Interior magnet rotary machines are well known in the art and include interior magnet motors and generators. Interior magnet rotary machines are a type of permanent magnet rotary machines and generally comprise a rotor having magnets built inside it. Examples of interior magnet machines may be found in U.S. Pat. Nos. 4,700,097 and 4,486,678.
Interior magnet machines have been widely investigated because their unique rotor structure combines synchronous and induction characteristics in a solid rotor that utilizes permanent magnets. Interior magnet machines may be used in different environments, with alternating current or direct current, and they provide a generally constant torque output throughout their speed range, which can include very high speeds due to the interior magnet rotor construction. Interior magnet machines are also preferred because of their contactless two-dimensional (perpendicular to the rotor axis) flux path between the rotor and stator.
Prior to the development of interior magnet machines, rotary machines were characterized by electrical windings on a rotor, the use of slip rings or brushes to accommodate the rotation of the windings or to commutate the flow of electricity, and the maintenance of a rotating electrical connection. This type of prior machine requires the rotor to be in electrical contact with the stator. Thus, slip rings and brushes are necessary even though they degrade operational efficiency through power loss from the electrical resistance of the brush contact, and through mechanical friction loss due to the drag of the brushes on the rotor. The windings on the rotor also significantly increase the mass of the rotor, necessitating slower rotational speeds or more energy from a prime mover. Moreover, the constant rotation, heating, and magnetic forces exerted on the coils and their insulation cause them to fatigue, crack, degrade, and ultimately fail with time.
The interior magnet machine solves these problems by mounting permanent magnets, rather than electromagnets, on the rotor. This eliminates the need for rotating electrical connections, saves the electrical power otherwise expended in exciting the field, lessens the amount of internal heat generation, and increases power density.
Notwithstanding the above advantages, interior magnet machines have not been widely used because a viable method for mass producing an interior magnet machine at low cost has heretofore not been available. See for example, U.S. Pat. No. 4,725,750, which describes an internal magnet rotor having a frame including a group of trough-shaped openings around the periphery thereof, extending parallel to the axis of the rotor frame, within the central portion thereof. A corresponding trough-shaped permanent magnet is inserted in each rotor frame opening.
Unfortunately, the rotor of the aforesaid U.S. Pat. No. 4,725,750 is difficult to efficiently mass produce because of the need to provide trough-shaped openings in the central portion of the rotor. In particular, the rotor frames must be made one at a time by a molding operation, or each trough-shaped opening must be individually cut from the central portion of the rotor frame. Molding or cutting are inherently slow and expensive, and as such, these rotors cannot be efficiently mass produced.
Moreover, besides being inefficient from a mass production standpoint, the machine described in the aforesaid U.S. Pat. No. 4,725,750 is inefficient, because of the flux leakage paths that are associated with the trough-shaped centrally located magnets. As is well known to those having skill in the art, flux leakage causes hysteresis loss, eddy current loss, heat production, and magnetic paths that operate at less than full saturation, resulting in low energy density and an increase in bulk and weight. Furthermore, it is very difficult, if not impossible, to uniformly magnetize these trough-shaped magnets, particularly at their respective ends.