GE SX TRANSISTOR CONTROL IC3645SR4U404N2 Manuel D’Utilisation

BASIC OPERATION AND FEATURES

SX TRANSISTOR CONTROL

Page 4

January 2000

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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RRE

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RTI

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ARMATURE CURRENT

Figure 1

SPEED

TORQUE

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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ART

ING

RRE

ARMATURE CURRENT

Figure 2

SPEED

TORQUE

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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ART

ING

RRE

ARMATURE CURRENT

Figure 3

SPEED

TORQUE

Aside from the constant horsepower characteristics

described above, there are many other features that

provide increased performance and lower cost. The