quick question about DC motor stepper motor current draw

[wakoko79]

I have stepper motors for a mobot class and I observed that the faster you make it spin, the lower current it draws (according to the power source reading). I really don't understand why its like that, I mean, you are just changing the switching frequency right? It doesn't make sense to me.

Is this also true for DC motors driven by PWM or is it the other way around? I'm confused if it makes sense since stepper motors are driven by the switching frequency while DC motors can be controlled by pulse widths.

With PWM, you are essentially controlling the DC voltage input to the motor, right? And as I understand it, DC motors (the ones used in robot arm joints, but i think they're the same as regular ones) draw more current if the voltage supplied to it is lower. I think I read that in some paper, it says its trying to maintain the power it is consuming.

By the way, I'm building a replacement driver box for a robot arm, a rather obsolete one (mitsubishi rv-m1). I'm worried that the bridge drivers I use will overheat or something like that and I don't want it blowing up on me.I would like to address that potential problem before actually building the unit.

thanks!^^

 

[alec_t]

The higher the switching frequency the higher will be the reactance of the motor coils, hence the lower the current. Read up about 'inductance'.

 

[wakoko79]

Oh, right... I totally forgot about that. >.<
I think I didn't thought about that since switch time was just about 3ms to 9ms, oh well. But what about DC motors?

 

[alec_t]

DC motors have coils, hence inductance.

 

[wakoko79]

Yes, of course.
I mean, do they really tend to draw more current when the input voltage is lower? The more I think about it, the more it seems absurd that lowering the voltage would cause the DC motor draw more current. Any intuitive explanation? Or is what I just said is plain wrong?

 

[alec_t]

At a given motor speed the higher the input voltage the higher will be the DC motor current. However if, for a given input voltage, the motor is slowed down below its free-running speed the back-emf it generates will be reduced and so the current will increase.