Re: emf, potential, voltage

[Bob Sciamanda <trebor@VELOCITY.NET>]

Points of information:

1) Is the motor a linear device?  If I apply various voltages across the
motor and plot V vs I will I get a straight line?  For instance, does not
the motor type matter (eg series or shunt field winding)?

2) An unloaded motor is of little use.  Is not the mechanical load a
parameter which will surely affect the V/I curve?

Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, March 16, 2001 07:27 AM
Subject: Re: emf, potential, voltage


> 4) Suppose I have a motor, and I apply 115V(AC RMS) to its terminals.
Then
> the average emf across its terminals is 115V.  That tells me the emf,
not
> the back emf.  If I measure the current under these conditions, that
gives
> me one point on the I-V curve.  Call this point (V1,I1).  One point is
not
> enough;  I need another point in order to pin down the first-order
> properties of this device.
>
> To get a second data point, I briefly open-circuit the terminals.  The
> still-spinning motor will act like a generator.  It will produce a
certain
> voltage (i.e. emf) on its terminals.  This is the open-circuit
> voltage.  Call this point (V0,I0) where I0=0 and V0 is called the "back
emf".
> Given these two points, (V1,I1) and (V0,I0), we can draw a straight line
> and have a nice first-order model of the behavior of the device.