rdubard mega contributor
People Skills : 3403 Registration date : 20090610 Age : 52 Location : Ransom Everglades, Miami, FL
 Subject: 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 isWhat am I missing?


rdubard mega contributor
People Skills : 3403 Registration date : 20090610 Age : 52 Location : Ransom Everglades, Miami, FL
 Subject: Re: Calculating Stall Current of Brushed Motors? Wed Oct 05, 2011 7:35 pm  
 Oh, and I almost forgotWhat 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.


rdubard mega contributor
People Skills : 3403 Registration date : 20090610 Age : 52 Location : Ransom Everglades, Miami, FL
 Subject: Re: Calculating Stall Current of Brushed Motors? Thu Oct 06, 2011 7:54 pm  
 I just read your post more carefullyand 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 ampsunder the 2.4 peak limit.
yet another possibility. 

zwtipp05
People Skills : 2558 Registration date : 20100521
 Subject: 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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 Subject: Re: Calculating Stall Current of Brushed Motors?  
 
