Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-07-12
2002-09-24
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S051000, C310S066000, C310S077000, C310S152000, C310S254100, C310S261100
Reexamination Certificate
active
06455975
ABSTRACT:
TECHNICAL FIELD
This invention relates to a permanent magnet generator (PMG) through which: 1) the physical airgap between the stator and rotor can be varied to obtain a desired voltage; and 2) the shaft/hub can be used as a braking mechanism.
BACKGROUND OF THE INVENTION
A permanent magnet generator (PMG) features a constant magnetic field. Consequently, its voltage output is dependent upon two factors:
(a) Its rotational speed, according to the formula:
e=Nd&phgr;/dt
where:
e=generated voltage
N=number of turns
d&phgr;/dt=change in flux with respect to time
d&phgr;/dt≡speed (DC field)
Therefore, with every other parameter held constant, speed is the only factor that affects voltage output (at no load), because it serves to vary d&phgr;/dt; and,
(b) the load it is powering, according to the phasor diagrams:
The above diagrams show that, when the internal resistance (R
A
) and reactance (X
A
) of the machine are factored with the load current (I
L
), the terminal voltage (E
T
) is equal to the vectorial sum of generated voltage (E
C
) and the machine internal voltage drops. Thus, the output voltage of a simple PMG can be stabilized by stabilizing speed and loading. However, most real-world applications, such as, for example, a turbine engine that is also used for propulsion, require that regulated power be delivered by a generating system which can operate over a wide range of speeds and loads.
Therefore, generator designers have long been challenged by the problem of regulating the output of a PMG, and have formulated various designs over the years. These designs, or methods to regulate a PMG, are well known in the prior art, but each suffers from some disadvantage. The following describes prior art attempts to regulate PMG output.
A. Power Converters
The prior art includes devices such as power conversion units and power conditioning units that utilize electronic components to regulate the output of a PMG whose load and speed vary. However, the complexity of such devices increases dramatically if they are required to operate over wider ranges of voltage. For example, a power converter that operates over a 3:1 voltage range is several times more complex than one that handles a 2:1 voltage range. In addition, although power converters may be employed successfully for low power ratings (up to 2,000 Watts), power converters are rarely suitable for higher power requirements because they are more complex, costly to produce, and cause installation problems due to their relatively large size.
B. Load Regulation
Other prior art methods have attempted to regulate PMG output by imposing on it a parasitic or artificial load. The magnitude of this load is adjusted to maintain a constant terminal voltage, and counteract voltage fluctuations caused by changes in speed and load. This method does indeed regulate PMG output, although the applied parasitic load is detrimental to efficiency and wastes energy in the form of heat.
C. Rotor Displacement
Yet another prior art method has addressed regulation of PMG output by a method commonly known as “rotor displacement”. As its name implies, this scheme involves displacing the rotor and stator magnetic centerlines from each other, thereby altering the effective air gap of the alternator. Stated differently, since the stator and rotor are cylindrical in shape, there is an annular air gap there between. Axial displacement of the rotor results in a change to the magnetic flux transferred from the rotor to the stator. This change in the magnetic flux is noticeable, but only after an appreciable displacement of the rotor has occurred.
This method works to some extent, but it suffers from at least two significant drawbacks. First, substantial relative movement must occur before any perceptible effect is detected in the output voltage. Longer rotors require more displacement than shorter-length rotors. The second drawback is that because the change in output voltage does not instantaneously change with rotor displacement, the regulation response time is inadequate for most applications.
SUMMARY OF THE INVENTION
our concept is an improvement over the prior art “rotor displacement” PMG. Our invention departs from the conventionally cylindrical rotor and stator design, in favor of a “tapered” or conical design. AS fully described herein, our design results in more efficient and effective voltage regulation, and also in better space efficiency when dealing with certain applications.
DESCRIPTION OF THE INVENTION
Our invention is primarily a tapered stator surface that faces a tapered rotor surface. The terms taper and conical can be used interchangeably and have the same meaning.
The PMG according to our invention can be in either one of two embodiments; each embodiment also capable of multiple variations as will be discussed.
The first embodiment is a stator having a tapered inner diameter and a rotor having a tapered outer diameter. The second embodiment is a rotor having a tapered inner diameter and a stator having a tapered outer diameter.
Our invention comprises a magnet assembly or magnetic field, which interacts with an armature through relative angular motion. The magnet assembly, i.e. rotor, may be located within the armature assembly, i.e. stator, (first embodiment), or the armature may be located within the magnetic field, for an equal electromagnetic effect (second embodiment). Our design can comprise a magnetic field located within an armature assembly, or an armature assembly located within a magnetic field.
The rotor and shaft are supported by bearings, arranged in such a manner as to provide rigid radial support, but allow unimpeded axial movement within a desired range. This effect may be achieved by a variety of means, such as utilizing bearings which can “slide” within their journals or bearing support; or use roller bearings with extra long races to permit the rotor to travel axially, or a combination thereof.
Another means for permitting rotor axial movement while maintaining rigid radial support is by the use of a “flexing bearing support”. As its name implies, the flexing bearing support is a device that is employed to keep the bearing centered on the axis of rotation, while allowing it to be moved axially. It consists of a hub, a plurality of tangential spokes, a mechanism to adjust the length of each spoke, and a outer web that anchors the spokes and refers them to the stator. The hub contains the bearing. Because the spokes are arranged tangentially, they permit two degrees of freedom. The first degree of freedom is circumferential, but against the direction of rotation. Thus, when the hub is angled back, the spokes become too long and the hub must be displaced axially to compensate for the added radial length. The opposite is also true. If the hub is displaced axially, it must also turn against the direction of rotation to allow the span length of the spokes to increase. Thus, is established the second degree of freedom, which is axial. Axial displacement causes the physical airgap to change, thereby regulating the voltage output of the generator.
Our design permits shaft movement in an axial direction. A slight displacement of the shaft in our PMG design is sufficient to significantly alter the physical airgap, or distance between the tapered surfaces of the rotor and stator. The physical airgap affects flux leakage between poles.
Like its cylindrical counterpart, a tapered PMG comprises a magnet assembly or field, and an armature where the usable power is produced. Because the armature must be connected to the load, the armature is usually the stator, while the magnetic field assembly is the rotor. It is to be understood that the rotor may be located either within the stator, or around it, with equal effectiveness, depending on the particular application.
The following description describes the first embodiment mentioned at the beginning of this section where the outer surface of the rotor is facing the inner surface of the stator; i.e. with the magnetic field inside the armature.
The armat
Kasdan Leon
Raad Bernard
Chabot Ralph D.
Mullins Burton S.
Pacific Scientific Electro Kinetics Division
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