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

[Larry Elie <lelie ford com>]

Steve wrote;

----------
From:   Steven Naugler[SMTP:snaugler earthlink net]
Sent:   Tuesday, January 12, 1999 9:27 PM
To:     Larry Elie; elec-trak discussion list; Max Hall
Subject:        Re: (ET) AC motor theory, dynamic braking, and regeneration

;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

Actually Steve, almost ALL inverters have really ugly waveforms....

;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%).

I think I said 5 to 15%, which you quoted below.  The reason I gave a 
range is that 
the inverter waveform may look more like a square-wave than a sine-wave, 
depending 
on how well it was made.  Yes, they can indeed be that bad.  I'm not sure 
how to do 
the calculation without looking at both the current and voltage waveforms.

;    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

Sure, DC is negledgeable.  The WORST AC case is not the inductive loads, 
but the 
capacitve ones (large switching power supplies), at least as far as the 
ones I have 
seen.  You can't add a cheap capacitor to re-set the phase; you have to do 
active 
correction in order to make a power supply that will work on normal 
circuits.

-----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
>
>
>
>