Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-11-19
2001-01-16
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S156030, C310S114000, C310S185000, C310S186000, C310S210000, C310S254100, C310S268000
Reexamination Certificate
active
06175178
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates generally to the design of electrical machines, and more particularly to the design of alternators, generators and motors having low inductance in the armature circuits with particular emphasis on use in flywheel energy storage systems
2. Description of the Prior Art
As is well understood by those skilled in the art, electrical machines have an internal impedance that interacts with other system impedance to determine the performance of the combined system. In a motor, the inductance is that portion of the internal impedance related to magnetic energy storage within the electrical machine as it is energized to deliver mechanical work. The electrical system driving the motor must deliver the energy to be stored in the inductor in addition to the energy for the mechanical work to be performed. This necessitates increases in the capacity of generators, wiring and transformers needed to supply the motor.
In alternators and generators the internal impedance is, perhaps, even more important. The alternator or generator impedance combines with the load impedance to determine the performance of the whole system. As the internal impedance of an alternator or generator is made to be a smaller fraction of the total impedance, the output voltage of the alternator or generator becomes a larger fraction of the ideal (pre-loss) voltage provided by the idealized source. In the current art care is generally taken to provide low resistance pathways in the copper windings of an alternator or a generator in order to minimize internal resistance and to minimize the power lost in the alternator or the generator and the waste heat that needs to be dissipated.
Another factor in the impedance of the alternator or generator is the inductance of the output windings. This inductance is a direct result of winding the output coils around magnetic pathways in the alternator or generator, this being the technique usually used to generate the output voltage. Any output current in such windings will store magnetic energy in the same magnetic pathways, as is well understood. The inductance, “L” of the circuit is related to this stored energy by the equation
L=2*(Energy Stored)/(Current
2
)
The inductance of the output windings is part of the internal impedance and acts to filter the output voltage applied to the load. As frequencies get higher this inductive impedance blocks an increasing proportion of the ideal voltage provided by the alternator or generator and prevents it from acting on the load. While this has not been much of an issue for 60 Hz synchronous generators, it becomes a substantial design challenge for high frequency alternators. This has been known for some time; for example Griffing and Glockler present the design of a “High Frequency Low Inductance Generator” in U.S. Pat. No. 3,858,071.
High frequency alternators or generators are desirable in that high levels of output power can be achieved with physically small magnetic paths; resulting in physically compact units. Claw pole alternators are typical of the physical design of high frequency generator devices and achieve high frequency by having a plurality of alternating poles. A disadvantage of these physically small claw pole units is that the close proximity of multiple poles and multiple magnetic pathways allows for unnecessary storage of substantial amounts of magnetic energy, resulting in high output inductance.
High output inductance causes several difficulties in the operation of high frequency alternators or generators. The impedance, Z of the inductor grows directly with the operating frequency (co, rad/sec) as shown in the following formula:
Z(&ohgr;)=j*&ohgr;*L(j=imaginary operator)
The higher the frequency, the greater the impedance and filtering. To overcome this filtering, the ideal voltage must be increased as the frequency is increased. The ideal (pre-loss) voltage is usually increased by increasing the magnetic excitation level of the field, leading to higher magnetic intensity levels in the magnetic pathways. Since core losses due to eddy current generation are proportionate to both the frequency squared and the magnetic intensity level squared it will be understood that the need for extra excitation to overcome the inductive impedance of the output will lead to high core losses at high frequency operation. At the limit, when the excitation levels reach the point where magnetic pathways become saturated, further excitation is precluded, and the output of the device drops off with further increases in operating frequency.
As a counterpoint to this, if internal inductance were negligible, then the output voltage would rise with increasing frequency due to the increased change of flux with time . The excitation levels could then be reduced as the frequency increased, leading the device away from saturation. The decrease in excitation would offset the increase in frequency such that the core losses would remain nearly constant with operating frequency.
It is therefore an objective of this invention to provide an electrical machine which may be used as a high frequency alternator with low output inductance.
Furthermore, high frequency alternators are often poly-phased devices used with solid state circuits to rectify, switch, commutate or chop the output and reform it into DC or desired power frequency (50 or 60 Hz, etc) AC forms. In such devices individual alternator output phases are turned on and off at high frequencies, again invoking the filtering of the output inductance. Also, it is common for the output inductance of one phase to be linked by mutual inductance to the output of other phases so that the sudden change in current (switching) in one phase produces unwanted voltage transients in the other phases.
It is therefore a further objective of this invention to provide an electrical machine which may be used as a poly-phase high frequency alternator with minimal adverse effects caused by mutual inductance between phases.
As noted for the typical high frequency alternators, such as the claw pole type, the close proximity of multiple poles and magnetic paths gives rise to the unnecessary storage of large amounts of magnetic energy. This is important in the field excitation circuit as well as the output circuit because of the saturation and core-loss issues already mentioned. It should be noted that the majority of magnetic energy is stored in the high reluctance air spaces which are interconnected by the low reluctance ferro-magnetic pathways in which the saturation and core-losses phenomenon occur. So called “leakage flux” passes through the air spaces to complete magnetic circuits without going through the intended pathways which link output coils. In physically compact machines where many poles and pathways are arranged in close proximity the leakage flux can become a high percentage of flux, making the machine inefficient.
Alternators and generators based on armatures with poloidal windings around a stator shaped as an annular ring (or toroid) have long been know. Kirkley and Smith present a design based on radial air gaps in U.S. Pat. No. 4,087,711. Further improvements were presented by Radovsky in U.S. Pat. No. 5,798,594 in which a brushless synchronous machine is presented with axial air gaps completing the magnetic circuit through an annular ring stator in a fashion that greatly limits the leakage flux from the field. The rotors in these designs are relatively complex, present difficulties in establishment of air gap clearances, and do not address the issues of output inductance or mutual inductance between phases.
It is therefore a further objective of this invention to provide a low inductance electric machine which may be used as a brushless alternator with low leakage of the field flux combined with simplified rotor construction and provision for independently establishing multiple air gaps.
Recent work by Groehl, disclosed in U.S. Pat. No. 5,565,836, presents methods for achieving the nullification of unnecessary
Tupper Christopher N.
Wood Duncan G.
Atwood Pierce
Caseiro Chris A.
Perez Guillermo
Ramirez Nestor
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