Alternators are commonly employed as the basic source of electrical power in airplanes, trucks, automobiles, and other vehicles; the alternator output is rectified and is used to charge the vehicle battery and to operate a wide variety of electrical devices. The size, weight, and cost of the alternator are all critical; all should be held to a minimum. Even more important, the alternator must operate with little or no attention over long periods of time, often in a very dirty environment.
Thus, the service requirements imposed upon a vehicle alternator are also quite severe. The load may vary over wide extremes, depending upon the number of electrical devices on the vehicle currently in use (e.g., heater, air conditioner, lighting, instrumentation, radio, etc.). The temperature may range from below 0.degree. F. to over 200.degree. F. Over all of these extremes, the alternator should exhibit good self-regulation and should accommodate rapid changes in any of the operating conditions.
The rotary electromagnetic excitation structure of a vehicle alternator usually comprises a series of alternate north and south rotor poles disposed concentrically with a multi-pole stator and separated from the stator poles by a small air gap. Most vehicle alternators utilize an excitation coil mounted on the rotor, with the exciting current for the coil applied through brushes and slip-ring connections. Vehicle alternators of so-called "brushless" construction are also known; in these, the excitation coil is stationary and the slip-rings and brushes are eliminated. A brushless alternator thus eliminates the wear and maintenance problems almost inevitably associated with brushes and slip-rings. On the other hand, the brushless construction introduces an additional air gap into the magnetic structure of the alternator, with some possible resultant reduction in efficiency.
One excellent example of a brushless alternator construction is set forth in Barrett U.S. Pat. No. 3,493,800, issued Feb. 3, 1970. The bearing arrangement shown in that patent, especially in the embodiment illustrated in FIG. 10, affords superior performance for a variety of applications, particularly in vehicle alternators. However, the magnetic structure is not as efficient as it might be, at least for some critical applications, due to losses in the air gaps between the rotor and the stationary portion of the magnetic excitation structure. Moreover, the interleaved, axially extending "Lundell" type rotor poles used in that machine, which face the stator across a radial air gap, may produce excessive noise due to the continuous fluctuations occurring in the magnetic attraction between the rotor and stator. Furthermore, centrifugal force on the rotor poles tends to change the rotor-stator air gap as a function of rotational speed, adding to the noise problem and imposing a limitation on the maximum permissible rotational speed.
An even better brushless construction is described in Barrett U.S. Pat. No. 3,953,753, issued Apr. 27, 1976, which incorporates the advantageous bearing arrangement of the earlier Barrett patent in an alternator having an unusual stator winding configuration combined with a rotor-stator pole relationship substantially different from conventional machines. The overall combination is more efficient and exhibits better self-regulation than conventional brush-type alternators. However, some problems of noise, cost and performance remain, in part due to continuing use of a radial stator-rotor air gap.