The need for efficient conversion of electrical energy into mechanical energy or mechanical energy into electrical energy is so well established as to require no elaboration. The disscusion that follows will be in terms of engine-driven alternators, it being understood that the same general considerations apply to electric motors.
In many applications, the mechanical energy input is provided by the reciprocation of a positive displacement mechanical element such as the piston in a Diesel, Brayton, or Stirling engine. The most common alternators are rotary machines which utilize a kinematic converter to transform the reciprocal motion of the positive displacement element to the rotary motion required by the alternator. However, the kinematic converter is subject to parasitic friction and life-shortening wear, with most such converters requiring separate bearings and lubrication systems using specially formulated lubricants. Thus, the requirement of such a kinematic converter adds cost, weight, and bulk of the machine.
Furthermore rotary machines tend to be relatively heavy and inefficient. For example, a typical 10-kilowatt alternator is 12 inches in diameter, 18 inches in length, weighs approximately 100 pounds, and has an efficiency on the order of 80%. To be sure, there are rotary machines having a lower weight/power ratio and a higher efficiency, but the machines exhibiting these desirable characteristics tend to be central station machines in the 100 megawatt range.
Some of the above deficiencies may be eliminated by employing a linear configuration wherein a reciprocating electromagnetic element is directly connected to the positive displacement element of the free-piston engine. These linear electromagnetic machines may be classified generally into two groups. First, are the Henry-type machines in which the magnetic field reciprocates relative to an armature coil, thereby inducing voltage in the coil by the well-known Henry law of induction. Second, are the Faraday-type machines in which the magnetic flux imposed on the armature coil is made to vary periodically with time, thereby inducing voltage in the coil by the well known Faraday law of induction. The prior art is replete with examples of both types of machines.
Representative of Henry-type machines are those disclosed in the following U.S. Patents:
Ostenberg, U.S. Pat. No. 2,362,151 PA1 Martin, U.S. Pat. No. 2,842,688 PA1 Dickinson, U.S. Pat. No. 2,944,160 PA1 Stauder, U.S. Pat. No. 3,024,374 PA1 Cutkosky, U.S. Pat. No. 3,465,161. PA1 Christian, U.S. Pat. No. 2,928,959 PA1 Schmidt, et al., U.S. Pat. No. 2,992,342 PA1 Wysocki, U.S. Pat. No. 3,094,635 PA1 James, Jr., U.S. Pat. No. 3,105,153 PA1 Dawes, U.S. Pat. No. 3,206,609 PA1 Colgate, U.S. Pat. No. 3,234,395 PA1 Montpetit, et al., U.S. Pat. No. 3,443,111 PA1 Wills, U.S. Pat. No. 3,629,595.
Representative of Faraday-type machines are those described in the following U.S. Patents:
However, while the elimination of the kinematic converter overcomes some of the problems discussed above, the voltage output of prior art linear alternators renders such machines unsuitable for certain applications. More particularly, any AC power generation equipment that is to be connected to a power grid must provide a substantially pure sinusoidal output waveform at precisely controlled frequency and phase. While the Faraday-type machines of the prior art tend to be simpler to construct than the Henry-type machines, they are variable reluctance machines which are subject to pronounced cogging action. This renders them very difficult to control with a free-piston engine, so that a sinusoidal output voltage is virtually unattainable.