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At 02:30 PM 3/16/01 -0500, Bob Sciamanda wrote:is
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,
a very serviceable model for a motor.predictions?
-- Can anybody offer a simpler model that makes comparably good
-- Can anybody offer a comparably simple model that makes betterpredictions?