Traction System; Theory Of Operation - JLG 1932RS/6RS Service And Maintenance Manual

SECTION 3 - CHASSIS & SCISSOR ARMS

3.11 TRACTION SYSTEM

Theory of Operation

The armatures (rotating windings) of the separately-excited
drive motors are wired in parallel to the Power Module's -T and
+B terminals (ZAPI) . The +B Terminal is always at the same
voltage as the +B (Battery Voltage when the Line Contactor is
closed) and allows the module to measure current with the
internal shunt (extremely low impedance). The -T Terminal is
pulled to Ground by the Armature Switch MOSFET's (con-
nected to -B Terminal).
To provide variable speed control, the Armature MOSFET tran-
sistors switch On and Off at high frequencies (pulse-width
modulation; 16kHz). The Duty Cycle (On & Off time) is varied to
control the voltage applied to the Armatures. When the MOS-
FET's spend 50% of the period On and 50% Off, approximately
½ of the available Battery Voltage will be applied to the Arma-
tures. Similarly, the MOSFET are On continuously (100% Duty
Cycle) to apply all available Battery Voltage to the Armatures
(as in Driving at Full Speed).
Instead of permanent magnets, the separately-excited drive
motors use electro-magnets (called Field Windings) located in
the stator (non-rotating) portion of the motor. Field windings
are preferable to permanent magnets because the Power
Module can adjust the stator's magnetism for optimum motor
performance. When climbing a grade a low speeds, the Power
Module may apply as much as 40A to the field windings for
more electro-motive force. On level terrain, the Power Module
+ +
Right
Left
Armature
Armature
3-14
Line Contactor
BF1 BF2
+B
T
Armature
MOSFET
Shunt
Power Module
Figure 3-20. Traction Control Circuit - ZAPI Power Module
will apply as little as 14A to the fields for higher rotational
speeds and better electrical efficiency.
The Field Windings also provide direction reversal for traction.
When driving forward, MOSFET switches 1 and 4 turn On to
apply positive potential to F2 and ground potential to F1. In
reverse, MOSFET switches 2 and 3 turn On to apply positive
potential to F1 and ground potential to F2. Theses switches
are pulse-width modulated by the Power Module to maintain
a fixed relationship between Field and Armature Current (also
called the Field Map).
Since the two 24V Armatures are wired in parallel, the drive
motors will attempt to rotate at the same speed under all con-
ditions. If one wheel slips, the wheel with traction will demand
more current as it slows slightly (under load). In this manner,
the system provides effective traction control with no added
complexity.
It is essential that the same amount of field current is supplied
to both drive motors, or one wheel will pull the vehicle (motor
overheating and excessive tire wear would result). The vehicle
uses 12V field windings wired in series to ensure proper distri-
bution of current.
Two electrically-released parking brakes are mounted to the
rear drive motors. The Ground Module energizes the two 24V
electro-magnets when appropriate to allow vehicle motion.
The parking brakes can be released electrically for emergency
vehicle towing.
24V
+ +
+
F2
B
F1
Field
MOSFETS
+
Right Field Winding
Left Field Winding
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