SECTION 3 - CHASSIS & SCISSOR ARMS
31215074
3-1
SECTION 3. CHASSIS & SCISSOR ARMS
3.1
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 mod
-
ule to measure current with the internal shunt
(extremely low impedance). The -T Terminal is pulled to
Ground by the Armature Switch MOSFET's (connected
to -B Terminal).
To provide variable speed control, the Armature MOS
-
FET transistors 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 MOSFET's spend 50% of the
period On and 50% Off, approximately ½ of the avail
-
able Battery Voltage will be applied to the Armatures.
Similarly, the MOSFET are On continuously (100% Duty
Cycle) to apply all available Battery Voltage to the Arma
-
tures (as in Driving at Full Speed).
Instead of permanent magnets, the separately-excited
drive motors use electro-magnets (called Field Wind
-
ings) located in the stator (non-rotating) portion of the
motor. Field windings are preferable to permanent mag
-
nets 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
will apply as little as 14A to the fields for higher rota
-
tional 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 modu
-
lated by the Power Module to maintain a fixed relation
-
ship 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 conditions. If one wheel slips, the wheel with
traction will demand more current as it slows slightly
(under load). In this manner, the system provides effec
-
tive 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). Independent field control would require a
more complex Power Module, and parallel field wind
-
ings would require impedance matching. For simplicity,
the vehicle uses 12V field windings wired in series to
ensure proper distribution of current.
Two electrically-released parking brakes are mounted to
the rear of the drive motors. The Ground Module ener
-
gizes the two 24V electro-magnets when appropriate to
allow vehicle motion. The parking brakes can be
released electrically for emergency vehicle towing
Figure 3-1. Traction Control Circuit - ZAPI Power Module
+
24V
+
+
+
Line Contactor
Right Field Winding
Left Field Winding
Power Module
Left
Armature
Right
Armature
+
+
BF1
BF2
B
+B
T
F1
F2
Field
MOSFETS
Armature
MOSFET
Shunt
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