Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-01-10
2003-09-30
Le, Dang Dinh (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06628032
ABSTRACT:
The present invention relates to electromagnetic devices, that is single- or multi-phase generators and motors of unlimited stroke or of limited stroke (actuators), each device comprising, for each phase, at least two relatively-movable sets of teeth of soft magnetic material, one set of teeth being associated with the stator and the other with the moving part that moves with rotary or linear motion (hereinafter “rotor”). Each set of teeth can comprise a plurality of teeth and the number of teeth can be different between the stator and the rotor. In the limit, one or even both sets could have only one tooth.
In many devices of this type, the pitch between the teeth is substantially constant and substantially the same for both sets of teeth. The reluctance opposing the passage of the magnetic field between these sets of teeth, and consequently the permeance which is the inverse of reluctance, varies during displacement. When one end of a stator tooth and one end of a rotor tooth face each other, they define between them an air-gap of minimum width E. In the devices to which the invention applies, the unit displacement dx (cf.
FIG. 5
) of a rotor tooth is parallel to the tangent to the end of a stator tooth, thus distinguishing such devices from electromagnets where displacement takes place in the minimum air-gap direction.
Rotary or linear motion devices having the above-mentioned characteristics can be motors, actuators, or generators with variable reluctance, that is without a permanent magnet, or motors, actuators, or generators that are hybrid or having “polarized reluctance”, that is including at least one permanent magnet in the stator or the rotor.
In its most commonplace rotary versions, the device comprises a rotor that is generally cylindrical in shape, being constituted by at least one coaxial assembly comprising at least one rotor pole piece fixed on a shaft, each pole piece of the rotor presenting a set of teeth formed by radial teeth disposed along its periphery at a uniform pitch. The device also comprises a stator which comprises a magnetic circuit portion of soft magnetic material, which is generally annular in shape being disposed coaxially around the rotor, and constituted by a peripheral portion and a plurality of stator pole pieces. Each stator pole piece is powered by at least one electrical coil and comprises one or more teeth directed radially so as to face the teeth of the rotor. If there are two or more teeth per pole piece, then the teeth of the stator are disposed substantially at the same pitch as the teeth of the rotor, one rotor tooth and one stator tooth placed facing each other defining between them a radial air-gap having a minimum width E. Variants also exist in which the air-gap is axial, or oblique.
Such electromagnetic devices have been known for several tens of years. Hybrid types are in widespread use, particularly in the form of two- or three-phase stepper motors. Descriptions of such devices can be found for example in the book “Stepping motors and their microprocessor controls” by Takashi Kenio and Akira Sugavara, Clarendon Press, Oxford, 1994, 2nd edition, pp. 28 to 36 for variable reluctance motors, pp. 37 to 44 for hybrid motors, or in the Treatise on Electricity of “l'Ecole Polytechnique Fédérale de Lausanne”, Vol. IX, entitled “Electromécanique” [Electromechanics], by Marcel Jufer, Presses polytechniques et universitaires romandes, § 11.2.5 “Moteur réluctant à simple circuit” [Single circuit reluctance motor] and § 11.2.11 “Moteur réluctant polarisé” [Polarized reluctance motor]. Linear motion variants correspond to rolling rotary motors out flat and are described, for example, on page 33 of the above-specified work by T. Kenjo and in § 11.13 in the above-cited work by M. Jufer.
Numerous theoretical studies have been done on such devices, cf. in particular the article by Marcel Jufer and Gunter Heine “Hybrid stepper motor torque and inductance characteristics with saturation effects” published in “Incremental Motor Control Systems and Devices (IMCSD) Proceedings”, Fifteenth Annual Symposium, 1986, pp. 207-211, and the references cited in that article.
In the traditional design of such devices, it is considered that the width of the air-gap between two facing teeth should be as narrow as possible in the light of the technical constraints that stem from manufacturing tolerances in terms of diameter, concentricity, centering, burring, and other sources of inaccuracy. T. Kenjo states this clearly on page 30 of the above-cited work in its chapter entitled “Air-gap should be as small as possible”. That concept has been supported by the theory. The well-known fundamental expression for calculating force or torque in electromagnetism and derived from the expression for the magnetic energy stored in the air-gap, for two sets of teeth in relative displacement with degree of freedom &agr; states that the torque C that is generated will be proportional to:
ⅆ
A
ⅆ
α
U
2
where U is the magnetic potential difference applied between the two sets of teeth, and A is the permeance between them. In a variable reluctance motor, this can be constituted by a magnetic potential difference due solely to the ampere-turns generated by one or more coils carrying electric currents, placed in various possible ways, or in a hybrid motor due to the algebraic sum of the magnetic potential difference U
a
polarizing the air-gap under the influence of the permanent magnet plus the magnetic potential difference U
ni
generated by the above-mentioned coil(s).
The derivative of the permeance dA/d&agr; can be developed in the form of a Fourier series, as can the permeance itself:
A=a
0
+a
1
sin(
N&agr;
)+
a
2
sin(2
N&agr;
)
dA/d&agr;=Na
1
cos(
N&agr;
)+2
Na
2
cos(2
N&agr;
)
where N is the number of teeth around the rotor, or if the rotor is incompletely fitted with teeth, the ratio 2&pgr;/(angular pitch) of the teeth that exist.
The first term of the derivative of this expression relative to &agr;, known as the fundamental term, is Na
1
cos(N&agr;). In a motor or an actuator for controlling movement, or in a generator from which an accurately sinusoidal voltage is expected, with the number N of teeth being fixed, it is desirable to increase the amplitude a
1
of the fundamental and to reduce as much as possible the amplitudes a
2
, a
3
, . . . of the harmonics cos(2N&agr;), cos(3N&agr;), . . . . The fundamental term of the torque is then given by expression [1]:
C
=
ⅆ
A
ⅆ
α
U
2
=
Na
1
U
2
cos
(
N
α
)
[
1
]
It is well known that the term a
1
increases with decreasing air-gap. Since the torque C is proportional to this term, it would appear to be logical to select an air-gap that is as small as possible compatible with the manufacturing method.
For a hybrid motor of ordinary size (known as size “23”, giving a diameter ≈51 millimeters (mm), length ≈51 mm), the usual minimum air-gap is about 0.07 mm to 0.08 mm, giving rise to severe constraints on manufacturing tolerances and therefore increasing manufacturing costs. In practice, the air-gap E of conventional motors of this size is always ≦0.1 mm.
For such a hybrid motor, the maximum potential difference U
max
that appears in the above formula for torque is U
max
=U
ni(max)
+U
a
. For the above-mentioned size and under steady conditions, the coil provides a maximum potential U
ni(max)
=85 ampere-turns (At) between teeth, for example. Since the torque due to the current is at a maximum when U
a
≈U
ni(max)
, U
a
is also set to be about 85 At, so U
max
=170 At. Ignoring magnetic potential losses in the soft magnetic materials of the stator and of the rotor, the induction B in the air-gap is given by:
B=&mgr;
0
U
max
/E.
[2]
If it is desired to set a limit of B=2 teslas (T) because of saturation of the material of the magnetic circuit, then E=1.07×10
−4
m
Oudet Claude
Urwyler Jean-François
Browning Clifford W.
Dinh Le Dang
MMT S.A.
Nguyen Hanh
Woodard Emhardt Moriarty McNett & Henry LLP
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