BASIC OPERATION AND FEATURES
SX TRANSISTOR CONTROL
Page
4
ollowing description provides a brief introduction to some
f
f these features.
o
ection 1. 2 Solid-State Reversing
S
The direction of armature rotation on a shunt motor is
etermined by the direction in which current flows through
d
the field windings. Because of the shunt motor field,
typically only requires about 10% of the armature current at
ull torque, it is normally
f
cost effective to replace the
ouble-pole, double-throw revers
d
ing contactor with a low
power transistor H-Bridge circuit (Figure 4).
By energizing the transistors in pairs, current can be made
to flow in either direction in the field. The field and
armature control circuits typically operate at 12KHZ to
15KHZ, a frequency range normally above human hearing.
his high frequency, coupled with the elimination of
T
irectional contactors, provid
d
es for very quiet vehicle
peration.
o
The line contactor is normally the only contactor required
or the shunt motor traction ci
f
rcuit. This contactor is used
or both pre-charge of the line capacitors and for
f
mergency shut down of the
e
motor circuit, in case of
roblems that would cause a full motor torque condition.
p
he line can be energized and de-energized by the various
T
logic combinations of the vehicle, i.e. activate on key, seat
r start switch closure, and de-e
o
nergize on time out of idle
ehicle. Again, these options
v
add to the quiet operation of
the vehicle.
ection 1. 3 Flexible System Application
S
Because the shunt motor controller has the ability to
control both the armature and field circuits independently,
the system can normally be adjusted for maximum system
efficiencies at certain operating parameters. Generally
speaking, with the ability to independently control the field
and armature, the motor performance curve can be
maximized through proper control application.
Section 1. 4 More Features with Fewer Components
Field weakening with a series wound motor is
accomplished by placing a resistor in parallel with the field
winding of the motor. Bypassing some of the current
flowing in the field into the resistor causes the field current
to be less, or weakened. With the field weakened, the motor
speed will increase, giving the effect of “overdrive”. To
change the “overdrive speed”, it is necessary to change
the resistor value. In a separately excited motor,
independent control of the field current provides for
infinite adjustments of “overdrive” levels, between the
motor base speed and maximum weak field. The
desirability of this feature is enhanced by the
elimination of the contactor and resistor required for
field weakening with a series motor.
With a separately excited motor, overhauling speed
limit, or downhill speed, will also be more constant. By
its nature, the shunt motor will try to maintain a
constant speed downhill. This characteristic can be
enhanced by increasing the field strength with the
control. Overhauling load control works in just the
opposite way of field weakening, as armature rotation
slows with the increase of current in the field. An
extension of this feature is a zero-speed detect feature
which prevents the vehicle from free-wheeling down an
incline, should the operator neglect to set the brake.
Regenerative braking (braking energy returned to the
battery) may be accomplished completely with solid-state
technology. The main advantage of regenerative braking is
increased motor life. Motor current is reduced by 50% or
better during braking while maintaining the same braking
torque as electrical braking with a diode clamp around the
armature. The lower current translates into longer brush
life and reduced motor heating. Solid state regenerative
braking also eliminates a power diode, current sensor and
contactor from the circuit.
For GE, the future is now, as we make available a new
generation of electric traction motor systems for electric
vehicles having separately excited DC shunt motors and
controls. Features that were once thought to be only
available on future AC or brushless DC technology vehicles
systems are now achievable and affordable.
Section 2. FEATURES OF SX FAMILY OF TRANSISTOR
MOTOR CONTROLLERS
Section 2.1 Performance
Section 2.1.1 Oscillator Card Features
FUSE
LINE
CAP
ARM
F2
F1
Q3
Q4
Q5
Q6
Q1
POS
NEG
Figure 4
A1 +
A2 -
Q2
Updated Sept 2019
