Why do motors draw more current when stalled?

[mstacho]

One of those "just out of curiosity" things: do motors draw more current when they're stalled than when they're moving?  If that's true, how is it possible?  A motor has a set resistance, and if V = IR, and I'm not changing the voltage, current should also be constant whether it's under load or not.

Unless the resistance changes somehow?  I'm assuming the motor is DC, just because AC electronics math would almost make sense as to why the current draw can change (unless of course a DC motor still draws like an AC motor while moving?)  OOh, science :-P

MIKE

[omkar]

check out this link
http://robots.freehostia.com/Motors/Motors.html

[waltr]

Motors use coils which have a Reactance and not a pure Resistance when the Current is changing. This is why Ohm's Law does work as a running motor is not a DC circuit.

The Stall condition is the DC condition and will follow Ohm's Law. Measure the DC resistance of the motor and multiplying by the Stall current will get close to the applied voltage.
Its when the motor is running that the Current decreases due to the factors in the link omkar posted. Kinda backwards from the way you were thinking.

[mstacho]

Yes, you're right, I did indeed have it backwards.  It's not that the motor doesn't follow Ohm's law when stalled, it's the opposite: it doesn't follow the law while moving.

Neat.  Thanks to you both!

MIKE

[FOIWATER]

no, they follow ohm's law at all times.

The motor armature does not ever draw a different amount of current for a same voltage.. you had it right

BUT...  the counter EMF is what you aren't remembering.  Every motor experiences generator action as it turns, this is what we call CEMF.   and it's overall effect is to limit the armature current, because it is induced at opposite polarity with respect to the source voltage..

This is why motors draw more current at starting... and at stall... because the armature is not turning, and very little CEMF is induced, therefore less limitation to your source current.

[rbtying]

no, they follow ohm's law at all times.

The motor armature does not ever draw a different amount of current for a same voltage.. you had it right

BUT...  the counter EMF is what you aren't remembering.  Every motor experiences generator action as it turns, this is what we call CEMF.   and it's overall effect is to limit the armature current, because it is induced at opposite polarity with respect to the source voltage..

This is why motors draw more current at starting... and at stall... because the armature is not turning, and very little CEMF is induced, therefore less limitation to your source current.

That's not quite true--the commutator of the motor converts the DC input into AC as it goes into the coils, such that the polarity of the coils switch (and thereby move the rotor). Since the direction of current in the coils changes, the reactance needs to be taken into account, which Ohm's law doesn't.

Ohm's law is only really valid for resistive (or close to resistive) loads--anything with a changing current tends to be non-ohmic. Once you get more complex, the only thing Ohm's law shows is the constitutive equation of a resistor (V = IR). The rest of the circuit needs to be modeled by taking the other components' respective equations into account.