Filed of Invention
The invention relates to the area of brushless exciter used for brushless
synchronous generators for obtaining self excitation during black start operation
of the generator. The hvention in particular relates to a brushless exciter
capable of providing self-excitation power during field flashing enabling black
start operation of brushless synchronous generators.
Background of the Invention
The brushless exciter is an electrical machne and is used to excite the generator
field whdhg. The brushless exciter of present art generally has wound field coils as
stater winding and three-phase rotor winding. When a d.c supply is given to this
stator windhg, a magnetic field is set up h the air-gap. The three-phase rotor
whding will accordingly have three-phase induced voltages because of mechanical
rotation. These three-phase voltages are converted to d.c. by a three-phase
rotating diode bridge and fed to the field winding of the generator. The exciter can
generate output only when the field windings are excited by d.c current through
the static regulator. The excitation power is normally obtained from a permanent
magnet generator (PMG), a station auxiliary supply or a station battery. In the
absence of PMG, auxiliary supply or station battery, it is not possible to excite the
exciter field windhg and h turn the man generator can not be excited for
producing electrical power. Sometimes, the residual magnetism available in the
silicon steel stampings (used to make excier poles) can create initial magnetisation
or excier field flashing. If the machine is not used for long time, the residual
magnetism is lost and it will not be possble to carry out field flashing via the
automatic voltage regulator in order to have black start operation of generator.

The patient GB1415376 dated 26.11.1975 discloses an arrangement of magnets on
a cylindrical structure including a separate machine oiher than the brushless exciter
to effect self exclation.
Objects of Invention
It is therefore an object of the invention to propose a brushless exciter capable
of providing self-excitation power during field flashhg enablhg black start
operation of brush less synchronous generators.
Another object of the invention is to propose a brushless exciter capable of
providing self-excitation power during field flashing enabling black start operation
of brushless synchronous generators which will make the generator more reliable
and can be started any time without the availability of auxiliary supply or station
battery.
Summary of the Invention
Accordingly there is provided a brushless exciter capable of providing initial
excitation power during field flushing enablhg black start operation of brushless
synchronous generators comprising a plurality of stator poles having pole
surface; atteast one d.c. field winding for excitation; a three phase armature
winding covering the rotor. A trapezoidal groove is configured on the pole
surfaces in which a plurality of trapezoidal shaped permanent magnets being
fixedly disposed. The magnets disposed north and south locations on albernabe
stator poles produce a magnetic flux in the air gap on providing a d.c field
winding which causes the armature winding to generate sufficient voltage to
provide the initia exclation power to the exciter.

The proposed brush less exciter with self excitation has field coils as stator winding
and three-phase armature winding. When a d.c supply is given to this stater
winding, a magnetic field is set up in the air-gap. The rotor has three phase
armature winding. The embedded magnets produces flux, which passes through
the rotor and armature winding without any d.c supply being given to the stater
winding/field cols. The initial excitation power is obtained from the flux produced
by embedded magnet. The developed brush less exciter has also been loaded to
check the demagnetisation effect of the armature current. The exciter field winding
is excited with d.c field current and it has resulted h increased output (excitation
power for the generator). The reversal of filed current has reduced the initial
excitation obtained by the embedded magnet and zero excitation level can also be
achieved to obtain smooth control.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 shows a schematic diagram of a brush less exciter.
Fig. 2 shows the brushless exciter construction without self excitation.
Fig.3 shows the constructional details of a brushless exciter with embedded
magnets for self excitation according to the present invention.
DETAILED DESCRIPTION OF A PREFFERED EMBODIMENT OF THE
INVENTION
The brushless exciter, an electrical machine, is used to excite the generator field
winding (Schematic diagram shown in Fig.1). When a d.c supply is given to its
stator winding (3), a magnetic field is set up in the air-gap. The three-phase
rotor winding will have three-phase induced voltages because of mechanical
rotation. These three-phase voltages are converted to d.c. by a three-phase
rotating diode bridge (9) and fed to the field winding (3) of the generator. The

exciter can generate output only when the field windings (3) are excited by d.c
current The excitation power is normally obtained from a permanent magnet
generator (10), station auxiliary supply (11) or station battery (12). In the
absence of PMG (10) or auxiliary supply (11), it is not possble to excite the
exciter field winding (3) and in turn the main generator can not be excited for
producing electrical power. Sometimes the residual magnetism available in the
silicon steel stampings of exciter stater poles (item 2) is used to create initial
magnetisation or exciter field flashing. If the machine is not used for long time
the residual magnetism is lost and it will not be possible to carry out field
flashing vla the automatic voltage regulator/ static regulator.
The proposed brush less exciter with self excitation has salient stator poles made
of silicon steel stampings (item 2) and has d.c. filed winding (3) for excitation. A
trapezoidal groove is milled on the pole surface and trapezoidal size of
permanent magnet (8) is pasted in the groove. The thickness and size of the
magnet is decided based on the initial voltage required from the exciter armature
winding (6). These trapezoidal magnets (8) form alternate north and south
poles. During black start operation, the magnets (8) (north and south on
alternate poles) will produce flux in the air-gap and the exciter armature winding
(6) will generate sufficient voltage (about 5-10 %) to start the initial excitation
and energising the automatic voltage regulator (11)/static regulator (12) . As the
generator rotates and reaches full speed the AVR (11) will supply d.c excitation
to the exciter field winding (3) from the station auxiliary supply (11) in addition
to the field generated by the magnets (8).
As the generator rotates and reaches full speed, the AVR (11) will supply d.c
excitation to the exciter field winding (3) to produce main flux in addition to the generated

by the permanent magnets (both the fields aiding each other). This will avoid loss
of excitation because of increase in the exciter field current during voltage build-up
after field flashing.
ln case there is requirement to reduce the excitation power to zero value, the
exciter field colls current has to be reversed in order to counter the filed/flux
produced by the magnet pieces embedded in the stator poles.

WE-CLAIM:

1. A brushless exciter capable of providing initial excitation power during
field flushing enabling black start operation of brushless synchronous
generators comprising:
- a plurality of stator poles (2) having pole surface;
- atleast one d.c. field winding (3) for excitation;
- a three phase armature winding (6) covering the rotor(1);
characterized in that a trapezoidal groove is configured on the pole surfaces in
which a plurality of trapezoidal shaped permanent magnet (8) being fixedly
disposed, and in that the magnets (8) disposed north and south locations on
alternate stator poles (2) producing a magnetic flux in the air gap on providing a
d.c field winding (3) which causes the armature winding (6) to generate
sufficient voltage to provide the initial excitation power to the exciter.
2. A brushless exciter capable of providing initial excitation power during field
flushing enabling black start operation of brushless synchronous generators as
substantially described herein and illustrated with reference to the accompanying
drawings.