Sensorless BLDC regen unreliable on engine-driven starter-generator application

[RMIShane]

Sensorless BLDC regen/current braking unreliable on engine-driven starter-generator application

Summary

We are using a VESC 6 MKVI HP in a starter-generator application where the same BLDC motor is used both to crank/start a small engine and then generate current back into the battery while the engine is running. While also being able to use positive current to help the engine reach max power when needed.

We are seeing unreliable regen/braking behavior in BLDC sensorless mode. In some cases regen works correctly, but in many cases commanded braking results in positive battery current draw instead of negative current/regen. We have also observed direction-dependent behavior in Current mode that seems asymmetric.

We are looking for guidance on whether this is an expected limitation of sensorless BLDC mode/current braking at this ERPM range, a configuration issue, or potentially a firmware issue. We are very motivated to get this working correctly and can provide detailed logs, test data, and possibly a hardware/test-stand setup if that would help reproduce the issue.

Hardware / setup

• Controller: VESC 6 MKVI HP
• Firmware/Software: VESC Tool 7.0, VESC Firmware 7.0
• Battery: 2x 6S LiPo packs in series
• Engine: 29 cc 2-stroke, single-cylinder
• Motor: 42-pole brushless motor coupled directly to the crankshaft
• Motor mode: Sensorless BLDC mode
• Application: motor starts the engine, then engine drives the motor as a generator to recharge the batteries

Operating speed

With the engine running, mechanical RPM is in 2500–8000 mechanical RPM range

With 21 pole pairs, it's about 50000-170000 ERPM

Goal

Our goal is to use current control with reliable braking/regen while the engine is running.

Expected behavior:

• At neutral throttle / PPM ~1500: approximately 0 current
• Below neutral: negative current / regen braking
• Above neutral: positive current / motoring

Observed behavior: Current No Reverse Brake mode

With the engine off, when running the motor electrically and then commanding brake in Current No Reverse Brake mode:

• Sometimes the controller regens nicely all the way to full stop.
• But often, instead of regen, it starts drawing approximately +30 A and brakes hard with a high-pitched noise.

With the engine running in Current No Reverse Brake mode:

• At PPM around 1500, current is 0 A as expected.
• When PPM is reduced below 1500, we expect negative current from regen braking.
• Instead, current goes to approximately +5 to +7 A and stays positive.
• We have not been able to get reliable negative current/regen in this mode while the engine is running.

Observed behavior: Current mode

In regular Current mode:

• We were able to get regen some of the time.
• However, it is unreliable.
• Sometimes it randomly starts drawing approximately +30 A instead of regenerating.

There is also a direction-dependent behavior that seems important:

1. With engine off, in Current mode:

   • Spin the motor in the reverse direction using PPM below 1500.
   • Then command the opposite direction using PPM above 1500.
   • Regen/braking works correctly.

2. But doing the opposite:

   • First spin the motor forward using PPM above 1500.
   • Then command the reverse/braking direction using PPM below 1500.
   • Regen does not work correctly.

We would expect the behavior to be symmetric in both directions, but it is not.

We can provide:

VESC Tool motor/app configuration files
Realtime data logs
Fault logs
Additional controlled tests if specific settings or debug points would be useful