Title of Invention

"AN IMPROVED ROTOR ASSEMBLY ADAPTABLE TO A HIGH-RATING INDUCTION MOTOR"

Abstract

An improved process for building a rotor cage of an induction motor comprising the steps of: - arranging a pre-determined groove in a short-circuit ring of the rotor construction; - inserting a plurality of rotor bars in a rotor core of said rotor construction; - correspondingly machining both ends of said plurality of rotor bars so as to maintain a uniform level; - placing said rotor vertically over said short-circuit ring such that all said plurality of rotor bar ends rest uniformly inside said groove of said short circuit ring; - concurrent induction brazing of all the joints between said plurality of rotor bars and said short circuit ring such that said bar ends remain submerged in a pool of molten brazing alloy, said groove in said short- circuit ring being consistently filled in with said molten brazing alloy arising out of said induction brazing, thereby forming a uniform connection between said plurality of rotor bars and said short-circuit ring.

Full Text

F1ELD OF INVENTION The invention relates to an improved design of 3-phase induction motor with brazed rotor construction for traction application. The invention in particular relates to a modified design of 850Kw, 3 phase induction motor for application on 6000 HP electric locos. The new design also involves construction of rotor cage through brazing process instead of hitherto used Metal Inert Gas (MIG) welding process. BACKGROUND OF THE INVENTION The conventional motors for traction application are manufactured with Metal Inert Gas (MIG) welding on the rotor cage. The current carrying rotor bars are MIG welded to the short-circuit ring to form a squirrel cage on the rotor. Due to excessive heat generated during the MIG welding process, said rotor bars and said short-circuit ring in the rotor structure loose their mechanical properties. Moreover, MIG welding being a intricate process with many external and internal factors affecting the weld joints, there is enough likely-hood of resulting in an unsatisfactory quality of weld with blow holes and inadequate fusion of parent material. In the recent years, Railways have introduced high powered electric locomotives with 3 phase drive technology. In such locomotives, the hauling is done with 3 phase induction motors instead of DC series motors. The motors are operated with voltage variable frequency supply from the converter invertor bank. These induction motors are of 850KW ratings with a compact design having insulation of high temperature class i.e. Class 200 resulting in a high power to weight ratio. The induction motor is of squirrel cage type. In the existing design, the rotor cage is formed through copper Metal Inert Gas (MIG) welding In many of these induction motors, a phenomenon of rotor cage burning has been observed during operation. The problems were analysed and it was found that due to excessive heat generated during the MIG welding process, the mechanical properties of the rotor bars and short circuit ring are in very poor state and as a result, due to centrifugal forces and vibrations generating during the operation, cracks develop in the joints of rotor bars and short circuit ring. These cracks do not allow proper current flow in the rotor cage and the joints get over heated at higher load operations. Eventually the joints burn out. The failure is revealed through a heavy current inrush at the stator side being indicated by the protection circuitry. It is therefore an object of the invention to propose a modified design of 850KW, 3 phase induction motor with changed construction of rotor cage. In this design of motor, the rotor construction has been modified with new type of rotor bars and short circuit ring. The joining of rotor bars to short circuit ring has been achieved through induction brazing process. This design of motor with rotor cage built through induction brazing is being done for first time for application on 6000HP electric locos with 3 phase drive technology. Another object of the invention is to propose an improved process for building a rotor cage of an induction motor, through induction brazing, in which the process temperature is maintained at an optimum degree, thereby reducing deterioration level of the mechanical properties of the rotor bar and short circuit ring of the rotor cage Yet another object of the invention is to propose an improved process for building a rotor cage of an induction motor wherein there is no spatter or consequential deteriorating effect on the adjacent components such as rotor stamping and rotor shaft by the heat developed during the induction brazing. A further object of the invention is to propose an improved process for building a rotor cage of an induction motor which consumes lesser time and does not require any post cleaning step. These and other objects of the invention will be apparent from the ensueing description made in conjunction with the accompanying drawings. SUMMARY OF THE INVENTION. Thus, according to this invention there is provided an improved design of 3 phase induction motor for traction application with rotor cage being built through induction brazing process comprising of following steps: - arranging a pre-dimensioned groove in a short-circuit ring of a rotor construction: - inserting a plurality of rotor bars in a rotor core of said rotor construction: - correspondingly machining both ends of said plurality of rotor bars so as to maintain a uniform level: - placing said rotor vertically over said short circuit ring such that all said plurality of rotor bar ends rest uniformly inside said groove of said short circuit ring. - concurrent induction brazing of all the joints between said plurality of rotor bars and said short circuit ring such that said bar ends remain submerged in a pool of molten brazing alloy, said groove in said short circuit ring being consistently filled in with said molten brazing alloy arising out of said induction brazing, thereby forming a uniform connection between said plurality of rotor bars and said short-circuit ring. This process is carried out on both ends of the rotor. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS. Fig. 1 Groove preparation for MIG welding in a rotor cage according to the conventional process. Fig. 2 Shows a MIG welded section of a rotor cage of prior art. Fig. 3 Shows a constructional arrangement of a rotor with a rotor cage in the prior art. Fig. 4 Shows the cross section of a brazed rotor cage according to the present invention. Fig. 5 Shows a brazed rotor assembly, the rotor cage being produced according to the present invention. Detailed Description of the Invention In the improved process according to the invention, the MIG welding process of rotor cage building has been replaced with induction brazing. Design calculations for rotor bar (1) and short circuit ring (2) construction have been carried out and accordingly the rotor cage design with induction brazing has been evolved. The process temperature is much less than MIG welding and there is much less deterioration of mechanical properties of rotor bars and short circuit ring. There is no spatter and consequential effect on the near by components of the rotor unlike MIG welding where the excessive heat also affects the rotor stampings and shaft. The induction brazing is also a neat and tidy process requiring a minimum of post process cleaning. According to the invention, an improved induction motor with ratings for application on 6000HP locomotives having an improved construction of rotor cage has been developed. In this design, the joints between rotor bars (1) and short circuit ring (2) are achieved through induction brazing process. After inserting the bars (1) in the rotor core (3), the end of bars are machined to maintain a uniform level. The rotor is then vertically placed over the short circuit ring (2) where all the rotor bar ends uniformly rest inside a groove (5) in the short circuit ring. All the joints between rotor bars and ring are then concurrently brazed through a special induction brazing machine, while brazing, a pool of molten brazing alloy fills the groove in the ring and all the bar ends are submerged inside the pool. When the brazing process is completed, a uniform connection is formed between the rotor bars and ring through the brazing material. This is very neat and clean process and does not affect the surrounding components. There is no deformation observed in either the rotor bar or ring or any other component of the rotor Required design calculations have also been carried out to check the adequacy with respect to current flow in the rotor and also considering the mechanical forces expected on the rotor construction. As per the calculation, a support ring (retaining ring) (4) is provided over the shorting ring (2) for protection from centrifugal forces We Claim 1. An improved process for building a rotor cage of an induction motor comprising the steps of: - arranging a pre-determined groove in a short-circuit ring of the rotor construction; - inserting a plurality of rotor bars in a rotor core of said rotor construction; - correspondingly machining both ends of said plurality of rotor bars so as to maintain a uniform level; - placing said rotor vertically over said short-circuit ring such that all said plurality of rotor bar ends rest uniformly inside said groove of said short circuit ring; - concurrent induction brazing of all the joints between said plurality of rotor bars and said short circuit ring such that said bar ends remain submerged in a pool of molten brazing alloy, said groove in said short- circuit ring being consistently filled in with said molten brazing alloy arising out of said induction brazing, thereby forming a uniform connection between said plurality of rotor bars and said short-circuit ring. 2. The process as claimed in claim 1, wherein before said step of concurrent induction brazing of joints, a retaining ring is provided over said short circuit ring to combat excessive centrifugal force generated during said induction brazing. 3. An improved induction brazed rotor of 850KW induction motor produced by the method as claimed in claim 1 for application on 6000HP electric locomotives.

Documents:

1063-del-2003-abstract.pdf

1063-del-2003-claims.pdf

1063-del-2003-correspondence-others.pdf

1063-del-2003-correspondence-po.pdf

1063-del-2003-description (complete).pdf

1063-del-2003-drawings.pdf

1063-del-2003-form-1.pdf

1063-del-2003-form-13.pdf

1063-del-2003-form-19.pdf

1063-del-2003-form-2.pdf

1063-del-2003-form-3.pdf

1063-del-2003-form-4.pdf

1063-del-2003-gpa.pdf