Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-06-10
2002-06-25
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S261100, C310S270000, C310S156020, C310S256000, C310S254100
Reexamination Certificate
active
06411002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electric machines or motor/generators and, more specifically, to permanent magnet, axial field electric machines.
2. Description of the Related Art
An electric motor/generator, referred to in the art as an electric machine, is a device that converts electrical energy to mechanical energy and/or mechanical energy to electrical energy. Since electric machines appear more commonly as motors, the ensuing discussion often assumes that electric energy is being converted to mechanical energy. However, those knowledgeable in the art recognize that the description below applies equally well to both motors and generators.
Electric machines generally operate based on Faraday's law,. which can be written as e=BLv, and the Lorentz force equation, which is often written as F=BLi. In electric machines that utilize rotational motion, these equations can be written as e=k
1
BL&OHgr; and T=k
2
BLi respectively. Faraday's law describes the speed voltage or back EMF (electromotive force), e, that appears across motor conductors due to the geometrically orthogonal interaction of a magnetic field having flux density B with conductors of length L traveling at a rotational speed &OHgr;. The Lorentz force equation describes the torque T generated by the geometrically orthogonal interaction of a magnetic field having flux density, B, with conductors of length L carrying current i. The coefficients k
1
and k
2
are constants that are a function of motor geometry, material properties, and design parameters.
A variety of electric machine types exist in the art based on how they generate the magnetic field and on how they control the flow of electrical energy in the conductors exposed to the magnetic field. The present invention pertains to electric machines where the magnetic field B is primarily generated by permanent magnets affixed to the rotating assembly, or rotor of the machine; whereas the conductors are affixed to the stationary assembly, or stator of the machine and electronic circuitry is used to control the flow of electrical energy. In the art this type of machine is commonly called a brushless DC motor or a brushless permanent magnet machine. In addition, such electric machines can be modified to use induction to generate the magnetic field. In this case the machine is commonly called an induction motor.
Electric machines that produce rotational motion are classified as either radial field or axial field. Radial field machines have a radially directed magnetic field interacting with axially directed conductors, leading to rotational motion. On the other hand, axial field machines have an axially directed magnetic field interacting with radially directed conductors, leading to rotational motion. Of these two machine topologies, the axial field machine appears much less often. In the art, axial field machines are most often found in applications where: (i) there is insufficient axial length to accommodate a radial field machine, (ii) relatively little torque is needed, and (iii) motor energy conversion efficiency is not a primary concern. The reasons why axial field machines generally appear less often than radial field machines include: (a) more familiarity with radial field machines, (b) the desire to minimize cost by reusing existing radial field machine tooling, and (c) the lack of market incentive to address manufacturing issues unique to axial field machines.
In terms of quantity produced, the spindle motor in computer floppy disk drives is the most commonly appearing axial field electric machine. In this application minimizing cost is the most critical design goal. As a result, this motor does not utilize materials, design steps, or construction techniques that lead to high efficiency over a broad range of speeds, high motor constant, or high power density. The floppy disk spindle motor uses an axial field topology solely because there is insufficient axial space available inside the floppy disk housing to use a radial field motor. This motor is typically manufactured with one rotor element and one stator element, with the stator element being constructed from a steel-backed printed circuit board upon which the stator windings and motor electric drive circuitry are connected.
The present invention discloses design aspects for axial field machines that offer greater performance than common axial field machines and performance that meets, exceeds, or is competitive with radial field machines. Performance in this case includes the measures of: (i) energy conversion efficiency; (ii) motor constant, (iii) gravimetric power density, (iv) volumetric power density, (v) manufacturing cost, and (vi) construction flexibility due to modular construction.
Energy conversion efficiency describes how well an electric machine converts energy. For a motor, efficiency can be written as
&eegr;=(Power Out)/(Power In)=(
T
&OHgr;)/(
T&OHgr;+P
r
+P
c
+P
m
) (Eq. 1)
where T is torque, &OHgr; is rotational speed, P
r
is resistive loss i.e., the so called I
2
R loss, which represents power converted to heat by the resistance of the current carrying conductors in the motor, P
c
is the core loss, which represents power converted to heat due to hysteresis and eddy current losses in the conductive and magnetic materials used in the motor, and P
m
is the mechanical loss, which includes bearing loss, windage, etc. Core and mechanical losses generally increase with the square of speed, so efficiency typically increases from zero at zero speed, to some peak value at some rated speed, then decreases beyond that rated speed. For constant speed applications, achieving high peak efficiency at a constant rated speed is all that is important. For variable speed applications, however, it is important to maximize the range of speeds over which maximum efficiency can be achieved. As defined in Eq. 1, efficiency is unitless and is often expressed as a percentage, where 100% efficiency reflects the ideal electric machine.
Referring to
FIG. 30
, a graph is presented showing the efficiency of a typical electric machine known in the art at various speeds and torque. The operation of the electric machine is bounded by a peak speed, a peak torque, and a maximum power output. In this example, the electric machine has a peak efficiency of 90% at a particular operating point (i.e., at a particular rated speed and torque). At other operating points, however, the efficiency drops off precipitously as indicated by the contours of constant efficiency. In a traction application, for example, when the electric machine is operated at different operating points on the graph, the average efficiency will be much lower than peak efficiency.
In servomotor applications where a motor does not turn continuously but rather starts and stops frequently, efficiency is not a good measure of motor performance because efficiency is zero at zero speed, i.e., &OHgr;=0. Under these conditions, the ability to produce torque with minimum losses is important. In the art the term motor constant describes the motor characteristic. Motor constant can be written and simplified as
K
m
=
T
P
r
=
K
T
I
I
2
R
=
K
T
R
(
Eq
.
2
)
where K
T
is the motor torque constant, I is the net motor current, and R is the net motor resistance. Core loss and mechanical loss are not included in the motor constant because these losses are zero at zero speed. The square root of P
r
is used in Eq. 2 because it makes the motor constant independent of current, which makes it independent of any motor load and makes it easier to compare the performance of different motors.
Based on Eq. 1 and Eq. 2, it is clear that a motor exhibiting high efficiency will generally exhibit a high motor constant. Likewise, if a motor exhibits minimal core loss and mechanical loss, a motor having a high motor constant will also exhibit high efficiency.
Gravimetric and volumetric power density are defined as th
Shenkal Yuval
Smith Stephen H.
Gonzalez R. Julio
Kenyon & Kenyon
Ramirez Nestor
Smith Technology Development
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