1. Field of the Invention
This invention relates generally to the production of power frequency (such as 50 Hz, 60 Hz or 400 Hz) AC power directly from single-pole type generators without the use of rectifiers, inverters, or other secondary processing of the output current and, more particularly, to an efficient method of modulating the generator output into a power frequency sinusoid via field modulation using resonant circuit techniques.
2. Description of the Prior Art
The production of power frequency (50 Hz or 60 Hz) AC power using high frequency alternators allows the engine speed to be independent of the output frequency, which allows the engine to operate at its most efficient or convenient speed, and due to the high frequency, allows small units to generate high output power. Typical systems to accomplish this include the use of inverters to shape the rectified (DC) output of the high frequency alternator, or the use of power frequency conversion switching electronics to shape the high frequency output power without explicit rectification. Because these system require hard switching and manipulation of the full output current, they involve large amperage semiconductors and the associated costs, inefficiencies and heat dissipation requirements.
Hilgendorf, in U.S. Pat. No. 3,916,284, discloses a method for producing low frequency (power frequency) AC directly from a poly-phase high frequency alternator through manipulation of the field excitation. In this method the field is modulated at the desired AC power frequency and the modulated, rectified, high frequency output is commutated, with respect to the load, via soft switching at the zero crossing points of the AC power frequency output. This method eliminates the cost and losses involved with inverter or power frequency conversion manipulation of the full output current, while retaining the light weight and speed independence of high frequency alternators. Tests on the method proposed by Hilgendorf show that it suffers several major problems: the typical cores of high frequency alternators proposed by Hilgendorf consume inordinate amounts of power through eddy currents and core losses; the high inductance typical of alternator cores require high driving voltages in order to quickly charge and discharge the magnetic energy in the field; and, as proposed by Hilgendorf, the energy exciting the field in each cycle is dissipated and wasted and must be replaced in the following cycle. Furthermore, the method of Hilgendorf does not account for the effect of residual magnetism in the alternator core, so the actual output voltage never does reduce to zero.
Tupper, in U.S. Pat. No. 6,051,959, issued Apr. 18, 2000, and entitled “Apparatus for resonant excitation of high frequency alternator field,” discloses an efficient method using resonant circuit techniques to modulate the field of a high frequency poly-phase alternator built with a low loss magnetic core. This overcomes the limitations of the Hilgendorf method and produces a sinusoidal output. High efficiency generator systems on this system have been built, demonstrated and commercialized to provide high quality 60 Hz alternating current power incidental to the operation of variable speed prime movers used in motor vehicles.
The natural rectification methods inherent in the proposals of Tupper and Hilgendorf avoid the drawbacks of hard switching but still require the use of semiconductors, often diodes or SCRs, with attendant voltage-drops which translate into power losses of several percent of the output power. These losses show up as heat, which must be dissipated. The rectification process also leaves a small rectification ripple, which may be removed by filtering but which represents a nuisance complication. Furthermore, the rectification steps necessitate a subsequent commutation step to reverse the output polarity every other half cycle. This commutation requires further complications of the output electronics. (Commutation is used here in the sense of a switch or apparatus to reverse the direction of electrical current within a circuit and the commutation process typically involves complications from the inductance effects of both the generator armature and the load circuit.) Additionally, the rectification of the polyphases of the high frequency alternator, inherent in the methods of Tupper and Hilgendorf, requires high speed switching between the multiple phases. The natural inductance of each phase of the alternator output resists high frequency switching of current between these phases, and, at higher frequencies (above a so called “pole” frequency established by the values of the phase inductance and the load resistance), this effect lowers or attenuates the output voltage in proportion to the increases in alternator frequency or shaft speed. To overcome such attenuation in output voltage, additional excitation is needed in the field magnetic circuit. The saturation limits of practical magnetic materials impose a limit on how much output inductance can be overcome by additional excitation.
Furthermore, the rectification electronics inherent in the poly-phase high frequency alternators provide an impediment to the return flow of current back into the alternator. Such return-current flow would normally occur during a portion of each cycle when driving loads with power-factors different from 1.0 (i.e. when driving complex or reactive loads). Complicating adjustments to the operations of the rectifier system are often required to accommodate reactive loads.
Inherent in the present techniques is the use of high frequency alternators, which use high rotational speed and multiple magnetic poles to produce high rates of flux change, primarily from the rapid switching of magnetic fields from “north” to “south”. These techniques also use multiple output phases, often three phases connected in a “wye” or “delta” arrangement. The outputs of these high frequency phases are combined by various means to produce the desired lower power frequency voltages and currents. It will be appreciated that in the present techniques, the “high frequency” signals are rectified into a quasi-constant uni-polar output, much like “DC” but at a level that is modulated at the power frequency so that the level varies relatively slowly over time when compared to the high frequency alternating signals. There is a limit on how quickly the power frequency can be modulated without interfering with the high frequency alternator operation. In present practice the typical frequencies are 400 Hz or greater for the high frequency alternations of the multiple phases, and 50 or 60 Hz for the power frequency modulations or the rectified output. Some electronics for avionics applications use 400 Hz power frequency equipment. It would be quite problematic to use the referenced modulation technologies to generate 400 Hz AC power with a variable-speed, high-frequency, poly-phase generator.
Typically, motors and generators can be described by the number of magnetic poles involved in providing relative motion between the magnetic field and the armature conductors. There is no known single-pole magnet, every magnet having an inherent “north” and “south” end. However, for the purposes of this disclosure, a generator can be described as a single-pole type generator if the electric generation is due to the relative motion between armature conductors and a generally steady level of magnetic flux from a single pole (or more correctly a single pole-pair), i.e., where the flux intensity-level, polarity and direction does not vary due to relative motion. An early example of such a generator is the well-known Faraday disk, sometimes called a uni-polar or homo-polar generator. These devices are, typically, associated with DC electrical production of low voltage and high current, or with pulses of high amperage and low voltage. The inductance of any magnetizing coils in such a machine is quite high and it is difficult to quickly change the level of magnetic excitation. This works out advantageously in many of these devices as the excitation is generally held steady in order to produce a “uniform” magnetic field for producing DC output. Permanent magnets are often associated with this technology
In addition to the disk type homopolar generators, like that of Faraday, there exist drum or cylindrical type homopolar generators; one example of a device designed for short pulses of uni-polar current, and for which the design details and rationale are well illustrated, is given by Weldon, et. al. In U.S. Pat. No. 4,246,507, “Removable brush mechanism for a homopolar generator,” issued Jan 20, 1981.
Tupper, as cited above, also discloses the need to minimize eddy current losses in the magnetic core of generators used with resonant excitation. Ohst, in United States Statutory Invention Registration H838 “Variable resistivity slip ring,” issued Nov. 6, 1990 discloses the need to minimize eddy current losses in the slip rings and armature circuitry of homopolar generators used with pulsed (time varying) excitation currents.
As a point of clarification, a single-pole type of alternator might have several different stages in each of which the flux of a single- pole operates locally on armature conductors and such stages may be combined in series or parallel combinations without changing the single-pole type of operation.