FIELD OF THE INVENTION
The present invention is generally relates to electrical machines and in particular to
permanent magnet machines (PMM). More particularly, the invention relates to an
improved rotor structure of permanent magnet machine for increasing flux weakening
capability and reducing the risk of demagnetization of the permanent magnets.
BACKGROUND OF THE INVENTION
As the permanent magnet flux in a PMM is fixed, air gap flux weakening can be
achieved by applying large demagnetizing current in the stator winding. The large
amount of demagnetizing current however increases the risk of demagnetization of
permanent magnets in addition to an increase in the conduction losses. The flux
weakening capability of a PMM has been investigated in the prior art literature to
increase its speed range with reduced negative effects.
A rotor structure of a PMM was published in IEEE paper "A new design concept of
permanent magnet machine for flux weakening operation", IEEE Trans, on Ind.
Applicat, vol. 31, no. 2, pp. 373-378, Mar/Apr 1995", which taught that the wide flux
weakening capability needs a complex rotor structure.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an improved rotor structure of
Permanent Magnet Machines for increasing flux weakening capability and reducing the
risk of demagnetization of permanent magnets.

SUMMARY OF THE INVENTION
According to an aspect of the invention, an improved rotor structure of a permanent
magnet machine is proposed in order to increase the flux weakening capability with
reduced risk of demagnetization of permanent magnets. The disclosed rotor structure is
significant in that at least two slots are provided for accommodating permanent
magnets per pole.
In another aspect of the invention, the rotor structure is significant in that a flux barrier
is created by making a free slot in between said permanent magnet to prevent the
leakage of permanent magnetic flux lines through the space between the permanent
magnet slots.
In still another aspect of the invention, the rotor structure of PMM is significant in that
the thickness of the free slot is provided in between the permanent magnet slots must
be at least two times higher than the air gap in order to limit the leakage of flux lines
through the space between the permanent magnet slots.
According to a further aspect of the invention, the rotor structure of PMM is significant
in that the thickness of the free slot is made smaller than the thickness of the
permanent magnet slots in order to limit the flow of demagnetizing flux through the
permanent magnet slots. Thus, a number of the flux lines of demagnetizing flux due to
large demagnetizing current now take the path through the space between the
permanent magnet slots as the reluctance in this path is less compared to the
reluctance in the path through the permanent magnet 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 is existing rotor structure of permanent magnet machine.
Fig. 2 is flux line plot of permanent magnet machine with existing rotor structure at no
load.
Fig. 3 is flux vector plot of permanent magnet machine with existing rotor structure at
no load.
Fig. 4 is flux line plot of permanent magnet machine with existing rotor structure at
load.
Fig. 5 is flux vector plot of permanent magnet machine with existing rotor structure at
load.
Fig. 6 is new rotor structure of permanent magnet machine.
Fig. 7 is flux line plot of permanent magnet machine with new rotor structure at no
load.
Fig. 8 is flux vector plot of permanent magnet machine with new rotor structure at no
load.
Fig. 9 is flux line plot of permanent magnet machine with new rotor structure at load.
Fig. 10 is flux vector plot of permanent magnet machine with new rotor structure at
load.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Permanent magnet machines has advantages like more efficiency, high power density
etc. and hence they have a wide variety of applications. One of the important
application is traction applications where variable speed operation is required. The

variable speed operation of PMM can be achieved by flux weakening method where the
air gap flux is reduced in order to increase the speed of PMM. As the permanent
magnet flux in PMM is fixed, the air gap flux weakening is achieved by applying
demagnetizing current in the stator winding. In constant power and variable speed
applications demanding wide range of speed, a large demagnetizing current is required
to weaken the permanent magnet flux which will increase the risk of demagnetization
of permanent magnets in addition to the increase in the conduction losses. Therefore
in the present invention, the flux weakening capability of permanent magnet machine
(PMM) is investigated and a new modified rotor structure is proposed for increasing flux
weakening capability by lowering the risk of demagnetization of permanent magnets.
In the present invention, the major modification brought out in the improved rotor
structure shown in Fig. 6 compared to the existing rotor structure shown in Fig. 1 is
that one permanent magnet slot is made into two permanent magnets slots for
accommodating permanent magnets to create a magnetic pole with a flux barrier in
between the permanent magnet slots in order to reduce the permanent magnet flux
leakage through the space between the permanent magnet slots and to reduce the
passage of the demagnetizing flux through the permanent magnet slots.
The flux line plot and flux vector plot of PMM with existing rotor structure shown in Fig.
2 and Fig. 3 are obtained for no load condition assuming permanent magnets in the
permanent magnet slots whereas the flux line plot and flux vector plot of PMM with
existing rotor structure shown in Fig. 4 and Fig. 5 are obtained for load condition
assuming air in the permanent magnet slots in order to see the effect of demagnetizing
flux lines on the permanent magnets. Thus, when large demagnetizing current in the
stator winding is applied the demagnetizing flux lines are passing through the
permanent magnet slots as shown in Fig. 4 and Fig. 5 which will increase the risk of
demagnetization of permanent magnets.

The rotor structure proposed in the present invention is significant in that a flux barrier
is created as shown in Fig. 6 by making a free slot in addition to the two slots created
for accommodating permanent magnets. The purpose of the flux barrier is to prevent
the leakage of permanent magnetic flux lines through the space between the
permanent magnet slots as shown in Fig. 7 and Fig. 8. In order to accomplish the
reduction of leakage of permanent magnetic flux lines through the space between the
permanent magnet slots, the thickness of free slot provided in between permanent
magnet slots must be greater enough compared to two times the air gap.
The new rotor structure proposed in the present invention is also significant in that a
flux barrier provided in between the permanent magnet slots offers less reluctance
compared to that in the path through the permanent magnet slots. In order to achieve
this, the thickness of the free slot provided in between the permanent magnet slots
should be smaller enough compared to the thickness of permanent magnet slots. Few
of the flux lines of demagnetizing flux due to large demagnetizing current now take the
path through the space between the permanent magnet slots as the reluctance in this
path is less compared to the reluctance in the path through the permanent magnet
slots as shown in Fig. 9 and Fig. 10.

WE CLAIM:
1. An improved rotor structure of Permanent Magnet Machines for increasing flux
weakening capability and reducing the risk of demagnetization of permanent
magnets, comprising :
at least two slots provided on the rotor structure for accommodating
corresponding number of permanent magnets to create a magnetic pole;
a free slot constructed in between the said permanent magnet slots to prevent
the leakage of magnetic flux lines through the space between the permanent
magnet slots;
wherein the thickness of the free slot constructed between the permanent
magnet slots is maintained at least two times higher than the air gap to limit the
flux line leakage, and wherein the thickness of the free slot is selected to be
lower than that of the permanent magnet slots to limit the flow of demagnetizing
flux through the permanent magnet slots.