Page 184 - Electric Drives and Electromechanical Systems

178 Electric Drives and Electromechanical Systems

will determine their detailed construction and hence analysis. The main characteristics
of a sinewave-wound brushless motor include:
The air gap flux is sinusoidal and is generated by a number of specially shaped
rotor magnets.
The windings have a sinusoidal distribution.
The motor is supplied with three-phase sinusoidal current.

While the analysis is more complex than for BDCM, it needs to be considered in some
detail if the operation of these motor is to be fully understood. The cross section of an
idealised sinewave-wound brushless motor is shown in Fig. 6.10. If the number of turns
per pole, N P , is given by,
N
N p ¼ (6.14)
2
and given an ideal winding distribution, the number of conductors within the angle dq of
a p -pole-pair motor, at a position q, is given by,
N sin pq dqp
(6.15)
2
The sinusoidal flux distribution within the air gap is provided by specially shaped
rotor magnets. The rotor-flux distribution is centred on the north axis, which can be
considered to be displaced by a radians from the axis of the stator winding; and is given
by,

BðqÞ¼ Bcosðpq aÞ (6.16)
b
Using the above relationships, it is possible to determine the torque and speed
characteristics of a sinusoidally wound permanent-magnet motor by the application of
conventional electromagnetic theory.

FIG. 6.10 The cross section of an idealised sinewave-wound permanent magnet machine.