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Jun 29, 2016, 12:01 AM
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Feiyu reluctance

Finally decided to reduce the Feiyu to the simplest possible circuit: a single P MOSFET feeding an N MOSFET through a single motor winding. This gave the same knee graph, current getting less negative as the N MOSFET duty cycle approached the P MOSFET duty cycle, then stalling at around 870. The stall region occupied more of the curve as the deadband increased.

The only way to see all the voltages involved was compositing multiple 2 channel scope views to make 4 channels. On the unknown Feiyu part, the MOSFETs are inverted so they're off if the gate is 0.

Strange physics (0 min 56 sec)

The time of N turning on is ramped from before P turning on to after P turning off. Reverse current happens when both MOSFETs are off, then breaks down when the N off time hits a certain point after P turning on. For later N on times, the behavior is expected. The length of both N & P at 6V increases because there's no place for the charge to go when both MOSFETS are off. The reverse current didn't happen with a purely resistive load.

Basically, the reverse current is a function of the duration of the current flowing from P to N & breaks down accordingly when the P to N is reduced. When N is on with P off, flux in the motor core is trying to send current from N to P, which only flows when N turns off. This is reluctance cogging.
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