Out of Step

BLDC Lurches Under Load Debug

Symptom

Under load the shaft averages the commanded 600 RPM but lurches, swinging between 0 and 1200 RPM twice a revolution. Phase current peaks at 18 A.

The starter's measured trace under the heavy load: the speed averages 600 RPM but swings between 2 and 1217 RPM, far outside the 100 RPM band, and the phase current peaks at 17.9 A, above the 15 A line

Circuit

Block diagram: U1, an STM32F4, drives U2, a three-phase bridge that turns M1, a brushless motor with a rotor position sensor, and prints on a serial console; below it the bridge's registers with offset, access and units

Pass condition

From 300 ms the shaft stays within 100 RPM of 600 RPM and no fault sets. From 600 ms phase current stays under 15 A (10 A at the lighter load).

Notes

The bench writes the mRPM speed command to SRAM at 0x2000FF00 and sets the load. The run is 1000 ms. USART1 (Datasheet tab) runs at 9600 baud: BRR = 0x683.

Related challenges

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