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Mar 07, 2017, 11:02 PM
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Death of a stepper motor


With the 18A cannibalized for fixing the Ruckus, the stepper motor switched to a 35A which didn't work at all.


The 1st problem was the same locking up of the FETs on startup & burnout. It wasn't a defect. Current from the UART retained the register contents when the ESC was off. It never happened in practical vehicles, probably because all signals were off at the same time. Zeroing the registers as soon as it boots up got it to work, sometimes.


The required deadband when switching MOSFETs was 24us on the 35A instead of 9us on the 18A. Probing the gates revealed an incredibly slow rate of turning off the P FETs. It just seemed to be a fact of life with cheap hardware. The simonk firmware doesn't PWM the P FETs, so it's not an issue for commutating motors. The stepper motor does PWM the P FETs. Simok also turns off all the FETs 1/3 of the time to detect back EMF.

The stock 35A P FET turned off faster than the stepper motor P FET, so there's some variability.
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