Appeal No. 1997-3279 Page 2
Application No. 08/240,702
motor suffers several disadvantages. Specifically, the
excitation torque generated at the stable point position of
the motor's permanent
magnet rotor is small. Because a magnetic balance has to be
destroyed to start the motor, moreover, the magnetic pole
shape thereof becomes special, and flux leakage is increased,
which reduces the motor's efficiency. In addition, the
different magnetic pole shapes of the motor's stator yokes
complicate assembly.
The inventive brushless DC motor includes a rotor having
magnetic poles, a stator having magnetic poles, and a field
coil. The ratio of the open angle of the stator's magnetic
poles to the open angle of the rotor's magnetic poles is
between 0.75 and 1. Such a ratio ensures that the stable
point position of the rotor is in vicinity of a maximum point
of torque generated by the field coil, which provides a high
starting torque. The stator's magnetic poles are alternately
disposed at regular intervals in the circumferential direction
with a constant air gap from the rotor magnet. The constant
air gap limits flux leakage into the gap, which increases the
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