Re: (ET) Reduced power in reverse

["daveb" <daveb seanet com>]

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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------- End of Original Message -------