DC Battery amps vs. Motor Amps

General topics and discussions about the VESC and its development.
devin
Posts: 244
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 05 Jul 2017, 17:42

On second glance it looks like "total current -- blue line" is averaging about 3 amps -- was your battery amp limit setting still 3 amps when you commanded it draw 3.5 amps?

battery amps = motor amps x (% duty cycle / 100)

therefore:

3 battery amps = 13.5 motor amps * (22.22% duty cycle / 100)

(~13.5A^2) * 0.38ohm = 69.25W = 5.13V Effective PWM Volts * 13.5A Motors Amps = 3A Battery Amps * 23.08V DC Battery

From your graph it looks like:

0.38 Ohms
3A Battery Amps
13.5A Motor Amps
23.08V DC Supply
22.22% Duty Cycle
5.13V Effective PWM Volts
69.25 Watts


Image

rew
Posts: 926
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: DC Battery amps vs. Motor Amps

Postby rew » 05 Jul 2017, 20:31

maxkinz wrote:Wow rew I hope you have an additional application other than helping me...
Screwing. It's a screwdriver, right?

Some people recommended these drills (and their cousins doing other stuff) for the reasonable battery and fair price. And I was a bit annoyed at having to take out the corded-drill for screwdriving. And it wasn't good at it. So when I landed on a "buy here" page searching for your drill, I just did. Latest news: they are going to deliver it to my office next saturday and when they find me not-home they will send it back and my money will be returned (that's what customer support told me).

devin
Posts: 244
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 05 Jul 2017, 21:48

:D

rew wrote:Screwing. It's a screwdriver, right?

Some people recommended these drills (and their cousins doing other stuff) for the reasonable battery and fair price. And I was a bit annoyed at having to take out the corded-drill for screwdriving. And it wasn't good at it. So when I landed on a "buy here" page searching for your drill, I just did. Latest news: they are going to deliver it to my office next saturday and when they find me not-home they will send it back and my money will be returned (that's what customer support told me).


devin wrote:On second glance it looks like "total current -- blue line" is averaging about 3 amps -- was your battery amp limit setting still 3 amps when you commanded it draw 3.5 amps?

battery amps = motor amps x (% duty cycle / 100)

therefore:

3 battery amps = 13.5 motor amps * (22.22% duty cycle / 100)

(~13.5A^2) * 0.38ohm = 69.25W = 5.13V Effective PWM Volts * 13.5A Motors Amps = 3A Battery Amps * 23.08V DC Battery


From your graph it looks like:

0.38 Ohms
3A Battery Amps
13.5A Motor Amps
23.08V DC Supply
22.22% Duty Cycle
5.13V Effective PWM Volts
69.25 Watts


see... it's as easy as 1 - 2 -3 ! :mrgreen:

Image

Image

maxkinz
Posts: 29
Joined: 30 Jun 2017, 14:41

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 06 Jul 2017, 07:34

Hey guys,
Thanks for the help again !
What you said about that being the motor and battery current with the subsequent calculations can not be the case however. The lab supply is set to 10V@3A max so it can't actually draw more than 30W of power.
Here is the realtime data for this and new current graph tab
CurrentMeasurement_RealtimeData.png
CurrentMeasurement_RealtimeData.png (77.79 KiB) Viewed 2936 times
CurrentMeasurement2.png
CurrentMeasurement2.png (148.35 KiB) Viewed 2936 times

The current plot was recorded during the "on-phase" of the realtime data plot. Battery current says its half an amp as you can see.
The problem is it is also inconsistent when I tell it to throw out 3A the torque i got was consistent before the reverse polarity event and now is not any more. You can actually hear the difference in the clicking when the motor makes it's effort to spin against the sensor. Also the lab supply shows me once It'll draw 0.5A the next time it draws a full 1A - same setup, same blocked motor, same command sent by BLDC tool. I am pretty sure i messed up my VESC except that it is working... My battery max. was set to 20A which is far more than the lab supply can do as I did not want that to limit anything.

Hope you guys can point me to what may be wrong.
Max

maxkinz
Posts: 29
Joined: 30 Jun 2017, 14:41

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 06 Jul 2017, 07:55

So... I just ordered a new VESC as I can use it for other things later anyway. Could not find any german reseller right now and the guy who soldered up mine appearently does not sell them any more.(was ebay) - so I ordered from china... hope that thing even works .... :(
I'll keep you posted
Max

devin
Posts: 244
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 06 Jul 2017, 08:37

max wrote:Hey guys,
Thanks for the help again !
What you said about that being the motor and battery current with the subsequent calculations can not be the case however. The lab supply is set to 10V@3A max so it can't actually draw more than 30W of power.
Here is the realtime data for this and new current graph tab
CurrentMeasurement_RealtimeData.png
CurrentMeasurement_RealtimeData.png (77.79 KiB) Viewed 4 times
CurrentMeasurement2.png
CurrentMeasurement2.png (148.35 KiB) Viewed 4 times

The current plot was recorded during the "on-phase" of the realtime data plot. Battery current says its half an amp as you can see.
The problem is it is also inconsistent when I tell it to throw out 3A the torque i got was consistent before the reverse polarity event and now is not any more. You can actually hear the difference in the clicking when the motor makes it's effort to spin against the sensor. Also the lab supply shows me once It'll draw 0.5A the next time it draws a full 1A - same setup, same blocked motor, same command sent by BLDC tool. I am pretty sure i messed up my VESC except that it is working... My battery max. was set to 20A which is far more than the lab supply can do as I did not want that to limit anything.

Hope you guys can point me to what may be wrong.
Max


I'd try the following:

-Your target seems to be 30W

-Connect a 12V lead acid battery to VESC to verify supply current limitation is not the source of the problem

30W = 2.5A Battery Amps * 12V DC Supply = (8.88A^2 Motor Amps) * 0.38ohms = 8.88A Motor Amps * 3.37V Effective PWM Volts

(3.37V Effective PWM Volts / 12V Battery Volts) x 100 = 28.08% Duty Cycle

Make sure your Amp Limit Settings are the following:

Battery Amp Limit: 3A

Motor Amp Limit: 9A

Absolute Amp Limit: 15A

Command it to draw 2.5 Battery Amps

If everything works right you should see:

2.5 Battery Amps
8.88 Motor Amps
30 Watts
28.08 % Duty

The motor should "see" avg 8.88 amps @ 3.37V the whole time (30w), while the battery should "see" ~8.88 amps @ 12V for 28.08 % of the time (30w) -- which is 2.5 Battery amps @ 12V on average (30w).

I'm not an expert on lab power supplies but you did say it was limited to 3A @ 10V, while the VESC should have been politely asking it for 8.88amps DC @ 10V -- 33.7% of the time -- which is 3A on average.

maxkinz
Posts: 29
Joined: 30 Jun 2017, 14:41

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 06 Jul 2017, 09:29

Hey devin,

I'm sorry but I think you misunderstood me.
I am trying to establish the Motor constant KT (https://en.wikipedia.org/wiki/Motor_constants)
by applying a controlled current using the VESC and measuring the torque using the torque sensor. I can't simply calculate it as there are friction losses in the planetary gearset. In case of the picture I was trying to supply 3.5 A using the VESC which previously worked out and I got repeatable torque corresponding to the current. I tried that with 2,3,4,5,6A motor current respectively and repeated each 25 times (I used a program to control the measurements) with consistent results before I accidentally reversed polarity input on the VESC's input. As you can see the in the realtime window image the target is a motor current of 3.5A - I do not care about battery current for the measurement. It stopped working after my mistake with the reverse polarity on the VESC as now the torque is inconsistent. Also an inline ampmeter FLUKE 867B was measuring a current corresponding well to the desired value before which also does not work any more as well as the supply's amperage indicator which did not read exactly the same as battery current in BLDC tool but at least consistently same amperage for the same command.
I am pretty sure my VESC is measuring current incorrectly after the reverse polarity mistake - question is just why and what part may cause this ?

devin
Posts: 244
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 06 Jul 2017, 09:45

maxkinz wrote:Hey devin,

I'm sorry but I think you misunderstood me.
I am trying to establish the Motor constant KT (https://en.wikipedia.org/wiki/Motor_constants)
by applying a controlled current using the VESC and measuring the torque using the torque sensor. I can't simply calculate it as there are friction losses in the planetary gearset. In case of the picture I was trying to supply 3.5 A using the VESC which previously worked out and I got repeatable torque corresponding to the current. I tried that with 2,3,4,5,6A motor current respectively and repeated each 25 times (I used a program to control the measurements) with consistent results before I accidentally reversed polarity input on the VESC's input. As you can see the in the realtime window image the target is a motor current of 3.5A - I do not care about battery current for the measurement. It stopped working after my mistake with the reverse polarity on the VESC as now the torque is inconsistent. Also an inline ampmeter FLUKE 867B was measuring a current corresponding well to the desired value before which also does not work any more as well as the supply's amperage indicator which did not read exactly the same as battery current in BLDC tool but at least consistently same amperage for the same command.
I am pretty sure my VESC is measuring current incorrectly after the reverse polarity mistake - question is just why and what part may cause this ?


Have you verified the capacitor discharge from the vesc into your lab supply hasn't damaged the supply itself leading to the inconsistency?

This is why it might be advisable to use a plain old 12V battery to verify if the VESC is still working properly... possibly with a simple watt meter attached between the battery & vesc.
Last edited by devin on 06 Jul 2017, 09:54, edited 2 times in total.

devin
Posts: 244
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 06 Jul 2017, 09:52

PS you mentioned your lab supply is set to 10v... but also 3A limit....

what action does the supply take when a load attempts to exceed the 3A limit? drop the voltage below 10v? I dont know.


Maybe since the VESC actually wants 8.88A for some percentage of the time depending on the supply voltage, and since you mentioned the 3A limit setting on your lab supply, maybe while the VESC is pulse width modulating, the lab supply voltage is dipping wildly to stay under 3a...

then when you look at the amp graph in the vesc... what might not be obvious is that the voltage may be wildy inconsistent across the amp graph, leading to the inconsistent measurements.

maxkinz
Posts: 29
Joined: 30 Jun 2017, 14:41

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 06 Jul 2017, 10:07

Thanks for the answer,
Yes the lab supply would just drop the voltage so it would switch from CV to CC mode if the 3A limit was hit.
The cap discharge can not have hurt the lab supply as the reverse voltage occured when I was setting up to reproduce the results using a battery so the lab supply was not involved in the accident. I just switched back to the lab supply afterwards to see if there was any damage done. Also the FLUKE was definately not damaged as it was also not involved - worked before switched polarity - does not work anymore ...


Return to “General”

Who is online

Users browsing this forum: Bing [Bot] and 1 guest