FIELD OF THE INVENTION
The present invention generally relates to electrical machines and in particular to
the permanent magnet machine. More particularly, the invention relates to a
permanent magnet machine (PMM) with enhanced air gap flux density and
operable as permanent magnet motor or generator.
BACKGROUND OF THE INVENTION
In order to increase the efficiency of electrical machines, many permanent
magnet machines are constructed by using different shapes of permanent
magnets in different rotor configurations. Based on configuration of rotor and
stator for PMM and the shape and grade of the magnets used, the value of peak
air gap flux density in PMM varies. The more the value of air gap flux density,
the more will be the rated power of the PMM. In order to increase the air gap
flux density in PMM, certain modifications are to be made such that the
maximum flux from the permanent magnet must be diverted towards the air gap
reducing the flux leakage. Thus each permanent magnet machine has maximum
air gap flux density based on its rotor and stator configurations and the shape of
the permanent magnets used in it. Thus, there is always a scope for increasing
the output power of a permanent magnet machine by improving the air gap flux
density.

OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a permanent magnet
machine with enhanced air gap flux density and operable as permanent magnet
motor or generator which provides increased power output.
Another object of the invention is to propose a permanent magnet machine with
enhanced air gap flux density and operable as permanent magnet motor or
generator which is provided with heat transfer means.
SUMMARY OF THE INVENTION
According to the present invention, a permanent magnet machine is constructed
with trapezoidal permanent magnets symmetrically distributed in the rotor such
that the spaces between the magnets form the north pole and the south pole is
maintained to minimum.
According to the invention, holes are made on the rotor core between the
trapezoidal permanent magnets which offer high reluctance to the magnetic field
and so as to minimize the leakage flux at the bottom side of the trapezoidal
permanent magnets. The holes further act as ventilation holes and transfer the
heat from the permanent magnet machine to the ambient by reducing the
temperature of the permanent magnet machine.

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 according to prior
art rotor configuration wherein rectangular permanent magnets are inserted
symmetrically.
Figure 2 is the flux plot of a permanent magnet machine with an existing rotor
configuration.
Figure 3 is the plot of air gap flux density for permanent magnet machine shown
in figure 2.
Figure 4 is a sectional view of a permanent magnet machine with modifications
of rotor configuration shown in figure 1 wherein trapezoidal permanent magnets
according to the invention are inserted symmetrically and holes are made on the
rotor core.

Figure 5 is a flux plot of the permanent magnet machine modified in respect of
rotor configuration.
Figure 6 is a plot of air gap flux density for permanent magnet machine shown
in figure 5.
DETAILED DESCRIPTION OF THE INVENTION
A prior art permanent magnet machine with rotor configuration in which
rectangular permanent magnets 101 are inserted symmetrically in as shown in
Figure 1, The thickness and width of the rectangular permanent magnets 101
used in the prior art rotor configuration are 12 mm and 35 mm. The rectangular
permanent magnets 101 used in this rotor configuration are the ones that are
uniformly magnetized along their thickness.
Electromagnetic analysis using known FEM software has been carried out for a
prior art permanent magnet machine with rotor configuration shown in figure 1,
to determine and plot the flow of flux lines (as shown in figure 2) and the air gap
flux density (as shown in figure 3). Approximately only 50% of the flux lines are
crossing the air gap and the remaining 50% of the flux lines are taking alternate
path in the flux plot of the prior art permanent magnet machine (see figure 2)
The maximum air gap flux density that has been achieved by the permanent
magnet machine with rotor configuration shown in figure 3 is 0.6 T.

A permanent magnet machine designed according to the present invention with
rotor configuration in which trapezoidal permanent magnets 401 are inserted
symmetrically such that the thickness of the trapezoidal permanent magnets 401
at the top is more than the thickness of the trapezoidal permanent magnets 401
at the bottom, and a plurality of holes 402 are made on the rotor core 404
between the trapezoidal permanent magnets 401 at the bottom (see figure 4).
The thickness of the trapezoidal permanent magnets 401 at the top is 13 mm
and at the bottom is 11 mm and the width of the trapezoidal permanent
magnets 401 to 35 mm. The trapezoidal permanent magnets 401 used in this
rotor configuration are the ones that are uniformly magnetized along their
thickness.
The holes 402 that are made on the rotor core 404 between the trapezoidal
permanent magnets 401, offer a high reluctance to the magnetic field and
reduce the leakage of flux at the bottom side of the trapezoidal permanent
magnets 402. Because of the holes 402 made on the rotor core 404, the majority
of the flux lines divert towards the air gap and increase the air gap flux density.
The dimensions of the holes 402 depend on the space available between the
trapezoidal permanent magnets 401 at the bottom and can be determined by
carrying out electromagnetic analysis and stress analysis using FEM software.
Stress analysis is required to maintain a minimum gap between the holes 402
and between the trapezoidal permanent magnets 401 and the holes 402 such
that no part of the rotor core is subjected to higher stress.

The holes 402 made on the rotor core 404 between the trapezoidal permanent
magnets 401 also act as ventilation holes and transfer the heat from the
permanent magnet machine to ambient by reducing temperature of the
permanent magnet machine of the present invention.
Electromagnetic analysis using FEM software has been carried out for the
permanent magnet machine with rotor configuration in which holes 402 are
made on the rotor core 404 as shown in figure 4 to plot the flow of flux as
shown in figure 5 and the air gap flux density as shown in figure 6.
Approximately 70% of the flux lines are crossing the air gap and the remaining
30% of the flux lines are taking alternate path in the flux plot of the modified
permanent magnet machine as shown in figure 5. Thus holes 402 made on the
rotor core 404 between the trapezoidal permanent magnets 401 at the bottom
are diverting the flux lines towards the air gap. The maximum air gap flux
density that has been achieved by the permanent magnet machine of the
present invention is 0.78 T as shown in figure 6.
The air gap flux density achieved in the permanent magnet machine of the
present invention is greater than the air gap flux density achieved in the prior art
permanent magnet machine. As the gap air flux density is more for the
permanent magnet machine of the present invention, the power density of the
permanent magnet machine is more compared to that of the prior art
permanent magnetic machine.

Though a ten-pole machine is shown in figure 1 and figure 4 for the purpose of
illustrating the invention, the present invention can be extended to machines
having more or less than ten poles. The present invention can also be used for
both integral and fractional slot permanent magnet machine. The dimensions of
the slots for the permanent magnets in permanent magnet machine will vary
according to the machine design constraints.

WE CLAIM :
1. A permanent magnet machine with enhanced air gap flux density and
operable as permanent magnet motor or generator comprising:
- at least a rotor having a rotor core with a plurality of slots;
- a plurality of permanent magnets each disposed in one of said slots; and
- multiple holes constructed in a spaced apart manner at the bottom of the
rotor core between the permanent magnets;
wherein the permanent magnets are inserted in the slots such that the
thickness of the magnets at the top is more than that at the bottom.
2. The machine as claimed in claim 1, wherein the permanent magnets are
of trapezoidal shape.
3. The machine as claimed in claim 1, wherein a minimum gap between the
magnets is maintained, and wherein the holes act as heat transfer means.

4. The machine as claimed in any of the preceding claims, wherein the
dimensions of the holes made on the rotor core between the trapezoidal
permanent magnets at the bottom are selected such that the path for the
flux at the bottom must be narrowed down.
5. The machine as claimed in any of the preceding claims wherein the air
gap flux density is achieved upto 0.78 T.

ABSTRACT

The invention relates to a permanent magnet machine with enhanced air gap
flux density and operable as permanent magnet motor or generator comprising
at least a rotor having a rotor core with a plurality of slots; a plurality of
permanent magnets each disposed in one of said slots; and multiple holes
constructed in a spaced apart manner at the bottom of the rotor core between
the permanent magnets; wherein the permanent magnets are inserted in the
slots such that the thickness of the magnets at the top is more than that at the
bottom.