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Re: emf, potential, voltage



Google turned up this URL
http://www.engin.umich.edu/group/ctm/examples/motor/motor.html

I'm just beginning to digest it, and right away I don't see the
justification for the last statement in this quote:

"The motor torque, T, is related to the armature current, i, by a constant
factor Kt. The back emf, e, is related to the rotational velocity by the
following equations:

Torque = Kt *Armature Current,
Back emf = Ke* Rotational Velocity

In SI units (which we will use), Kt (armature constant) is equal to Ke
(motor constant). "

Should this be obvious? Can someone explain?


Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, March 16, 2001 8:18 PM
Subject: Re: emf, potential, voltage


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?