Page 182 - Electric Drives and Electromechanical Systems

176 Electric Drives and Electromechanical Systems

therefore to maximise the output. The operation of resolvers and absolute optical po-
sition encoders was considered in Chapter 4. The use of this approach, while beneficial
in certain applications, does negate the relative simplicity of brushless d.c. motors. If a
higher performance approach is required, then a sinewave-wound machine should be
considered.

6.1.4 Commutation logic

In practical applications the correct switching sequence of the power circuit is required
to obtain optimum performance. The switching sequence is developed from the design
of the motor, from the required direction of motion, and is based on the positional
information obtained from the rotor-position-measurement system. The logic to decode
the sensor output can either be implemented as discrete logic, or more commonly as a
customised logic-gate array. In a commercial device, a number of additional features are
normally provided, particularly the ability to operate with brushless d.c. motors of
different construction, for example, three-phase motors with the Hall-effect devices
separated by either 30, 60, or 240 electrical degrees, and four-phase motors with a
separation of 90 electrical degrees. In addition, most commutation-logic devices provide
a facility for totally disabling the power bridge. In addition, the majority integrated
circuits for brushless d.c. motor are either capable of directly driving power-bridge de-
vices with a minimum of additional circuitry or in the case of small motors directly
power the motor.

6.1.5 Controller
The control of a. brushless d.c. motor velocity is undertaken by the control of the motor’s
terminal voltage; this is normally achieved by PWM of the supply voltage. In a common
approach the pulse width modulated switching waveform can be directly gated with the
commutation switching pattern; but, in practice, only the lower devices need to
be controlled. As with d.c. brushed-motor servo amplifiers, pulse width modulation can
be undertaken by either subharmonic or current-controlled hysteresis techniques, as
discussed in Section 5.3.4. As discussed earlier, the characteristics of brushless d.c.
motors are very similar to those of brushed motors; hence it is possible to control these
motors over a wide speed and torque range using a conventional analogue control loop.
The power circuit for a brushless d.c. motor drive consists of a conventional
six-device, three-phase, power bridge, as shown in Fig. 6.7, the devices used will depend
on the rating of the drive, but for small applications MOSFETs (metal-oxide semi-
conductor field-effect transistors) predominate, as with the power bridge discussed in
Section 5.3.4. The power circuit provides a number of auxiliary circuits to ensure
protection against over-voltages, under-voltages, fault currents, and excessive device
temperatures. When the motor regenerates, the energy which is returned will cause the
bus voltage to rise; this excess energy can be dissipated by the use of a conventional
bus-voltage regulator, as discussed in Section 5.4.