Composite magnet structure for rotor
An interior permanent magnet electric motor. A rotor comprising a slot radially spaced from its longitudinal axis of rotation extending parallel to the axis. First and second magnets are positioned in the slot and extend parallel to the axis. A first magnet is positioned between a second magnet and the axis.
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The present invention generally relates to an electric motor rotor design. More particularly, the present invention relates to an interior permanent magnet rotor design wherein strontium ferrite and neodymium-iron-boron are positioned in a common slot in the rotor core.
BACKGROUND OF THE INVENTIONInterior permanent magnet (IPM) rotor designs using strontium ferrite (ferrite) and neodymium-iron-boron (neo) are known in the art.
In one prior art design, the rotor has a core with long thin slots having neo in each slot. This design does not make use of ferrite. The slots are formed by using a punch press on the rotor core. In order to increase die life, decrease the core weight, and reduce flux leakage, the slots are oversized. The oversized slots allow air spaces around the neo which cause the motor to have high windage noise at high speeds. These motors can have a sinusoidal back electromagnetic flux (EMF) which is desirable.
Another option is to use ferrite in an IPM rotor design. Ferrite is less expensive and can be used to fill large slots. This results in very small air spaces which correspond to a quieter motor. The problem with ferrite is that it does not have a sufficiently high flux density to make an efficient motor.
The combination of neo and ferrite in a single rotor design has been the solution. Large slots near the center of the rotor are filled with ferrite, and smaller slots closer to the edge of the rotor have pieces of neo in them. A motor employing this design is somewhat quieter than a motor using neo alone (i.e. has less windage noise), but generally has a non-sinusoidal back EMF (i.e., it is harmonically rich). Also, the die used in manufacturing this type of rotor has a short lifespan due to the small size of the neo slot.
SUMMARY OF THE INVENTIONEmbodiments of the invention include IPM rotor designs with small air spaces and large slots in order to achieve a quiet motor and improved die life. Embodiments of the invention also include IPM rotor designs that demonstrate a near sinusoidal back EMF.
In accordance with one aspect of the invention, an electric motor rotor is provided. A core has a central longitudinal axis and a slot radially spaced from the longitudinal axis extending parallel to the axis. First and second magnets are positioned in the slot and extend parallel to the longitudinal axis. The first magnet is positioned between the second magnet and the longitudinal axis.
In accordance with another aspect of the invention, a method is provided for producing an electric motor. A slot is formed in a rotor core material having a central longitudinal axis. A first magnet is inserted in the slot. A second magnet is inserted in the slot such that the first magnet is substantially between the second magnet and the central longitudinal axis. The rotor core is inserted into a stator having windings. The windings of the stator are connected to a commutation circuit.
In accordance with another aspect of the invention, an electric motor is provided. A rotor includes a core and first and second magnets. The core has a central longitudinal axis and a slot radially spaced from the longitudinal axis extending parallel to the longitudinal axis. The first and second magnets are positioned in the slot and extend parallel to the longitudinal axis. The first magnet is positioned between the second magnet and the longitudinal axis. A stator having windings is in magnetic coupling relation to the rotor. A commutation circuit is electrically connected to the windings of the stator.
Alternatively, the invention may comprise various other methods and apparatuses.
Other objects and features will be in part apparent and in part pointed out hereinafter.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
Corresponding reference characters indicate corresponding parts throughout the drawings.
DETAILED DESCRIPTION OF THE INVENTION Referring to
Generally, motors employing the invention have a substantially sinusoidal back EMF whereas motors known in the art using ferrite and neo magnets have a harmonically rich back EMF. Motors employing the invention generally have a lower minimum inductance than motors known in the art, and the ratio of maximum inductance to minimum inductance is generally higher which improves the contribution of reluctance torque. Motors employing the invention also generate less noise at high speeds than motors known in the art because there are less air spaces in the rotor.
Motors employing the invention are generally less expensive to manufacture than those known in the art, but there are compromises between cost and noise. Rectangular neo magnets are less expensive than neo magnets of other shapes, but they allow some air spaces when used with an arc shaped ferrite magnet. Two small neo magnets generally conform to the arc shaped ferrite magnet better than one large neo magnet. However, using two small magnets may require a die used to form slots in a rotor core to have intricate details which means that the die will not last as long as a die that has less intricate details. Die life can be increased by not conforming to every detail of the magnets, but this will allow for air spaces which will increase acoustic noise when the motor is operating at high speeds. Because of their reduced cost, reduced acoustic noise, and reduced electrical noise, motors according to the invention may be advantageously applied in consumer appliances such as horizontal washing machines, dish washers and clothes dryers.
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In yet another embodiment, the present invention is a method of manufacturing an IPM motor having a rotor wherein a ferrite magnet and a neo magnet are both located in the same slot. One or more slots are formed in a cylindrical rotor core having a central longitudinal axis about which the core rotates. The neo magnet is inserted in the slot. The ferrite magnet is placed in the slot between the neo magnet and the central longitudinal axis of the cylindrical core. The ferrite magnet is arc shaped when viewed in cross section relative to the central longitudinal axis. The neo magnet is rectangular when viewed in cross section relative to the central longitudinal axis. The slot may be precisely complementary to the outline of the combined ferrite and neo magnets so as to minimize air spaces, or it may have a trapezoidal area around the rectangular neo magnet. The rotor core is secured within a stator having windings, and a commutation circuit energizes the windings. A magnetic field of the stator interacts with the magnets in the rotor causing the rotor to turn.
It is contemplated that aspects of the embodiments described above may be combined in numerous ways without deviating from the invention. For example, the embodiment shown in
Some embodiments of the invention have advantages over other embodiments. For example, using two rectangular (i.e., viewed in cross section) pieces of neo magnet allows small air spaces than one larger piece of neo magnet because they better conform to the curvature of the ferrite magnet. Embodiments of the invention utilizing a trapezoidal slot will generally have a higher maximum inductance than embodiments utilizing a precision slot because a precision slot tends to increase leakage flux. Embodiments using lobed rotor cores generally have a lower cogging torque and more sinusoidal back EMF than embodiments using cylindrical rotor cores. Also, embodiments with a neo magnet further from the center of the rotor than the ferrite magnet tend to develop a higher maximum inductance than embodiments with neo magnets closer to the center than the ferrite magnet.
The above description is also applicable to other motor configurations such as inside out motors and/or motors having windings in the rotor and permanent magnets in the stator, and visa versa. For example, embodiments of the invention in an inside out motor include neo and ferrite magnets located in a single slot. Magnet configurations and air space considerations are similar to those of the above described rotor designs.
This description refers to ferrite and neo throughout, but one skilled in the art will recognize that magnetic materials other than neo and ferrite may be used without deviating from the invention and more than one piece of neo and/or ferrite may be used in each slot. One skilled in the art will also notice that different shapes of neo magnets, ferrite magnets, and slots are possible without deviating from the invention. The cylindrical rotor core may be made with steel or some other material. The description refers to an IPM motor rotor throughout, but one skilled in the art knows that an electric motor may be configured as a generator.
Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.
The order of execution or performance of the methods illustrated and described herein is not essential, unless otherwise specified. That is, it is contemplated by the inventors that elements of the methods may be performed in any order, unless otherwise specified, and that the methods may include more or less elements than those disclosed herein. For example, it is contemplated that executing or performing a particular element before, contemporaneously with, or after another element is within the scope of the various embodiments of the invention.
When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.
As various changes could be made in the above products and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
Claims
1. An electric motor rotor comprising:
- a core having a central longitudinal axis and a slot radially spaced from the longitudinal axis and extending parallel to the axis; and
- first and second magnets positioned in the slot and extending parallel to the longitudinal axis, wherein the first magnet is positioned between the second magnet and the longitudinal axis.
2. The rotor of claim 1 wherein said core has an outer surface parallel to said longitudinal axis, said outer surface having a lobe.
3. The rotor of claim 2 further comprising an air space adjacent to the first or second magnet.
4. The rotor of claim 1 wherein, when viewed in cross section, the first magnet is arch-shaped, having a convex surface facing the central longitudinal axis and a concave surface facing the second magnet.
5. The rotor of claim 4 wherein at least one of the following:
- (1) when viewed in cross section, the second magnet has a convex surface generally complementary to and in contact with the concave surface of the first magnet; and
- (2) when viewed in cross section, at least a portion of the first magnet contacts the concave surface of the second magnet.
6. The rotor of claim 1 further comprising a third magnet of the same material as the second magnet sized and shaped substantially the same as the second magnet.
7. The rotor of claim 6 wherein, when viewed in cross section, the second and third magnets are substantially rectangular and at least a portion of each is in contact with the first magnet.
8. The rotor of claim 1 wherein one of the first or second magnets is strontium ferrite, the other of the first and second magnets is neodymium-iron-boron, and the core is steel.
9. A method of producing an electric motor comprising:
- forming a slot in a rotor core having a central longitudinal axis;
- inserting a first magnet in the slot;
- inserting a second magnet in the slot, wherein said first magnet is substantially between the second magnet and the central longitudinal axis;
- inserting the rotor core into a stator having windings; and
- connecting the windings of the stator to a commutation circuit.
10. The method of claim 9 wherein the first magnet is strontium ferrite, the second magnet is neodymium-iron-boron, and the core is steel.
11. An electric motor comprising:
- a rotor including: a core having a central longitudinal axis and a slot radially spaced from the longitudinal axis and extending parallel to the axis; and first and second magnets positioned in the slot and extending parallel to the longitudinal axis, wherein the first magnet is positioned between the second magnet and the longitudinal axis; a stator in magnetic coupling relation to the rotor having windings; and a commutation circuit electrically connected to the windings of the stator.
12. The rotor of claim 11 wherein said core has an outer surface parallel to said longitudinal axis, said outer surface having a lobe.
13. The motor of claim 12 further comprising an air space adjacent to the first or second magnet.
14. The motor of claim 11 wherein, when viewed in cross section, the first magnet is arch-shaped, having a convex surface facing the central longitudinal axis and a concave surface facing the second magnet.
15. The motor of claim 14 wherein at least one of the following:
- (1) when viewed in cross section, the second magnet has a convex surface generally complementary to and in contact with the concave surface of the first magnet; and
- (2) when viewed in cross section, at least a portion of the first magnet contacts the concave surface of the second magnet.
16. The motor of claim 11 further comprising a third magnet of the same material as the second magnet sized and shaped substantially the same as the second magnet.
17. The motor of claim 16 wherein, when viewed in cross section, the second and third magnets are substantially rectangular and at least a portion of each is in contact with the first magnet.
18. The motor of claim 17 wherein, when viewed in cross section, the second and third magnets are spaced apart.
19. The motor of claim 11 wherein the rotor has at least two slots equally spaced radially and circumferentially about the longitudinal axis.
20. The motor of claim 11 wherein one of the first or second magnets is strontium ferrite, the other of the first and second magnets is neodymium-iron-boron, and the core is steel.
Type: Application
Filed: Dec 19, 2005
Publication Date: Jul 12, 2007
Applicant: Emerson Electric Co. (St. Louis, MO)
Inventor: Gary Horst (Manchester, MO)
Application Number: 11/311,798
International Classification: H02K 21/12 (20060101);