Re: (ET) Reverse draws a lot of current: Why?

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

On 25 Feb 2002 at 6:48, KevinC927 aol com wrote:

> My E20, which I suspect of many problems and confirm at least one, has
> always shown max I meter deflection when backing uphill. 

This rang a little bell in my head.  (I wondered what was causing those 
headaches.  <g>)

I went back and reread CZ's original post:

> If I am in reverse and floor it from a dead stop, current goes
> off the scale, and the tractor moves *forward* slowly. Go reverse 
> and step through the speeds and the tractor goes backwards just fine.

This would suggest that when Chris "floors" it, there is ~no~ series 
field current at that moment.  It also reinforced my memory of the E20 
motor design.

The E20 motor (and possibly other tractors', I don't know) is actually a 
compound-wound motor.  The series field is set up so that it stabilizes 
the tractor's speed under heavy load.  It also would have the effect of 
moderating the motor braking (regeneration) effect, which might be 
desirable.

The series field is internally wired, rather than being brought out to 
external terminals.  Thus it is not reversed when the motor is reversed.  
So, its effects will be reversed when the tractor is in reverse -- less 
torque under heavy load, and probably more regen.

This could be the reason that an E20 would have less torque and higher 
current for a given speed in reverse than in forward.  How about some of 
the other E20 owners?  Have you noticed this effect?  Is it normal and 
typical?  Is yours as dramatic an effect as Chris's and Kevin's?

Now, about "flooring" it.  If there is no shunt field current, the series 
field will provide enough field gauss strength to move the tractor 
forward slowly.  It goes forward because the series field has not been 
reversed, and the shunt field is not up to power.

But with such a weak field, armature current will be dangerously high.  
And if the tractor is in neutral, there is a danger of the motor 
overspeeding itself.  The reason it goes slowly when in gear is that 
torque is very low.  But with no load, a series motor can easily rev 
itself to death, and it's ever more likely to happen with a weakened 
field.

I do wonder if Chris's controls are working right.  There should be no 
way to apply armature current before applying field current, even (and 
especially) if you "floor" it. But since I don't have an E20, I have no 
idea whether GE actually designed in such a safeguard.


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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