Re: (ET) AC motor theory, dynamic braking, and regeneration

["Steven Naugler" <snaugler earthlink net>]

Larry,
    In an AC circuit, having a low power factor does not theoretically
increase power wasted, it only increases the current needed to deliver that
power.  Where you waste power when power factor is low is due to your I
squared R (resistive) losses.  If your current is needlessly higher, all of
your voltage drops are higher and you have greater unwanted electric
resistance heating.
    If Dan's power factor is 0.8 (80%), a reasonable number for even a 
cheap
inverter at high loads, the current through the AC parts of the circuit 
will
be 1.25  (1 / 0.8 = 1.25) times higher than theoretically needed.  On the 
DC
side of the inverter you will have some DC current with probably some 
really
ugly AC waveform superimposed on top of it.  All of this increases the I
squared R losses, but as they are quite low to start, Dan probably won't 
see
them in his application.  If I is 1.25 times theoretical, I squared is 1.56
times theoretical.  If, for example, Dan's resistive losses were 5% of 
total
power delivered if his power factor were 1 (100%), then with a power factor
of 0.8 (80%) his resistive losses would be 1.56 times higher, or 7.81%  (5 
%
x 1.56 = 7.81%).
    When we in industry are penalized for low power factor it is generally
for three reasons:
1.  We are subsidizing the power company's purchases of larger than
theoretically needed transformers and other power delivery equipment.
2.  Where installed equipment is at maximum capacity and not easily upsized
(all of New York City) they make it very expensive to waste capacity.  If
power factor in their transmission lines is 100% they can sell 25% more
electricity than if their power factor is 80%.
3.  Of the three,this is frequently of the lowest concern.  They don't want
to waste electricity due to I squared R losses.
    One last note to those of you who don't understand power factor.  I 
will
not be giving an unsolicited lecture on power factor.  The GE stuff is DC
where power factor has no significant effect.  My wife already thinks I
spend to much time on this discussion group.  I'll pass for now.

Steve Naugler

-----Original Message-----
From: Larry Elie <lelie ford com>
To: elec-trak discussion list <elec-trak cosmos5 phy tufts edu>; Max Hall
<maxo iname com>
Date: Tuesday, January 12, 1999 10:55 AM
Subject: RE: (ET) AC motor theory, dynamic braking, and regeneration


>Steven Naugler replied to Dan in accurate detail about using AC with an
inverter for a battery vehicle.  Yes, it
>can be done, but I suspect there is one more thing to worry about;
power-factor.  An AC motor is not a
>pure resistive load.  It costs you extra power because of a phase angle.
There are 3 ways to deal with the
>problem:  1.) Ignore it, and have 5 to 15% of your batteries energy go out
without doing any work (actually,
>with most inverters, you already loose that much so you might not care).
2.)  Add a capacitor to delay
>things so the power factor is right under SOME load.  The down side is 
>that
you will be varying the load
>and the power factor some.  This difference may be small.  3.)  Active
power factor (Nola or equiv.) correction.
>These are all doable.  BTW, for CONTROL reasons, the Ranger EV uses a AC
traction motor/generator, and
>yes, you can do regenerative breaking.  The circuit is complex and
patented.  The GE is fun, but you can do
>things a lot better today than when it was designed.
>
>Larry Elie
>
>
>
>