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Re: (ET) Alltrax install!

At 08:22 PM 2/22/2006 -0500, David Roden (Akron OH USA) wrote:

On 22 Feb 2006 at 19:56, Robert Adsett wrote:

> At 04:43 PM 2/22/2006 -0800, Rob Brockway wrote:
> >I did not think the unit was truelly regen capable.  Does anyone know if
> >it really is ?

I specifically asked Steve about this.  The Alltrax ET controller does NOT
provide regenerative braking.  It does have plug braking which is activated
when power is removed from the enable input.
That seems very odd. It takes some extra effort to prevent regen with a separately excited setup. 


For those who don't know what plug braking is, it's similar to dynamic
braking, but all the kinetic energy is dissipated in the motor itself instead of 
in an external resistor.  If you have an electric push mower, most likely 
it
uses plug braking to stop the blade immediately when you shut the mower 
off.
Umm we apparently are working with different definitions of plug braking. In my experience it has always referred to reversing the motor under load. See for example 

http://www.toyotaforklift.com/about_us/product_focus/technologicallysuperior.aspx

or

http://www.repp.org/discussion/ev/200201/msg00717.html
I can't imagine why you would even want to do that with a separately excited motor except at near zero speed. You are going to need a very weak field to keep the torque low enough not to cause problems.
I rather expect the electric push mower uses either regen braking or an electrically released mechanical brake. Generally the circuitry required to reverse the motor is too expensive for that application.
I don't know for sure why Alltrax did it this way. I could say that it might be 
because that prevents battery damage from trying to regen into a fully-
charged battery, but that would be pure speculation.
Unless you are planning on starting at the top of a mountain and braking all the way down this doesn't make sense. The energy you are putting in the battery comes from the battery in the first place, add in the inevitable losses and...
Overvoltage can be a concern in heavy braking on a (nearly) full battery but that can be taken care fairly straightforwardly. Actually the only really somewhat dicey situation is regenning into an open (such as into a blown power fuse) and since any over speed is going to put you in regen immediately (milliseconds at most) you have to deal with that case anyway. The only way to prevent regen on overspeed is to weaken the field and that will take longer than the motor response time. 

Robert
" 'Freedom' has no meaning of itself. There are always restrictions, be they legal, genetic, or physical. If you don't believe me, try to chew a radio signal. " -- Kelvin Throop, III 
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