Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

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

Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby johnny77lee » 11 Aug 2017, 03:34

Hi ,
I used Maytech's VESC with a embedded system broad to control a Traxxas RC car
The RC Car Include 2S Li-po Battery , SensorLess BLDC (Velineon 3500) and Servo motor(Steering Control).

The MayTech's VESC is already upload FW to "FW ver 2.18 with servo out "version

After I connected the battery and the BLDC to VESC and use BLDC-Tool to control the BLDC motor ,
I try to use keyboard control the BLDC Spin up but the motor seems spin up very hard .

During the motor spin up , the accelerate of motor spin seems not smoothly and hardly for spin to Hi speed .

After more try and error found , if change the Minimum duty cycle over 0.28 and Maximum duty cycle to 0.3
Motor spin to Hi Speed will be easily but sometimes Motor spin up still hardly in low Speed

In the RealTime data page of bldc tool , I could found the Motor Current signal shake during the motor spin up like bellow picture
https://drive.google.com/file/d/0B9VBaNsv09YrVmstTmFvOHNJbXM/view?usp=sharing

My Question :
1. How to make the motor accelerate smoothly during spin up and low speed ?
2. How to make the motor work in low speed ?

ps. I used the config xml file from mit-racear project for my motor configuration base,bellow is the link of xml file:
https://github.com/mit-racecar/hardware/blob/master/vesc/6.141_bldc_old_hw_30k_erpm.xml

Thanks

johnny77lee
Posts: 4
Joined: 24 Jul 2017, 16:44
Location: Taipei

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby johnny77lee » 11 Aug 2017, 09:15

Add the picture " Motor Current signal shake during the motor spin up"
Screenshot from 2017-08-10 11-31-22.png
Screenshot from 2017-08-10 11-31-22.png (235.91 KiB) Viewed 2419 times

linsus
Posts: 35
Joined: 24 Apr 2016, 10:20

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby linsus » 11 Aug 2017, 13:45

Hi!

My guess would be that the problem is running on low voltages such as 2s makes it hard for the controller.
If I'm not misstaken 3S-12S was originally the recommended span of operation for the VESC. Bigger motor maybe? ;)

Running sensored will probably make the startup smoother.

//Linus

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

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby rew » 12 Aug 2017, 14:11

The maximum voltage on a motor causes it to spin at a certain speed that the motor is happy with.

In the old days you could stall a motor and it would not overheat. But nowadays I have a 16mOhm motor rated at 50V 50A. Put 50V on the motor like that and the current would be over 3000A.

Calculate the max RPM at 2S and use that as the RPM limit. Or just set the maximum dutycycle at 66% and use 3S.

Things go wrong with 2S because there is a low-voltage-cutoff that happens around 8V.... (you can't drive power MOSFETs very well with lower voltages).

johnny77lee
Posts: 4
Joined: 24 Jul 2017, 16:44
Location: Taipei

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby johnny77lee » 14 Aug 2017, 08:32

Hi rew ,

As I know , the Voltage of 2S Lipo Battery is 8.4V after it charge full and I do the test with this kind of battery .
When the Motor work not smoothly , I see the Battery Voltage still over 8V by Realtime Data of BLDC tool .

I do bellow test about low voltage input with VESC :
1. Try to change the Battery cut off end field in Voltage Limits to 7V ,but the BLDC Motor seems still work not smoothly during spin up .
2. About the 2S Lipo battery , I try to use the motorcycle battery to connect VESC , it seems still not smoothly and it will the motor always couldn't spin to high speed .

If the root cause is output voltage of 2S Lipo Battery too low for VESC , after change the battery to motorcycle battery(12 V) the bldc motor should be work smoothly during spin up . But It seems still work abnormally during spin up .

About the speed control with VESC , would you know what is the key to control it ? Duty cycle? or Driving Current ?

Would you provide how to calculate the max rpm with 2S Battery ?

Thanks

linsus
Posts: 35
Joined: 24 Apr 2016, 10:20

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby linsus » 14 Aug 2017, 09:56

Guessing that rew is refering to the KV ratio on the motor.

Read bens explaination on KV here : http://vedder.se/2014/10/chosing-the-ri ... kateboard/

Essentially KV x Volt = RPM, i fI'm not misstaken.

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

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby rew » 15 Aug 2017, 05:49

KV times volts gives you the MAXIMUM RPM that this battery voltage will allow your motor to run at.

1. Try to change the Battery cut off end field in Voltage Limits to 7V ,but the BLDC Motor seems still work not smoothly during spin up .
If you try to drive powermosfets with a gate voltage lower than about 10V, they start performing worse, and heat up. This heat can damage the mosfets, So to protect the mosfets the HARDWARE has an "under voltage lock out" implemented. It should kick in close to about 8V. You cannot change that limit by configuring the software to attempt to work below 8V.

subodh.malgonde
Posts: 4
Joined: 17 Sep 2018, 13:17
Location: India

Re: Some question about sensorless BLDC(Velineon 3500) spin up not smoothly

Postby subodh.malgonde » 25 Sep 2018, 10:55

@ johnny77lee Were you able to figure out the issue? I am facing the same problem.


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