Funny behavior

General topics and discussions about the VESC and its development.
joe90
Posts: 49
Joined: 15 Dec 2015, 16:52

Funny behavior

Postby joe90 » 24 May 2016, 17:11

I had to ride my bike without battery and I discovered that the esc wake up and power accessories without any batteries connected to it !
Michel Cote
Twin v4.7 + SK3 6374 149KV
Bicycle
2500W peak/unit on 12S

Shack
Posts: 7
Joined: 03 Apr 2016, 11:58
Location: Denmark

Re: Funny behavior

Postby Shack » 25 May 2016, 13:38

The motor works as a generator and the current flows through the body diodes of the mosfets, powering the esc.. Which then should do active rectification :)

rew
Posts: 928
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Funny behavior

Postby rew » 25 May 2016, 18:51

Shack wrote:The motor works as a generator and the current flows through the body diodes of the mosfets, powering the esc.. Which then should do active rectification :)

I don't think it WILL do active rectification (it could, but doesn't). It will leave the mosfets "unpowered" when you don't command it to do anything with the motor. (all PWM signals are "off" when the motor is supposed to be "freewheeling").

scienceworks
Posts: 29
Joined: 27 Mar 2016, 16:27

Re: Funny behavior

Postby scienceworks » 27 May 2016, 18:31

so the vesc can basically bootstrap itself without a battery? thats good to know. is it also doing active freewheeling?

rew
Posts: 928
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Funny behavior

Postby rew » 28 May 2016, 07:15

In theory that would be possible. If I hit "release motor" on the BLDC_Tool, and then turn the rotor I see it tracking the movement with dutycycle and all.

My first thought was to command a 0.0001 current so that it would be tracking and aiming for a very small forward current. However, There are a few minimums: A minimum current and duty cycle. Below that VESC considers the motor "off".

On the other hand, I'm not sure I understand your term "active freewheeling" correctly. What are you trying to do?

When the VESC is bootstrapped like this, the capacitor(s) will get some voltage from the motor, maybe the software can "hold out" when you stop for a few seconds. But you can't power anything.... And braking is completely out of the question: You'll charge the capacitor until either the capacitor, the fets or the DRV chip blows. Whichever comes first....

Hmmm. This gives me an idea. We could have the VESC software command an external switch that connects it to the battery. So you can have it "autosleep" when nothing happens for 5 minutes, but it can connect itself to the battery after having been bootstrapped by movement on the motor side.... Would be useful on skateboard. The downside is you need a hefty switch in the power lines.

scienceworks
Posts: 29
Joined: 27 Mar 2016, 16:27

Re: Funny behavior

Postby scienceworks » 29 May 2016, 12:11

My first thought was to command a 0.0001 current so that it would be tracking and aiming for a very small forward current. However, There are a few minimums: A minimum current and duty cycle. Below that VESC considers the motor "off".

On the other hand, I'm not sure I understand your term "active freewheeling" correctly. What are you trying to do?


i think you describe exactly what active freewheeling means, instead of losing power over the diode drop the esc switches the mosfets to allow the currents to flow with less loss.

aimthiazz
Posts: 5
Joined: 23 May 2016, 06:50
Location: India

Re: Funny behavior

Postby aimthiazz » 13 Jun 2016, 08:00

I have experienced the same phenomenon, and it happens because the BEMF voltage is rectified via body diode of the MOSFET. Body diode works even though the MOSFET is turned off. Now the rectified voltage appears on +ve and -ve terminal of the VESC thereby powering on ST uC and DRV chip.

rew
Posts: 928
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Funny behavior

Postby rew » 14 Jun 2016, 06:30

Yes! And when there is a battery connected to the "bus power", there is NO WAY that the motor controller can prevent you from charging the battery....


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