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

["Christopher Zach" <czach computer org>]

Hm. This is quite interesting...

I decided to crack open the Elec Trak Owner's manuals and see what they
said. And you're right: According to them, the E15 does reverse the field
current via REV and REV'.

Actually, the E15's design is quite unique: The E12 and E20 both rely
exclusively on the relays to switch power to the interlocks. The interlocks
handle all the high-current switching and provide timing control by their
natural mechanical latency. The only task the "electronic" card does is to
bypass the field weakening resistors when the shunt current exceeds a
threshold (thus keeping the motor from overloading).

The E15 on the other hand appears to only have solenoids for speeds one and
two (bypassing the nichrome resistors to the armature) and the L master
interlock solenoid. Everything else is done by the electronics in card 1.
This removes the four solenoids and interlocks needed to switch the
high-current armature with a reversing relay for the rather low current
field as well as three relays for interlocking and cruise control.

This also explains to me why the E15's max torque is in speed #3, while the
E20's max torque is speed #4 (E20 has three levels of resistance and three
solenoids instead of the two on the E15). Also explains why E15 people
complain of blown relay contacts when reversing on the fly; on the E20 that
sort of hit would blow up one of the solenoids, not the reverse relay 
(which
simply triggers the solenoids to fire)

Interesting. I wonder if the E15 was the last model to come out from GE.
This design was probably a lot cheaper to make, requiring less high current
wire and less expensive parts. Any thoughts on this?

Side note: The E8 and E10 had PM magnets for the field. Thus none of the
above really applies.

But anyway, there doesn't appear to be anything in the reversing circuit on
an E20 that involves the field circuits (with the exeption of a bypass
around the FW resistors in reverse). Thus even if the reverse relay failed,
the only impact is that I would have speeds 5-8 while running in reverse. 
So
it's got to be something else... I can always hook a VOM to the field
connections on the motor and see if it's fully energized volt-wise in FW1
and REV1.

Maybe motors like to run in a certain direction with polarity matching in
the field and the armature? And when you reverse the armature but keep the
field the same the motor runs differently? If you flip the armature and the
field, does the motor continue to run backwards?

Anyone with an E20 want to try driving in reverse up a hill at full speed?
See if the ammeter buries itself, and see if the tractor moves faster in
reverse than forward.

Chris

----- Original Message -----
From: "David Roden (Akron OH USA)" <roden ald net>
To: <elec-trak cosmos phy tufts edu>
Sent: Saturday, February 23, 2002 8:13 PM
Subject: Re: (ET) Reverse draws a lot of current: Why?


> On 23 Feb 2002 at 18:51, Christopher Zach wrote:
>
> > Actually, I checked the wiring schematic, and don't think the reversing
> > relay is the problem.
> >
> > On an E20, field current is not reversed when you reverse the tractor.
>
> I have an E15, not E20.  On mine reverse is accomplished by reversing the
> field.  I believe that some E20s reversed the field also, despite what
> the diagram says; not sure.  Check your own tractor.
>
> Regardless, the symptoms you describe -- high current and high speed with
> little torque -- strongly suggest low or no field current.  They are
> occuring in reverse only.  So, if it's not the reverse relay, and I still
> suspect that it is, you'll have to trace things out to discover what is
> causing low/no field current in reverse.
>
> Perhaps the reverse relay operates, but doesn't actually reverse the
> field?  Odd, but possible ...