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

[Daystar Energy Services <daniel laser net>]

Steve, 

Thank you for all your time and effort. Very informative stuff. If you
find yourself in another generous mood, I would like to know how these
two "lectures" differ from series wound motors.

In terms of regen braking, I wouldn't think that this would be practical
on the mower side of things anyway, as opposed to regen from the
traction motor traveling downhill. My concern is simply to stop the
spinning blades as quickly as possible. I have a circ saw with this
feature and I find it emotionally comforting (I sleep so much better
now).

I'm sure that you are right about my inverter not being able to take a
back feed through the ac out. Battery charging is done through a
separate set of terminals and the literature is very specific about not
connecting ac out to ac in. Do the over the counter ac braking devices
necessarily back feed the line? Do they have an amp draw when the motor
is at rest? Or effect the running motor's efficiency? Do they have a
name? I assume they are sized, and priced, for specific loads.

As far as 1500w being insufficient, you, and apparently quite a few
others, may be right. However, I see it as worth a try. Here's why:

1) They laughed at Robert Fulton.

2) They scoffed at Orville and Wilber.

3) They told Evel Knevel that he couldn't jump the Snake River Canyon
(He couldn't, but he made a lot of money anyway).

4) The inverter is a done deal, installed and operational. An ac motor
of whatever size will be cheaper, smaller and easier than an equivalent
dc motor and wiring and fuseing set up. If the inverter is too small, I
can then go dc.

>    Here is why I don't think that it is big enough.  1.5 kW x 0.85 % 
>efficiency x 1.341 hp/kW = 1.71 hp.  Multiply by 3 to get gasoline 
>horsepower and you get 5.13 hp.  When 22 inch
>push mowers have 3.5 hp, you'd at best get enough power for a 32 inch 
>deck.  And that's assuming a 100% duty cycle at 100% power for that 
>inverter.  Also, 5-7.5 hp 120 VAC single
>phase AC motors are somewhat hard to find.

Trace claims 94% maximum efficiency however that is dc to ac efficiency.
It puts out the full 1500w continuosly with a 4800w surge capablity. I
dont accept electric motor to gas engine hp comparisons, but using  a 6
hp engine and the same ratio I get a 38" deck. Fine. And no, I dont need
a 5-7.5hp 120 vac motor (you're mixing electric and ICE ratings), I need
a 2 hp 120vac motor, 12.5 amp. From a circ saw or electric chain saw.

Look, I'm not trying to be argumentative. And I'm preparing myself for
absolute failure. Maybe there are efficiencies to be had with better
bearings, larger blades (to store rotational momentum), higher blade
speeds. Maybe not. Anybody else have some real world ammeter readings
for your mower blades? 

Larry Elie wrote:

>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.

Just so we are clear, my traction motor is a series wound 24v, 3 hp
driven by a curtis 1204, 275amp controller. I have no misconceptions
about harvesting regen from either traction or mower motors. I simply
want the mower blades to stop when I say stop. 

I knew there was a monster called power factor but was afraid to ask.
Anyone want to attempt the power factor 101 lecture?

Thanks again for all of the input. 

Dan Lunceford