> 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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