FIELD OF THE INVENTION
The present invention generally relates to electrical machines. More particularly,
the invention relates to the dual excited round pole synchronous machine
comprising permanent magnets and excitation windings.
BACKGROUND OF THE INVENTION
In order to provide magnetic flux for machine operation, many synchronous
machines are known which either use excitation windings, and fed by a regulated
source, or use permanent magnets. Synchronous machines 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 the synchronous
machines, permanent magnets can be used for providing magnetic flux instead
of excitation windings but permanent magnet synchronous machines can not be
easily regulated inspite of having higher efficiency.
The advantages of the excitation control of conventional synchronous machines
and the advantages of higher efficiency of permanent magnet based
synchronous machines can be combined by a dual excited synchronous
machines.
US Patent No. 6,509,664 and US Patent No. 6,037,691 describe a process for
regulation of a synchronous machine by providing mixed excitation (excitation by
both permanent magnets and winding coils) but the synchronous machine
cannot be regulated over a wide range. In case, this prior art synchronous
machine is upgraded to regulate over a wide range, the permanent magnets may
get demagnetized and loose its characteristics. Also the voltage waveform shape

of this hybrid synchronous machine is not as good as that of either conventional
synchronous machine or permanent magnet based synchronous machine.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a dual excited round pole
synchronous machine, which is regulated over a wide range.
Another object of the invention is to propose a dual excited round pole
synchronous machine, which is capable to prevent permanent magnet from
getting demagnetized by the magnetic field of excitation windings.
Still another object of the invention is to propose a dual excited round pole
synchronous machine, which is enabled to generate sinusoidal output voltage
waveform with less harmonic content.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved dual excited round pole synchronous
machine generating sinusoidal voltage waveform with characteristics of wide
range regulation, comprising a shaft having one rotor core with permanent
magnets disposed over a predetermined length and excitations windings along
the other rotor core, uniform gap being maintained between the two rotor cores
along the rotor shaft.
The improved dual excited round pole synchronous machine can be regulated
over a wide range by using a bi-directional excitation source to excite the
excitation windings and selecting the length of the rotor with excitation windings
based on the range of regulation specified.

Demagnetization of the permanent magnets is eliminated as the permanent
magnets and excitation windings are arranged in two different rotor cores of the
dual excited round pole synchronous machine.
Since the permanent magnetic field and the magnetic field of excitation windings
do not interact with each other and so the sinusoidal distribution of magnetic flux
density across the air gap does not change, the output voltage waveform of a
dual excited round pole synchronous machine is sinusoidal in shape with less
harmonic content.
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 dual excited round pole synchronous machine
with permanent magnets and excitation windings according to the invention.
Figure 2 is a sectional view of a shaft, a rotor core with permanent magnets and
a stator core of a second embodiment of the dual excited round pole
synchronous machine.
Figure 3 is a sectional view of a shaft, a rotor core with excitation windings and
the stator core of a third embodiment of the dual excited round pole
synchronous machine.
DETAILED DESCRIPTION OF THE INVENTION
A dual excited round pole synchronous machine includes a shaft (101) having a

first rotor core (102) with permanent magnets disposed along a predetermined
length and a second rotor core (103) with excitation winding interposed along
the remaining length of the shaft with certain gap in between them is shown in
Fig.l. The first rotor core (102) with permanent magnets and the second rotor
core (103) with excitation windings are separated by a gap for providing
sufficient space for overhang portion (104) of the excitation windings.
The first rotor core 102 with the permanent magnets is slightly extended
compared to one end of the stator core (105) and the second rotor core (103)
with the excitation windings is projecting out at the other end of the stator core
(105) as shown in Fig.l to reduce the fringing effect. The features of the dual
excited round pole synchronous machine shown in Fig.l, are (101)- shaft, (102)-
Rotor core with permanent magnets, (103)-Rotor core with excitation windings,
(104)-Rotor excitations windings overhang, (105)-Stator core, (106)-Stator
windings overhang, (107)-Stator frame, (108)-Air gap.
Though a six-pole machine is shown in Fig.2 and Fig.3 for the purpose of
simplicity, the present invention can be extended to machine having more than
six poles. The width and depth of slots (203),(303) for the permanent magnets
and excitation windings vary according to the machine design constraints.
Features of the different embodiments of the dual excited round pole
synchronous machine shown in figures 2 and 3 are (201)-Stator core, (202)-
Shaft, (203)-Slot for per permanent magnet (204)-rotor core, (205)-Stator
frame, (206)-Air gap. Stator frame (302)-Stator core, (303) Slot for excitation
windings, (301) Rotor core, (305) Air gap, (306)-Shaft.
A bi-directional excitation source can be used either to direct the magnetic field
of excitation windings in the direction of the permanent magnetic field or in the
opposite direction of the permanent magnetic field. The length of the second
rotor (103) with excitation windings is selected based on the range of regulation

specified. For example, the length of the second rotor (103) with excitation
windings is generally 25% of that of the first rotor (102) with the permanent
magnets if the range of regulation specified is -25% to +25% of the rated value.
Thus a dual excited round pole synchronous machine can be regulated over a
wide range.
In the present invention, since the excitation windings and the permanent
magnets are arranged in two different rotor cores unlike the prior art US Patent
No. 6,509,664 and US Patent No. 6,037,691, where both excitation windings and
the permanent magnets are arranged in the same rotor core, there is no
possibility for magnetic field of excitation windings to demagnetize the
permanent magnets.
The output voltage of the dual excited round pole synchronous machine is
substantially a combination of the output voltage of the permanent magnet
synchronous machine and that of a conventional synchronous machine. The
shape of output voltage waveform in the present case is sinusoidal with less
harmonic content because the permanent magnetic field and the magnetic field
of excitation windings do not interact with each other and so the sinusoidal
distribution of magnetic flux density across the air gap does not change whereas
in the US Patent No. 6,509,664 and US Patent No. 6,037,691, the magnetic fields
of the permanent magnets and excitation windings interact with each other
making it difficult to obtain a perfect sinusoidal distribution of magnetic flux
density across the air gap. Accordingly, a perfect sinusoidal shape of output
voltage waveform like in case of either permanent magnet synchronous machine
or conventional synchronous machine is not generated in case of prior art.

APPLICATIONS:
The present invention is applicable for synchronous machines to be regulated
over a wide range with a perfect sinusoidal output voltage containing less
harmonic content.

WE CLAIM
1. An improved dual-excited round pole synchronous machine generating
sinusoidal voltage waveform with wide-range regulation characteristic,
comprising:
- a rotor shaft having one each rotor core, the first rotor core
accommodating a plurality of permanent magnets, the second rotor
core incorporating corresponding excitation windings, the two rotor
cores separated by a gap of predetermined length on the rotor shaft;
- a stator core with a first end and a second end, the first rotor core
extending slightly beyond the first end of the stator core, the second
rotor core projecting out in respect of the second end of the stator
core;
- a bi-directional excitation source directing the generated magnetic
field of the excitation windings in the direction of the permanent
magnets or in a direction opposite to the permanent magnet field;
wherein a plurality of slots configured on the rotor shaft for the
permanent magnets and the excitation windings, the size and width of the
slots being variable corresponding to the capacity of the synchronous
machine, and wherein the length of the second rotor core is substantially
around 25% of that of the first rotor core when the range of regulation is
selected between -25% to +25%.

ABSTRACT

The invention relates to an improved dual-excited round pole synchronous machine generating sinusoidal voltage waveform with wide-range regulation characteristic, comprising a rotor shaft having one each rotor core, the first rotor core accommodating a plurality of permanent magnets, the second rotor core
incorporating corresponding excitation windings, the two rotor cores separated by a gap of predetermined length on the rotor shaft; a stator core with a first end and a second end, the first rotor core extending slightly beyond the first end of the stator core, the second rotor core projecting out in respect of the second end of the stator core; a bi-directional excitation source directing the generated
magnetic field of the excitation windings in the direction of the permanent magnets or in a direction opposite to the permanent magnet field; wherein a plurality of slots configured on the rotor shaft for the permanent magnets and
the excitation windings, the size and width of the slots being variable corresponding to the capacity of the synchronous machine, and wherein the length of the second rotor core is substantially around 25% of that of the first
rotor core when the range of regulation is selected between -25% to +25%.