BRUSHLESS D.C. MOTORS (Motors And Drives)

Much of the impetus for the development of brushless d.c. motors came from the computer peripheral and aerospace industries, where high performance coupled with reliability and low maintenance are essential. Very large numbers of brushless d.c. motors are now used, particularly in sizes up to a few hundred watts. The small versions (less than 100 W) are increasingly made with all the control and power electronic circuits integrated at one end of the motor, so that they can be directly retrofitted as a replacement for a conventional d.c. motor. Because all the heat-dissipating circuits are on the stator, cooling is much better than in a conventional motor, so higher specific outputs can be achieved. The rotor inertia can also be less than that of a conventional armature, which means that the torque-inertia ratio is better, giving a higher acceleration. Higher speeds are practicable because there is no mechanical commutator.
In principle, there is no difference between a brushless d.c. motor and the self-synchronous permanent magnet motor discussed earlier in this topic. The reader may therefore be puzzled as to why some motors are described as brushless d.c. while others are not. In fact, there is no logical reason at all, nor indeed is there any universal definition or agreed terminology.
Broadly speaking, however, the accepted practice is to restrict the term ‘brushless d.c. motor’ to a particular type of self-synchronous permanent magnet motor in which the rotor magnets and stator windings are arranged to produce an air-gap flux density wave which has a trapezoidal shape. Such motors are fed from inverters that produce rectangular current waveforms, the switch-on being initiated by digital signals from a relatively simple rotor position sensor. This combination permits the motor to develop a more or less smooth torque, regardless of speed, but does not require an elaborate position sensor. (In contrast, many self-synchronous machines have sinusoidal air-gap fields, and therefore require more sophisticated position sensing and current profiling if they are to develop continuous smooth torque.)
The brushless d.c. motor is essentially an inside out electronically commutated d.c. motor, and can therefore be controlled in the same
way as a conventional d.c. motor (see topic 4). Many brushless motors are used in demanding servo-type applications, where they need to be integrated with digitally controlled systems. For this sort of application, complete digital control systems, which provide for torque, speed and position control are available.

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