### Author Topic: Calculating current through motors?  (Read 1025 times)

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#### pYr0rAGE

• Beginner
• Posts: 3
##### Calculating current through motors?
« on: February 07, 2012, 10:22:41 PM »
Hello all

I am planning on building a robot centering around this chassis and this controller board. The bot kit comes with 6 motors, each with a stall current of 6.6A. I know that the board only allows 15A of current to go to the motors, but I've heard that it's a bad idea to drive motors to full stall current, anyways.

My question is this: I'm not quite sure how to calculate the required battery size needed to drive the bot around. It seems unreasonable that we'd use the full 15A all the time, but we also won't be using the no-load current of 650mA the whole time either. I figure that the amount of current we'd be using is a function of the bot's weight, but I don't have any idea how to crunch the numbers. Can anyone point me in the right direction?

-pYr0

#### newInRobotics

• Supreme Robot
• Posts: 1,015
• N.I.R.
##### Re: Calculating current through motors?
« Reply #1 on: February 08, 2012, 01:20:36 AM »
I figure that the amount of current we'd be using is a function of the bot's weight, but I don't have any idea how to crunch the numbers. Can anyone point me in the right direction?
Knowing mass of your bot and diameter of it's wheels gives torque required to move it. Find datasheet of motors in question and in there find Torque vs Current chart - here's Your answer.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### zeeshan

• Jr. Member
• Posts: 11
##### Re: Calculating current through motors?
« Reply #2 on: February 08, 2012, 01:31:11 AM »
if my bot is about 7kgs and the wheel diameter is 78mm and the bot has four wheels driven by a chain drive with two motors...how much torque would be required?

#### newInRobotics

• Supreme Robot
• Posts: 1,015
• N.I.R.
##### Re: Calculating current through motors?
« Reply #3 on: February 08, 2012, 02:19:01 AM »
To move the bot each motor has to output atleast 13.65kg-cm torque.

Torque Converter
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### pYr0rAGE

• Beginner
• Posts: 3
##### Re: Calculating current through motors?
« Reply #4 on: February 08, 2012, 08:17:53 AM »
That makes sense, except for the fact that the datasheet for these particular motors do not have a torque vs. current chart, only the no-load and the stall currents. Is there a way to figure it out without the chart?

• Supreme Robot
• Posts: 11,702
##### Re: Calculating current through motors?
« Reply #5 on: February 08, 2012, 10:31:42 AM »
the datasheet for these particular motors do not have a torque vs. current chart, only the no-load and the stall currents.
And that's why the motors are cheap

Quote
Is there a way to figure it out without the chart?
You do a physical experiment . . . or if you don't have the motors, you guess assuming the curve is linear.

You might also find this helpful:
http://www.societyofrobots.com/battery_calculator.shtml