drive and run it without a mechanical load. This isolates the electrical and magnetic ripple from mechanical resonance.

Start at low speed, such as 10 to 50 rpm. Listen for audible noise. Use a tachometer or encoder feedback to verify the speed is constant. If the speed fluctuates, the torque ripple is significant.

Increase the speed in steps, such as every 100 rpm. Listen and feel for vibration. At high speed, cogging torque becomes more noticeable. A well-designed motor should have a smooth speed profile.

Red flags:

• Audible whine or buzz at low speed.
• Visible speed fluctuation at steady-state operation.
• Vibration that increases with speed but does not stabilize.

What to look for:

• Silent or near-silent operation at low speed.
• Constant speed on the tachometer readout.
• Minimal vibration on the motor frame, which can be felt by hand or measured with a contact accelerometer.

Run a Full-Load Test

Attach a load that simulates your application. This can be a brake, a flywheel, or a mechanical coupling to a test spindle. Run the motor at rated torque and rated speed.

Torque ripple under load is often higher than under no load. The magnetic forces are stronger, and the mechanical stress can cause the rotor to deflect slightly. This changes the air gap and increases cogging.

Measure the torque with a sensor if possible. If you do not have a sensor, monitor the current drawn by the drive. A fluctuating current at a fixed speed indicates torque ripple.

Red flags:

• Significant current fluctuation at rated speed and load.
• Audible noise that changes with the load.
• Vibration that is much higher under load than under no load.

What to look for:

• Stable current reading on the drive display.
• Minimal vibration on the motor and load shaft.
• A torque ripple value, if measured, that is within your application’s tolerance.

Measure Vibration Quantitatively

If the application has strict noise or vibration limits, use a measurement tool. A contact accelerometer bolted to the motor frame is the standard method. A laser vibrometer can be used if you need non-contact measurement.

Measure vibration in millimeters per second, RMS, or in g’s. Compare the result to your application’s limit. Many industrial standards use 0.5 to 2.0 mm/s for precision machines, but the limit depends on the specific system.

Take measurements at multiple speeds. Plot the vibration level against speed. Look for peaks that correspond to the cogging frequency or the mechanical resonance of the frame.

Red flags:

• Vibration levels that exceed the application limit at any speed.
• A sharp peak in the vibration spectrum at a specific speed.
• Vibration that is higher than a similar motor from another manufacturer.

What to look for:

• A flat vibration profile across the operating speed range.
• Vibration levels that are well below the application limit.
• A clear correlation between speed and vibration, which helps identify the cause.

Compare with Reference Data

If you are evaluating multiple motors, run the same test on each one. Use the same drive, cables, and load. This gives a direct comparison.

Record the torque ripple percentage, vibration level, and noise level for each motor. A table is useful for this.

Test Parameter	Motor A	Motor B	Application Limit
Torque Ripple at Rated Speed	0.5%	1.2%	1.0%
Vibration at Rated Speed	0.3 mm/s	0.8 mm/s	1.0 mm/s
Noise at Rated Speed	45 dB	52 dB	50 dB

This table shows that Motor A is within the limit for all parameters, while Motor B exceeds the noise limit. This kind of comparison helps you make a data-driven decision.

Red flags:

• A motor that passes the datasheet but fails the bench test.
• A motor that is cheaper but has higher ripple or noise.
• A motor that performs well on one test but poorly on another.

Review the Documentation and Support

The final step is to review the documentation. Check if the manufacturer provides a torque ripple graph or a test report. A test report from a third party is more credible than a manufacturer’s claim.

Contact the manufacturer’s technical support. Ask how they measured the torque ripple. Ask if they can provide a sample unit for bench testing. A reputable manufacturer will provide this information and may even arrange a test.

Red flags:

• No test report available.
• The manufacturer cannot explain how the torque ripple was measured.
• The manufacturer refuses to provide a sample unit for testing.

What to look for:

• A third-party test report or a detailed in-house test report.
• A clear explanation of the measurement method.
• A willingness to provide a sample unit and support the testing process.

Final Decision Criteria

Use the checklist to make a go/no-go decision. If the motor fails any of the critical checks, do not purchase it. If it passes all checks, proceed with the purchase.

Document the results. Keep the test report, the datasheet, and the vibration measurements. This documentation will be useful for future troubleshooting and for the next project.

Torque ripple is a specification that is easy to overlook and hard to fix after the fact. A motor with high ripple can cause position error, wear, and noise. Verifying it before purchase saves time and money.