Title of Invention	A COPPER ALUMINIUM DIE CAST ROTOR FOR AN INDUCTION MOTOR
Abstract	A copper aluminium die cast rotor for an induction motor wherein 30% of the rotor slots are filled with aluminium and the balance number of rotor slots are filled with copper bars, already introduced before die casting; the end rings being formed in aluminium by die casting, whereby a through short circuiting of end rings with the copper/aluminium bars is ensured.

Full Text

This invention relates to a copper aluminium die cast rotor for an induction motor. The rotor proposed herein, apart from being cost effective, improves efficiency of the induction motor during operation. It increases production by reason of improved efficiency. Rotors with either copper bars or aluminium as conductors are known. When copper bars are used as conductors they are joined at both ends of the rotor stack with either forged copper end rings or with thin copper plates either Tig welded or braced with special silver alloys. When aluminium is used as conductors the rotor is die cast with aluminium and the end rings get formed integrally with the aluminium conductors by gravity or pressure die casting. Copper has a conductivity of 162% of Aluminium making it as the world's first choice as an effective conductor. Hence using copper as a conductor in the rotor slots reduces the resistivity to 60% of that of aluminium. This gives rise to greater speed and lower losses. However the cost of copper is very high compared to aluminium. Using aluminium die cast rotor may be cheaper but the rotor loss will be heavy which will affect the efficiency of the motor. Since conservation of energy at all levels and fields has become imperative the world over, the use of energy efficient motors has almost become mandatory. This calls for a reduction of losses in the motor. Of the various losses in an A.C. electric motor, stator and rotor copper loss constitute a major factor which forms 25% of losses in motor. The use of copper in rotor will reduce the losses by minimum 10% in all motors making it energy efficient. Die casting with copper in place of aluminium has been attempted to reduce losses and to improve efficiency. In the case of submersible pumps the size of the rotor is so small that invariably the length will be at least 3 times its diameter which makes the copper die casting process very difficult. The temperature for making the copper flow is very high around 1350°C making the process difficult and expensive. This high temperature results also in oxygen pickup resulting in air pockets in the copper making the copper die casting to some extent ineffective. Copper being hygroscopic picks up oxygen from atmosphere and having a melting point near 1350°c requires special technology for die casting and also special die material to withstand the high temperature. Internationally either copper rotors with copper bars inside the slots of rotor stampings - with copper end rings or aluminium die cast rotors are now used. Since copper die casting is in the infant stage of development and due to the inherent difficulties of copper die casting and its increased cost, we have considered only the conventional copper rotor and the aluminium die cast rotor for our analysis. In an attempt to optimize the reduction of rotor loss and the cost a combination of copper and aluminium as conductors, with aluminium as end rings have been experimented and found to be successful. Both aluminium and copper are used as conductors in the rotor proposed herein. 30% of the rotor slots are filled with aluminium and the balance with copper bars already introduced before die casting. The end rings are formed in aluminium by die casting. This ensures a thorough short circuiting of end ring with copper / Aluminium rotor bars and is better than welding. The new combination of copper and aluminium as conductors in the rotor of A.C. induction motors, takes advantage of higher conductivity of copper and easier die castability of aluminium. In our experimentation we took a IHP 4" submersible pump motor. The technical details of the rotor are: Rotor Dia = 45mm Length of rotor - L2 =180mm Slot shape -wedge shaped Slot area - Ag = 32 sq.mm No. of slots - S2 = 14 Shaft OD - D = 18mm End ring area - Ar =75 sq.mm for copper rotor Ar to be adjusted for getting same resistance for the copper Aluminium die cast rotor.. End ring OD Dr=44mm Bar area - Ab = 95% of slot area for aluminium and 90% of slot area for copper conductor for easy assembly. The motor is a 2 pole motor. We will consider the Rotor resistance for both conventional copper rotor and copper aluminium die cast rotor. Resistance constant = 0.693m (C.kw)2 x 10-2 where C = Total number of series conductors per phase. kw = Winding factor. and m = Number of phase. For single phase m = 2 Since (Ckw)2 is the same for both type of rotors the rotor resistance is proportional to the resistance (bar + ring) in both cases. Hence the resistance constant is not considered in our exercise. L2 = Length of rotor where SK = skew of the rotor which is nil in our case. Cdbr = deep bar correction for resistance and in our case it is 1. S2 = Number of slots in rotor Ab= Area of bar. Cp/100 is 1 for copper and 0.6 for Aluminium, where Dr = Diameter of ring Ar = Area of ring Kring = Ring factor = 0.85 P = Pole = 2 (in our case) The Aluminium fills 95% of slot area since it has a higher coefficient of linear expansion in high temperature leading to contraction at room temperature. Substuting the values the equation 11 becomes Taking the case of a copper rotor. Length of rotor - L2 = 180mm Copper rotor end ring area - Ar= 75 sq.mm Copper bar area Ab= 32 x 0.9 sq.mm For copper rotor Substituting in Equation III Ring = 0.135 D/Ar = 0.08 (Since Cp/100 for copper =1) Resistance (Bar + Ring) = 0.446 + 0.08 = 0.526 For copper Aluminium die cast rotor with 11 bars in copper and 3 bars in Aluminium and end ring in Aluminium. = 0.471. (Since Cp /100 for Aluminium = 0.6) The gap between copper rod and rotor slot is 0.05 and this will be filled up by Aluminium during Aluminium die casting process and hence taken in to consideration for computing Bar resistance. For getting the same total resistance for the copper Aluminium die cast rotor the resistance of Ring (Aluminium) Resistance = 0.526 - 0.471 = 0.055 Resistance Ring = 0.055 = 0.135 Dy(Ar Cp/lOO) = 180.63 sq.mm Since - Cp /100 for Aluminium = 0.6 Cost Comparison between the two types of Rotors Price of copper bar = Rs.230/- kg Price of Aluminium = Rs. 100/- kg Price of copper ring =Rs.430/-kg Cost of copper scrap = Rs.l30/- kg The reason for extra cost of copper ring is due to low utilization factor of copper sheet. The ring is either made of copper sheets or forged where tiie utilization factor is as low as 33%. Copper end ring = [cost of copper sheet x 1/U.F]- [(1/U.F -1) cost] of scrap. where U.F = Utilization factor =1/3 Cost of ring = 3x230-(3-l)xl30 =690 - 260 = Rs.430/-. kg Now cost of copper rotor = cost of Bar + cost of ring Length of copper Bar = 180+15+15 = 210 mm (since at both ends the bars have to project by 15mm for welding with the rings) Weight of copper Bar = 14x210x32x0.9x8.9 / 1000x1000 -0.754 kg (where 8.9 is the specify gravity of copper) Ring Dr - O.D of ring = 44mm Bore of Ring = 19mm Dm Mean dia =31.5 mm (for 2 rings one on either side) Material cost for copper rotor = 0.754x230+0.132x430 = Rs.230.18 Labour cost for copper rotor = Rs.40 Total cost of copper rotor = Material + labour = 230.18 + 40 = Rs.270.18 or Rs.270/- (This does not take into consideration the cost of rotor stamping) Cost of copper Aluminium die cast rotor: Bar cost of copper bars in 11 slots Length of copper bar = 180 + 10 + 10 = 200mm (Since the end ring is Aluminium die cast the extension of bar on either side is limited to 10mm) Cost of Aluminium = Rs.5.58 (Sp gravity of Aluminium 2.85) Weight of 2 rings = 2XTTX3 1.5x180x2.85x100/106- Rs.10.15 Total material cost = 129.70+5.58+10.15 = 145.43 Labour for die casting = Rs.30/- rotor Total cost of copper Aluminium die cast rotor = 145.43+30.00 = 175.43 or Rs.175.50 In the case of copper and aluminium die cast rotor there is no scrap generation since the runners, risers and flash outs can be remelted and used. The cost of copper rotor = Rs.270.00 Cost reduction per rotor = Rs.94.50 which is almost 35% The total resistance in both the rotor is the same. The advantages of this copper aluminium die cast rotor are: 1. High productivity - As the process is simple and does not require complicated high cost machinery. 2. Zero defect rotors - As there an no air pick ups due to high temperature for melting of copper. 3. Cost of manufacture reduced by nearly 30% of copper die cast rotor. 4. Equipment for aluminium die casting is simple and the existing machinery can be used - conventional die casting system can be adopted without modification. 5. For copper die casting the machinery required is costly as a very high temperature more than 1300° C is to be maintained as the melting temperature of copper is very high compared to the melting temperature of aluminium. 6. Due to bar tightness in copper aluminium die cast rotor -stray losses reduced. 7. Higher efficiency due to lower rotor resistance compared to conventional aluminium die cast rotor. 8. Difficulty in copper die casting due to oxygen pick up is avoided. 9. Performance equivalent to copper rotor. 10.Noise and vibration reduced very much. In the copper aluminium die cast rotor for an induction motor, according to this invention, 30% of the rotor slots are filled with aluminium and the balance number of rotor slots are filled with copper bars, already introduced before die casting;the end rings being formed in aluminium by die casting, whereby a thorough short circuiting of end rings with the copper / aluminium bars is ensured. The method of manufacture of a copper aluminium die cast rotor for an induction motor, according to this invention, comprises the steps of assembling stampings to required core length, guided with a mandrel and pressed to specified core length; inserting wedge shaped copper rods in all rotor slots excepting three slots which are substantially equally spaced: loading the pressed rotor stampings along with the mandrel into a pressure die casting machine and locking the same inside the machine, the injection pressure being predetermined, depending upon the said core length; pouring molten aluminium at 900 degree C into the die casting machine and injecting into the rotor already locked inside the machine with the said predetermined pressure; releasing the said die cast rotor from the said machine and air cooling the same; removing the mandrel; cleaning the die cast rotor and finishing the same by machining. Reference will now be made to the accompanying drawings which illustrate by way of example, and not by way of limitation, one of possible embodiments of the rotor proposed herein. Fig. 1 illustrating the embodiment in section and Fig. 2 illustrating the said embodiment in exploded view. In the drawings the following numerals indicate the corresponding parts noted below: Aluminium conductors 1 Copper conductors 2 End ring 3 Rotor stamping 4 Rotor shaft 5 The rotor proposed herein is manufactured in the following manner: Rotor stampings are assembled to required core length and guided with a suitable mandrel and pressed to specified core length. Wedge shaped copper rods are inserted in all rotor slots excepting three slots which are substantially equally spaced. The pressed rotor stampings along with the mandrel will be loaded into a pressure die casting machine and locked inside the machine. Depending upon the core length the injection pressure is pre-set in the machine. Pure aluminium ingots are melted in a separate furnace to a temperature of 900oC. The molten aluminium at 900o0 is poured into the receptacle of the die casting machine and injected into the rotor already locked inside the machine with a pre-set pressure by automatic control. The die cast rotor is released from the machine after 30 seconds and then air cooled. The mandrel is removed and the runners and risers and flash out removed. The die cast rotor is then cleaned and taken for machining. As the melting temperature of aluminium is 900oC only compared to 1350o0 of copper no special and complicated arrangements for withstanding high temperature is needed in the conventional die casting machine. It will be appreciated from the foregoing that various other embodiments of the rotor proposed herein are possible without departing from the scope and ambit of this invention. We Claim: lA copper aluminium die cast rotor for an induction motor wherein 30% of the rotor slots are filled with aluminium and the balance number of rotor slots are filled with copper bars, already introduced before die casting; the end rings being formed in aluminium by die casting, whereby a thorough short circuiting of end rings with the copper / aluminium bars is ensured. 2.A copper aluminium die cast rotor for an induction motor substantially as herein described with reference to, and as illustrated in the accompanying drawings. 3.A method of manufacture of a copper aluminium die cast rotor for an induction motor comprising the steps of assembling stampings to required core length, guided with a mandrel and pressed to specified core length; inserting wedge shaped copper rods in all rotor slots excepting three slots which are substantially equally spaced: loading the pressed rotor stampings along with the mandrel into a pressure die casting machine and locking the same inside the machine, the injection pressure being predetermined, depending upon the said core length; pouring molten aluminium at 900 degree C into the die casting machine and injecting into the rotor already locked inside the machine with the said predetermined pressure; releasing the said die cast rotor from the said machine and air cooling the same; removing the mandrel; cleaning the die cast rotor and finishing the same by machining. 4.A method of manufacture of a copper aluminium die cast rotor for an induction motor substantially as herein described with reference to and as illustrated in the accompanying drawings. Dated this the 23RD June 2005

Documents:

0787-che-2005 claims duplicate.pdf

0787-che-2005 description (complete) duplicate.pdf

787-che-2005-abstract.pdf

787-che-2005-claims.pdf

787-che-2005-correspondnece-others.pdf

787-che-2005-correspondnece-po.pdf

787-che-2005-description(complete).pdf

787-che-2005-drawings.pdf

787-che-2005-form 1.pdf

787-che-2005-form 26.pdf

787-che-2005-form 9.pdf