Direction Interlock

k. Drive Power Circuit. The drive motor operates

speed and therefore prevents excessive surge currents.

With the accelerator pedal at normal (up), the speed

when the power switch assembly transistors are turned

interlock switch contacts are held actuated as shown in

on by the pulses from the control unit. The direction of

the schematic diagram. This permits normal operation.

motor operation is determined by the direction of current

With the accelerator pedal depressed, the contacts

in the drive motor field winding. The schematic diagram

become normalled and the operate path to the control

shows one transistor circuit of a single power switch

relay is open.

module. Each module contains two circuits. There are

h. Direction Interlock.

The direction interlock

twelve such modules in the power switch assembly,

contacts on the forward and reverse contactors prevent

making a total of twenty-four circuits.

These are

both contactors from being energized at the same time.

connected in parallel in order to handle the high current

Assuming an initial forward direction and a switch to

required by the drive motor.

I. Capacitor and Diode Assembly.

reverse, the collapsing field of the F coil creates a

current through the coil and its parallel diode, keeping

(1) During the off time of the power switch

the contactor energized momentarily. The F interlock

assembly, the collapsing magnetic field of the drive

contacts keep the circuit to the R contactor open at this

motor field winding tends to induce a current through the

time. When current has diminished sufficiently in the F

motor armature in the proper direction. Diode DI acts as

coil, the contactor releases, allowing the path to be

a free-wheeling diode to pass this current. This keeps

completed to the R contactor.

average drive motor current at a higher level than

i. Key Switch Bypass.

The key switch is

average battery current.

connected in parallel with two pairs of normally open

(2) When the power switch assembly cuts off

direction selector contacts. As long as a direction is

after each drive pulse, there is a brief period before

selected and the accelerator pedal is depressed, this

diode D1 can sustain the motor conduction. During this

prevents the key switch from being used to turn off

period diode D3 conducts, charging the 1000 microfarad

power. If the key switch is turned off, power is not

capacitors. The capacitors discharge through the 0.33

removed until the directional control lever is moved to the

ohm limiting resistor each time the power switch

neutral position or the accelerator pedal is released.

assembly is turned on.

This protects against a loss of bias voltage to the power

(3) Diode D2 connects across the drive motor

switch until the directional contactor has been de-

armature and functions only during rapid reversals of

energized.

direction.

During such a reversal, the armature

j. Control Unit.

The control unit provides

continues to operate in the previous direction for a brief

period. The armature acts as a generator at this time.

controlling voltages for the power switch assembly,

Diode D2 carries the generated current and therefore

monitors available battery voltage for an undervoltage

prevents an excessive voltage from appearing across the

condition, and monitors drive current consumption for an

armature.

overcurrent condition.

(4) The two 5500 microfarad capacitors

(1) With a direction selected, the setting of the

connect between B2- and B2+. These act as a filter for

speed potentiometer determines the voltage provided at

any voltage transients caused by switching in the system.

control unit terminal 5. This in turn determines the width

of the drive pulses supplied at terminal 3.

The

3-3. General Requirements for Test and Adjustment

transistors of the power switch assembly are on during

Precautions to be followed throughout all electrical

each drive pulse,(-5 volt input). The transistors are kept

testing and adjustments are as follows:

off in the absence of a drive pulse (+3 volt input) by the

a. Always check battery polarity.

bias voltage provided at terminal 1. This voltage is

b. To shut down power, disconnect battery and

approximately +5 volts dc with respect to battery positive.

discharge capacitors according to paragraph 2-5.

(2) The control unit senses battery voltage

c. Always disconnect battery and discharge

between terminal 8 and B2+ . If the battery voltage

capacitors before taking resistance readings.

between these points drops to about 24 volts, the drive

d. If battery is connected, never manually operate a

pulses are cut off until voltage is restored.

directional contactor unless key switch is on and control

(3) Drive motor current for the truck is through

system is operating.

the current sensing shunt. The voltage developed

e. Do not ground or short any heat sinks (finned

across the shunt is monitored by the control unit between

assemblies) as they are integral parts of circuits. Never

B2+ and B1 +. If the current becomes excessive, the

eliminate or modify heat sinks.

control unit cuts off drive pulses. The current is sensed

f. Do not operate truck without the complete

with each pulse so that normal operation is restored

compliment of power switch transistors, except in

when the overcurrent condition is removed.

extreme emergencies.

Operation without the full

3-2