Patent Application
20050073210
Permanent magnet electric motors have been available for some time and have been found generally satisfactory for certain tasks. Needed improvements have been noted however in certain design features.
Initially, a slotless stator design is preferred. With the elimination of the need for teeth on the steel back iron, more easily manufactured tolerances can be employed. In addition, cogging of the motor is eliminated.
Secondly, a substantially improved back iron is provided. The back iron is formed of steel but in a highly unconventional manner. A continuous coil much in the nature of a “slinky” is formed. This reduces iron losses, noise generation, and power draw as compared with sectional back irons of the conventional type. Moreover, the use of the “slinky” design accommodates a most desirable feature whereby the grain of the steel, preferably grain oriented silicon steel, can be aligned with the direction of rotation. This also enhances motor efficiency.
By encapsulating the back iron in a pair of mating molded plastic members in face-to-face relationship a number of requirements are met. The molded plastic provides circumaxially spaced separators accommodating a convenient and efficient method of winding stator wire about the back iron. Use of the molded plastic members also provides insulation between back iron and the wire, which eliminates the need for additional coating. Finally, the molded members accommodate a press fit between one of the members and a main housing of the motor. This provides a positive structural link and the necessary precise alignment between the stator and the rotor, which is also supported by bearings mounted in a bearing tower supported in the housing.
Winding of the stator wire about the back iron is accomplished with sections 25,25 of wire wound between the separators on the molded plastic members. Each section of wire 25,25 is would helically with each coil in closely spaced relationship with each adjacent coil. With a number of sections of wire 25,25, for example twelve [12], there are of course a large number of lead wires or wire ends, twenty-four [24] in the present example. Preferably, each lead or wire end is attached to a conductive pin, which is mounted in one of the plastic members. The pins, in turn, are connected to a P.C. board, which connects all of sections of wire in appropriate relationship. Preferably, the board is of copper construction and has a second conventional board associated with it. The circuit boards also carry additional circuitry, thermistors, hall sensors and connectors.
Further in winding the wire sections, and particularly when wire of relatively large diameter is required, Litz wire is preferred. This avoids excessive eddy current losses otherwise encountered.
While the helically wound wire sections may create more heat than other types of windings, they also provide a unique opportunity for cooling the motor. Portions of the windings inside the back iron are essentially unused electrically but provide a convenient heat sink for the remainder of the windings. By designing the motor with air moving blades on the rotor and openings directing airflow through the center of the back iron and over these portions of the windings substantial cooling of the motor is achieved. Contamination problems are avoided since the air is not directed to flow through the air gap externally of the back iron and windings. Additionally, the inner portions of the windings provide a convenient location for thermistors which engage the wire and can be directly attached to the P.C. board. A second level of protection is thus provided with the thermistors set to turn off when temperature exceeds a preset limit.
The use of a copper P.C. board provides a substantial reduction in electrical resistance as well as a convenient motor cooling system. The copper of the board which connects the winding sections has a resistance much lower than the wire itself or a trace on a standard P.C. board. This of course substantially enhances motor efficiency.
With regard to cooling, the copper board serves as a heat sink for the winding sections and mounts or the FETS (Field Effect Transistors). By inducing a cooling airflow over the copper board, the winding sections and FETS are indirectly cooled. Finally, the stator may be encased in molded plastic. This allows the motor to be in airflow as in a blower installation. The smooth plastic rather than the relatively rough surfaces of the winding sections are disposed in the airflow and this avoids depositing debris on the windings.
Referring initially to
Each winding section 25 of the stator comprises a length of wire wound helically about the back iron between adjacent separator 16, 16. Twelve (12) winding sections 25, 25 are shown in
Reverting to
A copper P.C. board 27 is illustrated in
In
Finally, in
As will be apparent from the foregoing, a number of improvements in permanent magnet motors have been achieved with the result substantial improvement in both motor performance and sound attenuation.
