FIELD OF INVENTION:
This invention relates to the field of permanent magnet machines in general and
to an optimized permanent magnet rotor of a pilot exciter in particular. The
invention also relates to an improved method of mounting magnets, manufacture
and assembly of permanent magnet machine rotor being used as pilot exciter for
power plant generators.
BACKGROUND AND PRIOR ART OF THE INVENTION:
A permanent magnet generator (PMG) is a rotating electrical machine with
permanent magnets mounted on rotor that establishes the field required to
generate electromotive force. The PMG when used to when used to excite the
brushless exciter of an industrial generator or power station generator is called a
pilot exciter. Efforts are now being made to replace these Aluminium-Nickel
Cobalt alloy (ALNICO) magnets with high energy Neodymium-Iron-Boron alloy
(NdFeB) magnets in order to reduce weight and size. The reduction of weight
and size reduces vibration and noise. However, in prior art the mounting of these
high-energy magnets on the rotor of a permanent magnet machine of medium
and large size poses a lot of technical challenge for the machine manufacturer.
The high energy permanent magnets can get attracted to any nearby iron part of
the rotor and can fly like a bullet and cause injury to the working personnel. This
sudden movement of magnets during assembly can also lead to the magnets
breaking into pieces as they are very fragile. A compact 70 kW PMG with
innovative features has been developed by the applicant to replace the PMG with
Alnico magnets for pilot exciters of turbogenerators used in power station. The
present invention provides for the compact and innovative rotor design and

improved rotor assembly method for the compact PMG, which overcomes the
drawbacks of the prior art as started above.
OBJECTS OF THE INVENTION:
An object of this invention is to provide an improved method of mounting
permanent magnets on a magnet assembly holder alongwith pole shoes.
Another object of this invention is to provide an improved method of mounting
permanent magnet assemblies on permanent magnet machine rotor which is
safer for the working personnel.
A further object of this invention is to propose an improved permanent magnet
machine assembly which is highly cost effective.
A still further object of this invention is to propose a compact permanent magnet
generator which is superior mechanically and offers lower size and weight.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows PMG rotor with magnet assembly and stator.
Fig. 2 shows the trapezoidal shaped non-magnetic holder assembly.
The invention will now be described in detail with an exemplary embodiment of
the invention as depicted in the drawings. However, there can be several other
embodiments of the invention, all of which are deemed covered by this
description .
DESCRIPTION OF THE INVENTION:
According to this invention there is provided a permanent magnet generator
comprising:
A permanent magnet rotor assembly (1) which has a magnet sleeve/hub (2).
The sleeve (2) has a special trapezoidal slot (3), which has a plurality of tapped
holes (4). A magnet holder assembly (5) with magnet cartridge (6) & pole shoes

(7) bolted/ fixed with each other using plurality of screw type of bolts, also
having a plurality of bolting arrangement with high strength non-magnetic
bolts(8) to suit tapped holes (4) and plurality of magnets (9) placed in the slots
provided in the magnet cartridge (6) & bonded with adhesives; the complete
magnet holder assembly (5) along with magnet cartridge and pole shoe (Fig.2) is
positioned in the slot (3). A stator (10) wound with three phase fractional slot
winding (11) is provided in the permanent magnet generator.
The permanent magnet generator (PMG) is provided with NdFeB alloy magnets
mounted on the rotor sleeve (2) and fixed with non-magnetic bolts and pole
shoes (7) and tested successfully. The PMG has a 3-phase stator and permanent
magnet rotor, wherein the magnets are mounted on the rotor sleeve having flat
surfaces equal to number of poles on the outer diameter. The rectangular
magnet assemblies are mounted by using the required non-magnetic bolts.
Permanent Magnet Generator (PMG) in accordance with the invention is a
rotating electrical machine with permanent magnets assembled on the rotor
having trapezoidal milled surfaces and tapped hole to mount the high-energy
magnets along with magnet cartridge & pole shoes. In the present invention a
pilot exciter which is a PMG has been developed using the proposed high energy
magnet (9) and magnet holder assembly (5) which is bolted with very few non-
magnetic bolts. In contrast, pilot exciters of the prior art have large number of
non-magnetic bolts.
The magnet cartridge made of SS316 material holds the magnets (9) alongwith
pole shoes (7) and also acts as flux barrier. The permanent magnet assemblies
are mounted on the magnet sleeve (2) by using a special tool and non-magnetic

bolts (8). The optimized electromagnetic design along with the special magnet
cartridge assembly maximizes the main flux and reduces the leakage flux. The
PMG in accordance with the invention has been tested in the laboratory for70 kW
of output power. The optimized rotor design and improved rotor magnet
assembly has not only simplified the manufacture of the permanent magnet rotor
but has also reduced the rotor weight by 50%. This has resulted in lower
overhang weight and improved machine performance in addition to saving in
labour and material cost.

WE CLAIM:
1. A permanent magnet generator comprising a permanent magnet rotor
assembly (1) having a magnet sleeve (2), the magnet sleeve being
provided with slot (3), magnet holder" assembly (5) and a magnet
cartridge (6) with permanent magnets (9) positioned and bonded with
adhesives in slot provided in the magnet cartridge (6) and pole shoes
(7) ; the complete magnet holder assembly (5) along with magnet
cartridge and pole shoe (Fig.2) being positioned in slot (3) and fixed by
bolts (8) through tapped holes (4) and stator (10) having fractional slot
winding (11), characterized in that the magnet holder assembly (5)
prevents permanent magnets (9) from escaping.
2. A permanent magnet generator as claimed in claim 1, wherein the slot (3)
is trapezoidal in shape.
3. A permanent magnet generator as claimed in claim 1, wherein the
magnet cartridge (6) and pole shoes (7) act as a flux barrier.
4. A permanent magnet generator as claimed in claim 1, wherein the
magnets (9) are made of Neodymium-Iron-Boron (NdFeB) alloy to make
them light.
5. A method for permanent magnet rotor assembly comprising the steps of:
- Placing permanent magnets (9) in a magnet cartridge (6) which is
held in magnet holder assembly (5); and
- Placing the magnet holder assembly (5) and pole shoes (7) in the
trapezoidal slot (3) using a special tool and fixing with a plurality of
non-magnetic bolts (8).

A permanent magnet generator comprising a permanent magnet rotor assembly (1) having a magnet sleeve (2), the magnet sleeve being
provided with slot (3), magnet holder assembly (5) and a magnet cartridge (6) with permanent magnets (9) positioned and bonded with
adhesives in slot provided in the magnet cartridge (6) and pole shoes (7) ; the complete magnet holder assembly (5) along with magnet
cartridge and pole shoe (Fig.2) being positioned in slot (3) and fixed by bolts (8) through tapped holes (4) and stator (10) having fractional slot winding (11), characterized in that the magnet holder assembly (5) prevents permanent magnets (9) from escaping.