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

["Christopher Zach" <czach computer org>]

David:

This makes a lot of sense. If there is a series field in there with the
field windings, it would cause the net field to be different in reverse, 
and
when under heavy load in reverse (ie: backing up a steep hill) would cause
the effects of the field weakening circuits to be amplified.

As to why the field is down when I floor it, I'll take a look. Perhaps 
there
is an interlock or relay that is supposed to engage at speed 1A, however
when I go straight to 7a it doesn't have time to lock down before hitting 
5a
(at which point it might not engage since the E20 doesn't power the 
switches
for speeds 6,7,8). That could explain the collapse of the field current, as
well as why it only happens in reverse (the interlock would engage in the
forward speeds because 6,7,8 are active in fwd).

Will check into this tonight.
Chris


----- Original Message -----
From: "David Roden (Akron OH USA)" <roden ald net>
To: <elec-trak cosmos phy tufts edu>
Sent: Monday, February 25, 2002 12:14 PM
Subject: Re: (ET) Reverse draws a lot of current: Why?


> 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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