DC Battery amps vs. Motor Amps

General topics and discussions about the VESC and its development.
rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: DC Battery amps vs. Motor Amps

Postby rew » 03 Jul 2017, 20:32

OK. Back to your problem.

can you measure the current that flows through the motor when you're getting more torque using the original hardware?

Oh... What do you have your "max charge" setting set for? I've seen the VESC apply that limit when in fact there is no charging going on.

devin
Posts: 225
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 03 Jul 2017, 22:30

What is your lead to lead winding resistance, voltage of your battery, and desired DC amps from battery?

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: DC Battery amps vs. Motor Amps

Postby rew » 04 Jul 2017, 07:14

Most multimeters are not that accurate in measuring resistance below 1 Ohm. This is because 1) they don't want to damage your component by forcing say 1A through it, and 2) they don't have the powersource to force that 1A through it.

If you have a lab powersupply (I'm told you do :-) ) just set the current for 1A , stall the motor and measure the voltage across the motor terminals. This gives a much more accurate measurement than the multimeter by itself. If you have a second multimeter, use it to double check your lab supply's current setting. Note that wires are no longer ideal wires. This weekend someone left my lab powersupply at 3A shorting my (crappy) leads. there was still a significant voltage across the leads something like 3V IIRC.

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

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 04 Jul 2017, 09:17

Hey guys,

Thank you, I will try and measure once I get home from work.

I did, in the range below 10A, get pretty ok results so far (almost lin. relationship between torque and current) so if I can't figure it out I might just try if its sufficient for the drilling I need to do and go with that. It should however obviously be possible to "full throttle" the drill using the VESC.
I'll get back with the results.
Thanks so far !
Max

rew
Posts: 904
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: DC Battery amps vs. Motor Amps

Postby rew » 05 Jul 2017, 05:37

I bought your drill. I'll be able to test with it soon.

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

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 05 Jul 2017, 08:16

Hi guys,

Wow rew I hope you have an additional application other than helping me...
I did measure the resistance yesterday to be 0.38 Ohms. Almost destroyed my VESC this morning connecting it reversed polarity to a Battery - pulled back quick at first spark and it survived... but scary start to the day -shouldnt do that before the second coffee... so off to the office :D.

Has any of you guys ever measured RPM of a brushed motor by back EMF ? Do you think the VESC can do this ?
My first plan was to use a CNY70 optical sensor to count the fan blades of the drill when doing the test drilling and use that but it is tedious to install and I'm not sure its entirely necessary as the commutations of the brushes should leave me spikes to count. I'll try that using bldc tools current plot function and report back. Maybe I'll take the easy way out and use an external rpm logger....

Thanks so far
Max

devin
Posts: 225
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 05 Jul 2017, 08:29

0.38ohm

--------------

"Bland Mode"

10a DC draw desired from 12V Batt

120W

120W = I^2R

120W = (17.77a^2)*0.38ohm = 10a * 12vdc

Battery Amp Limit Setting: 10a

Motor Amp Limit Setting: 17.77a

Absolute Amp Limit Setting : 25a

-------------

"Power Mode"

20a DC draw desired from 12V Batt

240W

240W = I^2R

240W = (25.13a^2)*0.38ohm = 20a * 12vdc

Battery Amp Limit Setting: 20a

Motor Amp Limit Setting: 25.13a

Absolute Amp Limit Setting : 35a

----------------

"Ludicrous Mode"

If you'd simply like to apply the full 12v to the motor:

12v * 31.57a = 378.94W = (31.57a^2) * 0.38ohm

Battery Amp Limit Setting: 31.57a

Motor Amp Limit Setting: 31.57a

Absolute Amp Limit Setting: 40a

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

Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 05 Jul 2017, 14:21

sorry for my grammar - 0.38 Ohm :)
So I fiddled around some more with the setup and I'm unsure if I did or did not damage the VESC this morning. Everything is actually running fine and I have tried hooking up a small BLDC motor as well to make sure all MOSFETs survived the attack with no issue. Trouble is, when I tell BLDC tool to supply constant current of say 3A and hook up the ampmeter in series with my DC motor it showed pretty much the 3A I asked of it before that unintentional attack on my electronics. Now not so much anymore. BLDC Tool realtime data looks the same as before though. What is weird is that in the current plot, the Phase 1 Current shows to be much higher than anticipated. The following was recorded while driving the stalled cordless drill by telling it to drive 3.5A using BLDC Tool.
CurrentMeasurement.png
CurrentMeasurement.png (132.74 KiB) Viewed 2747 times

Is that normal ? I have had very repeatable results with my torque measurements before and did not particularly pay attention to this graph before.

What part could I have damaged ? A shunt resistor ? A Mosfets body diode ? but if the latter - wouldn't that cause a failure when driving the BLDC motor ? I do have a hot air station, a soldering station etc. so I could change the part at fault - it is just weird as like I said everything worked great using the small brushless I tried in BLDC mode after I thought there was some damage. Current measurements did seem ok with that too.
Right now BLDC tool tells me Motor current of 300mA even when the motor is detached and its told to do nothing.

Sorry for all the questions. Maybe I can give back later if anyone ever abuses the VESC as a measurement device again.

Thanks for the help guys !

devin
Posts: 225
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 05 Jul 2017, 17:06

maxkinz wrote:sorry for my grammar - 0.38 Ohm :)
So I fiddled around some more with the setup and I'm unsure if I did or did not damage the VESC this morning. Everything is actually running fine and I have tried hooking up a small BLDC motor as well to make sure all MOSFETs survived the attack with no issue. Trouble is, when I tell BLDC tool to supply constant current of say 3A and hook up the ampmeter in series with my DC motor it showed pretty much the 3A I asked of it before that unintentional attack on my electronics. Now not so much anymore. BLDC Tool realtime data looks the same as before though. What is weird is that in the current plot, the Phase 1 Current shows to be much higher than anticipated. The following was recorded while driving the stalled cordless drill by telling it to drive 3.5A using BLDC Tool.

CurrentMeasurement.png
CurrentMeasurement.png (132.74 KiB) Viewed 4 times


Is that normal ? I have had very repeatable results with my torque measurements before and did not particularly pay attention to this graph before.

What part could I have damaged ? A shunt resistor ? A Mosfets body diode ? but if the latter - wouldn't that cause a failure when driving the BLDC motor ? I do have a hot air station, a soldering station etc. so I could change the part at fault - it is just weird as like I said everything worked great using the small brushless I tried in BLDC mode after I thought there was some damage. Current measurements did seem ok with that too.
Right now BLDC tool tells me Motor current of 300mA even when the motor is detached and its told to do nothing.

Sorry for all the questions. Maybe I can give back later if anyone ever abuses the VESC as a measurement device again.

Thanks for the help guys !


What are your batt/motor/absolute amp limit settings? What is your battery voltage during the recording? From the "total current" blue line It looks like it's averaging the 3.5amps you asked for. I suspect the blue line is the battery amps and the pink line is the motor amps.

Simply 3.5a is drawn from the DC source while ~13.7a is supplied to the motor at lower effective pwm voltage.

devin
Posts: 225
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: DC Battery amps vs. Motor Amps

Postby devin » 05 Jul 2017, 17:34

Image

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

therefore:

3.5 battery amps = 13.5 motor amps (25.92% duty cycle / 100)

(~13.5A^2) * 0.38ohm = 5.13V Effective PWM Volts * 13.5A Motors Amps = 69.25W = 3.5A Battery Amps * 19.78V DC Battery

From your graph it looks like:

3.5A Battery Amps
13.5A Motor Amps
19.78V DC Supply
25.92% Duty Cycle
5.13V Effective PWM Volts
69.25 Watts


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