Section 4. PRINCIPLES OF OPERATION

SECTION 4

PRINCIPLES OF OPERATION

4-1.

GENERAL.

F contacts close. This establishes a voltage path

through fuse 12FU, the travel motor, normally-open F

contacts of the forward relay, the series field of the

4-2.

This section presents the principles of lift truck

travel motor, the normally-closed R contacts of the

control panel operation, and includes the distribution of

reverse relay, transformer IT primary and across

electrical power for

the operation of its various

capacitor IC and conducting rectifier 2SCR.

This

accessories. The control panel circuit controls the

permits capacitor IC to charge to battery voltage. As the

operation and speed of the travel motor, the hydraulic

capacitor charges, current through rectifier 2SCR

pump motor and the power steering motor through

reduces to zero, and when the capacitor voltage reaches

relays whose contacts are designed with high power

battery voltage rectifier 2SCR cuts off. The polarity of

handling capabilities. Secondary circuits, such as the

terminal 2 of IC is positive with respect to terminal 1.

lights and horn are switch operated. The following

During pre-charge, the travel motor does not receive

material is presented using figures 4-1 and 4-2 as

enough power to produce torque.

reference. Figure 4-1 is a schematic diagram of the

control circuitry, and figure 4-2, a wiring diagram.

4-8.

TURN-ON. Turn-on occurs when the delayed

4-3.

POWER CIRCUIT CONTROL.

gate pulse from control pack 2CP reaches and opens

the gate of rectifier ISCR, allowing it to conduct.

Conduction of rectifier ISCR permits travel motor

4-4.

Power for the operation of the electric truck is

current to increase through the primary winding of

received from the 36-volt battery, whose application to

transformer IT to induce a voltage in the secondary.

the various circuits is controlled by master switch relay

This voltage source is in series with the battery. This

IMS. Two conditions are required to operate this relay;

induced voltage in the secondary produces a current in

(1) the battery switch mounted on the

battery

the direction of the dot. Capacitor IC thus reverses

compartment wall on tile lift side of the seat must be

polarity; its charging path completed through coil IL,

closed, and (2) the seat switch must be operated. This,

rectifier ISCR, transformer IT SEC and diode 3RC.

then completes the battery circuit through fuse 8FU, (lie

During turn-on time, the travel motor receives power to

battery switch, the panic switch, the seat switch, master

produce drive torque, the speed of the motor governed

switch relay IMS and the diode across terminals 4 and 5

by the width of the pulse. Pulse width, in turn, is

of suppresser 4SP to the positive side of the battery.

controlled by the speed potentiometer. For slow and

When relay IMS is energized, normally-open contacts

moderate speeds, pulse width is short; for higher

IMS close (pick up) to enable available power for

speeds, pulse width is longer.

electric truck operation.

4-5.

TRAVEL MOTOR CONTROL.

4-9.

TURN-OFF. Turn-off occurs when the gate

pulse is applied to rectifier 2SCR.

This permits

capacitor IC to discharge through it and rectifier ISCR in

4-6.

The travel motor is a series field traction motor

a direction opposite to the load current. When the

under the control of silicon controlled rectifiers (SCR) for

capacitor discharges, current through rectifier ISCR

creep and low speeds, and essentially directly across

reduces to zero, and the rectifier cuts-off. This permits

the battery for high speed travel. Operation of the travel

capacitor IC to charge again to battery voltage, and,

motor for creep and slow speeds is described under

when charged, causes rectifier 2SCR to cut-off.

three modes of operation; pre-charge, turn-on and turn-

off. Higher speed control follows in the latter part of the

4-10.

HIGH SPEED CONTROL. For full speed

text.

cruising, the solid state control circuit is bypassed by the

application of a voltage developed in control pack 2CP

4-7.

PRECHARGE. Pre-charge occurs when the

and applied across shorting contactor relay SC. This

directional controller is set in either the forward or

voltage is delivered across the relay when the 'final'

reverse position and the accelerator switch is depressed

contacts in the accelerator switch close. When tile relay

to the position where the INITIAL contacts close. This

energizes, its SC contacts close to permit travel motor

condition generates a short duration from control pack

current to flow through field weakening pack FWP. This

ICP which is delayed to rectifier ISCR but applied

effectively applies full battery power across the travel

directly to rectifier 2SCR, allowing it to conduct. With

motor. Additionally, a second set of SC contacts close

the directional switch in the forward position, a voltage

to permit battery voltage to energize field weakening

path is thus completed from control pack 2CP, line

relay FW. The circuit established is through fuse 6FU,

2PL6, the travel motor thermocouple, the brake switch,

the closed SC contacts, field weakening pack FWP and

the closed INITIAL contacts of the accelerator switch

the FW relay. When the relay energizes, its contacts in

and closed 1 CIRC FWD contacts of the directional

series

switch to the forward relay. When the relay energizes,

its normally-closed F contacts open and normally-open

4-1