FORM 2 THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
& The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
An axial gap electric motor
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Dr. Samsul Ekram of Crompton Greaves Ltd, APDTC,CG Global R&D Centre, Crompton Greaves Ltd., Kanjur (E), Mumbai 400042, Maharashtra, India, an Indian National and Dr. Min-Fu Hsieh, Associate Professor, Department of Systems and Naval Mechatronic Engineering of the National Cheng Kung University, No. 1 University Road, Tainan City - 701, Taiwan (ROC), a Taiwanese National

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:


FIELD OF THE INVENTION
The invention relates to an axial gap electric motor. BACKGROUND OF THE INVENTION
In a permanent magnet rotating machine such as an axial gap electric motor, a pair of rotors is disposed on the opposite sides of the stator and maintaining an air gap there between. Permanent magnets are provided on the outer surface of the rotor facing the stator. The permanent magnets are arranged in such a way that their North and South poles alternate along the surface of the rotor. When electricity is supplied to the stator windings of the stator, a rotating magnetic field is created thereby rotating the rotor. In such axial gap electric motors, a rotational torque ripple called cogging torque is produced leading to vibration, noise and degraded performance of the controllability of the motor. Conventionally, such cogging torque is reduced by arranging the permanent magnets offset or skewed in the direction of the circumferential direction of the rotor produce a skewing effect. Techniques for reducing the cogging torque are specific for the design and application of the motor. The amount of cogging torque being reduced in motors has always been insufficient with the conventional methods of theoretically deducing the appropriate skew angle of the permanent magnets. Skewing the permanent magnets to a skewing angle calculated by means of conventional techniques of theoretically deducing the skew angle do not take into account the effect of the skewing on the dynamic performance of the motor.
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OBJECTS OF THE INVENTION
An object of the invention is to provide an axial gap electric motor with a reduced cogging torque.
Another object of the invention is to provide axial gap electric motor with a decreased cogging torque without disturbing the motor dynamic performance. DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided an axial gap electric motor comprising a shaft axially extending through a motor housing, a stator disposed in the housing on the shaft and a pair of rotors disposed over the shaft on opposite sides of the statorf one rotor being fitted on one end of the housing and the other rotor to the other end of the housing, the rotors each comprising of a plurality of permanent magnets arranged
in a skewed configuration wherein the skewing angle of each magnet is —(R0 - R1)
where P is the number of pole pairs and R1 and R0 being the inner and outer radius of the permanent magnet from the centre of the housing.
The following is a detailed description of the invention with reference to the accompanying drawings, in which
Fig 1 is an isometric view of an axial gap electric motor;
Fig 2a is a perspective view of a stator and permanent magnet's arrangement in an axial gap electric motor;
Fig 2b is a front view of a stator and permanent magnets arrangement in an axial gap electric motor
Fig 2c illustrates a skew angle in the permanent magnets of the axial gap electric motor;
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Fig 1 is an isometric view of an axial gap electric motor.The axia! gap electric motor has a stator 1 mounted on a fixing plate 2. The fixing plate 2 is mounted over a stationary shaft 3. The stator 1 has stator slots 4 for housing the stator windings (not shown in the fig). A pair of rotors 5 and 6 is arranged coaxially to the shaft 3 and opposedly on both side surfaces of the stator 1 with a Predetermined gap between the stator 1 and rotors 5 and 6. Each rotor 5 and 6 comprises of a back iron plate 7 ancj 8 with permanent magnets 9 and 10 attached on its outer surface respectively such that the permanent magnets 9 and 10 face the stator 1. The north and south pole permanent magnets are alternately arranged on the back plates 7 and 8. The permanent magnets 9
and 10 of the back plates 7&8 respectively are symmerically arranged with respect to
each other. The rotors 5 and 6 and the stator 1 are all enclosed in a motor housing 11 having end shield plates 12 and 13 as removable closures t0 seai the housing 11 from the opposite sides thereof. The back plates 7 and 8 are affixed respective end shields 12 and 13. When electricity is supplied to the stator wingings of the stator 1, a rotating magnetic field is created thereby rotating the rotors 5 and 6 as well as the end shields 12 and 13 and the motor housing 11. The end shields 12 and 13 and the motor housing 11 is rotated by virtue of the back plates 7 and 8 being affixed to the respective end shields 12 and 13 and the end shields 12 and 13 being further affixed to the motor housing 11. Various other mechanisms including the stator I and rotors 5 and 5 are housed in the motor housing 11 which are not shown in the figure.
Fig 2a is a perspective view of a stator and permant magnet's arrangement in an axial gap electric motor. Fig 2b is a front view of the same A predetermined gap is maintained between the stator 1 and permanent magnets 9 and to of the rotors. The North
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and South Pole permanent magnets 9 and 10 are alternately arranged on the back plates 7 and 8. The permanent magnets 9 of the back plate 7 are symmetric with respect to the position of the permanent magnets 10 of the back plate 8. To counter the cogging torque, each of the permanent magnets 9 and 10 are offset or skewed. The skewing angle 0 is calculated by the formula
where P is the number of pole pairs and R1 and R0 being the inner and outer radius of the permanent magnets 9 and 10 from the centre of the housing 11. By means of the above equation, a skew angle of the permanents magnets can be calculated for any rating of the motor as well as for any pole slot combination. Further, a least cogging torque is generated in the motor wherein the skew angle of the permanent magnets is determined by means of the above equation.
According to the invention, the axial air gap electric motor wherein the skew angle is calculated by the above equation is same for the inner and outer radius of each permanent magnet. Therefore the size and shape of the all permanent magnets including the North and South poles is the same thereby eliminating the possibility of confusion in the poles of the magnetss while being manufactured. Farther, the skew angle can be calculated for any pole/slot ratio.
Instead of a symmetric arrangement of the per-manent magnets, asymmetric arrangement of the permanent magnets can be arrangeq. Further, the design and the configuration of the motor in general may vary. For example, instead of a stationary shaft, there can be a moving shaft. Instead of having a pair of rotors on either sides of the
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stator, there can be pair of stator on either side of the single rotor, as well as also possibility of combination of one rotor and one stator
Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the invention as defined.
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We claim:
1. An axial gap electric motor comprising a shaft axially extending through a motor
housing, a stator disposed in the housing on the shaft and a pair of rotors disposed
over the shaft on opposite sides of the stator, one rotor being fitted on one end of
the housing and the other rotor to the other end of the housing, the rotors each
comprising of a plurality of permanent magnets arranged in a skewed
configuration wherein the skewing angle of each magnet is —(R0 - Rf) where P
is the number of pole pairs and Rj and RQ being the inner and outer radius of the permanent magnet from the centre of the housing.
2. The motor as claimed in claim 1, wherein the motor housing comprises of a main body and a closure for the main body.
3. The motor as claimed in claim 1, wherein the permanent magnets on each of the rotor are symmetric to each other.
4. The motor as claimed in claim 1, wherein the permanent magnets on each of the rotor are asymmetric to each other.

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