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6

Brushless motors

Chapter outline
6.1 The brushless d.c. motor............................................................................................................ 168
6.1.1 Torque-speed characteristics............................................................................................ 172
6.1.2 Brushless d.c. motor controllers....................................................................................... 173
6.1.3 Rotor-position measurement ........................................................................................... 174
6.1.4 Commutation logic ........................................................................................................... 176
6.1.5 Controller ........................................................................................................................... 176
6.1.6 Sensorless control.............................................................................................................. 177
6.2 Sinewave-wound brushless motors.......................................................................................... 177
6.2.1 Torque characteristics ....................................................................................................... 179
6.2.2 Voltage characteristics...................................................................................................... 180
6.2.3 Torque-speed characteristics............................................................................................ 180
6.2.4 Control of sinewave-wound brushless motors............................................................... 181
6.3 Linear motors .............................................................................................................................. 182
6.4 Summary...................................................................................................................................... 184
References........................................................................................................................................... 184

The application of brushless motors, of all types, is becoming of considerable impor-
tance to the designers of machine tools and robotic systems. This is largely due to the
benefits that these types of motor bring to a system, particularly in improvements in the
reliability. If the term is used in its broadest sense, this classification will include
permanent-magnet motors, stepper motors, and alternating current (a.c.) induction
motors. This chapter is concerned with permanent-magnet brushless motors and their
associated controllers; a.c. asynchronous induction motors and stepper motors will be
considered in Chapters 7 and 8, respectively.
Within the market place, there appears to be a degree of confusion in the naming of
motors, with given constructions. To prevent confusion, the following motors and their
associated controllers are considered in this chapter.
Permanent-magnet synchronous motors with a trapezoidal winding distribution,
commonly known as brushless d.c. motors, commonly abbreviated to BDCM The
associated controller consists of a conventional three-phase bridge, whose switch-
ing pattern is normally determined by a low-resolution rotor-position sensor.

Electric Drives and Electromechanical Systems. https://doi.org/10.1016/B978-0-08-102884-1.00006-6 165
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