Why Servo Motor Testing Cannot Be Skipped
A servo motor that passes factory spot-checks can still fail in the field — because spot-checks rarely replicate the full operating envelope. A proper servo motor test bench runs the motor through every load point, every thermal regime, and every edge case before it ever reaches a customer’s machine.
This guide covers the 8 performance tests that a production-grade servo motor test bench must execute, the instruments behind each test, and what the data tells you. The test sequence follows the Chinese national standard GB/T 30549 (General Technical Conditions for Permanent-Magnet AC Servo Motors), which is now referenced by procurement teams in Europe, Southeast Asia, and the CIS region as a baseline.
What Is a Servo Motor Test Bench?
A servo motor test bench is an integrated system that couples the motor under test (MUT) to a load dynamometer through a precision torque-speed sensor. The test bench controls load, acquires electrical and mechanical data simultaneously, and generates standardized test reports.
A production-grade system typically consists of:
- Load dynamometer — AC servo motor acting as controllable load (absorbs or drives torque)
- Torque-speed sensor — inline shaft-mounted, accuracy ±0.1% to ±0.5% FS
- Power analyzer — measures 3-phase voltage, current, power factor, harmonics
- PLC control cabinet — automates load profiles and safety interlocks
- Industrial PC + software — real-time data acquisition, curve generation, report export
- Safety guard — mandatory enclosure for high-speed shaft rotation
Optional instruments for extended testing include a dynamic signal analyzer (for NVH), an LCR meter (winding resistance), a DC resistance meter, a signal generator, and a temperature data logger.
System schematic showing MUT connected to load dyno via torque sensor, with power analyzer on electrical input side
The 8 Essential Servo Motor Performance Tests
Test 1 — Load Characteristic Test
The load characteristic test is the backbone of servo motor validation. The test bench automatically steps through preset load points (torque-controlled mode), recording at each point:
- Input voltage, current, power, power factor
- Output torque and speed
- Input and output power
- Efficiency
- Temperature at predefined measurement points
- Harmonic content of voltage and current
The result is a comprehensive performance map across the motor’s rated operating range. Test software plots any parameter combination on X–Y axes and exports data tables for further analysis.
Key instruments: Load dynamometer, torque-speed sensor, power analyzer, data logger
Test 2 — No-Load Test
With the load dynamometer disengaged, the MUT runs at rated voltage and rated speed. The test measures:
- No-load current (iron loss indicator)
- No-load speed accuracy
- No-load power consumption
Elevated no-load current typically signals core lamination defects or demagnetization of permanent magnets — problems invisible on a static inspection.
Key instruments: Torque-speed sensor, power analyzer
Test 3 — Locked Rotor (Stall) Test
The load dynamometer applies increasing torque until the MUT reaches near-zero speed (stall condition). The test bench records stall torque and stall current simultaneously. Fixture design is critical: both MUT and load machine must be mounted on a sliding rail system to allow shaft alignment adjustment under load.
Stall torque is a key parameter for applications requiring high starting torque — robotic joints, press feeders, injection molding axes.
Key instruments: Load dynamometer, torque-speed sensor, power analyzer
Test 4 — T-N Characteristic Curve
The torque-speed (T-N) curve defines the motor’s operating envelope. The test bench sweeps speed from zero to maximum while recording output torque at each point, producing the characteristic curve that distinguishes the continuous duty zone from the intermittent peak zone.
A properly shaped T-N curve shows:
- Flat torque from zero speed up to base speed (constant torque region)
- Field-weakening region above base speed (constant power region)
- Peak torque capability (typically 2–3× rated torque for 10–30 seconds)
Key instruments: Load dynamometer, torque-speed sensor, power analyzer
Example T-N curve graph showing continuous zone, peak zone, and field weakening region
Test 5 — Efficiency MAP
The efficiency MAP (also called an efficiency cloud map or contour map) is now a mandatory deliverable for servo motors used in robotics, EV auxiliary drives, and precision machinery. The test bench sweeps through a grid of torque × speed operating points, measuring efficiency at each:
- Speed range: 0 to maximum rated speed
- Torque range: 0 to rated torque (some tests extend to peak torque)
- Grid resolution: typically 10×10 to 20×20 points
The result is a 2D contour plot showing efficiency islands — the high-efficiency operating zones where system designers should target normal operating conditions. For a well-designed servo motor, peak efficiency typically exceeds 90–95% near the rated operating point.
Key instruments: Load dynamometer, torque-speed sensor, power analyzer
Test 6 — Working Zone Validation
Working zone testing confirms that the motor can sustain both continuous and peak operating conditions within thermal limits. The test bench holds the motor at rated load for an extended period (typically 30–60 minutes per GB/T 30549), monitoring:
- Winding temperature rise (must not exceed insulation class limits)
- Bearing temperature
- Housing temperature
- Efficiency stability over time
A motor that passes short-burst tests may still fail sustained operation due to inadequate cooling design.
Test 7 — Temperature Rise Test
Separate from working zone validation, the dedicated temperature rise test uses a multi-channel data logger with thermocouples placed at winding end turns, bearings (DE and NDE), and housing. The test runs until thermal equilibrium is reached (temperature change <2°C per hour).
Temperature rise limits by insulation class:
| Insulation Class | Max Temperature Rise (K) | Max Absolute Temperature (°C) |
|---|---|---|
| Class B | 80 K | 130°C |
| Class F | 105 K | 155°C |
| Class H | 125 K | 180°C |
Key instruments: Multi-channel data logger, thermocouples, power analyzer
Test 8 — Cogging Torque Measurement
Cogging torque — the periodic torque variation caused by the interaction between permanent magnets and stator slots — is a critical quality parameter for servo motors used in precision positioning. Even a small cogging torque causes speed ripple at low speeds, degrading positioning accuracy.
The test bench uses a high-resolution dynamic torque sensor (kistler or equivalent, resolution to 0.01 N·m) combined with a high-precision angle encoder to map torque variation over one full electrical cycle. The system is mounted vertically to eliminate rotor weight effects on the measurement.
Cogging torque is typically expressed as a percentage of rated torque. Acceptance criteria for precision servo motors: <0.5% to <2% depending on application requirements.
Key instruments: Dynamic torque sensor (mN·m resolution), angle encoder, dynamic signal analyzer
Servo Motor Test Bench System Configuration Example
Below is a representative configuration for a 7.5 kW servo motor test bench covering all 8 tests above:
| Component | Specification | Purpose |
|---|---|---|
| Load Dynamometer | AC Servo, 15 kW, 0–6000 rpm | Controllable load / energy recovery |
| Torque-Speed Sensor | 0–50 N·m, ±0.1% FS, 10000 rpm | Shaft torque and speed |
| Power Analyzer | 4-channel, 600V / 20A, 0.05% accuracy | Electrical input measurement |
| Dynamic Signal Analyzer | 16-channel, 24-bit, 51.2 kS/s | NVH / vibration analysis |
| LCR Meter | 20 Hz – 1 MHz range | Winding impedance |
| Data Logger | 16-channel thermocouple input | Temperature mapping |
| Industrial PC | Intel Core i7, 16 GB RAM, 1 TB SSD | Test software, data storage |
| Test Software | Automatic load profiles, report generation | GB/T 30549 compliant reports |
Software: From Raw Data to Certified Test Reports
Modern servo motor test software does more than collect data. A well-designed system provides:
- Automatic load profile execution — pre-programmed test sequences with pass/fail criteria
- Real-time curve plotting — T-N curve, efficiency MAP, temperature rise curves
- Harmonic analysis — FFT of voltage and current waveforms
- Multi-format report export — PDF, Excel, CSV for quality records
- Standard compliance check — automatic comparison against GB/T 30549, IEC 60034
Screenshot of test software showing efficiency MAP contour plot and data table
Frequently Asked Questions
What is the difference between a servo motor test bench and a regular motor test bench?
A servo motor test bench is optimized for the specific characteristics of closed-loop servo motors: four-quadrant operation (motoring and regenerating in both directions), high-speed dynamic loading, and cogging torque measurement capability. A general motor test bench typically covers only single-quadrant load characteristics and lacks the dynamic signal analysis hardware needed for servo motor NVH and cogging torque tests.
How long does a full servo motor test cycle take?
A complete 8-test cycle on a 7.5–15 kW servo motor typically takes 4–8 hours per motor. The temperature rise test alone requires 60–120 minutes to reach thermal equilibrium. Automated test benches can run overnight with minimal operator supervision.
What torque sensor accuracy is needed for servo motor testing?
General performance tests (load characteristic, T-N curve, efficiency MAP) require torque sensor accuracy of ±0.1% to ±0.5% FS. Cogging torque measurement requires a dynamic sensor with resolution to 0.01 N·m or better — standard shaft torque sensors are insufficient for this test.
Can one test bench cover multiple servo motor sizes?
Yes, with proper fixture design. A modular test bench can accommodate motors from 0.5 kW to 50+ kW by changing the coupling adapter and adjusting load dynamometer parameters. The torque-speed sensor range is typically the binding constraint — it must be sized for the largest motor while maintaining accuracy for the smallest.
What national standards apply to servo motor testing?
The primary Chinese standard is GB/T 30549-2014 (Permanent-Magnet AC Servo Motors — General Technical Conditions). International equivalents include IEC 60034-1 (general rotating machinery) and IEC 60034-30-1 (efficiency classes). Many export markets now accept GB/T test reports alongside IEC documentation.
Does the test bench need energy recovery?
For motors above 7.5 kW, an energy feedback unit (AFE — Active Front End) is strongly recommended. It regenerates absorbed energy back to the grid instead of dissipating it as heat, reducing operating costs significantly for high-volume production testing. For motors below 3 kW, resistive load is acceptable.
Conclusion
A complete servo motor test bench is not a single instrument — it is a calibrated system of mechanical, electrical, and software components working together to replicate and document every operating condition the motor will face in service. The 8 tests described above form the foundation of any GB/T 30549-compliant validation program.
EconoTest designs and supplies servo motor test benches from 0.5 kW to 500 kW, covering all tests described in this guide. Our SFT series systems ship complete with calibrated sensors, test software, and full documentation.
→ Contact our engineering team for a test bench configuration matched to your motor range and test requirements.
