Rotary electrical device

Details Rotary electrical device Rotary electrical device

1999-08-05

2001-07-17

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S152000, C310S154090, C310S156030

Reexamination Certificate

active

06262508

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rotary electrical device driven by a three-phase power supply whose stator and rotor cores are provided with bonded permanent magnet strips, producing a high torque at slow operating speeds for use in electrical motors such as vernier motors.
2. Description of the Related Art
FIG. 1
is a schematic cross sectional view of a conventional vernier motor. A stator core
11
is provided with Z
1
slots
24
for installing three-phase windings to produce rotational magnetic fields of p magnetic pole pairs, and is also provided with permanent magnets
15
which are bonded to the inner surface of each slot
24
. It follows that there are Z
1
permanent magnets provided for the stator core
11
. For the rotor core
10
, there are Z
2
permanent magnets
16
provided equidistantly along its outer peripheral surface. The number of permanent magnets Z
2
on the rotor core and the number of permanent magnets Z
1
on the stator core are related by the following expression:
Z
2
=Z
1
±p
The permanent magnets on the stator core
11
and those on the rotor core
15
are arranged so that the magnetic poles are identically oriented in the radial direction. That is, if the polarities of the permanent magnets
15
on the stator core
11
are such that the outer side is the N-pole and the inner side is the S-pole, then the permanent magnets
16
on the rotor core
10
are arranged so that the polarities are also the N-pole on the outer side and the S-pole on the inner side.
Such a vernier motor is operated by rotating magnetic fields produced by the three-phase windings in the slots on the stator core, and exhibits a characteristic ability to produce a high output torque at low speeds, because of the effects of the permanent magnets having attracting poles across a clearance gap between the stator and rotor cores. The rotational speed &ohgr;v of such a vernier motor is given by:
&ohgr;
v=&ohgr;/Z
2
where &ohgr; is the angular speed of the rotating magnetic fields produced by the alternating current supplied to the three-phase windings. Therefore, the vernier motor can be operated at a rotational speed that is 1/Z
2
lower than the angular speed &ohgr; generated by normal rotating magnetic fields. Similarly, the rotational torque Tv is given by:
Tv
=(
Z
2
/p
)×&tgr;
where &tgr; is the normal torque produced in a permanent magnet type synchronous motor, so that an output torque produced is (Z
2
/p) times greater than that produced by a normal permanent magnet type synchronous motor.
Such vernier motors are, therefore, ideally suited to applications such as wafer polishing apparatus that requires a high turning power at slow rotational speeds. In a polishing apparatus to produce a flat and mirror polished surface on a semiconductor wafer, the top ring holding the wafer is pressed against the polishing surface (cloth or fixed abrasive materials) mounted on a turntable at a given pressure, and both are rotated at low speeds with a polishing solution (slurry or pure water etc.) at the contact interface of the wafer and the polishing surface, until polishing is completed. Such a polishing apparatus is operated typically at 15 r.p.m., and the motor is required to produce a high output torque to overcome a large frictional resistance generated at the contact interface of the wafer and the polishing surface. Normal induction motors do not easily exhibit the characteristics necessary for such low speed, high torque operations, so that vernier motors are much more suitable for such applications.
However, the vernier motor shown in
FIG. 1
presents the following operational problems. The first problem is that, because the permanent magnets
15
are placed at the inner opening of the slot
24
, magnetic resistance is increased in a slot section, and correspondingly, the total available magnetic flux is reduced. This has an effect of reducing the torque that can be generated by the motor.
The second problem is related to the design of the motor. Because the permanent magnets
15
are to be placed at the opening of each slot
24
of the stator core, the magnets must be bonded after the conductive wires of the windings are installed in the stator slots and fixated with varnish. In other words, after the wires are installed in the slots, varnish is used to enhance electrical insulation as well as the fixate the wires inside the slots. However, varnish applied to the permanent magnets
15
interferes with a bonding operation of the permanent magnets
15
to the inside surfaces of the stator core
11
with an adhesive agent, so that it becomes difficult to produce secure bonding of the magnets on the stator core. Even more important is a loss of the distinguishing feature, which is the narrow clearance gap between the stator core
11
and the rotor core
10
, because of the unavoidable variations that occur in the varnished conditions at the opening section. Such variations in the gap dimension is a primary cause of variability in the performance of vernier motors produced by the conventional assembly method.
Remedial steps that may be taken include masking of the bonding surfaces of the stator core beforehand to avoid application of varnish thereto, or removal of varnish from the bonding surfaces before bonding. These steps are very cumbersome and labor-intensive, and require careful process monitoring because any residual varnish can affect the gap size and consequently the motor performance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rotary electrical device that can be manufactured easily, and which produces a high output torque at low operating speeds.
To achieve the object of the present invention, there is provided a rotary electrical device comprising: a stator core having Z
1
stator slots for installing three-phase windings which generate rotational magnetic fields of p magnetic pole pairs in the stator core, and Z
1
permanent magnets provided in tooth sections fabricated between the stator slots around the stator core; and a rotor core having Z
2
permanent magnets being provided around the rotor core; wherein Z
1
, Z
2
and p are related by an expression Z
2
=Z
1
±p, and the direction of the magnetic poles of all the permanent magnets are aligned identically in a radial direction.
According to one aspect of the present invention, there is provided a method for manufacturing a rotary electrical device having a stator core and a rotor core, with the stator core having Z
1
stator slots for installing three-phase windings so as to produce rotational magnetic fields of p magnetic pole pairs and Z
1
permanent magnets, and with the rotor core having Z
2
permanent magnets such that Z
1
and Z
2
are related by an expression Z
2
=Z
1
±p, wherein the method comprises: preparing a plurality of stator slots for installing three-phase windings with tooth sections fabricated between the stator slots; affixing Z
1
permanent magnets in the tooth sections; installing the three-phase windings in the stator slots; and varnishing an assembled unit of the stator core, magnets and windings.
One of the features of the present invention is that the magnetic resistance to impede operation of the present rotary electrical device is reduced significantly compared with the conventional design of rotary electrical devices in which stator magnets are bonded in front of the opening of each stator slot, because the stator permanent magnets in the present device are moved away from the opening to lateral inner surfaces of the stator tooth sections such that these magnets are not radially between the center of the rotor and the slots, respectively. Available magnetic flux densities are thus increased because of the shorter magnetic path, and the output torque is increased for a given size of the motor.
Because the step of affixing the magnets can be performed before installing the electrical windings, the degree of freedom in the manufacturing proces

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