Page 143 - Electric Drives and Electromechanical Systems

136 Electric Drives and Electromechanical Systems

Printed-circuit motors.

Each of these motors has a number of advantages and disadvantages which need to
be considered when selecting a motor for a particular application. Brushed d.c. motors,
within certain constraints, can be controlled either with a linear or a switching amplifier.
As discussed in Chapter 2, Analysing a Drive System, the motor and its associated
amplifier need to be considered as a combined system if the maximum performance is to
be obtained.

5.1 Review of motor theory

The basic relationships for d.c., permanent-magnet, brushed motors whose equivalent
circuit is shown in Fig. 5.1A are given by

dI a dI a
V m ¼ u m K e þ I a R a þ L a ¼ E m þ I a R a þ L a (5.1a)
dt dt
(5.1b)
T ¼ I a K t
where I a is the armature current, R a and L a the armature resistance and inductance, u m

FIG. 5.1 The equivalent circuit and characteristics of a brushed d.c. motor. (A) Equivalent circuit. (B) The speed-
torque characteristics, showing the limiting values due to peak armature voltage, armature current and the
commutation limit.