FIELD OF THE INVENTION:
The present invention is generally related to the electrical machines. More
particularly, the invention relates to the hybrid excitation synchronous
machines comprising permanent magnets and excitation windings.
BACKGROUND OF THE INVENTION:
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 results 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 has advantage of good flux
regulation as compared to series hybrid excitation generator. 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. Few rotor

excitation topologies are discussed in the patents US 9356479B2, EP
259919681, JP 567364082, DE 112013000314T5 and WO2014006294A1.
Patent No. US935647982, titled (Hybrid excitation rotating electrical machine
teaches a hybrid excitation machine comprising a rotor having first and second
rotor cores with a gap in an axial direction, wherein first magnetic poles excited
by a permanent magnet and second magnetic poles not excited by the
permanent magnet are alternately arranged in a circumferential direction in
each rotor core. The first magnetic poles of the first and second rotor cores
have different polarities, and the first magnetic poles of one motor, core face
the second magnetic poles of the other rotor core in the axial direction. A stator
generating a magnetic field rotating the motor is placed radially outward of the
rotor and an exciting coil exciting the second magnetic poles is placed in the
gap, wherein the stator has, in the axial direction, first stator cores on both
sides and a second stator core having lower magnetic resistance than the first
stator core, in a center.
Patent No. EP259919681, titled "Synchronous rotary electric machine having a
double excitation rotor" teaches a machine (1) having a rotor (11) including
permanent magnets (PM) and field coils (EC). The magnets are housed in first
axial recesses (E1) distributed in a circumferential portion of the magnetic
body, thus defining circumferential polar sections. The coils are housed in
second axial recesses (E2) distributed in an intermediate portion of the
magnetic body and defining polar teeth (RT). The circumferential polar section
includes a third recess (E3) having a maximum width at the top thereof, and
the ratio of the maximum width of the third recess to a pole width of the
circumferential polar section has a value of around 0.13 mm to around 0.44
mm.
Patent No. JP567364082, titled "Hybrid type electric rotating machine" teaches
a excitation machine comprising a rotor having first and second rotor cores
with an axial gap between; a stator placed radially outward of the rotor; and an

exciting coil fixed to the stator placed in an air gap between the stator and
rotor to protrude radially inward from the stator. In an axial direction, an axial
end of a radially outer end of the first rotor core located on the second rotor
core side and the second rotor core are positioned on opposite sides of an axial
end of the exciting coil located on the first rotor core side, and an axial end of a
radially outer end of the second rotor core located on the first rotor core side
and the first rotor core ate positioned on opposite sides of the axial end of the
exciting coil located on the second rotor core side.
Patent No. DE112013000314T5, titled "Rotating electric machine with
synchronous motor" teaches a excitation machine comprising a rotor having
first and second rotor cores with an axial gap between; a stator placed radially
outward of the rotor; and an exciting coil fixed to the stator placed in an air gap
between the stator and rotor to protrude radially inward from the stator. In an
axial direction, an axial end of a radially outer end of the first rotor core located
on the second rotor core side and the second rotor core are positioned on
opposite sides of an axial end of the exciting coil located on the first rotor core
side, and an axial end of a radially outer end of the second rotor core located
on the first rotor core side and the first rotor core are positioned on opposite
sides of the axial end of the exciting coil located on the second rotor core side.
Patent No. WO2014006294A1, titled "Rotating electric machine with
compensation of armature magnetic feedback" teaches a machine of the type of
those comprising a rotor (2) having alternating north and south poles and
excitation coils (3) housed in first recesses (4) which are distributed regularly
around a shaft (5) of the rotor. These first recesses are arranged radially in the
rotor between a central part and a circumferential part of the rotor and extend
axially, in such a way as to define polar sections (6). Each excitation coil is
inserted around a core forming a partition between two first consecutive
recesses, the core being substantially aligned with a central radial axis (d) of

the corresponding polar section. In accordance with the invention, the rotor
furthermore comprises a plurality of permanent magnets (7) exhibiting a
substantially tangential direction of magnetization, which are arranged in
second recesses (8) and extend substantially in radial symmetry planes of the
polar sections (6), in such a way as to compensate the armature magnetic
feedback.
Hence in order to achieve the wide flux regulation in series hybrid excitation
alternator like in parallel hybrid excitation alternator, a new rotor configuration
is proposed in the present invention.

OBJECTS OF THE INVENTION:

It is therefore an object of the invention is to propose a series hybrid excitation
alternator which will have wide flux regulation like parallel hybrid excitation
alternator.
Another object of the invention is to propose an approach for accommodating
both the excitation windings and permanent magnets in the rotor core of series
hybrid excitation alternator which results in wide flux regulation thereby wide
regulation of output voltage.
SUMMARY OF THE INVENTION:

According to an aspect of the invention an approach for accommodating both
excitation windings and permanent magnets in the rotor of a series hybrid
excitation alternator is proposed for achieving wide regulation in the output
voltage.
The rotor of the series hybrid excitation alternator is significant in that the
permanent magnet slots are provided well inside the rotor unlike in the
conventional rotor where the permanent magnet slots are provided just below

the rotor outer diameter. The purpose of this arrangement is to allow the
magnetic flux leakage within the rotor. The space in between the permanent
magnet slots is also provided inside the rotor to aid the magnetic flux leakage
within the rotor under zero excitation.
The rotor of the series hybrid excitation alternator is significant in that it has
excitation windings accommodated in the slots provided at the bottom of the
extreme permanent magnet slots per pole. The rotor of the series hybrid
excitation alternator is also significant in that outer
slot openings and inner slot openings are provided for placing excitation
windings in its slots. The outer slot openings are above the extreme permanent
magnet slots per pole and the inner slot openings are provided in between the
extreme permanent magnet slots per pole and excitation winding slots.
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 shows a sectional view of the series hybrid excitation alternator
comprising both permanent magnets and excitation windings in accordance to
the invention.
Fig. 2 shows a flux line plot of the series hybrid excitation alternator when field
windings are positively excited in accordance to the invention.
Fig. 3 shows a air gap flux density plot of the series hybrid excitation alternator
when field windings are positively excited in accordance to the invention.
Fig. 4 shows output voltage of the series hybrid excitation alternator when field
windings are positively excited in accordance to the invention.

Fig. 5 shows a flux line plot of the series hybrid excitation alternator when field
windings are not excited in accordance to the invention.
Fig. 6 shows a air gap flux density plot of the series hybrid excitation alternator
when field windings are not excited in accordance to the invention.
Fig. 7 shows output voltage of the series hybrid excitation alternator when field
windings are not excited in accordance to the invention.
Fig. 8 shows a flux line plot of the series hybrid excitation alternator when field
windings are negatively excited in accordance to the invention.
Fig. 9 shows a air gap flux density plot of the series hybrid excitation alternator
when field windings are negatively excited in accordance to the invention.
Fig. 10 shows output voltage of the series hybrid excitation alternator when
field windings are negatively excited in accordance to the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS:
A 4-pole series hybrid excitation alternator is designed in the invention with
rotor configuration as shown in Fig. 1. Usually the permanent magnet slots for
arranging radial permanent magnets are provided in the rotor just below the
outer diameter of the rotor in order to have maximum air gap flux density. But
in the invention, the permanent magnet slots 101 are arranged across the
circumference which is approximately 10 mm away from the outer diameter of
the rotor. In addition to this, the space 102 in between permanent magnets is
also provided. The space in between permanent magnets and above permanent
magnet slots are provided in order to allow the leakage of magnetic flux within
the rotor thereby reducing the air gap flux linking the stator winding. Twelve
permanent magnet slots are made in the rotor to allocate three permanent
magnet slots for one pole. Exploiting the periodicity of hybrid excited
synchronous machine, only half of the cross section of series hybrid excitation
alternator is shown in Fig. 1.

The excitation winding slots 103 are provided below the extreme permanent
magnet slots per pole 104. The outer slot openings 105 are provided above the
outer diameter of the rotor and they are exactly above the extreme permanent
magnet slots per pole. The inner slot openings 106 are also provided in between
the extreme permanent magnet slots and excitation winding slots. Thus the
field winding is placed into the excitation winding slots through the outer slot
openings, extreme permanent magnet slots per pole and inner slot openings.
The permanent magnets is pasted into the permanent magnet slots after the
placement of field winding into the excitation winding slots.
When the field windings are not excited, some of the magnetic flux leak within
the rotor and only some of the magnetic flux will link the stator winding. When
the field windings are positively excited (electromagnet pole and permanent
magnet pole are same), the leakage flux within the rotor reduces and the air
gap flux will increase to the maximum value. Similarly, when the field windings
are negatively excited (electromagnet pole and permanent magnet pole are
different), the leakage flux within the rotor increases to the maximum value
and hence the air gap flux will decrease. Thus an approach proposed for
accommodating both excitation windings and permanent magnets in the rotor
of a series hybrid excitation alternator achieves wide regulation in the magnetic
flux density.
The transient analysis using FEM software has been carried out on the
proposed series hybrid excitation alternator with field windings positively
excited to determine and plot the flow of flux lines (as shown in Fig. 2), the air
gap magnetic flux density (as shown in Fig. 3) and the output voltage waveform
(as shown in Fig. a). The peak air gap magnetic flux density is 0.6 T. The RMS
value of phase output voltage is 425 V. The transient analysis using FEM
software has also been carried out on the proposed series hybrid excitation
alternator with field windings unexcited to determine and plot the flow of flux
lines (as shown in Fig. 5), the air gap magnetic flux density (as shown in Fig. 6)
and the output voltage waveform (as shown in Fig. 7). The peak air gap

magnetic flux density is 0.27 T. The RMS value of phase output voltage is 255
V. Similarly the transient analysis using FEM software has also been carried
out on the proposed series hybrid excitation alternator with field windings
negatively excited to determine and plot the flow of flux lines (as shown in Fig.
8), the air gap magnetic flux density (as shown in Fig. 9) and the output voltage
waveform (as shown in Fig. 10). The peak air gap magnetic flux density is 0.12
T. The RMS value of phase output voltage is 72 V.

We Claim-
1. An improved rotor configuration for an electric machine comprising,
- at least a permanent magnet slot(101) aligned on peripheral of the
rotor;
- at least an excitation winding slot (103) positioned below a permanent
magnet slot (104) at extremes of each pole;
- an inner slot opening provided to link the excitation winding slot (103)
with the permanent magnet slot (104);
- an outer slot opening is provided at the peripheral of the rotor to link
the permanent magnet slot (104);
wherein the permanent magnet is placed in the permanent magnet slot after
placing the field windings in the field winding slot;
wherein each permanent magnet slot is positioned at predefined intervals to
maintain space between slots which allows the magnetic flux leakage to remain
trapped within the rotor.
2. The improved rotor configuration for an electric machine as claimed in
claim 1, wherein the leakage flux within the rotor decreases and air gap flux
increases when the field windings are positively excited.
3. The improved rotor configuration for an electric machine as claimed in
claim 1, wherein the leakage flux within the rotor increases and air gap flux
decreases when the field windings are negatively excited.

4. The improved rotor configuration for an electric machine as
substantially described and illustrated herein with reference to the
accompanying drawings.
5. An improved rotor configuration for an electric machine as claimed in
claim 1, wherein the electric machine may be a hybrid excitation alternator.