Brushless DC motor

A brushless DC motor includes a rotor assembly and a stator assembly. The rotor assembly includes a plurality of rotor permanent magnets of a first polarity. The stator assembly includes a plurality of stator magnetic field assemblies facing the rotor assembly. Each stator magnetic field assembly includes a box-like magnetic core having an opening facing the rotor assembly. A permanent magnetic core is disposed within the box-like magnetic core having a surface facing the rotor assembly and possessing a second polarity at this surface. A field coil is wound in proximity to the box-like magnetic core for energizing the stator magnetic field assembly. A boosting permanent magnet is provided along an outer surface of the box-like magnetic core.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a brushless DC motor of the type in which 
a magnetic field of a rotor is generated by a rotor permanent magnet and a 
magnetic field of a stator is generated by a field magnetic coil. In 
particular, the present invention relates to a brushless DC motor in which 
the efficiency thereof is improved by providing a permanent magnetic core 
within a stator field magnetic coil core and by providing a boosting 
permanent magnet external this permanent magnetic core within the stator. 
The stator magnetic field is enhanced by the combination of the magnetic 
force of the field magnetic coil and the magnetic force of the boosting 
permanent magnet when the field magnetic coil is excited. 
2. Description of the Related Art 
In the conventional DC motor of the type in which the magnetic field of the 
rotor is generated by the rotor permanent magnet and the magnetic field of 
the stator is generated by the field magnetic coil, the efficiency is low 
relative the power consumption of the electric current supplied to excite 
the field magnetic coil to generate the magnetic field of the stator. 
SUMMARY OF THE INVENTION 
The present invention in conceived in due consideration of the low 
efficiency of the conventional DC motor. According to the present 
invention, even when a small amount of electric power is consumed, the 
efficiency of the motor is enhanced a considerable degree by the provision 
of a permanent magnetic core within the field magnetic coil core and a 
boosting permanent magnet external the permanent magnetic core to thereby 
enhance the strength of the stator magnetic field which is formed when the 
field magnet is excited utilizing the boosting permanent magnet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 and FIG. 2 illustrated the configuration of the present invention. A 
rotor (10) is provided on a rotating shaft (13) and supported by ball 
bearings (12) contained within a casing (11). Permanent magnets (14) are 
placed with magnetic poles along the circumference of the rotor (10) and 
are arranged at a same spacing from each other, for example, at every 
90.degree. angle as illustrated. In the illustrated example, the number of 
rotor poles amounts to 4. 
The stator (15) includes a plurality of stator magnetic field assemblies 
(20), each of which comprises a field coil (16) which forms the magnetic 
field of stator (15). The field coil (16) is wound onto a box-type core 
(17) having an open side facing the rotor (10). Within the core (17), a 
permanent magnetic core (18) is provided having a magnetic pole of a 
polarity contrary to that of rotor permanent magnet (14) facing toward the 
rotor permanent magnet (14). On the outside of box-type core (17) opposite 
the opening thereof, a boosting permanent magnet (19) is provided having a 
magnetic pole of a polarity identical to that of rotor permanent magnet 
(14) facing in the direction toward the rotor permanent magnet (14). 
Accordingly, a field magnetic coil (16), a permanent magnetic core (18) 
and a boosting permanent magnet (19) form the stator magnetic field 
assembly (20). The stator magnetic field assembly (20) will achieve a 
S-N-S magnetism arrangement by means of the boosting permanent magnet 
(19), if the magnetic pole polarity of rotor permanent magnet (14) is N, 
when the permanent magnetic core (18) polarity does not excite the field 
coil (16). The stator magnetic field formed by the stator field magnetic 
assembly (20) comprises 6 field poles. 
The field coils (16) of the stator magnetic field assembly (20) are 
connected in series with those field coils (16) which are in the 
diametrical opposed direction and accordingly the mechanical angle between 
phases is 120.degree.. 
As shown in FIG. 2, a position sensing rotor (21) is installed at the end 
of rotating shaft (13) on one side of the casing (11). The position 
sensing rotor (21) comprises a yoke (22) and a position sensing permanent 
magnet (23) which is arranged in the yoke (22) in the same manner as the 
rotor permanent magnet (14). A position sensing element (25) is installed 
in the cap-type case (24). The position sensing element (25) is a Hall 
element and the position at which it is placed is coincident with the 
mechanical angle of the stator. Such a position sensing element (25) is 
well-known in the art as is the position sensing rotor (21). The position 
sensing element (25) is connected to a well-known drive circuit of the 
motor to supply an intermittent electric current to the field coil (16). 
The operation of the present invention as configured in such a manner as 
described hereinabove is explained with reference to FIGS. 4A, 4B and 4C. 
Suppose that the relative positions of the rotor (10) and stator (15) are 
as shown in FIG. 4A when the DC motor is not operated. If the polarity of 
rotor permanent magnet (14) is N at this time, the magnetism of the 
permanent magnetic core (18) of the stator field magnetic assembly (20) 
will be in an SNS arrangement. Accordingly, the polarity of stator field 
magnetic assembly (20) is S and, since the polarity of rotor permanent 
magnet (14) and the polarity of stator field magnetic assembly (2) are 
opposed, there exists an attracting force between the two. When an 
electric current is supplied to the drive circuit of the motor, the 
position sensing element (25) senses the position of rotor permanent 
magnet (14) by means of the permanent magnet (23) of the position sensing 
rotor (21). The two field magnetic coils (16) are then excited by 
supplying an electric current thereto, which are opposed in the drive 
circuit in the diametrical direction, of the stator field magnetic 
assembly (20). 
Thus, the permanent magnetic core (18) of the stator field magnetic 
assembly (20) has the same N pole polarity as the rotor permanent magnet 
(14) as illustrated in FIG. 4B. Since a strong magnetism is effectively 
arranged in an N-S manner in the boosting permanent magnet (19), a 
repulsive force operates between the rotor permanent magnet (14) and the 
stator permanent magnetic core (18) and the rotor (10) thus begins to 
rotate (FIG. 4B). The corresponding permanent magnet (14) of the rotor 
(10) which rotates by means of the repulsive force against the stator 
permanent magnetic core (18) approaches the unexcited neighboring stator 
field magnetic assembly (20) and continues to rotate by means of the 
attracting force which operates between the permanent magnetic core (18) 
of this neighboring stator field magnetic assembly (20) and the rotor 
permanent magnet (14). Then, the field coil (16) of this neighboring 
stator field magnetic assembly (20) is excited and the rotor (10) rotates 
smoothly by operation of the resultant repulsive force (FIG. 4C). 
Thus, according to the present invention, an attracting force is effected 
with the rotor permanent magnet when the field coil is not excited, and 
the permanent magnetic core on which the repulsive force operates when the 
field coil is excited operates on the rotor as a magnetic force, which is 
boosted to a considerable degree by the boosting permanent magnet, to 
improve the efficiency of the motor by promoting its torque, while 
consuming a small amount of electric power. 
The present invention as described hereinabove includes four rotor poles 
and six stator field poles. However, it is possible to construct the 
present invention with the number of rotor poles being a multiple of 4 and 
the number of stator field poles being a multiple of 6. Additionally, the 
construction of the position sensing permanent magnets and/or the position 
sensing elements is changeable.