BASIC OPERATION AND FEATURES
IT400 TRANSISTOR CONTROL
Page
4
Section 1. INTRODUCTION
Section 1.1 Motor Characteristics
The level of sophistication in the controllability of
traction motors has changed greatly over the past
several years. Vehicle manufacturers and users are
continuing to expect more value and flexibility in
electric vehicle motor and control systems as they
are applied today. In order to respond to these
market demands, traction system designers have
been forced to develop new approaches to reduce
cost and improve functions and features of the
overall system. Development is being done in a multi-
generational format that allows the market to take
advantage of today’s technology, while looking
forward to new advances on the horizon. GE has
introduced a second generation system using
separately excited DC shunt wound motors. The
separately excited DC motor system offers many of
the features that are generally found on the
advanced AC systems. Historically, most electric
vehicles have relied have on series motor designs
because of their ability to produce very high levels of
torque at low speeds. But, as the demand for high
efficiency systems increases, i.e., systems that are
more closely applied to customers’ specific torque
requirements, shunt motors are now often being
considered over series motors. In most applications,
by independently controlling the field and armature
currents in the separately excited motor, the best
attributes of both the series and the shunt wound
motors can be combined.
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Figure 1
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As shown in from the typical performance curves of
Figure 1, the high torque at low speed characteristic
of the series motor is evident.
In a shunt motor, the field is connected directly
across the voltage source and is therefore
independent of variations in load and armature
current. If field strength is held constant, the torque
developed will vary directly with the armature current.
If the mechanical load on the motor increases, the
motor slows down, reducing the back EMF (which
depends on the speed, as well as the constant field
strength). The reduced back EMF allows the
armature current to increase, providing the greater
torque needed to drive the increased mechanical
load. If the mechanical load is decreased, the
process reverses. The motor speed and the back
EMF increase, while the armature current and the
torque developed decrease. Thus, whenever the load
changes, the speed changes also, until the motor is
again in electrical balance.
In a shunt motor, the variation of speed from no load
to normal full load on level ground is less than 10%.
For this reason, shunt motors are considered to be
constant speed motors (Figure 2).
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Figure 2
SPEED
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In the separately excited motor, the motor is operated
as a fixed field shunt motor in the normal running
range. However, when additional torque is required,
for example, to climb non-level terrain, such as
ramps and the like, the field current is increased to
provide the higher level of torque. In most cases, the
armature to field ampere turn ratio can be very
similar to that of a comparable size series motor
(Figure 3.)
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Figure 3
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November 2005
