DC Battery amps vs. Motor Amps

General topics and discussions about the VESC and its development.
maxkinz
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Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 07 Jul 2017, 08:46

Hey devin,

Thanks for the suggestion.
I am busy with other stuff today but I will go make some more experiments especially measure the voltage rails whenever I get to it. KB control does not influence the results. As the screenshot of the realtime graph shows the current is supposed to be 3.5 motor amps - translating to roughly 0.5 Battery amps. That is at least what should be happening which is why I was at a loss to explain the measurements in the current plot. I will retry using a battery later as well.
Thank you guys for all the help.

devin
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Re: DC Battery amps vs. Motor Amps

Postby devin » 07 Jul 2017, 09:40


maxkinz
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Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 10 Jul 2017, 10:24

Hey Devin,
Thanks for the work.
I will wait for my other VESC to arrive so I can do head to head comparisons - no value in guessing for me at this point - if It turns out the behaviour is specific to this VESC (which I think it will be) I know for sure that something is broken and will try and repair (probably replace DRV). If not I will investigate further.

Thank you again for all the effort !

rew
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Re: DC Battery amps vs. Motor Amps

Postby rew » 10 Jul 2017, 13:23

Just for your information, Devin seems to live in a universe where the rules of physics are slightly different.

You should not take for a fact when he says the VESC will plot the current it intended to push through the motor. What you see in the graph is what was actually measured. Sometimes when e.g. a sudden load change happens, you can see the "WHOA need to adjust things, the current is much more than requested". THis is then a transient and it happens pretty quickly, but if you see a steady "3A" line, that is what the VESC measured.

Of course you can fool it. If you mount say 2mOhm resistors instead of 1mOhm resistors, then the 3mV of measured voltage does not correspond to 3A, but only to 1.5A. So in that case, unless you confess to the VESC that you modfiied the resistors (= recompile with the right settings), the indicated current will be wrong.

devin
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Re: DC Battery amps vs. Motor Amps

Postby devin » 10 Jul 2017, 13:49


rew
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Re: DC Battery amps vs. Motor Amps

Postby rew » 10 Jul 2017, 17:25

The VESC (4.x hardware) has only 2 current sense channels. For BLCD and FOC you need to know all three currents. However there is a law in electrical engineering: all currents in a node add up to zero. And this holds even when you expand "node" to black box. So if we make the motor a black box, and we see 1A going in on one wire, 2A going in on the second wire, then we know there must be -3A going into the third wire (a negative amount going in equals current coming out)....

In DC motor mode, the two sensors are mounted on the two wires going into the DC motor. So in theory, if you measure 2.3A going into lead A, you should measure -2.3A going into lead C (i.e. that 2.3A coming out).

Imperfections make that you don't always exactly measure perfectly. This means that the third phase (Ib) that is calculated from Ia + Ib + Ic = 0 does not always end up being exactly zero. If you're seeing something "constant and not close to zero", then the calibration is off by a bit. (I wrote a terminal command dc_cal to re-calibrate it)

rew
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Re: DC Battery amps vs. Motor Amps

Postby rew » 10 Jul 2017, 18:02

I have patched my screwdriver. I now have both motor leads and the original drive lead outside of the screwdriver. I measured the current, and it went above 20A that my multimeter can handle. So it should be reasonably safe to tell the VESC that the motor can handle 20A.

There is quite a lot of friction in the gearbox: The motor requires 5A just to run at top speed.

P.S. The charger is not very good. Leave your battery on the charger for a month and it (the battery) will be dead. Try to remove the battery from the charger as soon as the light turns green.

maxkinz
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Re: DC Battery amps vs. Motor Amps

Postby maxkinz » 11 Jul 2017, 08:23

Hey rew,
Thanks for the update ! I know about the friction in the planetary. My drill requires about the same to get to top speed. I was going to subtract whatever current I need to get to a certain speed before calculating the torque from the "residual" current. I know about the charger and I didn't use it at all - got an iMax B6 here to fullfill most of my charging needs. There seems to be some electronics built into the battery (didn't open it) as when running off of that battery the VESC looses power after a while - do you observe this, too ? it repowers after reconnecting the battery. This does not happen on the lab supply.

devin
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Re: DC Battery amps vs. Motor Amps

Postby devin » 11 Jul 2017, 14:14


rew
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Re: DC Battery amps vs. Motor Amps

Postby rew » 12 Jul 2017, 06:00



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