DC motor current requirements

[Joe]

I am trying to find a motor controller for a motor for which I have no spec sheets, most importantly stall current. The current it draws seems to be extremely high, way too high to measure with an ammeter or supply with a bench supply. So, would I be correct to think measuring the resistance between the motor leads and dividing the voltage by that resistance will give me the maximum current the motor will draw? (eg 7.2V/0.5Ω = 14.4A) Actually, there will be 2 motors in electrical parallel working together against the same mechanical load. Does this mean that current draw will be doubled? (eg 18.8A)

[SomeSaba]

Hi, i am noob but i will try to answer parts of ur question

yes the current draw will double to 28.8Amps

but i dont know the validity of the method u used to find the current.

This is a random side question: wont the motor heat up as current runs through it? and if i remember correctly temperature is factored into resistance.. so wouldnt that change everything 

[Tsukubadaisei]

Your method will not work. I am almost sure you will get a very low resistance (if it is a conventional DC motor). When the motor is actually running the resistance will varies due to the movement of the magnets inside the motor, it will produce a current in the opposite direction reducing the total current. This phenomenon is called Lenz's Law.
For more info:
http://en.wikipedia.org/wiki/Lenz's_law
or a good physics book(recommended).

[bens]

Lenz's law wouldn't apply to the stall current, though, which seems to be the worst-case current draw Joe is interested in.  I see no reason why Joe's calculation wouldn't be valid for stall current.

[Admin]

So, would I be correct to think measuring the resistance between the motor leads and dividing the voltage by that resistance will give me the maximum current the motor will draw?

No this will not work. The reason is because your motor needs to be powered when doing this measurement, which means when you do a measurement you are not just putting leads across the motor, but also across the power source.

You are in effect measuring the parallel resistance of the motor AND power source, which is not what you want.

At 7.2V I wouldn't expect more than about 4A draw (per motor). You wouldn't want to run your motors at stall, anyway. Have you measured the zero load current draw?

[4u_allie]

 Okay, so I have this Voltage DC motor. But its from a remote control car toy. Yeah, I did take apart my brothers old toy for it, to find one! They're hard to find! Does anybody know if a Voltage DC motor can work for Robot? (I actually just barly found another DC in the toy car as I'm typeing this. Anybody tried using these for their Robot. Did it work?

[bens]

No this will not work. The reason is because your motor needs to be powered when doing this measurement, which means when you do a measurement you are not just putting leads across the motor, but also across the power source.

I'm not sure I understand.  Why does the motor need to be powered to measure its stationary resistance?  When a DC motor isn't moving (i.e. stalled), it's basically just a large coil of wires.  If I measure the resistance (R) of that coil and then put V volts across it, I can expect to have a current of V/R pass through that coil of wires so long as I hold it stalled, right?  Sure, you typically don't want to run your motors to stall, but stall current can still be a property of your motor that's important to know.

[Admin]

Why does the motor need to be powered to measure its stationary resistance?

Hmmmm I'm not sure actually, does the existence of a magnetic field create/change resistance in the motor coils? I don't remember my physics . . .

I think you are right, that resistance for an unpowered motor should be equal to the resistance of a stalled motor, with the assumption the inductive field isn't changing . . .

If I measure the resistance (R) of that coil and then put V volts across it, I can expect to have a current of V/R pass through that coil of wires so long as I hold it stalled, right?

Hmmm depends on what the power source is doing to your multimeter signal too, since its in parallel with your motor . . . you are in effect also including the resistance of the power source in your measurement.

stall current can still be a property of your motor that's important to know.

Yea, I usually rate my motor drivers around stall current (or a little above).

[Joe]

At 7.2V I wouldn't expect more than about 4A draw (per motor). You wouldn't want to run your motors at stall, anyway. Have you measured the zero load current draw?

I do not have access to an ammeter that will measure much more than an amp. Also, the only time I would want them to draw anywhere near the stall current is during the initial acceleration, since that is mathematically when the most force would be required, thus power, i think. My robot is quite a bit heavier than the RC car that it's built on. If the extra mass requires too much current to accelerate, I guess I'll have to get some new motors (and base). As for getting hot, these particular motors don't seem to get very hot when moving at low speed. Anyway, I think I'm going to get one of those Parallax HB-25s. Thanks for all the input and I will check out Lenz's law.

[bulkhead]

I believe you are supposed to measure the resistance of the motor with the shaft at various angles and take the average resistance of the motor.  Then divide V by that average resistance to get current (at stall).

Resistance increases with temperature, so if V is constant, then I (current) will decrease when the motor warms up.  Temperature in this case really only matters if the motor will be running in a colder environment than the one where the resistance measurements are done.

Lenz's law shouldn't affect stall torque because (theoretically) the motor is stalled.  It only affects the motor when it is at speed, in which case current draw would best be obtained by experimentation.

[Tsukubadaisei]

Lenz's law wouldn't apply to the stall current, though, which seems to be the worst-case current draw Joe is interested in.  I see no reason why Joe's calculation wouldn't be valid for stall current.

I see. Actually I thought that stall current meaned the current without load. I am not an English native speaker. (the only stall I know is for aeronautics)
And if "stall current" is the current when the motor cant rotate(if the motor is stopped the magnets cant move, no variable magnetic field, no lenz law, the resistance of the unpowered motor is the same of the resistance of a stalled motor), then the calculation will be correct. Just be careful not to burn the motor.

[Joe]

Awsome! Thanks again Society!