FIELD OF THE INVENTION:
The present invention generally relates to electrical machines and in particular
to the rotor core of a hybrid excitation alternator. More particularly, the invention
relates to the high speed operation of the rotor of a hybrid excitation alternator.
BACKGROUND OF THE INVENTION:
Conventional synchronous alternators have less efficiency but they can be
regulated whereas permanent magnet synchronous alternators cannot be
regulated but they have higher efficiency. This disadvantage of permanent
magnet alternators being not regulated can be overcome by a hybrid excited
synchronous machine which contains both excitation windings and permanent
magnets. The parallel hybrid excitation alternator (PHEG) has advantage of good
flux regulation as compared to series hybrid excitation alternator (SHEG). While
the series hybrid excitation alternator has a simple Structure, the parallel hybrid
excitation alternators proposed so far have either complex structures or large
size to overcome the drawbacks of series hybrid excitation alternator. Many
parallel hybrid excitation alternators with excitation sources in the rotor
proposed have utilized two rotor cores for flux regulation which increases the
size of the machine.
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.
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.

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.
From the above existing rotor excitation topologies of parallel hybrid excitation
alternators, 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 alternator which utilizes only one rotor
core to have wide flux regulation along with reduced volume is proposed in the
patent filed (APPLN_No. 982 KOL 2013 dated 26-08-13). Few rotor excitation
topologies are discussed in the patents US935647982, EP259919681,
JP567364082, DE112013000314T5 and WO2014006294A1.
Though the regulation of air gap flux density of proposed hybrid excitation
alternator is wide in the filed patent, it may not be able to operate at high speed
as it can become structurally weak due to centrifugal load at high speed. Hence,
in order to ensure the operation of parallel hybrid excitation alternator at high
speeds also, an improved rotor configuration has been proposed in the present
invention without degrading the performance characteristics of parallel hybrid
excitation alternator.
OBJECTS OF THE INVENTION:
It is therefore an object of the invention is to improve the rotor structure which
can withstand the centrifugal load at high speeds.
Another object of the invention is also not to lose the electrical performance
characteristics of a parallel hybrid excitation alternator while changing the rotor
configuration.
Yet another object of the invention is to propose a rotor configuration which can
also be developed without manufacturing difficulties.

SUMMARY OF THE INVENTION:
According to an aspect of the invention, the rotor of the high speed parallel hybrid
excitation alternator is significant in that the outer slot openings are provided
over the outer diameter of the rotor and they are provided exactly above the
permanent magnet slots. The rotor of the high speed parallel hybrid excitation
alternator is also significant in that the inner slot openings are also provided in
between the permanent magnet slots and excitation winding slots.
The rotor is also significant that slot openings provided will offer high reluctance
to the magnetic flux and hence it avoids the flux leakage through the sections
above and below the permanent magnet slots. Thus, the slot openings acting like
flux barriers in fact allow the increase in the thickness of the sections above and
below the permanent slots. The increase in the thickness of the sections above
and below the permanent magnet slots will further reduce the structural stresses
across these sections. The thickness of the section present between the inner
diameter and field winding slots can be increased based on the centrifugal load
the rotor should withstand.
ln the present invention, the above-mentioned modifications made in the rotor
of the high speed parallel hybrid excitation alternator ensure its operation at
high speed by withstanding the centrifugal load. At the same time it also ensures
good electrical performance characteristics of the alternator.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
It is to be noted, however, that the appended drawings illustrate only typical
embodiments of the present subject matter and are therefore not to be
considered for limiting of its scope, for the invention may admit to other equally
effective embodiments. The detailed description is described with reference to
the accompanying figures. Some embodiments of system or methods in
accordance with embodiments of the present subject matter are now described,
by way of example, and with reference to the accompanying figures, in which:

FlG.1 shows a sectional view of the hybrid excitation machine proposed in the
filed patent in accordance to the invention.
FlG. 2 shows a sectional view of the high speed parallel hybrid excitation
alternator with improved rotor configuration in accordance to the invention.
FlG.3 shows magnetic flux plot of high speed parallel hybrid excitation alternator
under positive excitation of field winding in accordance to the invention.
FIG.4 shows air gap flux density of high speed parallel hybrid excitation
alternator under positive excitation of field winding in accordance to the
invention.
FlG. 5 shows magnetic flux plot of high speed parallel hybrid excitation
alternator under zero excitation of field winding in accordance to the invention.
FIG. 6 shows air gap flux density of high speed parallel hybrid excitation
alternator under zero excitation of field winding in accordance to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION:-
The rotor excitation topology of hybrid excitation alternator which utilizes only
one rotor core is proposed in the patent filed (APPLN_No. 982KOL2013 dated 26-
08-2013) to have wide regulation of air gap flux density along with reduced
volume of the alternator. Exploiting the periodicity of hybrid excited synchronous
alternator, only half of the cross section of hybrid excited synchronous alternator
proposed in the filed patent is shown in Fig. 1. The rotor 101 of the hybrid excited
synchronous alternator in the filed patent is significant in that it has excitation
windings 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 invention in the prior art the variation in the air gap flux will be
achieved by controlling the excitation current of field windings. The rotor has
thin sections above and below the permanent magnet slots as shown in Fig. 1
and these are the sections that will resist the whole centrifugal load. Though the
desired electromagnetic performance of the hybrid excitation generator is
achieved, the existing rotor structure 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.
Accordingly, an improved rotor configuration is proposed as shown in Fig. 2.
Exploiting the periodicity of hybrid excitation machine, only one-half part of the
cross section is shown in Fig.2. Instead of providing the slot openings for field
winding slots over the inner diameter of the rotor, the outer slot openings 201
are provided over the outer diameter of the rotor. These slot openings are
provided exactly over the permanent magnet slots 202 and thus the field winding
should be inserted through the permanent magnet slots, inner slot openings 203
are also provided below the permanent magnet slots to provide the path to the
filed winding slots 204. As it is evident from the rotor configuration that there
are no slots between the inner diameter and field winding slots of the rotor. This
ensures the mechanical strength of the rotor. Unlike in the rotor shown in Fig.
1 the centrifugal load can be resisted by the thick section 205 as shown in Fig.
2. The tangential permanent magnets are to be placed in this rotor after the
placement of field winding in the relevant slots.
The transient analysis using FEM software has been carried out on the proposed
high speed parallel hybrid excitation alternator with field windings in unexcited
state to determine and plot the flow of flux lines (as shown in Fig. 3) and the air
gap magnetic flux density {as shown in Fig. 4). It can be observed from Fig. 3
that all the permanent magnet flux leaks through the rotor without linking the
stator winding. Thus, the average flux density is almost zero in the case of filed
winding unexcited.

The transient analysis using FEM software has also been carried out on the
proposed high speed parallel hybrid excitation alternator with field windings are
positively excited to determine and plot the flow of flux lines (as shown in Fig. 5)
and the air gap magnetic flux density (as shown in Fig.5), It can be observed from
Fig. 5 that all the permanent magnet flux links the stator winding without
leaking through the rotor. The average flux density is 0.5 T in the case of filed
winding positively excited. The other advantage with this proposed improved
rotor configuration in high speed parallel hybrid excitation alternator is that wide
flux regulation ranging from zero value to the rated value can be achieved
without the need to go for bidirectional excitation like in the rotor configuration
shown in Fig. 1. Yet there is another advantage of this proposed rotor
configuration compared to the rotor configuration shown in Fig. 1 that the slot
openings provided will also act as flux barriers and hence the thickness of the
sections above and below the permanent magnet slots can be increased. The
increase in the thickness of the sections above and below the permanent magnet
slots shown in Fig. l will reduce the performance characteristics of hybrid
excitation alternator by increasing the leakage flux and reducing the air gap flux.

WE CLAIM:
1. A modified hybrid excited synchronous generator that accommodates
excitation windings and permanent magnets in a rotor core for regulation of
output voltage and reducing risk of demagnetization of the permanent magnets,
wherein the rotor configuration is improved that comprises: -
- plurality of slot openings (201) at the peripheral of the rotor which is in
alignment with a permanent magnet slot (202); wherein the said permanent
magnet slot (202) has an inner slot opening (203) that is aligned to the field
winding slot (204).
2. The modified hybrid excited synchronous generator as claimed in claim 1,
wherein field winding slot (204) does not open to the inner diameter of the rotor.
3. The modified hybrid excited synchronous generator as claimed in claim 1 and
2, wherein the centrifugal load is resisted by the gap between the field winding
slot and inner diameter of the rotor.
4. A modified hybrid excited synchronous generator as substantially described
and illustrated herein with reference to the accompanying drawings.
5. The modified hybrid excited synchronous generator as claimed in claim 1,
wherein field windings are inserted into the excitation winding slot through the
permanent magnet slot.
6. The modified hybrid excited synchronous generator as claimed in claim 1 and
2, wherein the gap is altered depending on the centrifugal load.