Power consumption: RPM control vs duty cycle control

Discussions regarding the Field Oriented Control (FOC) implementation on the VESC.
andresv
Posts: 2
Joined: 10 Jan 2017, 11:30

Power consumption: RPM control vs duty cycle control

Postby andresv » 10 Jan 2017, 11:56

I did a test where motor is connected with a quite heavy inertial load.
I noticed that power consumption is rather strange if i run the motor with the same speed using RPM control and then duty cycle control.
In this test 0.52 dutycycle gives the same speed as 43200 RPM.
In the picture left side motor is running at 43200 using RPM control and you can see that it draws around 0.1 A from battery at 20V. Then i switched to 0.52 Duty control and motor seems to generate power which is very strange.

However actual power supply shows that with RPM mode it draws 0.42A @ 20V and with duty control it draws 0.5A. So it seems that current consumption is reported incorrectly in duty cycle mode?
I also noticed that the same thing happens if max duty cycle is hit. For example i have set max duty cycle to 0.55 and now if i increase RPMs over this limit where it must use the max duty cycle I can see that current consumption drops in BLDC tool but not in power supply. So there must be something wrong with reporting.

Firmware 2.18.

[img]bldctool.jpg[/img]
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pf26
Posts: 188
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Power consumption: RPM control vs duty cycle control

Postby pf26 » 10 Jan 2017, 15:57

Your current measurements are not accurate enough to display correctly.
Be aware that your values are much less than 1% of the full scale..
If you load your motor (not only inertial load, but also drag), then you will see relevant current graphs.

andresv
Posts: 2
Joined: 10 Jan 2017, 11:30

Re: Power consumption: RPM control vs duty cycle control

Postby andresv » 11 Jan 2017, 07:53

Alright I did another experiment with drag. So now currents are much higher, however the phenomenon is still the same.
On the picture left side motor is set to 40000 ERPM and then in the middle is commanded to 43000 ERPM. Notice that duty cycle is limited to 0.55. What happens is that if max duty cycle limit is hit, motor reports that it consumes less power however it is actually rotating faster (40900 EPM vs 40000 ERPM) and should consume more if switching is optimal.

[img]drag[/img]
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pf26
Posts: 188
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Power consumption: RPM control vs duty cycle control

Postby pf26 » 13 Jan 2017, 13:40

Seems like there is more drag on the left side than on the right (right = where the motor speed is just limitted by max duty).
More drag means more current.
What makes you think there should be more current with higher speed ?

rew
Posts: 601
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Power consumption: RPM control vs duty cycle control

Postby rew » 14 Jan 2017, 15:42

Suppose that running at 53% dutycycle results in 4500 RPM.

Now if we set the VESC for RPM control at 4500 RPM, we'll see the VESC report a significantly different current from when it runs at 53% dutycycle. Try it.

Stay at least a few percent away from "duty cycle limit". Funny things happen near that point too.


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