PMSM vs Induction Motor Testing: Key Differences and Test Requirements

By the EconoTest Engineering Team · Test bench manufacturer, Shanghai

Why Motor Type Changes the Test Approach

Permanent magnet synchronous motors (PMSM) and induction motors (IM) share the same basic electrical interfaces — three-phase, variable-frequency drive, same torque/speed output — but their internal physics are fundamentally different. Those differences cascade into the test procedure, the dynamometer requirements, the data channels needed, and the pass/fail criteria applied. Specifying a test bench without accounting for motor type is the most common and most expensive configuration mistake we see in RFQs.

Key Physical Differences That Drive Test Requirements

Parameter	PMSM	Induction Motor (IM)
Rotor excitation	Permanent magnets (no slip)	Induced rotor current (requires slip)
Speed at synchronous frequency	Exactly synchronous	Always below synchronous (1–5% slip)
Back-EMF at zero current	Non-zero (magnet flux)	Zero (no rotor magnetization)
Cogging torque	Present (pole-slot interaction)	Negligible (cage rotor)
Efficiency peak location	Mid-load, mid-speed (wide region)	Near rated load, rated speed
Thermal failure mode	Magnet demagnetization + winding	Winding insulation only

Tests Unique to PMSM

Back-EMF (BEMF) Test

The BEMF test spins the PMSM to a defined speed using the dynamometer while the motor terminals are open-circuit (no drive connected). The induced voltage at each phase pair is measured and compared against the design value. BEMF amplitude confirms magnet flux density and effective pole-pair count; BEMF waveform (FFT analysis) identifies magnet assembly defects, eccentricity, and demagnetized regions. This test is impossible on an induction motor — there is no residual rotor magnetization to generate a voltage.

Cogging Torque Test

Cogging torque is the periodic torque ripple caused by the interaction between permanent magnets and stator slots at zero current. To measure it, the dynamometer must rotate the motor at very low, constant speed (<5 RPM) while measuring instantaneous torque. Peak cogging torque and its harmonic content are extracted from the torque signal. A high-resolution torque transducer (±0.05% FS or better) and a low-noise amplifier are required. See our cogging torque measurement guide for the complete procedure.

Demagnetization Risk Test

NdFeB magnets lose flux irreversibly if exposed to excessive opposing magnetic field (demagnetizing current) above their knee-point temperature. The test applies short-circuit current at the maximum expected temperature and measures BEMF before and after. A permanent drop in BEMF amplitude indicates partial demagnetization. This test requires temperature-controlled conditions and careful current monitoring — the drive must be capable of controlled short-circuit current injection.

d-q Axis Current Angle Mapping

PMSM efficiency depends on the ratio of d-axis to q-axis current (the current advance angle γ). The maximum torque per ampere (MTPA) point shifts with speed and temperature. Full efficiency mapping of a PMSM requires the test bench software to sweep both torque setpoint AND current angle simultaneously — a two-dimensional test grid that produces an efficiency surface rather than a simple efficiency curve. Standard induction motor test software cannot do this.

Tests Unique to Induction Motors

Slip Measurement and Compensation

Induction motor speed is always below synchronous speed by the slip percentage: s = (n_sync − n_rotor) / n_sync. At rated load, slip is typically 1–5%. The test bench must measure actual rotor speed precisely to calculate slip — a ±0.1% speed measurement error causes ±2–5% power calculation error at low slip. Test software must compensate applied load torque for the motor’s actual slip characteristic, not assume synchronous operation.

No-Load and Locked-Rotor Tests

The classical no-load test (motor running freely at rated voltage) separates core losses and friction/windage from copper losses. The locked-rotor test (rotor held stationary at reduced voltage) characterizes leakage inductance and starting current. Both tests are standard for IMs and meaningless for PMSMs — a PMSM at zero current produces BEMF (not a no-load condition), and a locked PMSM cannot be characterized by locked-rotor voltage in the same way.

Thermal Class — Winding Only

Induction motors have one thermal failure mode: winding insulation breakdown. The test engineer focuses on phase resistance balance (to catch asymmetric heating from turn faults) and absolute temperature rise against the declared insulation class. There is no equivalent to PMSM demagnetization risk; the rotor cage is not temperature-sensitive in the same failure mode sense.

Shared Tests — But Different Acceptance Criteria

Efficiency Mapping

Both motor types require efficiency mapping for IE-class certification. However, PMSM efficiency peaks in a broader central region of the torque-speed map, while IM efficiency is strongly peaked near rated operating point. Test grids for PMSM therefore need more test points in the mid-range and fewer at extremes to resolve the efficiency surface accurately. IEC 60034-30-1 applies to both types but the supplementary measurement standard IEC 60034-2-1 has specific method sections for synchronous and asynchronous machines.

Torque Ripple

Torque ripple exists in both types but for different reasons. PMSM ripple is dominated by cogging (spatial) and current harmonic (temporal) components. IM ripple comes from rotor slot harmonics and asymmetric air gap. Measurement method is the same — high-resolution torque signal at operating speed — but the analysis frequency content differs. PMSM ripple is at multiples of pole × slot frequency; IM ripple is at multiples of rotor bar frequency.

Choosing the Right Test Bench Configuration

For a lab testing both PMSMs and induction motors, specify: (1) a low-speed, high-resolution torque transducer capable of resolving cogging torque (critical for PMSM, harmless extra for IM); (2) test software with both MTPA sweep and slip-compensation modes; (3) a dynamometer with position control for low-speed BEMF and cogging tests; (4) a high-accuracy power analyzer that correctly handles the non-sinusoidal waveforms produced by PWM inverters driving both motor types.

Our application engineers can specify a single test bench configuration that covers both PMSM and IM without compromising on either. Contact us with your motor specifications to get a recommendation.

Frequently Asked Questions

Can the same dynamometer test both PMSM and induction motors?

Yes. The dynamometer itself — whether eddy current, hysteresis, or AC — is motor-type agnostic. It applies load torque and measures speed. The differences lie in the test software, the torque transducer resolution needed for cogging measurement, and the need for position control at very low speed. A well-specified R&D bench with position-control mode, a high-resolution torque flange, and software that supports both MTPA sweeps and slip-compensation modes can test both motor types on the same hardware.

Why does PMSM testing require position control on the dynamometer?

Cogging torque and BEMF tests require the motor to rotate at 1–5 RPM with precisely controlled, constant angular velocity. Standard speed control loops on dynamometers are tuned for hundreds or thousands of RPM and have too much speed error at near-zero speed. Position control mode — where the dynamometer servo closes the loop on rotor angle rather than speed — achieves the low-speed precision needed to resolve the torque signal over one electrical cycle.

Is the efficiency test procedure different for PMSM vs IM?

Both use IEC 60034-2-1 as the base measurement standard, but the operating point grid differs. For IM, the standard grid sweeps from 25% to 125% of rated torque at rated speed. For PMSM (and IPMSMs specifically), the grid must also sweep speed from low to base speed and into field-weakening, because the efficiency surface changes significantly with speed. This means PMSM efficiency mapping takes 3–5× more test points and 2–3× more test time than equivalent IM mapping.

What is the biggest testing mistake for PMSM?

Running locked-rotor tests at full voltage. Unlike an induction motor where locked-rotor test at reduced voltage is benign, a PMSM with high current at locked rotor generates maximum demagnetizing field on the magnets at a time when the rotor is not rotating to spread the heat. Within seconds, local magnet temperature can exceed the irreversible demagnetization threshold. Always current-limit PMSM locked-rotor tests and monitor magnet temperature with an embedded sensor.