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 Calculating Stall Current of Brushed Motors?

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Alex
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PostSubject: Calculating Stall Current of Brushed Motors?   Wed Oct 05, 2011 4:26 pm

Hey guys I was trying to figure out the right esc/motor set up for my new ant weight. I plan on using 4 spark motors at 4s lipo (15ish v) two motors per side. Fingertech robotics list the stall current @ 6v on their site as 1.3 amps. How do I find the stall current at 4s lipo?


I assumed like this:

R = V/I
R = 6/1.3
R = 4.616 ohms

I = V/R
I = 14.4/4.616
I = 3.119 amps <----Stall current per motor.
I = 6.24 amps <---Stall current for both motors per side

The fingertech tinyesc's are only good for 1 amp continuous and 2.4 amp peak. Did I do the math wrong or something? I read that MHRobotic's antweight rudy runs two spark motors at 5s on a single tiny esc, how is this possible?


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rdubard
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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Wed Oct 05, 2011 7:28 pm

From what I know about series wound DC motors, you have done it correctly. At stall, it is only the resistance of the windings that presents a load to the ESC. Thus, you can take the rated voltage and divide it by the stall current to figure the coil resistance. Then you can divide the actual applied voltage by this resistance to get the stall current.

Which is what you did. So it seems you are right on the math. However, it's possible the rating on the ESCs is continuous, and you most likely would not be at stall for long, because the traction of the wheels is typically less than the force that the gearmotor can generate, so the wheels slip against the floor and thus you are no longer at stall. Also, you probably wouldn't keep pushing against an immovable object for too long.

Is this a 3lb. bot? How many wheels? Where is the center of gravity in relation to these wheels? What is the coefficient of friction of the wheel material on the floor surface? What is the wheel diameter? What is the gearing ratio? What is the stall torque of these motors? Answer these questions, and you can figure out if the wheels (and thus the motors) actually stall at all.

Of course, a smart and experienced bot builder like yourself knows all this already, so I guess the question I should be asking is--What am I missing?
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rdubard
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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Wed Oct 05, 2011 7:35 pm

Oh, and I almost forgot--What is the internal resistance of the batteries? See, if the sum of the motor winding resistance and the internal resistance of the batteries at a given current drops the applied voltage to the motor enough, then you are looking at a different voltage to divide by the coil resistance. I know that seems like a circular argument, and it is (sorta), but it can make a real difference in some cases.

However, I would guess it's the wheel slipping thing, combined with the difference between the continuous and momentary current capabilities of the ESC that are making the difference, because modern LiPos have really low internal resistance.
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Alex
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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Wed Oct 05, 2011 8:35 pm

Alright thanks!

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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Thu Oct 06, 2011 7:54 pm

I just read your post more carefully--and I see you were looking at the peak current (not cont.), so that's clearly not it.

But then I wondered, is your info for rudy for one side of the robot? In other words, are the two sparkmotors running together on a single sided ESC, or are you saying the tiny ESC is for both left and right sides of the bot?

Because, if they are on two different output circuits, then it must be that the wheels never really stall, but if they are on the same circuit, then it's possible they are wired in series, giving an effective resistance of 9.232 ohms. At 5s (18.5V), this gives a current of only 2 amps--under the 2.4 peak limit.

yet another possibility.
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Alex
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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Thu Oct 06, 2011 8:11 pm

rdubard wrote:
I just read your post more carefully--and I see you were looking at the peak current (not cont.), so that's clearly not it.

But then I wondered, is your info for rudy for one side of the robot? In other words, are the two sparkmotors running together on a single sided ESC, or are you saying the tiny ESC is for both left and right sides of the bot?

Because, if they are on two different output circuits, then it must be that the wheels never really stall, but if they are on the same circuit, then it's possible they are wired in series, giving an effective resistance of 9.232 ohms. At 5s (18.5V), this gives a current of only 2 amps--under the 2.4 peak limit.

yet another possibility.


From what I read about the robot he uses 1 esc for 2 motors (ie one for left one for right). I was considering stall/peak current (are they the same?)

If they were wired in series then what ever one has less resistance would spin and the other would stop/slow right? So if one wheel gets lifted the out would move the robot? Am i thinking about this the right way?

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zwtipp05



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PostSubject: Re: Calculating Stall Current of Brushed Motors?   Thu Oct 06, 2011 10:29 pm

I run 4 Pololu 10:1 HP motors (2 on each side) in my fleaweight. At 2S, it comes out to 4A stall per side and my esc is rated for 1A continuous and 3A peak, but I haven't had any problems yet.

I think you can be alright using a lower rated ESC sometimes when your wheels are more likely to break traction than stall.
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