Introduction; Motor Characteristics - GE IC3645SR7A353T3 Installation And Operation Manual

BASIC OPERATION AND FEATURES
SX TRANSISTOR CONTROL
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 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.
SPEED
TORQUE
ARMATURE CURRENT
Figure 1
As shown in 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).
SPEED
TORQUE
ARMATURE CURRENT
Figure 2
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.)
SPEED
TORQUE
ARMATURE CURRENT
Figure 3
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
some of these features.
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Updated Sept 2019