(ET) Power pulse--Ah!

["Christopher Zach" <czach computer org>]

Ok, now I see the logic behind 2A, and the power pulse:

If you sit on the tractor and start going, both resistors are in series 
with
the armature. When you go to speed 2, one resistor is cut out by closing 
1A.
If you go to speed 3, the second resistor will be cut out if (and only if)
either of the following two conditions are met:

1) The motor is spinning fast enough to warrant speed 2
2) The tractor is not in reverse.

Condition 1 prevents you from going from a dead stop to full power and 
keeps
you from popping wheelies. Condition 2 ensures that reverse has only two
speeds (and can't go as fast as forward).

The power pulse button allows you to deliver full armature voltage from a
stop or in speed 1. Needed if you are under load and can't get the tractor
moving in speed 1 (due to the drop across resistor 2).

The E20 doesn't need this since you go through three resistors to get to
full speed, and by that time the lag in the contactors closing prevents the
wheelie effect. You also have a spring loaded pedal. Reverse on the E20
allows all four speeds, but cuts out the FW circuit (which is a really good
idea since the field is already weakened due to the compound windings in 
the
E20 motors.

Chris