Re: (ET) moving armature sense from 6 to 8...

[Larry Chace <RLC1 etnainstruments com>]

Chris asked about the use of wire 6 versus wire 8 as the feed into 
terminal 7 on card 1 of an E15.  (Whew!)

I've done a bit of experimenting and would like to provide the 
following as my best guess about some of the secrets of card 1.

The purpose of the input at terminal 7 appears to be to prevent 
swithing from "off" to electric speed #1 when the motor is still 
turning ("fast enough").  It also prevents switching from forward 
into reverse when the motor is still turning.  In both cases, the 
tractor may be moving or stationary.

Both of these events will occur when the 1A and 2A contactors are 
open (and toaster elements R1 and R2 are in series with the motor 
armature); in addition, the L contactor will be open, so that the 
only voltage appearing at either wire (6 or 8) will be the result of 
generator action as the motor is being turned by its own momentum or 
by the tractor's momentum, with the armature turning through the 
remaining magnetic field from the field coi.

I measured the voltage at wire 8 when the motor was turned off and 
allowed to coast, the tractor being stationary. Using a digital 
storage oscilloscope, I saw the voltage on wire 8 drop from about 36v 
to 0v over a period of about 1 second.  I did not measure the voltage 
on wire 6, but it would clearly be somewhat less than the voltage on 
wire 8.  However, the bulk of the voltage drop will be across the 15 
or so ohms of the field coil; the toaster elements will have a very 
much smaller voltage drop.

Some of the GE wiring diagrams show wire 6 connecting to card 1 
terminal 7, while others show wire 8 connecting there.  Perhaps 
(perhaps!) it really makes little difference.

Regarding the reverse relay, as long as there is "enough" voltage on 
terminal 7,  transistor Q105 will serve to disable any attempt to go 
from forward to reverse.  The reverse switch will feed 36v to SCR106, 
but its gate voltage will be held at "0v" by Q105.  When terminal 7 
has dropped low enough, Q105 will turn off and the SCR106 will fire, 
latching itself on and energizing the reverse relay.

That relay will remain energized no matter what the motor is doing 
until the reverse switch is opened (for forward motion).  At that 
point, no matter what, the reverse relay will drop out, switching the 
field polarity.  That would seem dangerous if the motor is turning 
fast and acting as a generator feeding the field coil *in the 
previous polarity direction*.

Capacitor C106, 20uf, is connected such that it will keep the relay 
energized even after the reverse switch opens, but only for a few 
milliseconds.  A much bigger capacitor would be needed to keep the 
relay engaged "long enough" for the  motor to slow down and stop 
being a generator.  Something on the order of 4000 uf might do the 
trick (for some values of "might").

It seems strange that GE would go to the effort to avoid switching 
from forward to reverse while moving but would not provide a similar 
protection for switching from reverse to forward.

These tests were in the context of examining the effect of letting up 
on the speed control on the I-5 (CA version), where there is no 
dynamic braking effect as there is on the E15.  The I-5 coasts for 
about twice as long, and on steep downhills will gain speed, 
something that the E15 is much less likely to do.

From a few tests, it seems that adding a single diode from wire 8 to 
the "top" of the field coil will give the I-5 (CA) a practical 
dynamic braking effect.  (This applies to tractors that use the 
two-pole contactors R and L contactors for reversing the armature 
polarity.)

"Believe at your own risk..."

Larry Chace, Ithaca, NY  I-5 and E15