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
Squirrel cage AC induction motor
APPLICANTS
Crompton Greaves Limited. CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian company
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF INVENTION
This invention relates to a squirrel cage AC induction motor.
BACKGROUND OF INVENTION
AC inductor motors are generally classified into squirrel cage induction motors and wound-rotor induction motors based on the type of rotor construction. Induction motors have a variety of industrial applications as prime movers, for example, in pumps, crushers, compressors, fans or mills. A squirrel cage induction motor comprises a cylindrical rotor rotatably disposed within a stator having stator windings. The rotor comprises a rotor body having a plurality of electrical grade steel laminations press formed together. The rotor body is mounted on a shaft with end plates pressed against the ends of the rotor body. The end plates each comprises a plurality of spaced apart upwardly directed straight teeth at the outer circumference thereof.
The rotor also comprises a plurality of longitudinal conductor slots extending axially along the inner circumference of the rotor body in radially spaced apart relationship with one another and a plurality of rotor conductors extending through the conductor slots and protruding out from the ends of the rotor. The rotor further comprises a pair of short circuiting rings, each ring being disposed at each end of the rotor body and comprising a plurality of slits transversely extending through the circumference thereof matching with the protruding ends of the rotor conductors. Each of the short circuiting rings is engaged to the protruding ends of the rotor conductors at each end of the rotor through the slits and welded and short circuited to the respective protruding ends of the rotor conductors.

During operation of the motor, a current is passed through the stator windings to induce a current in the rotor and create a magnetic field around the rotor to cause the rotor to rotate. Induced current in the rotor flows through the rotor conductors and short circuiting rings. The rotor rotates at high speeds and develops high centrifugal forces. High current also passes through the rotor conductors and short circuiting rings generating excess heat at the surface area of contact or joint between the short circuiting rings and rotor conductors. Unless the joint between the conductor ends and short circuiting rings is mechanically strong and electrically enduring, it will easily fail under the high centrifugal forces and excess heat development.
The short circuiting rings are generally welded to the conductor ends by induction brazing using copper as the weld material. Although high temperatures of the order of 450°C are developed during induction brazing, because of the good thermal and mechanical properties of copper, the copper conductors and rings can be welded together by induction brazing. However, induction brazing is expensive and time consuming and cumbersome to carry out. Besides, although the surface area of contact or joint between the conductor ends and rings is narrow and limited, because of the good thermal and mechanical properties of copper, the narrow and limited surface area of contact between the rotor conductors and short circuiting rings serves as a mechanically strong and electrically enduring joint between the rotor conductors and short circuiting rings to withstand the adverse effects of high centrifugal forces and excess heat development.
Copper is an expensive metal and is becoming scarce day by day. Although aluminium is known as an alternative to copper as a cheap conductor material, to the best of our knowledge and information, squirrel cage induction motor rotors comprising aluminium rotor conductors and

aluminium short circuiting rings are not known to be used probably because of the low mechanical strength, low resistivity and low melting properties of aluminium and unsuitability of aluminium for induction brazing at high temperatures. Aluminium will melt and fuse at such high temperatures. Furthermore, because of the low mechanical strength, resistivity and melting properties of aluminium, a narrow and limited joint between aluminium short circuiting rings and rotor conductors would not be mechanically strong enough and electrically enduring to withstand the adverse effects of high centrifugal forces and excessive heat development at the surface area of contact between the rings and conductor ends during operation of the motor. There is thus need for squirrel cage AC inductor motors based on aluminium rotors.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided a squirrel cage AC induction motor comprising a cylindrical rotor rotatably disposed within a stator, wherein the rotor comprises a rotor body having a plurality of electrical grade steel laminations press formed together and mounted on a shaft with end plates pressed against the end of the rotor body, the end plates each comprising a plurality of spaced apart upwardly directed straight teeth at the outer circumference thereof and wherein the rotor further comprises a plurality of longitudinal conductor slots extending axially along the inner circumference of the rotor body in radially spaced apart relationship with one another and a plurality of rotor conductors extending through the conductor slots in the rotor body and protruding out from the ends of the rotor and a pair of short circuiting rings, each ring being disposed at each end of the rotor body and having a plurality of slits transversely extending through the circumference thereof matching with the protruding ends of the rotor conductors and each ring being engaged to the respective protruding ends of the rotor conductors

through the slits and short circuited to the respective protruding ends of the rotor conductors and wherein the rotor conductors and short circuiting rings are made of aluminium and the short circuiting rings are short circuited to the respective protruding ends of the rotor conductors by metal inert gas welding with aluminium as the weld material.
According to an embodiment of the invention, the rotor conductors each comprises an L-shaped cut at the protruding end thereof defining a lower face and a raised face and the short circuiting rings are engaged to the protruding ends of the rotor conductors through the slits in the rings and located against the raised face at the protruding ends of the rotor conductors.
According to an embodiment of the invention, the short circuiting rings and protruding ends of the rotor conductors are cut circumferentially partially and the outer face of the rings is simultaneously formed with depressions to increase the surface area at the outer face of the rings to provide a thick and large of weld joint between the rings and respective protruding ends of the rotor conductors continuously around the outer face of the rings flush with the outer face of the rings and thereby to provide a mechanically strong and electrically enduring weld joint between the short circuiting rings and the respective protruding ends of the rotor conductors.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS In the accompanying schematic drawings :
Fig 1 is a cross-sectional view of a squirrel cage induction motor according to an embodiment of the invention;

Fig 2 is an isometric view of the rotor of the induction motor of Fig 1 according to an embodiment of the invention;
Figs 3 and 4 are enlarged scrap views at A and B in Fig 2, respectively;
Fig 5 is an isometric view of the rotor body of the rotor of Fig 2;
Fig 6 is an isometric view of an end plate of the rotor of Fig 2;
Fig 7 is an isometric view of the rotor body of Fig 5 and end plate of Fig 6 mounted on a shaft;
Fig 8 is an isometric view of the rotor body of Fig 7 with rotor conductors extending through the rotor body and protruding out of the ends of the rotor body;
Fig 9 is an isometric view of the rotor body of Fig 8 with the protruding ends of the rotor conductors formed with L-shaped cuts;
Fig 10 is an enlarged scrap view at C in Fig 9;
Figs 11 and 12 are side view and isometric view of a short circuiting ring of the rotor of Fig 2, respectively;

Fig 13 is an isometric view of the rotor body of Fig 9 assembled with the short circuiting rings of Figs 11 and 12 at the ends thereof;
Fig 14 is an enlarged scrap view at D in Fig 13;
Fig 15 is an isometric view of the rotor body of Fig 13 with an L-shaped notch formed at the outer face of the short circuiting rings; and
Figs 16 and 17 are enlarged scrap views at E and F in Fig 15, respectively.
DESCRIPTION OF EMBODIMENTS OF INVENTION
As illustrated in Fig 1 of the accompanying drawings, the squirrel cage AC induction motor 1 comprises a cylindrical rotor 2 rotatably disposed within a stator 3 with stator windings 4. The motor works in known manner. The rotor 2 as illustrated in detail in Figs 2 to 17 of the accompanying drawing comprises a rotor body 5 having a plurality of electrical grade steel laminations 6 press formed together and mounted on a shaft 7 with end plates 8 pressed against the ends of the rotor body. The end plates each comprises a plurality of spaced apart upwardly directed straight teeth 9 at the outer circumference thereof. Shaft hole and air vents in the rotor body are marked 10 and 11, respectively. Matching shaft hole and air vents in the end plates are marked 12 and 13, respectively. A plurality of longitudinal conductor slots 14 extend axially along the inner surface of the rotor body in radially spaced apart relationship with one another. A plurality of rotor conductors 15 extend through the conductor slots and protrude out from the ends of the rotor. The rotor also comprises a pair of short circuiting rings 16 each disposed at

each end of the rotor. Each ring has a plurality of slits 17 transversely extending through the circumference thereof matching with the protruding ends of the rotor conductors.
Each of the rotor conductors comprises an L-shaped cut 18 at the protruding end thereof defining a lower face 19 and a raised face 20 (Figs 9 and 10). The L-shaped cut is formed at the protruding ends of the rotor conductors by machining, preferably, turning in a lathe machine (not shown). The short circuiting rings are engaged to the protruding ends of the respective rotor conductors through the slits 17 in the rings 16 and located against the raised faces 20 at the protruding ends of the rotor conductors (Figs 13 and 14). The short circuiting rings and protruding ends of the rotor conductors are cut circumferentially partially and the outer face of the rings are simultaneously formed with depressions 24 (Figs 15 - 17). The depressions 24 in the rings are simultaneously formed by machining, preferably, by turning in a lathe machine (not shown). Following this, the short circuiting rings 16 are welded and short circuited to the protruding ends of the rotor conductors by forming a weld joint 25 therebetween in the depressions 24 by MIG (metal inert gas) welding using aluminium as the weld material.
According to the invention the rotor conductors 15 and short circuiting rings 16 are made of aluminium, preferably, by forging. The short circuiting rings are short circuited to the respective protruding ends of the conductors by MIG welding with aluminium as the weld material. The depressions 24 formed at the outer face of the short circuiting rings increase the surface area at the outer face of the rings to provide a thick and large weld joint 25 between the rings and respective protruding ends of the rotor conductors continuously around the outer face of the rings flush with the outer face of the rings and thereby to provide a mechanically strong and

electrically enduring weld joint between the short circuiting rings and respective protruding ends of the rotor conductors.
Because of the aluminium rotor conductors and short circuiting rings there is substantial material cost benefit for the motor of the invention. Temperatures developed during MIG welding are of the order of 150°C. Because of the depressions formed at the outer face of the short circuiting rings, surface area for deposition of weld material is substantially increased to provide correspondingly large and thick mechanically strong and electrically enduring weld joint between the protruding ends of the rotor conductors and short circuiting rings. The L-shaped cuts 18 at the protruding ends of the rotor conductors help to reduce the effective outer diameter of the rings and the overall weight of the rotor. MIG welding of aluminium conductors and short circuiting rings is also easier and cheaper as compared to induction brazing of copper conductors and short circuiting rings.
It is understood that the mechanically strong and electrically enduring weld joint between the protruding ends of the aluminium rotor conductors and aluminium short circuiting rings can be formed in a different manner. Such a variation of the invention is obvious to a person skilled in the art and should be construed and understood to be within the scope of the invention.

We claim :
1. A squirrel cage AC induction motor comprising a cylindrical rotor rotatably disposed within a stator, wherein the rotor comprises a rotor body having a plurality of electrical grade steel laminations press formed together and mounted on a shaft with end plates pressed against the end of the rotor body, the end plates each comprising a plurality of spaced apart upwardly directed straight teeth at the outer circumference thereof and wherein the rotor further comprises a plurality of longitudinal conductor slots extending axially along the inner circumference of the rotor body in radially spaced apart relationship with one another and a plurality of rotor conductors extending through the conductor slots in the rotor body and protruding out from the ends of the rotor and a pair of short circuiting rings, each ring being disposed at each end of the rotor body and having a plurality of slits transversely extending through the circumference thereof matching with the protruding ends of the rotor conductors and each ring being engaged to the respective protruding ends of the rotor conductors through the slits and short circuited to the respective protruding ends of the rotor conductors and wherein the rotor conductors and short circuiting rings are made of aluminium and the short circuiting rings are short circuited to the respective protruding ends of the rotor conductors by metal inert gas welding with aluminium as the weld material.
2. The AC induction motor as claimed in claim 1, wherein the rotor conductors each comprises an L-shaped cut at the protruding end thereof defining a lower face and a raised face and the short circuiting rings are engaged to the protruding ends of the rotor

conductors through the slits in the rings and located against the raised face at the protruding ends of the rotor conductors.
3. The AC induction motor as claimed in claim 2, wherein the short circuiting rings and the protruding ends of the rotor conductors are cut circumferentially partially and the outer face of the rings is simultaneously formed with depressions to increase the surface area at the outer face of the rings to provide a thick and large weld joint between the rings and respective protruding ends of the rotor conductors continuously around the outer face of the rings flush with the outer face of the rings and thereby to provide a mechanically strong and electrically enduring weld joint between the short circuiting rings and the respective protruding ends of the rotor conductors.
4. The AC induction motor as claimed in claim 1, 2 or 3, wherein the aluminium rotor conductors and short circuiting rings are made by forging.