torque and electric motors (slightly OT)

let me try an explanation - perhaps this will clarify. suppose I have a box with a shaft coming out of it. I hook up a 12V battery (DC) and the shaft starts to turn. Is the box a DC motor?

OK, the right answer could be almost anything - I would say "it might be".

So now, let's open the box and look inside.

Inside is some electronic circuit (let's not worry about how it works - trust me on this one, it's not rocket science) that takes DC power and makes

Now, let's look at the question again - is the box a DC motor? Do you see why I said "it might be" - in this case, haveing looked inside, I'd now say "no, it is not". What would you say?

hope this helps snip-------

may I respectfuly disagree with George -

If "it" has a constant polarity - tells you nothing about the motor technology.

The motor in the disk drive in your computer, and in the "DC" fan in your computer both have permanent magnet armatures (if you don't believe me, take one apart). The armatures are drive by a rotating magnetic field created by a set of coils driven by a speicial purpose IC, with feedback generated by a hall effect device. These motors are polyphase AC motors. You can put DC to the coils of these motors all you want and they won't rotate.

The motor that moves the head in your disk drive in your computer has a (very strong) samarium cobalt magnet or two - the motor is a "voice coil" motor, a true DC motor, used with an analog current source, feedback is from a head position signal read from the surface of the disk (at least on a modern disk drive)

The window motors in your car have permanent magnet stators, and are brush motors with the windings in the armature - these are true DC motors in the classic sense. This is perhaps the type of motor to which George was referring

I guess I was being obtuse. It's the max torque at 0 rpm that befuddles me. Hence my quip about being more productive when sleeping in. Zero motion, max output. Or something...

...Kevin