FIELD OF THE INVENTION:
The present invention generally relates to the electrical machines and in particular to the rotor core of a hybrid excitation generator. More particularly, the invention relates to a process for modification of rotor core to increase structural strength of the rotor core of a hybrid excitation generator
BACKGROUND OF THE INVENTION:
From the existing rotor excitation topologies of parallel hybrid excitation generators [Ref. 2], [Ref. 3], [Ref. 4], it is observed that two rotor cores are being used in the machine for flux regulation, which increases the volume of the machine. Hence, the rotor excitation topology of hybrid excitation generator which utilizes only one rotor core to have wide flux regulation along with reduced volume is known from Indian patent (APPLN_No. 982/KOL/2013 dated 26-08-13). This prior art discusses about the approach for accommodating both the excitation windings and permanent magnets in the same rotor core of generator to have wide flux regulation. The slots configuration followed in the rotor structure for exploiting the advantage of avoiding the risk of demagnetization of permanent magnets while achieving the flux regulation normally leads to reduction of mechanical strength of the rotor structure there by limiting the operation of the hybrid excitation generator at lower speeds only. According to this prior art, although the desired electro-

magnetic performance of the hybrid excitation generator can be achieved, the existing rotor structure however, is likely to have more structural deformation when subjected to the higher structural stresses as the rotor structure is not strong enough to withstand the mechanical load because of slot configuration used. Hence the hybrid excitation generator known from prior art must be operated at higher speeds which requires an increase in the mechanical strength of the rotor structure through necessary modification of the rotor structure.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention is to propose a process for modification of rotor core to increase structural strength of the rotor core of a hybrid excitation generator in which, the modifications are made without allowing any loss of electromagnetic performance of the hybrid excitation generator.
A further object of the invention is to propose a process for modification of rotor core to increase structural strength of the rotor core of a hybrid excitation generator which enables the hybrid excited generator to be operated at higher speeds with reduced structural deformation.
SUMMARY OF THE INVENTION:
Accordingly, there is provided a process for modification of rotor core to increase structural strength of the rotor core of a hybrid excitation generator.

As disclosed in the present invention, in one aspect of the invention, the rotor structure is having holes of required diameter throughout the length of the rotor, where corresponding number of circular rods of same diameter with length more than the rotor length, are inserted. Two plates of required thickness, ID and OD, having number of holes identical to those configured in the rotor structure, are fixed to the end sides of the rotor, by inserting the plates through said circular rods already placed in the rotor structure and fastening with a plurality of bolts to the circular rods.
A structural analysis of the modified rotor structure has been carried out by FEM software as well as that of the prior art rotor structure to compare the maximum shear stress and the total deformation developed in the rotor structures. It is observed that the maximum shear stress developed in the modified rotor structure is less compared to that in the existing rotor structure. It is also observed that the total deformation developed in the modified rotor structure is less compared to that in the existing rotor structure as shown.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS-
The advantages of the present invention will become apparent from the following detailed description of embodiments with reference to the accompanying drawings, in which:
Fig. 1 is a view of 3D model of a prior art rotor structure of a hybrid excitation generator.
Fig. 2 is plot of maximum shear stress of the prior art rotor structure of the hybrid excitation generator.

Fig. 3 is plot of total deformation of existing rotor structure of the hybrid excitation generator.
Fig. 4 is a view of 3D model of a rotor structure of hybrid excitation generator with holes on the rotor structure according to the invention.
Fig. 5 is a view of 3D model of the rotor structure of the hybrid excitation generator of Figure 4 with circular rods inserted into the rotor structure.
Fig. 6 is a view of 3D model of a completely modified rotor structure of the hybrid excitation generator according to the invention.
Fig. 7 is plot of maximum shear stress of the modified rotor structure of the hybrid excitation generator.
Fig. 8 is plot of total deformation of the modified rotor structure of the hybrid excitation generator.
DETAILED DESCRIPTION OF EMBODIMENTS:
The rotor excited hybrid excitation generator which utilizes only one rotor core for both permanent magnet excitation and field winding excitation is disclosed in Indian patent (APPLN_No. 982/KOL72013 dated 26-08-13) which exhibit wide regulation of air gap flux density along with reduced volume of the generator. The slots configuration followed in the rotor structure of prior art as shown in Fig. 1 for exploiting the advantage of avoiding the risk of

demagnetization of permanent magnets while achieving the flux regulation normally leads to the reduced mechanical strength of the rotor structure. The field winding slot is made below the permanent magnet slot with 2 mm gap in between them and the permanent magnet slot is made 2 mm inside from the outer diameter of the rotor. As the thickness over the permanent magnet slot and in between the permanent magnet slot and field winding slot in the rotor structure is small, the rotor structure undergoes more structural deformation when subjected to higher structural stresses, causing the operation of hybrid excitation generator at lower speeds only.
Therefore in the present invention, applicable modifications to the rotor structure have been proposed in order to allow operation of the hybrid excitation generator disclosed in the prior art higher speeds including an increase in the mechanical strength of the rotor structure. According to one aspect of the invention, a plurality of holes of required diameter are made on the rotor structure throughout the length of the rotor as shown in Fig. 4. A corresponding member of circular rods of diameter same as that of hole diameter with length more than the rotor length are inserted into the holes of the rotor structure as shown in Fig. 5. Two plates of required thickness, ID and OD having number of holes identical to that constructed in the rotor structure are fixed to the end sides of the rotor structure by inserting the plates through the circular rods placed in the rotor structure and fastening the plates with bolts to the circular rods as shown in Fig. 6. The number of holes, diameter of the holes, thickness, ID and OD of the plates are selected corresponding to the size of the rotor structure and the speed at which the rotor rotates including associated pressure loads acting on the rotor in order to provide sufficient mechanical strength to the rotor structure.

The structural analysis has been carried out by FEM software on both the existing rotor structure and modified rotor structure to show the reduction in the maximum shear stress and the total deformation in the case of latter compared to the case of former due to the modifications proposed to the rotor structure. While the maximum shear stress developed in the rotor structure in the former case is 234 MPa as shown in Fig. 2, the maximum shear stress developed in the rotor structure in the latter case is 108 MPa as shown in Fig. 7 which means there is reduction of maximum shear stress developed in the rotor structure due to proposed modifications to the rotor structure. While the maximum total deformation developed in the rotor structure in the former case is 69 micro meters as shown in Fig. 3, the, the maximum total deformation developed in the rotor structure in the latter case is 22 micro meters as shown in Fig. 8 which means there is reduction of total deformation also developed in the rotor structure due to proposed modifications to the rotor structure. The
level of reduction in the total deformation developed in the rotor structure depends on the thickness, ID and OD of the plates fixed to the rotor end sides and the diameter of the circular rods and the number of circular rods that are used.

WE CLAIM :
1. A process for modification of rotor core to increase structural strength of the rotor core of a hybrid excitation generator, comprising the steps of:
- constructing a plurality of holes along the length of the rotor structure, the diameter of the holes depending on the size of the rotor structure, corresponding to the capacity of the hybrid excitation generator;
- providing circular rods corresponding to the number of said holes having
diameter equal to that of the holes, the length of the circular rods being
higher than that of the rotor;
- providing at least two plates configured with holes numbering equal to that
constructed on the rotor structure;
wherein the circular rods are inserted into the holes of the rotor structure, and wherein said plates are releasably fixed to the ends of the rotor structure by means of bolts.

2. The process as claimed in claim 1, wherein the maximum shear stress developed in the modified rotor structure is 108 MPa and wherein the maximum total deformation generated is 22 micro meters.