The present invention is generally related to the electrical machines.
More particularly, the invention is related to the hybrid excited
synchronous machines comprising permanent magnets and excitation
windings.
BACKGROUND OF THE INVENTION:-
Conventional synchronous generators have less efficiency but they can
be regulated whereas permanent magnet synchronous generators cannot
be regulated yet they have higher efficiency. This disadvantage of
permanent magnet generators being not regulated 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
the series hybrid excitation generator (SHEG). While the series hybrid
excitation generator has a simple structure, the parallel hybrid excitation
generators proposed so far have either complex structures or large size to
overcome the drawbacks of 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
increases the size of the machine. So the rotor excitation topology of
hybrid excitation generator which utilizes only one rotor core is proposed
in the patent filed (Appln.No. 982KOL2013 dated 26.08.13) to have wide
regulation of air flux density along with reduced volume of the generator.
Though the regulation of air gap flux density of proposed generator is
wide in the filed patent, the generator is almost unutilized while
unexcited and the range of flux regulation cannot be varied for given
magnet, shaft, rotor and stator dimension. So in order to have desired

range of flux regulation and utilization of generator even while unexcited,
a modification to the hybrid excited generator is required.
In order to provide magnetic flux for machine operation, many
synchronous generators are designed by using either excitation
windings, which is fed by a regulated source, or permanent magnets.
Synchronous generators containing excitation windings to produce
magnetic flux can be regulated over a wide range but winding losses
occurs in the wound field machines which result in decreasing the
machine efficiency. In order to improve the efficiency of synchronous
generators, permanent magnets can be used for providing magnetic flux
instead of excitation windings but permanent magnet synchronous
generators cannot be regulated though higher efficiency can be achieved.
The advantages of the excitation control of conventional synchronous
generators and the advantages of higher efficiency of permanent magnet
synchronous generators can be attained by a hybrid excited synchronous
generators comprising both excitation windings and permanent magnets
in the rotor core.
The parallel hybrid excitation generator (PHEG) has advantage of good
flux regulation as compared to series hybrid excitation generator (SHEG).
The PHEG can be further categorized into two types according to location
of the excitation source, namely the parallel hybrid rotor excitation
topology generators (PHRETG) with excitation source located in the rotor
core and the parallel hybrid stator excitation topology generators
(PHSETG) with excitation source located in the stator core as discussed
in Y.Wang.Z.Deng, "Parallel hybrid excitation machines and their control
schemes for DCgeneration system." IET Elctr. Power Appli. 2012, Vol. 6,
Iss. 9, pp. 699-680.
The parallel hybrid rotor excitation topology machine 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 anistropic part as
discussed in 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.
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
as discussed in 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.
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
as discussed in 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. Eng.,
2009, 29, (33), pp. 83-89.
From the above existing rotor excitation topologies of parallel hybrid
excitation generators as discussed in '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,

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 and
'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.' , 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 as
disclosed in the prior art (APPL_No. 982KOL2013 dated 26.08.13). So in
order to have desired range of flux regulation and utilization of generator
even while unexcited, a modification to the hybrid excited generator in
the filed patent is proposed in the present invention.
OBJECT OF THE INVENTION:-
It is therefore an object of the invention to propose an improved hybrid
excited generator compared to the prior art in order to have desired
range of flux regulation.
Another object of the invention is to propose a hybrid exited generator
with improved utilization of machine even in unexcited state.
SUMMARY OF THE INVENTION:-
The rotor of the hybrid excited synchronous generator discussed in the
prior art is significant in that it has excitation windings accommodated

in the slots provided at the bottom of the permanent magnets as shown
in Fig. 1.
According to an aspect of the invention, a modification to the hybrid
excited generator in the field patent is made for achieving desired range
of regulation in the output voltage and utilization of machine even while
it is unexcited.
The hybrid excited synchronous generator is significant in that the rotor
core and shaft are separated by a non-magnetic sleeve of whose
thickness depends on the desired range of flux regulation as shown in
Fig. 6.
Thus the thickness of the non-magnetic sleeve plays a crucial role in the
design of the compact hybrid excited synchronous generator with desired
range of flux regulation and the amount of utilization of the generator.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:-
The advantage of the present invention will become apparatus from the
following detailed description of embodiments with reference to the
accompanying drawings, in which:
Fig.1 shows the sectional view of the hybrid excited synchronous
generator of the prior art.
Fig.2 shows the flux plot of the hybrid excited synchronous generator
when field windings are not excited.
Fig. 3 shows the air gap flux density of the hybrid excited synchronous
generator when field windings are not excited.
Fig. 4 shows the flux plot of the hybrid excited synchronous generator
shown in Fig. 1 when field windings are excited.
Fig. 5 shows air gap flux density of the hybrid excited synchronous
generator shown in Fig. 1 when field windings are excited.

Fig. 6 shows a sectional view of the hybrid excited synchronous
generator in the present invention after modification.
Fig. 7 shows the flux plot of the hybrid excited synchronous generator in
when field windings are not excited.
Fig. 8 shows air gap flux density of the hybrid excited synchronous
generator when field windings are not excited.
Fig. 9 shows the plot of the hybrid excited synchronous generator when
field windings are excited.
Fig. 10 shows air gap the flux plot of the hybrid excited synchronous
generator when field windings are excited.
DETAILED DESCRIPTION OF EMBODIMENTS:-
The rotor excitation topology of hybrid excitation generator which utilizes
only one rotor core is discussed in the prior art to have 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 is
shown in Fig. 1. The rotor of the hybrid excited synchronous generator in
the filed patent is significant in that it has excitation windings
accommodated in the slots provided at the bottom of the permanent
magnets and the slot openings 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. Though the regulation of air gap flux
density of the generator is wide, the generator is almost unutilized while
it is unexcited and the range of flux regulation cannot be varied for given
magnet, shaft, rotor and stator dimension.

Fig. 6 shows the modified excited synchronous machine designed
comprises a specific rotor configuration in which plurality of permanent
magnets (64) are inserted symmetrically. Corresponding number of slots
(65) for placing the field windings are made at the bottom of the
permanent magnets (64). The permanent magnets (64) used in the rotor
configuration are uniformly magnetized along their thickness and placed
in such a way that the spaces between the rectangular permanent
magnets (64) from north and south poles remain alternatively. The rotor
(61) of the hybrid excited synchronous machine is designed such that the
slot openings (66) for placing excitation windings in the slots (65) is at
the bottom of the permanent magnets (64) in the rotor core (61).
The modification to the hybrid excited generator in the filed patent is
proposed for achieving desired range of flux regulation and utilization of
generator even while it is unexcited by providing a non-magnetic sleeve
(62) in between the rotor core (61) and the shaft.
The function of the non-magnetic sleeve (62) is to offer high magnetic
resistance between the rotor core (61) and the shaft (63) so that the
magnetic flux lines from permanent magnets (64) of the generator will
cross the air gap and consequently the air gap flux density of the
generator while it is unexcited increases. The non-magnetic sleeve (62)
also controls the range of flux regulation from unexcited case to the rated
excited case. For example, the thicker the non-magnetic sleeve (62),
lesser will be the flux regulation of the generator. Thus the arrangement
of non-magnetic sleeve (62) between the rotor core and shaft increases
the utilization of the generator even while it is unexcited and the desired
range of flux regulation of the generator can be obtained by non-
magnetic sleeve (62) of suitable thickness. Thus, the thickness of the
non-magnetic sleeve (62) plays a crucial role in the design of the compact
hybrid excited synchronous generator with desired range of flux
regulation and the amount of utilization of the generator.

The transient analysis using FEM software has been carried out on the
modified hybrid excited synchronous generator herein and the hybrid
excited synchronous generator in the prior art with field windings
unexcited and excited (2000 At) to determine and plot the flow lines (as
shown in Fig 2, Fig.7,Fig.9) and the air gap magnetic flux density (as
shown in Fig. 3, Fig. 5, Fig. 8, Fig. 10).
The RMS value of air gap magnetic flux density of the proposed hybrid
excited synchronous generator in the prior art varies from 0.03 T in
unexcited case to 0.41 T in excited case. Thus, the generator has wide
regulation but the generator is almost unutilized while it is unexcited.
The RMS value of air magnetic flux density of the proposed modified
hybrid excited synchronous generator in the present invention varies
from 0.30 T in unexcited case to 0.39 T excited case. Thus the generator
has a particular range of regulation and at the same time, the generator
is being utilized even while it is unexcited.
The range of flux regulation and the amount of utilization of the modified
generator while unexcited depend on the thickness of the non-magnetic
sleeve (62). As the thickness of the non-magnetic sleeve (62) increases,
the range of flux regulation decreases and the amount of utilization of
the generator while unexcited increases. The desired range of flux
regulation can be achieved by choosing appropriate thickness of non-
magnetic sleeve (62) and the hybrid excited synchronous generator can
be utilized even while it is unexcited.
Though a four-pole hybrid excited synchronous generator is used for the
purpose of illustrating the invention, it can be extended to generators
having more or less than four poles based on the design.

We claim:-
1. A modified hybrid excited synchronous generator that accommodates
excitation windings and permanent magnets in the rotor core (61) for
wide regulation of the output voltage and reduces risk of demagnetization
of the permanent magnet (64) comprising:-
- atleast a permanent magnet (64) disposed in rotor core (61) of the
machine that includes excitation windings;
- atleast a slot (65) placed underneath the said permanent magnet (64),
wherein the slot (65) comprises slot opening (66) provided over inner
diameter of the rotor core (61), wherein the excitation windings are
disposed in the slot (65)
characterized in that
a non magnetic sleeve is disposed between the slot opening (66) and a
shaft (63) to offer high magnetic resistance between the rotor core (61)
and the shaft (63) forming a pathway for the magnetic flux lines from the
permanent magnets (64) to the stator core.
2. A modified hybrid excited synchronous generator as claimed in claim
1, wherein the thickness of the non magnet sleeve regulates the flux
regulation.
3. A modified hybrid excited synchronous generator as claimed in claim
1, wherein the generator generates magnetic flux density in the
unexcited state as well.