Re: (ET) E12S and a curtis controller??

["David Roden (Akron OH USA)" <roden ald net>]

On 27 Jun 2003 at 17:51, Steve Richardson wrote:

> The Alltrax controller automatically adjust the field strength to keep it
> within a percentage of the of the armature current.

I take it that means it will ^not^ perform field weakening based on pedal 
position. Too bad. Our GE motor was designed to use field weakening 
(though 
obviously it doesn't require it).  I'd like to retain FW on mine. Others 
may 
feel differently, of course.


> We believe the tractors will probably see an increase in top ground speed
> because of the efficiency of the controller over the GE control 
> system.  

With the GE, top speed (before field weakening) is with the battery 
connected directly to the motor.  I don't see how you can get any more 
efficient than that!  Semiconductors always have a forward voltage drop.  
Unless you have a bypass contactor, I think you'll find your non-FW top 
speed is slightly ^lower^ than the original speed (assuming 36 volts).  
Not 
that it would be a big deal.


>  Another option, one I'm going to try, is use a 48v battery pack ...

Mark Hanson did that with his homebrew ET controller.  He just dropped in 
some 8 volt golf car batteries.  He didn't use FW either, and said the 
higher voltage just about made up for the loss of top speed.

There are some problems with using 8v batteries though.  

First, you'd probably need some way to derive 36 volts for the mower deck 
and other accessories.   Mark built a separate controller just for that 
purpose.  He's a pretty smart engineer, so if he felt it was necessary I'm 
not about to try running my mower deck on 48 bolts.

Second, you'd also need a DC:DC to run the lights -- 8v is too low, 16v is 
too high.  So figure more $$$ for that, though it's a good idea anyway.  
You 
might be able to get away with just a simple regulator circuit (unless 
you're worried about wasting the energy as heat).  

Third, you'd need a different charger.  Again perhaps a good thing as the 
original is kind of brutish -- but I suspect few users will be very 
interested in this kind of investment.

I'm sure the lift will work fine on 16 volts, so no problem there.

An alternative to the 8v golf car batteries is to use four smaller 12 
marine 
batteries instead of 6 golf car batteries.  Then you could retain pack 
tapping (but still need a new charger).  Downside is that you'd lose cycle 
life with those batteries.  Range also might then become a problem, 
depending on the user's needs.

Maybe you could configure the controller as a buck/boost type.  Then the 
user could use 36 volts input, and get 0 to 48 volts at the motor.  I 
don't 
know how much this might add to the cost.


> The field weakening
> method used on the Elec-Trac's is to insert resistance in series with the
> field winding which turns a lot of energy into heat. A modern controller 
> uses
> PWM to create varying voltages for the field, creating very little heat.

The field current is fairly low, so IMO the amount of loss from the ET's 
resistive field control is trivial.  The loss from the resistive armature 
control is much, much more significant.  

But even at that, I hear relatively few complaints about limited range or 
inefficiency on this list.  I may be mistaken, but I think most 
experienced 
ET users know enough to pick a gear that allows them to run the tractor 
most 
of the time with the armature resistors out of the circuit.

The main reason I want a "real" controller is to get fine control without 
having to shift into LL.  Range isn't an issue for me.  The stock ET has 
plenty of range for my needs, even with lower battery capacity than was 
standard.  I currently use East Penn 8G24 group 24 gel batteries (6 in 
series-parallel), and I've never come close to running them flat.  But, 
again, others might benefit from increased range.


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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