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Braking current after high-speed coast

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Last seen: 1 year 1 month ago
Joined: 2020-07-24 01:10
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Braking current after high-speed coast

I mentioned my experience in a different thread, but wanted to repeat it here, since this felt like a serious event.

While logging data I was coasting downhill with a little more speed (perhaps 10 %) than the drive can normally produce, and started regen braking a bit (using FOC).  The actual brake was a lot stronger than requested!  Link to graph (28k eRPM = 25 km/h):

Fortunately there was no damage.  My settings have max regen current of -11.6 A and max brake current of -33A.  The graph shows actual regen current of -37 A, and phase current of -52 A.  Hardware is original VESC 6 MKIV.  The control "app" is a customization of app_adc.c that uses a second throttle for braking and reverse w/o button (thanks district9prawn).  There is also an automatic throttle holding feature, but that's inactive during coast.  Link to code (ctrl_type = ADC_CTRL_TYPE_CURRENT_REV_BUTTON_BRAKE_ADC):  

I vaguely remember reading that high speed coasting presents a problem for the controller, because the battery voltage can't match the EMF.  But even if proper FOC is unavailable, there must be ways (dangerous words, I know) to get controlled braking current.  Incurring some torque ripple seems preferable to slamming the brake.

To work around this and since I have a mechanical brake, my app_adc.c will probably just check duty cycle before applying the e-brake.