The Min Speed of BLDC with VESC ??

General topics and discussions about the VESC and its development.
johnny77lee
Posts: 4
Joined: 24 Jul 2017, 16:44
Location: Taipei

The Min Speed of BLDC with VESC ??

Postby johnny77lee » 25 Jul 2017, 02:08

Hi ,

I want to follow the Jetson RaceCar build Step(from Jetson Hacker) to build a RaceCar with VESC .
Because VESC seems could be provide the low speed control with the BLDC .
But I have a question about the BLDC control by VESC ,
If I used the VESC to control the BLDC(ex. TRAXXAS Velineon 3500 ) ,
is it have min speed limitation from still to rotation ?

Thanks
Johnny77Lee

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: The Min Speed of BLDC with VESC ??

Postby rew » 26 Jul 2017, 06:20

To be able to commutate at the right moments, the controller has to measure the phase voltages. These voltages are small and drown in the electrical noise when the speed of the motor is low. So, below a certain speed, the controller is "blind" if your motor doesn't have sensors.

In theory if you assume that the torque on the motor at low speed is very low you can drive the motor "open loop" without sensors. You'll be wasting a bit of energy, but it should work. This is software that hasn't really been implemented yet. (in FOC mode there is foc_openloop, but it is not integrated into the normal "run slow" part of the performance range..)

Hummie
Posts: 110
Joined: 10 May 2016, 04:05

Re: The Min Speed of BLDC with VESC ??

Postby Hummie » 30 Jul 2017, 00:56

Rew do u know how much more efficient a slow speed high torque motor would be with or without sensors? It's hard to get a reliable temp reading on a motor but they seem to run ten to fifteen degrees cooler with sensors. That's a lot


Also at what rpm will it switch out of sensor mode to plain bldc when running hybrid mode and is it adjustable ..and how best to decide what speed to have it switch? With a hub motor and 14 poles even at max speed I'm at a very low erpm

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: The Min Speed of BLDC with VESC ??

Postby rew » 30 Jul 2017, 11:25

In open-loop mode you'd have to make the current significantly higher than the max required torque. CURRENT is what causes losses in both the motor and the controller. If I'd put 5A through the motor at "below-BEMF sensing threshold" speed, I'd potentially cause I2.R of losses. I have a 20mOhm motor and a 3mOhm controller (2*1.5), so you do the math....

The switch over RPM is configurable. With low-KV motors like hub motors, that can be lowered considerably from the default setting because the back EMF is larger than on what the default was set for. Actually, this threshold would be better to set at a percentage of the max measurable range. Once the back EMF exceeds that voltage, reliable sensing would be possible and you can switch to sensorless mode.

Things are a bit difficult if you want to have a throttle control the current through the motor. You cannot get (enough) feedback from the motor to sense how fast it's going to e.g. emulate a current-control-mode. So you could possibly use this in RPM control mode, but most people expect the throttle to control the current. That feels most natural.


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