Or were you saying that the tractor suddenly has a lot less power in reverse?
>>> "daveb" <daveb seanet com> 3/10/2004 9:58:05 PM >>>
so it's not a problem but a feature! :^)
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---------- Original Message -----------
From: Chris Zach <czach computer org>
To: Joanne Hodges <sqfarm msn com>
Cc: Elec-trak cosmos phy tufts edu
Sent: Wed, 10 Mar 2004 21:01:45 -0500
Subject: Re: (ET) Reduced power in reverse
> > Can anyone tell me why I have drastically reduced power in reverse?
>
> Sure.
>
> The motor in an E20 is not really a pure shunt wound motor. In addition
> to the shunt field there is a small field that is in series with the
> main motor. This apparently is needed to help the motor provide constant
> power under heavy load (amps go up in armature, field current gets
> stronger due to the increased small series field). Good idea.
>
> However when you throw an E-20 in reverse, you're reversing the armature
> power but *not* the field coils. And along with the armature
> switching it's direction, the little series field changes as well.
> And the little series field is now opposite of the main field and is
> making the overall motor field a bit *weaker*
>
> You can see this by putting the tractor in Neutral and spinning up
> the motor. The motor will sound faster in reverse than in forward
> under no load. This is because the field is slightly weakened.
>
> Now: The real fun comes when you apply a load to the motor in
> reverse. Instead of getting more power, the field gets weaker
> because the small series field is drawing more current along with
> the armature opposite the main field. Thus the field weakens and
> power keeps going *down*. Thus your tractor motor pulls a *lot* of
> current without much power going to the wheels. Bad thing.
>
> This also explains why you only have 3 speeds in reverse: You really
> don't want to even think about weakening the main field further; the
> motor would melt down. So the E-20 control system locks out field
> weakening while in reverse.
>
> Honestly, it's not really a design problem as it is a simple matter
> of a design trade-off. You really want the thing optimized for
> forward power, so you put all the effort into that. Reverse is more
> for cosmetic purposes and to get you out of a jam.
>
> Chris
>
> >
> >
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