Page 392 - Power Electronics Handbook

Electrical machine control 381

T& are switched on and off, there being a short period in between to prevent
both transistors being on simultaneously and causing a ‘shoot through’ of the
supply.
Figure 14.31 shows a fixed frequency variable mark space mode of
operation, although fixed mark and variable frequency can also be used. For
a low mark to space ratio, as in Figure 14.31(b), the voltage across the motor
(VA - V,) is low and the motor current is also low. To increase the output,
and hence the motor speed and torque, the mark to space ratio is increased,
as in Figure 14.31(a).
The direction of rotation of the motor can be changed by keeping transistor
TR3 on and switching TR, and TR2 on and off alternately, giving waveforms
similar to those of Figure 14.31 but reversed in direction. For both directions
of operation it is necessary to close one of the transistors in the H-bridge
whilst operating the other in an on-off mode, so as to provide a return path
for the inductive current in the motor. The power transistors used can carry
current in either direction.
Charge pump circuits are formed by P, and P2 which drive the gate of the
DMOS transistors above the supply to turn them on. The circuit shown in
Figure 14.30 also incorporates current and temperature sensing, with the
ability to vary the mark to space ratio so as to limit the current. When the
temperature reaches a preset value a warning is given via a flag and if no
action is taken to reduce the load, and the temperature increases further, the
control circuit steps in and turns the system off.

14.3.3 Electronic commutator d.c. motors
The d.c. motor is by far the most popular variable-speed machine, due to
its excellent speed-torque characteristics and variable-speed capability. Its
one major drawback is its mechanical commutator which places design and
environmental limitations on its operation. It is feasible to use an a.c.
motor drive system when a mechanical commutator is undesirable but, as is
shown later, a d.c. motor has many advantages over a.c. motors in some
applications. It is in order to maintain these characteristics, whilst
overcoming the limitations of a mechanical commutator, that electronic
commutator, or brushless, motors are used.
In this section the construction of electronic commutator d.c. motors is
described in more detail. These motors are not commonly used in large
sizes but have gained wide acceptance in high-performance small motor
drive systems. Transistors are now the switching device most commonly
used.

14.3.3. I The electronic commutator
In a d.c. motor torque is produced by the interaction of the stator and rotor
fields. Usually the stator contains salient poles and interpoles, if provided,
which are energised by a field coil, and the rotor carries armature current
supplied by an external d.c source. To maintain unidirectional torque
between the rotor and stator it is necessary to switch the rotor current
periodically so as to keep the two fields as closely perpendicular to each
other as possible. A conventional d.c. machine uses a mechanical
commutator and brushes to achieve this. The commutator switches the