FIELD OF THE INVENTION
The present invention generally relates to electrical machines and in particular
to permanent magnet motor/generators. More particularly, the invention relates
to permanent magnet machines with asymmetric rotor construction to increase
peak air gap flux density in the machine.
BACKGROUND OF THE INVENTION
In order to increase the efficiency of electrical machines, permanent magnet
machines (PMM) are designed by using permanent magnets in different rotor
configurations. Based on the designed configuration of the rotor and stator of
PMM and the shape and the grade of magnets used in the designed
configuration, the value of peak air gap flux density in the PMM varies. The more
the value of peak air gap flux density, the more will be the rated power of the
PMM. In order to increase the air gap flux density in the PMM, either certain
modifications to the rotor and stator configurations or to the magnets are to be
made. Thus, each permanent magnet machine has different peak air gap flux
density based on its rotor and stator configurations including the shape and the
grade of permanent magnets used in the rotor. Thus, there is always a scope for
increasing the output power of a permanent magnet machine by improving the
peak air gap flux density, which can be achieved by new design configuration of
the rotor.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose permanent magnet machines
with asymmetric rotor construction to increase peak air gap flux density in the
machine.
Another object of the invention is to propose permanent magnet machines with
asymmetric rotor construction to increase peak air gap flux density in the
machine, which enhances output power and power density of the machines.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a completely new rotor configuration
which is asymmetric in nature is designed such that a first set of permanent
magnets forming a V shape are separated by a second set of permanent
magnets to form the north pole and the south pole around the rotor
alternatively.
The first set of permanent magnets forming the V shape in the rotor are the
actual permanent magnets that are intended to form the poles around the rotor.
Due to asymmetric design configuration of the rotor, the value of peak air gap
flux density under the north pole is not same as that of the south pole.
Accordingly, the second set of five permanent magnets which are thinner than
the first set of permanent magnets used in V shape are inserted between the V
shapes and a first plurality of flux equalizers of for example, 2 mm free holes are
made inside the V shapes to make the peaks of air gap flux density under the
north pole and the south pole equal.

A second plurality of Flux barriers are made on the rotor core at the bottom of
the permanent magnets of the first set, forming the V shape. These second type
flux barriers (free holes) offer high reluctance to the magnetic field which inter
alia reduce the leakage of flux that take place at the bottom side of the
permanent magnets.
The second type flux barriers (free holes) on the rotor core at the bottom of the
first set of permanent magnets and the first type of flux equalizers made on the
rotor core between the V shapes also act as ventilation holes and transfer the
heat from the permanent magnet machine to the ambient reducing the
temperature of the permanent magnet machine.
The air gap flux density is more for the permanent magnet machine with new
asymmetric rotor design configuration in the present invention thereby increasing
the output power and the power density of the permanent magnet machine
compared to that of the prior art machines.
The present invention can be used for both integral and fractional slot machines
and it can be extended to any number of poles. The thickness of the first set of
permanent magnets inserted between the V shapes and the thickness of the first
plurality of flux equalizers made on the rotor core inside the V shape vary
according to the design constraints.

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 a permanent magnet machine with prior art rotor
design configuration in which permanent magnets are inserted such that the
thickness of the permanent magnet is along the radial direction.
Figure 2 is the flux plot of the permanent magnet machine according to prior art-
Figure 3 is the plot of air gap flux density for permanent magnet machine shown
in figure 1.
Figure 4 is a sectional view of a permanent magnet machine with new
asymmetric rotor design configuration according to the invention.
Figure 5 is the flux plot of the permanent magnet machine of figure 4.
Figure 6 is the plot of air gap flux density for permanent magnet machine of
figure 4.

DETAILED DESCRIPTION OF EMBODIMENTS
The present invention is compared with a conventional embedded type design
configuration for presenting the innovative wok of the present invention by
carrying out electromagnetic analysis of permanent magnet machine with a
conventional embedded type design configuration along with the electromagnetic
analysis of permanent magnet machine with proposed rotor design configuration.
The air gap and the dimensions of stator core and rotor core are kept same in
both the design configuration for easy comparison between the power densities
that can be achieved by both existing and proposed design configuration.
A permanent magnet machine with an existing rotor design configuration in
which permanent magnets 101 are inserted such that the thickness of the
permanent magnet is along the radial direction is as shown in figure 1. The
thickness and width of the permanent magnets used in this rotor design
configuration are 12 mm and 35 mm. The permanent magnets used in this rotor
design configuration are the one that are uniformly magnetized along the
thickness of the permanent magnets.
Electromagnetic analysis using FEM software has been carried out for the
permanent magnet machine with an existing rotor design configuration shown in
figure 1. The flux lines from the north pole in rotor core (103) are entering into
the south pole in rotor core (103) after passing through the stator core (102) as
shown the flux plot of PMM with an existing design configuration shown in
figure 2.

The peak air gap flux density that has been achieved by the permanent magnet
machine with an existing rotor design configuration is 0.62 T as shown in the
plot of the air gap flux density of PMM with an existing design configuration in
figure 3.
In the present invention, a completely new rotor design configuration which is
asymmetric in nature is designed such that a first set of permanent magnets
(401) forming a V shape are separated by a second set of permanent magnets
(402) to form the north pole and the south pole around the rotor alternatively as
shown in figure 4.
The first set of permanent magnets (401) forming the V shape in the rotor core
(406) shown in figure 4 are the actual permanent magnets that are intended to
form the poles around the rotor. The thickness and width of the first set of
permanent magnets (401) used for forming the V shape for example, are 12 mm
and 33 mm. The angle between the first set of permanent magnets (401)
forming the V shape varies based on the number of poles to be used in the rotor
of PMM.
Due to asymmetric design configuration of the rotor, the value of peak air gap
flux density under the north pole is not same as the value of peak air gap flux
density under the south pole. So, a second set of five permanent magnets (402)
which are thinner than the first set of permanent magnets (401) are inserted
between the V shapes. A first plurality of thick flux equalizers (free holes) (403)
are made inside the V shapes to make the peaks of air gap flux density under
the north pole and the south pole equal.

The first set of permanent magnets (401) used to form the V shape in the new
asymmetric rotor configuration for the PMM are the ones that are uniformly
magnetized along their thickness wherein the second set of permanent magnets
(402) that are used between the V shapes are the ones that are radially
magnetized.
A second plurality of flux barriers (free holes) (404) are made on the rotor core
(406) at the bottom of the first permanent magnets (401) forming the V shape
as shown in figure 4. These second type flux barriers (free holes) (404) offer
high reluctance to the magnetic field and reduce the leakage of flux at the
bottom side of the first set of permanent magnets (401). Because of the second
plurality of flux barriers (free holes) (404) made in the rotor core (406), the
majority of the flux lines divert towards the gap thereby increasing the air gap
flux density. The dimensions of the second type flux barriers (free holes) 404 at
the bottom of the first set of permanent magnets (401) are selected such that
the leakage of the flux at the bottom of the first set of permanent magnets (401)
must be maximally prevented.
Due to core losses and copper losses, high heat gets generated in the permanent
magnet machine which increases the temperature of the permanent magnet
machine. The second flux barriers (free holes) (404) made on the rotor core at
the bottom of the first permanent magnets (401) and the first plurality of flux
equalizers (free holes) (403) made on the rotor core between the first

permanent magnets (401), according to the invention, additionally act as
ventilation holes, and transfer the heat from the permanent magnet machine to
the ambient reducing the temperature of the permanent magnet machine.
Electromagnetic analysis using FEM software has been carried out for the
permanent magnet machine with new asymmetric rotor configuration shown in
figure 4. The flux lines from the north pole in the rotor core (406) are entering
into the south pole in the rotor core (406) after passing through the stator core
(405) of PMM of the invention, is shown in figure 5. The peak air gap flux density
that has been achieved by the permanent magnet machine of the invention is
0.8 T as shown in the plot of the air gap flux density in figure 6.
The air gap flux density achieved in the PMM of the present invention is greater
than that of the air gap flux density achieved in the prior art PMM of figure 1. As
the air gap flux density is more for the permanent magnet machine of figure 4,
the output voltage including output power and the power density of the
permanent magnet machine of the invention are more compared to those of the
prior art permanent magnet machines. Since the power density of the permanent
magnet machine of the invention is more compared to the prior art permanent
magnet machine, the new permanent magnet machine is more compact. As the
permanent magnet machine of the present invention is compact, its weight and
cost of manufacturing also get reduced.
The present invention can be used for both the integral and the fractional slot
machines and it can be extended to any number of poles. The thickness of the
second set of permanent magnets (402) inserted between the V shapes and the

thickness of the first type flux equalizers (free holes) (403) vary according to the
design constraints. The present invention can be improved further by using the
shaped magnets to reduce the harmonic content in the output voltage and the
cogging torque and the torque ripple.
Through a ten-pole machine is shown in figure 1 and figure 4 for the purpose of
explaining the invention, the present invention can be extended to machines
having more than or less than ten poles. The dimensions of the slots for the
permanent magnets in permanent magnet machine will vary according to the
machine design constraints.

WE CLAIM:
1. Permanent magnet machines with asymmetric rotor construction to
increase peak air gap flux density in the machine, comprising a rotor
assembly having an asymmetric configuration of the rotor core (406) with
a first set of permanent magnets (401) disposed to form a V-shaped
spaces and separated by a second set of permanent magnets (402)
disposed between the V-shaped spaces to form the north pole and the
south pole of the rotor, the north pole and the south pole generating
different values of peak air gap flux density due to asymmetric design of
the rotor; a first plurality of flux equalizers (403) constructed inside the V-
shaped spaces to equalize the peak air gap flux density of the north and
south pole; and a second plurality of flux carriers (404) formed on the
rotor core (406) a the bottom of the first set of permanent magnets (401)
which provide a high reluctance to the magnetic field and reduce the
leakage of flux through the bottom of the magnets (401).
2. The permanent magnet machine as claimed in claim 1, wherein the
thickness and the width of each of the first permanent magnets is about 12
mm and 33 mm respectively and wherein the number of second permanent
magnets is five.
3. The permanent magnet machine as claimed in claim 1, wherein the magnets
of the first set (401) are uniformly magnetized along their thickness, and
wherein the magnets of the second set (402) are radially magnetized.

4. The permanent magnet machine as claimed in claim 1, wherein the first
plurality of flux equalizers (403) and the second plurality of flux barriers
(404) additionally act as ventilation holes and wherein the equalizers
(403) and the barriers (404) are constructed as free holes.

ABSTRACT

The invention relates to Permanent magnet machines with asymmetric rotor
construction to increase peak air gap flux density in the machine, comprising a
rotor assembly having an asymmetric configuration of the rotor core (406) with
a first set of permanent magnets (401) disposed to form a V-shaped spaces
and separated by a second set of permanent magnets (402) disposed
between the V-shaped spaces to form the north pole and the south pole of
the rotor, the north pole and the south pole generating different values of
peak air gap flux density due to asymmetric design of the rotor; a first plurality
of flux equalizers (403) constructed inside the V-shaped spaces to equalize the
peak air gap flux density of the north and south pole; and a second plurality
of flux barriers (404) formed on the rotor core (406) at the bottom of the first
set of permanent magnets (401), which provide a high reluctance to the
magnetic field and reduce the leakage of flux through the bottom of the magnets
(401).