Re: emf, potential, voltage

["John S. Denker" <jsd@MONMOUTH.COM>]

At 02:30 PM 3/16/01 -0500, Bob Sciamanda wrote:

> Is the motor a linear device?

To first order, it's linear :-)

> If I apply various voltages across the
> motor and plot V vs I will I get a straight line?

There is an OTBE issue here (Other Things Being Equal).

--- If you hold the rotation rate constant, you should get a reasonably
decent approximation of a straight line.  See below for more on this.

--- If you don't hold the rotation speed constant, you can get horribly
complicated results.  For instance, if you have a stalled motor, applying
additional voltage may get it unstalled, resulting in *less* current.

>   For instance, does not
> the motor type matter (eg series or shunt field winding)?

Why should it?

> An unloaded motor is of little use.

Agreed.

>   Is not the mechanical load a
> parameter which will surely affect the V/I curve?

Yes!  That's exactly the point.

Given a point (V1, I1) on the I-V curve, it is *much* smarter to model it
as a back emf of V0 and an impedance of
        Z := (V1-V0)/I1                 (equation 1)
        I1 = (V1-V0)/Z                  (equation 2)
rather than as no back emf and a larger impedance
        V1/I1

In particular, changing the rotation speed will change the back emf in
equation 2, leaving Z unchanged to a good approximation.

Bottom line:  equation 2, with a back emf that depends on rotation rate, is
a very serviceable model for a motor.

 -- Can anybody offer a simpler model that makes comparably good predictions?
 -- Can anybody offer a comparably simple model that makes better predictions?