Page 124 - Electric Drives and Electromechanical Systems

Chapter 4 Velocity and position transducers 117

rotor, composed of wires held together by fibreglass and a polymer resin and has a low
moment of inertia which ensures that the system performance is not compromised,
similar to that of the ironless-rotor d.c. machine, discussed in Chapter 5, Brushed direct-
current motors. In addition to the low inertia and the low ripple content of the output,
the axial magnets ensure that the tachogenerator length is small. In practice, this could
add as little as 5e10 mm to the length of the overall package. A further refinement is the
provision of frameless designs, which allows the system designer the option to mount
the tachogenerator directly on the shaft whose speed is to be measured, thus further
reducing the size of the overall package.
The performance of a brushed tachogenerator depends on it being used within its
specified operating capabilities; the linearity of the output will suffer if the load resis-
tance, R L , is allowed to fall below the manufacturer’s recommended value. From
Fig. 4.10, it can be shown that,
E g ¼ R a I þ R L I (4.10)
where R a is the armature resistance; hence the terminal voltage, V, is given by,
R L K g N
V ¼ (4.11)
R a þ R L
The load resistance should be as large as possible to ensure that the terminal voltage
is maximised; however, the current which is drawn should be sufficiently high to ensure
that the commutator surface does not become contaminated.

4.2.2 Brushless d.c. tachogenerators
With the increasing use of brushless d.c. motors in servo systems, motor speeds are no
longer limited by brushes; this leads to shaft speeds approaching 100 000 rev min 1 in
some high-performance machine tools. The maximum speed of a brushed tachogen-
erator is limited to the speed at which aerodynamic lifting of the brushes occurs, and by
increased armature-core losses which result in the output linearity deteriorating.
Brushless tachogenerators have been developed as a response to these problems. The
principle of their operation is identical to that of brushless motors (as discussed in

FIG. 4.10 The equivalent circuit of a brushed tachogenerator, with its associated load resistance.