(ET) Card #1 vs Reverse

[Larry Chace <RLC1 etnainstruments com>]

The switching logic on card #1 (E15 and E12S) seems rather complex, 
but it was probably the best the GE engineers could do at the time 
(and within the given budget).  I just made some measurements to see 
what the card does in the context of Reverse while moving Forward.

The reverse switch is a simple SPST microswitch that is normally open 
(for Forward) or can be closed (for Reverse).  That on/off signal 
reaches card #1 at its #8 terminal and provides power to a typical 
SCR circuit that, when energized, latches itself into the ON position 
and also energizes the Reverse relay, whose DPDT contacts will then 
reverse the motor field polarity and thereby reverse the motor's 
roation.

To avoid nastiness, there is another part to this circuit.  The 
voltage at the motor armature is fed to card #1 via its terminal #7 
and is routed to two circuits, each of which can *disable* an SCR 
latching circuit.  There are 3 interesting voltage levels.

With 16v or less on the armature, the motor is turning slowly, if at 
all, and the disablement circuits are inactive; both the Start and 
the Reverse switches will operate normally.  In this state, the Start 
and Reverse switches work just as if the motor wasn't turning.  This 
is the normal condition when the tractor starts to move.

At 20v, the Start switch logic is disabled.  I'm not sure why they 
are concerned about this condition.

At 26v or more, the motor is turning quickly, and the disablement 
circuits are acive.  Neither the Start nor the Reverse switches can 
do anything.

So, if the motor is turning slowly, you can switch from foward into 
reverse at will.  If the motor is turning quickly, then you cannot 
switch from forward into reverse.  HOWEVER, you can *always* switch 
from reverse into forward!!!  (Opening the Reverse switch turns off 
power to its SCR and therefore to the Reverse relay, which then 
happily drops back into forward polarity.)

There is Varistor wired across the motor field winding, and its job 
is to short out the inductive kickback that is generated when the 
field is turned off or reversed, so as to protect the reverse relay's 
contacts.  Varistors, however, have a limited lifespan and will 
ultimately wear out, probably shorting and then converting themselves 
into smoke.  The reverse relay contacts are now unprotected from the 
kickback, which can then lead to the dreaded welded contacts, etc.

I'm guessing that the GE engineers felt it was more likely that 
someone would try to use reverse to stop unwanted forward motion than 
that they would try to use forward to stop unwanted reverse motion.

The card does have a few timing circuits using capacitors and 
resistors connected to Unijunction transistors (a GE invention). 
These transistors are like comparators, turning on when their control 
input reaches a certain percentage of the applied voltage.  They in 
turn trigger SCRs (another GE invention) that provide high-current 
switching *and* latch themselves up until their power is turned off.

Larry Chace, I-5 and E15