The present invention is based upon the free pole theory. In a permanent magnet, the atoms consist of very small atomic nuclei surrounded by clouds of electrons. In some kinds of atoms, there are more electrons circling in a clockwise direction around the atom axis than are circling in a counter-clockwise direction. A rotational motion of electrons around the nucleus thus results which creates a current and develops a magnetic field. These atoms are, in effect, little permanent magnets with external fields, just like the field of a dipole.
It is a well known phenomenon that when two like poles of permanent magnets are facing each other, strong repulsion occurs therebetween. The magnetomotive force (mmf) in the middle of the two like poles becomes greater than the original mmf. Moreover, because of opposing magnetic fields, negative divergence occurs near the center of the two like pole magnets. The negative divergence of a magnetic pole at a point inside of the body equals the pole strength per unit volume at that point. These diverged poles are called free poles. In the case of unlike poles which attract each other, all of the free poles are diverged to the opposite side of the magnet which intensifies the flux density at both ends of the magnet.
The present invention, while having application to all dc electric motors, is primarily directed toward conventional, inexpensive, cylindrical, permanent magnet brush-type dc electric motors. Such brush-type dc motors typically utilize fairly inexpensive ceramic magnets (ferrite magnets). These type motors find application in a variety of different fields including electric boat trolling motors, automobile motors, sub-fractional motors for battery powered tools and the like.
These conventional motors typically are inefficient and noisy. Moreover, these conventional motors tend not to be very powerful. While existing technologies and materials are available to increase the efficiency and performance of these motors, those technologies and materials are costly. Therefore, because these motors are generally mass produced for commercial applications, it is not cost effective or practical to employ such known technologies in these type motors. For example, to improve the performance and efficiency of a motor of the foregoing type, one could utilize the brushless type motor and rare earth cobalt magnet. However, the electronic control device for brushless type motors typically costs more than the motor itself. Therefore, the modification would be impractical.
In order to design a quiet and efficient permanent magnet dc motor, many important design parameters must be considered. Some of these design parameters include: maximizing the flux density at the pole-arc faces, i.e. the inner periphery of the stator magnets, while at the same time maximizing reluctance; decreasing the inner pole magnetic flux leakage; and, decreasing the magnetic flux leakage at the outer periphery of the stator magnets.
Conventional two pole dc motors have large interpole working air gaps. The greater reluctance of the large air gap reduces the total flux; however, a shorter interpole working air gap is also undesirable as it tends to increase the magnetic flux leakage between the poles because there is little difference in the reluctance of the useful flux and the leakage path in the interpole region. Additionally, the flux density waveform distortion in the interpole region, in turn, results in armature fluctuations and slot harmonics. These results tend to make conventional motors quite noisy.
Although the magnetic flux can be encouraged to follow a ferromagnetic path and to cross a working air gap, it cannot be confined completely to that path because the ferromagnetic material exhibits a non-linear relationship between flux density B and the magnetizing force H. This situation is known as a saturation phenomenon. Moreover, when the magnetic flux path is encouraged to pass through a ferromagnetic material, such as a pole piece, a substantial amount of reluctance is lost, in turn resulting in a less efficient motor.
Prior art designs have attempted to increase the useful flux density between main stator magnets and reduce magnetic leakage. For example, Mishima, U.S. Pat. No. 4,376,903 issued Mar. 15, 1983 discloses a direct current dynamo electric machine including a non-magnetic cylindrical rotor and a non-cylindrical stator. The stator comprises oppositely disposed permanent magnets externally of the cylindrical coreless rotor and a series of blocking magnets for blocking the leakage magnetic flux of the magnetic field of the permanent magnets by a reverse magnetic field. The blocking magnets include primary blocking magnets and auxiliary blocking magnets. The auxiliary blocking magnets divert the leakage from the primary blocking magnets and the main magnets along a path through the air gap between the auxiliary blocking magnets. This path causes a loss in reluctance, and thus, a corresponding loss of mmf. The orientation of the auxiliary blocking magnets also is disadvantageous because it generates an additional leakage flux. The auxiliary blocking magnets do not function as interpolar magnets because they do not provide proper magnetic orientation or a proper flux density waveform between the two main stator magnets. Also, that field system is designed for a non-magnetic cylindrical rotor type motor having a large interpole gap and a rectangular configuration, and thus cannot practically and effectively be applied to an inexpensive permanent magnet dc electric motor having copper windings on a permeable iron rotor contained within a cylindrical housing.
Mishima, U.S. Pat. No. 4,243,903 issued Jan. 6, 1981, discloses another direct current dynamo electric machine described as an improvement over the device of U.S. Pat. No. 4,376,903. This field system has two main stator magnets having an extra large interpole gap and two main blocking magnets located between the interpole region. The blocking magnets apply a reverse magnetic field against the main magnets. The two blocking magnets each produce a magnetic field opposite in direction but equal in intensity to the magnetic field between the main magnets. At the vicinity of the boundary area where the two opposing poles meet together a very high intensity magnetic flux is created. While this may be good as useful reluctance for the armature, these boundary areas emit a strong magnetic field through the yoke as wasted magnetic leakage.
Another previous field system for reducing leakage magnetic field and increasing flux density is disclosed in de Graffenried U.S. Pat. No. 3,906,268, issued Sep. 16, 1975. This field system is designed for a non-magnetic cylindrical rotor and a non-cylindrical type housing having a large air space between the rotor surface and iron housing wall. The large space serves to accommodate a plurality of magnets disposed in contact with magnetically permeable pole pieces on many sides to increase the air gap flux density. The magnetic flux path passes through the magnetically permeable pole piece, to the air gap, and then to the armature. This path through the magnetically permeable material causes a loss in overall reluctance. Because mmf is the product of the magnetic flux and the reluctance, substantial mmf is lost resulting in the motor having an overall lower efficiency. Moreover, it is impractical to apply this field system to existing cylindrical type permanent magnet dc electric motors because of the limited spaces between the motor housing wall and the armature surface in such conventional motors.
Another previous invention relating to increasing flux density and reducing magnetic leakage is disclosed in Miyamoto U.S. Pat. No. 4,459,500, issued Jul. 10, 1984. This field system includes a pair of auxiliary magnets mounted on the axial end surface of a pole piece to reduce the magnetic flux leakage from the pole piece for increased magnetic force. The positional relationship between the pole piece, the permanent magnets, and the repulsive auxiliary magnets disposed around the pole piece, is such that the diverged magnetic poles are diverged toward the pole piece, thereby intensifying the magnetic flux at the pole piece and providing a high flux density at the pole piece face. However, the field system has low reluctance and a high magnetic leakage at the outer surface.
Therefore, it is obvious that the prior art motors have increased the flux density at the cost of mmf efficiency per fixed magnet volume. Moreover, the prior art field systems cannot practically be applied to conventional inexpensive permanent magnet dc electric motors having copper windings on permeable iron rotors and disposed in a cylindrical housing. The present invention therefore satisfies a long-felt need to provide a permanent magnet construction wherein flux density of the main magnets is increased with low magnetic leakage.
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