By the EconoTest Engineering Team · Test bench manufacturer, Shanghai
Key Takeaways
- Servo motor quality lives in the details a basic load test misses: cogging torque, torque ripple, bandwidth and transient response.
- Hysteresis dynamometers are the reference load for servo work — frictionless torque from 0 RPM with ±0.2% FS accuracy (HB series).
- Measure cogging unpowered at very low speed; measure ripple under load across the speed range.
- Bandwidth and response-time tests need a low-inertia load and high-rate data acquisition, not just a steady-state dyno.
What Is Servo Motor Testing?
Servo motor testing is the measurement of a servo’s torque quality and control dynamics — cogging torque, torque ripple, starting torque, frequency response bandwidth and transient behavior — in addition to standard performance parameters like efficiency, temperature rise and speed regulation. It requires load equipment that applies smooth torque from zero speed.
Core Servo Test Items
Cogging Torque
Measured with the motor unpowered, rotated slowly while torque is recorded over at least one full revolution. The peak-to-peak variation is the cogging figure. Because values are small (often a fraction of a Nm), the load must be frictionless — any stick-slip would bury the signal. This is the defining application of the hysteresis dynamometer (0.1–30 Nm range, full torque at 0 RPM, no mechanical contact).
Torque Ripple
Measured powered, under load, across the speed range. Ripple combines cogging, current waveform and manufacturing effects, and is reported as a percentage of mean torque. High-bandwidth torque acquisition separates true motor ripple from drivetrain resonance.
Starting Torque and Locked Rotor
Both require rated braking torque at standstill. Hysteresis and magnetic powder dynamometers hold static load natively; eddy current brakes cannot and are the wrong tool here.
Bandwidth and Transient Response
Frequency response bandwidth, torque control response time, speed control response time and dynamic position tracking error characterize the servo loop, not just the motor. The bench applies command steps and frequency sweeps while a low-inertia load keeps the mechanical system from dominating the result. EconoTest motor testing systems include these dynamic test items alongside standard performance sequences.
Standard Performance Items
Rated load, efficiency, temperature rise, vibration and noise, overload capacity, speed regulation, back EMF and harmonic analysis complete the program — the same fundamentals covered in our dynamometer guide.
Choosing the Load: KB vs HB Series
| Criterion | KB (standard) | HB (high precision) |
|---|---|---|
| Accuracy | ±0.5% FS | ±0.2% FS |
| Max speed | up to 30,000 RPM (small models) | up to 25,000 RPM (most models) |
| Cooling | Air (most models); water on 5/7.5/15 Nm | Air+water, water+blower or air+blower by size |
| Best for | Production verification | Cogging, ripple, R&D characterization |
Production Line Servo Testing
For end-of-line verification at takt time, magnetic powder dynamometers (full torque at zero speed, 2–2,000 Nm, high duty cycle) and automated test sequences deliver fast pass/fail results — see our production test bench systems.
Need a servo test configuration? Send motor type, torque, speed and the test items above — engineering response within 24 hours via the contact page.
Frequently Asked Questions
What is cogging torque and how is it measured?
Cogging torque is the pulsating torque a permanent magnet motor produces from the magnetic attraction between rotor magnets and stator teeth, felt even with no current applied. It is measured by rotating the motor slowly against a hysteresis dynamometer while recording torque ripple — hysteresis brakes are used because they apply smooth, frictionless load from zero speed.
Why are hysteresis dynamometers preferred for servo motor testing?
Hysteresis dynamometers generate braking torque electromagnetically with no mechanical contact, so they deliver full rated torque from 0 RPM with no stick-slip or friction artifacts. This makes them the standard choice for cogging torque, torque ripple, starting torque and low-speed servo measurements where friction-based loads would corrupt the data.
What accuracy is required for servo motor torque measurement?
Precision servo characterization (cogging torque, torque ripple) typically requires ±0.2% full scale accuracy — EconoTest HB-series hysteresis dynamometers achieve ±0.2% FS. For routine production verification, ±0.5% FS (KB series) is generally sufficient.
What is torque ripple in a servo motor?
Torque ripple is the periodic variation of output torque around its mean value as the motor rotates, caused by cogging torque, current waveform distortion, and manufacturing asymmetries. It is expressed as a percentage of mean torque and measured under load across the speed range on a dynamometer with high-bandwidth torque acquisition.
What speed range do servo motor test benches cover?
EconoTest hysteresis dynamometers for servo testing cover 0.1–30 Nm with maximum speeds up to 30,000 RPM (KB series) or 25,000 RPM (HB series), with smaller models reaching the highest speeds. For servo spindles beyond these speeds, eddy current models extend testing to 50,000 RPM.
