Determining the current draw of a dc motor
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Determining the current draw of a dc motor

by mikeglass on Tue Feb 05, 2013 2:31 pm

I'm new to this stuff so bear with me. I want to run a old dc motor from my motor shield. It seems to run fine but I nave no idea what the specs are on the motor (I think it came from an old scanner) and I would rather not have it not kill my motor shield. How can I measure how much current the motor is drawing? I don't even know what voltage the motor is designed for. It pulled between 9-10 volts when running. I'm using a 12v 500ma wall wart for power. I plan on putting a gear box on it and running it slowly. There are no markings or numbers on the case. It is brushed and about 1 3/8" in diameter and 2 inches long.
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Re: Determining the current draw of a dc motor

by adafruit_support_bill on Tue Feb 05, 2013 2:49 pm

To measure the current that the motor is drawing in normal use, see this tutorial: http://learn.adafruit.com/multimeters/current
To measure the stall current (worst case current draw), just measure the resistance across the motor terminals and apply Ohm's Law. (I = V/R)

For example, if the resistance is 10 ohms and the supply voltage is 12v, the stall current is 12/10A or 1.2A which would toast the L293 chip on your shield. That assumes that your 12v supply is capable of delivering 1.2A. If it can't, the voltage will drop or the supply will shut-down.

The good news is that with your 500mA wall-wart, you probably won't be able to exceed the 600mA rating of the shield. :D
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Re: Determining the current draw of a dc motor

by mikeglass on Wed Feb 06, 2013 1:16 pm

Ok. I measured the current of the motor running with no load and it was 60 ma. I then stalled the motor mometarily with my fingers and it went to 350ma. The resistance of the motor is 12 ohms so the stall current should be 12v/12 ohms = 1 A which would fry my chip.

I'm building a treat dispenser for animals. At some point it will get jammed with food or dirty enough to stall the motor. You mentioned that my power supply would fail before I got near 1 A. What happens to the power supply? Does it die completely? Can I put a fuse in the curcuit to take care of any stalling problem?

Thanks
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Re: Determining the current draw of a dc motor

by adafruit_support_bill on Wed Feb 06, 2013 1:33 pm

It depends on the power supply. Many inexpensive wall-warts will just 'do the best that they can' and the output voltage will sag. If you are measuring a stall current of 350mA, the voltage has probably dropped to under 4v. This probably won't hurt the motor or the shield. But the wall-wart could potentially overheat and become a different kind of hazard. Some have active overload circuitry. Those usually just shutdown when they overload.

A fuse would protect that. But get a "slow-blow" type because the motor startup current is the same as the stall current - if only momentarily. A resettable circuit breaker might make sense if you expect frequent jams.
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Re: Determining the current draw of a dc motor

by dmpyron on Fri Feb 08, 2013 12:20 am

adafruit_support wrote:
A fuse would protect that. But get a "slow-blow" type because the motor startup current is the same as the stall current - if only momentarily. A resettable circuit breaker might make sense if you expect frequent jams.


Drop by a good auto parts place. You'll find almost any value of fuses in either standard or slo-blo. And plenty of cb's, too. Getting the mounts might be a little more problematic, but they're findable.
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