BASIC OPERATION AND FEATURES
IT400 TRANSISTOR CONTROL
Page
5
Aside from the constant horsepower characteristics
described above, there are many other features that
provide increased performance and lower cost. The
following description provides a brief introduction to
examples of some of these features.
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 of
independent field and armature, the motor
performance curve can be maximized through proper
control application.
Section 1. 2 Solid-State Reversing
The direction of armature rotation on a shunt motor is
determined by the direction in which current flows
through the field windings. Because of the of the
shunt motor field only typically requires about 10% of
the armature current at full torque, it is normally cost
effective to replace the double-pole, double-throw
reversing contactor with a low power transistor H-
Bridge circuit (Figure 4).
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 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.
By energizing the transistors in pairs, current can be
made to flow in either direction in the field. The
armature control circuit typically operates at 12KHZ
to 15KHZ, a frequency range normally above human
hearing. This high frequency coupled with the
elimination of directional contactors, provides very
quiet vehicle operation. The field control circuits
typically operate at 2 KHZ.
The line contactor is normally the only contactor
required for the shunt motor traction circuit. This
contactor is used for both pre-charge of the line
capacitors and for emergency shut down of the motor
circuit, in case of problems that would cause a full
motor torque condition. The line can be energized
and de-energized by the various logic combinations
of the vehicle, i.e. activate on key, seat or start switch
closure, and de-energize on time out of idle vehicle.
Again, these options add to the quiet operation of the
vehicle.
Section 1. 3 Flexible System Application
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, armature rotation slows with the increase
of current in the field.
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 more 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.
FUSE
LINE
CAP
ARM
F2
F1
Q3
Q4
Q5
Q6
Q1
POS
NEG
Figure 4
A1 +
A2 -
Q2
November 2005
