Why Did Your PMSM Motor Just Stall

Stall events remain one of the more frustrating challenges for engineers working with permanent magnet synchronous motor drives. You complete your initial tuning, the PMSM Electric Motor spins smoothly at no-load, but the moment you apply torque, the drive trips or the rotor stops. Understanding why this happens requires a closer look at rotor position accuracy and control loop integrity.

The Hidden Culprit: Electrical Angle Misalignment

Field-oriented control (FOC) depends entirely on accurate rotor position information. When a Permanent Magnet Synchronous Motor stalls under load after running well at no-load, the root cause is almost always incorrect electrical angle alignment.

At no-load conditions, torque demand is minimal. Even with slightly inaccurate position feedback, the motor can still rotate because the required torque is low. However, when load increases, the situation changes dramatically. The torque produced by a PMSM Electric Motor follows a sine function of the angle error between the rotor flux and stator current vector. As one technical forum discussion explains, if your electrical angle error reaches 30 degrees, you lose 50% of your available torque. At 45 degrees error, torque drops to 70% of commanded value. When the load torque exceeds this reduced output, the motor stalls.

Three Common Reasons for Stall Events

1. Incorrect Initial Offset Calibration

Many engineers use trial-and-error methods to find the initial angle offset. They adjust values until the motor runs smoothly at no-load, assuming the job is done. However, as load increases, small offset errors become critical. A resolver or encoder provides mechanical position, which must be converted to electrical angle using the pole pair count. Subtracting an initial offset aligns this with the motor's phase zero. If this offset is wrong by even 0.1 per-unit (36 electrical degrees), stall under load becomes likely.

2. Failure to Perform Proper Rotor Alignment

The correct method for finding zero offset requires forcing the rotor into a known position. Applying a DC current through the d-axis (with Iq set to zero) aligns the rotor with the stator field. Crucially, this must be done with sufficient current magnitude. Engineers sometimes use only 10% of rated current, which may not move the rotor if load is present. Near-rated current ensures positive alignment regardless of initial position.

3. PWM Saturation During Load Transients

When a PMSM Electric Motor stalls, monitoring the PWM outputs often reveals that duty cycles have reached maximum limits. This indicates that the controller is demanding full voltage but cannot maintain synchronism. The angle error causes the FOC algorithm to lose field orientation, and the motor pulls out of sync.

Verifying Your Position Calibration

Zhejiang Hechao Motor Co., Ltd. recommends a systematic verification process. primary, perform the DC alignment procedure with rated d-axis current and record the encoder or resolver reading. Use this as your permanent offset. Second, test at progressively higher torque levels while monitoring actual vs. commanded torque. If deviation exceeds 10%, your angle calibration requires refinement. Third, implement a method to store this offset permanently so it remains valid across power cycles.

Hardware and Software Interactions

Sometimes the issue is not purely angle-related. Current measurement errors can masquerade as position problems. If your PMSM Electric Motor draws excessive current at startup and triggers protection circuits, verify that phase current sensing is correct and that measured currents are positive into the motor. Additionally, ensure your PI controllers are tuned appropriately for loaded conditions. Gains that work at no-load may be insufficient under load due to the changed dynamics of the system.

A PMSM Electric Motor that stalls under load points directly to electrical angle inaccuracy. The solution requires precise initial offset determination using forced alignment with adequate d-axis current, verification under load, and proper handling of encoder index signals. Zhejiang Hechao Motor Co., Ltd. emphasizes that no-load operation alone never confirms correct tuning. Only loaded testing reveals the true quality of your position calibration and control setup. By addressing these fundamentals, you can eliminate unexpected stall events and achieve reliable performance across your entire operating range.