FIELD OF THE INVENTION
The invention generally relates to the electrical machines. More particularly, the invention relates to a non-uniform air gap hybrid excitation machine for reduction of harmonic content in the output voltage of the machine.
BACKGROUND OF THE INVENTION
Conventional synchronous generators have less efficiency but they can be flux-regulated whereas the permanent magnet synchronous generators cannot be regulated although having higher efficiency. This disadvantage of the permanent magnet generators can be overcome by a hybrid excited synchronous machine which contains both excitation windings and permanent magnets. The parallel hybrid excitation generator (PHEG) has advantage of good flux regulation as compared to series hybrid excitation generator ( SHEG). While the series hybrid excitation generator (SHEG). While the series hybrid excitation generator (SHEG) has a simple structure, the parallel hybrid excitation generator (PHEG) proposed so far have either complex structures or large size to overcome the drawbacks of the series hybrid excitation generator. The parallel hybrid excitation generators with excitation sources in the rotor proposed so far have utilized two rotor cores for flux regulation which increase the size of the machine.

The shape of the output voltage of the permanent magnet generator mainly depends on the shape of the air gap flux density which depends on many parameters like stator slot opening, magnet shape, direction of magnetization etc. The air gap flux density in the permanent magnet generator must be ideally sinusoidal to have less harmonic content in the output voltage of the permanent magnet generator. A hybrid excitation machine which has both winding and magnet as source of excitation will generally tend to have more harmonic content in the air gap flux density based on the complexity of rotor configuration.
The parallel hybrid rotor excitation topology machines proposed so far have utilized two rotor cores. A parallel hybrid excitation machine with rotor excitation topology was proposed based on permanent magnet synchronous machine and reluctance machine in 1998. One rotor core of this parallel hybrid excitation machine has permanent magnets and the other rotor core of this machine is axially laminated anisotropic part [Ref.1].
Another type of parallel hybrid excitation machine with rotor excitation topology was proposed based on permanent magnet synchronous machine and conventional salient-pole synchronous machine in 2001. One rotor core of this parallel hybrid excitation machine has permanent magnets and the other rotor core of this machine has excitation windings [Ref.2].

Another type of parallel hybrid excitation machine, which uses two rotor cores, with rotor excitation topology was proposed based on interior permanent magnet synchronous machine and brushless synchronous machine having two additional air gaps and electrical excitation in 2009 [Ref. 3].
From the above existing rotor excitation topologies of parallel hybrid excitation generators [Ref.1], [Ref.2], [Ref.3], it is observed that two rotor cores are being used in the machine for flux regulation, which increases the volume of the machine. So the rotor excitation topology of hybrid excitation generator which utilizes only one rotor core to have wide flux regulation along with reduced volume is disclosed in the Indian Patent Application No.982/KOL/2013 dated 26th August 2013).
Though the regulation of air gap flux density of proposed hybrid excitation generator is wide in the filed Patent, its air gap flux density has high harmonic content. Hence, in order to reduce the harmonic content in the air gap flux density a non uniform air gap hybrid excitation machine is proposed by shaping the outer diameter of the rotor.
It is therefore an object of the invention to propose a non-uniform air gap hybrid excitation machine for reduction of harmonic content in the output voltage of the machine.

Another object of the invention to propose a non-uniform air gap hybrid excitation machine for reduction of harmonic content in the output voltage of the machine which allows the machine to distribute the air gap flux density in a sinusoidal shape.
SUMMARY OF THE INVENTION
According to the invention, the rotor 507 of the hybrid excitation machine is significant in that each rotor pole has six faces 501, 502, 503, 504, 505 and 506 of equal width with faces 503 and 504 closer to the stator core 509 compared to the other faces 501, 502, 504 and 506 and with faces 502 and 505 closer to the stator core compared to the other faces 501 and 506.
The rotor of permanent magnet generator is also significant in that the ratio of the average air gap at the faces 501 and 506 of the rotor pole face to the average air gap at the faces 503 and 504 of the rotor pole face must be optimum value for sinusoidal distribution of air gap flux density. The ratio will be advantageous if it is between 2 and 2.5.
The rotor of the permanent magnet generator is also significant in that the ratio of the average air gap at the faces 502 and 505 of the rotor pole face to the average air gap at the faces 503 and 504 of the rotor pole face must be optimum value for sinusoidal distribution of air gap flux density. The ratio will be advantageous if it is between 1.5 and 2.

In the present invention, the above mentioned modifications made in the rotor of the permanent magnet generator make the air gap flux density almost in sinusoidal shape resulting in the output voltage with less harmonic content.
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 :
Figure 1 is a sectional view of the hybrid excitation machine proposed in the filed Patent.
Figure 2 is the flux plot of the hybrid excitation machine4 shown in Figure 1.
Figure 3 is air gap flux density of hybrid excitation machine shown in Figure 1.
Figure 4 is the FFT analysis of the air gap flux density of hybrid excitation machine shown in Figure 1.
Figure 5 is a sectional view of non-uniform air gap hybrid excitation machine proposed in the present invention.
Figure 6 is flux plot of the non-uniform air gap hybrid excitation machine shown in Figure 5.

Figure 7 is air gap flux density of the non-uniform air gap hybrid excitation machine shown in Figure 5.
Figure 8 is the FFT analysis of the air gao flux density of non-uniform air gap hybrid excitation machine shown in Figure 5.
DETAILED DESCRIPTION OF THE INVENTION
The rotor excitation topology of hybrid excitation generator which utilizes only one rotor core was proposed in the Indian Patent Application No.982/KOL/2013 dated 26th August 2013 which enables wide regulation of air gap flux density along with reduced volume of the generator. Exploiting the periodicity of hybrid excited synchronous generator, only half of the cross section of hybrid excited synchronous generator proposed in the cited prior art is shown in Figure 1. The rotor 101 of the hybrid excited synchronous generator disclosed in the cited prior art is significant in that the excitation windings of said generator were accommodated in the slots 102 provided at the bottom of the permanent magnets 103 and the slot openings 104 for the excitation winding are provided over the inner diameter of the rotor core, The permanent magnets are uniformly magnetized along their thickness and placed in such a way that the spaces between the rectangular permanent magnets form north and south poles alternatively.

According to the said prior art, the variation in the air gap flux is achieved by controlling the excitation current of the field windings. But the reduction of harmonic content in the air gap flux density of the hybrid excitation machine in said prior art was not addressed. Though the regulation of air gap flux density of the generator is wide, the air gap flux density has high harmonic content. Hence, in order to have less harmonic content in the air gap flux density, a non-uniform air gap hybrid excitation machine is proposed in the present invention.
The transient analysis using FEM software has been carried out according to the proposed invention, and a modified hybrid excited synchronous generator has been provided. As per the present invention, the field windings are excited (2000 At) to determine and plot the flow of flux lines as shown in Figure 2 including the air gap magnetic flux density as shown in Figure 3 as well as the harmonic content of air gap flux density as shown in Figure 4.
A non-uniform air gap hybrid excitation machine is configured as per the present invention with the same rotor configuration in which permanent magnets 508 are inserted symmetrically, excitation windings slots 510 are made at the bottom of the permanent magnets as shown in Figure 5. Exploiting the periodicity of hybrid excitation machine, only one-half part of the cross section is shown in Figure 5. The rectangular permanent magnets used in this rotor configuration are uniformly magnetized along their thickness and placed in such a way that the spaces between the rectangular permanent magnets form north and south poles alternatively.

In the present invention, the rotor 507 of the hybrid excitation machine is designed such that each rotor pole has six faces 501, 502, 503, 504, 505 and 506 of equal width face 503 and 504 closer to the stator core 509 compared to the other faces 501, 502, 505 and 506 and with faces 502 and 505 closer to the stator core compared to the other faces 501 and 506. The rotor of the hybrid excitation machine is also designed such that the ratio of the average air gap at the faces 501 and 506 of the rotor pole face to the average air gap at the faces 503 and 504 of the rotor pole face must be optimum value which can be between 2 and 2.5 to obtain the sinusoidal distribution of air gap flux density. The rotor of the permanent magnet generator is also designed such that the ratio of the average air gap at the faces 502 and 505 of the rotor pole face to the average air gap at the faces 503 and 504 of the rotor pole face must be optimum value which can be between 1.5 and 2 to obtain the sinusoidal distribution of air gap flux density.
In the present invention, the above mentioned modifications made in the rotor of the hybrid excitation machine make the air gap flux density almost in sinusoidal shaped resulting in the less harmonic content. Also the proposed shaped rotor to maintain non uniform air gap in the present invention can easily be manufactured with less cost.
The transient analysis using known FEM software has been carried out on the non-uniform hybrid excitation machine proposed in the present invention to determine and plot the flow of flux lines as shown in Figure 6, the air gap flux density as shown in Figure 7 and the harmonic content of the air gap flux density

as shown in Figure 8. It is found that the harmonic content of the air gap flux density of the non-uniform air gap hybrid excitation machine with shaped rotor proposed in the present invention is less than the harmonic content of the air gap flux density for the hybrid excitation machine proposed in the filed Patent.
References:
[1] Chalmers, B. J. Akmese, R, Musaba, L., “Design and field-weakening
performance of permanent-magnet/reluctance motor with two-part rotor,” IET Electr.Power Appl.1998, 145, (2), pp. 133-139.
[2] Naoe, N, Fukami, T, “Trial production of a hybrid excitation type
synchronous machine,” Proc. Int. Con on Electrical Machines and Drives (IEMDC), Cambridge, USA, August 2001, pp. 545-547.
[3] Zhuoran, Z., Jingjie, Z., Yangguang, Y., Bo, Z., ‘Construction and
operation principle of a novel coordinates structure hybrid excitation synchronous machine,” Proc. Chin .Soc. Electr. Engg.,2009, 29,
(33),pp.83-89.

WE CLAIM :

A non-uniform air gap hybrid excitation machine for reduction of harmonic content in the output voltage of the machine, comprising :
- a plurality of permanent magnets (508) symmetrically disposed along the rotor (507) north-south poles;
- A plurality of slots (510) provided at the bottom of the permanent magnets (508) to accommodate the multiple excitation windings;
- a stator with at least one stator core (509)
characterized in that each rotor pole has six faces 501, 502, 503, 504, 505 and 506 of equal width, wherein the faces 503 and 504 located closer to the stator core 509 to the remaining faces 501 and 506, and, and in that the ratio of the average air gap at the faces 501 and 506 of the rotor pole face and that at the faces 503 and 504 is maintained between 2 and 2.5.