The document discusses different types of generators used in power plants, focusing on Totally Enclosed Water to Air Cooled (TEWAC) generators. TEWAC generators circulate cooled air inside the enclosed generator through air-to-water heat exchangers to cool the rotor, unlike open-ventilated generators which draw in outside air. The document also describes the basic components of generators, including the stator, rotor, and exciter, which is a separate unit that provides current to the rotor to start power generation and control output.

1. CHARLES ISIADINSO
TOTALLY ENCLOSED WATER TO
AIR COOLED (TEWAC)
GENERATOR
THE GENERATOR
In a power plant, electricity is generated by the generator, which
works on Faraday’s law of electromagnetic induction. A turbine
(steam or gas) turns shaft to which a rotor is attached. The rotor
spins in a gap in a part called the stator. The law of
electromagnetic induction states that if a magnet is rotated close to
a conductor (e.g. copper wire), a current is induced in the
conductor. To vary the strength of the magnetic flux, the rotor is
an electromagnet (a magnet with windings of a conductor around
it) rotating inside the stator, which is a toothed metal ring shaped
structure with coils of a conductor between the teeth. To start the
process, current (excitation current) is passed into the conductor
coils of the rotor; this creates a magnetic flux in the air gap
between the rotor and stator. This magnetic flux then induces a
flow of electrons (current) in the stator coil thus generating
electricity. In some turbine systems, the generator is used to start
the turbine. An exciter module, which contains the generator
excitation system and start-up frequency converter (SFC) start
system, starts the gas turbine by temporarily converting the
generator into a motor, using the SFC. However, with GE’s 9E gas
turbine, a separate staring motor is used. The generator is
connected to the 9E after the exhaust system by a rigid metal shaft
that runs through the entire turbine and into the generator, and is
what the generator rotor is attached to. Rotation of the air
compressor and turbine blades causes a rotation in the shaft, which
then rotates the rotor in the stator, generating electricity.
The generator consists of a Stator consisting of a:
1. STATOR CORE: is a silicon steel frame, to which field
magnets are attached.

2. CHARLES ISIADINSO
2. FIELD MAGNETS: Each magnet is made up of a pole core
and shoe.
3. STATOR WINDING: Copper wire wound tightly round the
pole core of the field magnet, make up the stator winding.
And a rotor consisting of:
1. METAL CORE: The core is a cylindrical metal part that
houses the armature coils, which experiences a flow of
electrons when rotated through the magnetic flux created by
the field magnets’ pole coils.
2. COLLECTOR RINGS: or slip rings are used to transfer the
electricity generated from the stator to the rotor.
3. WINDING SYSTEM: made up of high conductivity wedge-
shaped copper segments, insulated from each other.
4. BEARINGS: Bearings are used to allow friction free
movement of the armature shaft.
5. RADIAL-FLOW FANS: there are two fans positioned one at
the commutator end, and the other at the armature end.
Their primary function is to provide cooling air for the stator
core and winding.
When in operation, the rotor is cooled by water, air or hydrogen.
There are four types of generators (differentiated by their cooling
systems):
1. Open-ventilated air cooled generator

3. CHARLES ISIADINSO
2. Totally enclosed water to air-cooled (TEWAC) generator
3. Hydrogen-cooledgenerators
4. Water-cooled generators
1. OPEN-VENTILATED AIR COOLED (OV)
GENERATOR:
Outside air is drawn into the generator through a filter system
to remove air contaminants, passed over the rotor, cooling it,
and then discharged back into the surroundings.
2. TEWAC GENERATOR:
Unlike the open-ventilated generator, TEWACs are cooled using
re-circulating air inside the enclosed generator, passed through
air-to-water heat exchangers, effectively cooling the air. Air
enter the generator through a filter system, this clean the air,
removing contaminants. Filtered air then passes through
silencers, for noise reduction. The stream is then directed over
the rotor and then over frame-mounted heat exchangers, which
extracts heat from the air, cooling it. Air from the heat
exchangers is then directed through another set of silencers,
and then out through an air outlet. Heat from the heat
exchangers is transferred to water, which then flows out of the
generator. At full load, TEWAC’s have efficiencies of

4. CHARLES ISIADINSO
approximately 98%. TEWAC’s are usually the chosen because of
their reliability and simplicity.
THE EXCITER
Induction generators need current to be present in the stator
windings to work, the process of passing current through the
windings, in order to starts the generator, is called excitation. In a
power station, there is a separate unit (called an exciter) used to
excite the generator. The exciter is also used to control the
generator output. Varying the current supplier to the rotor, which
varies the intensity of the magnetic flux, varies the current induced
in the stator core field magnets.
There are two basic types of exciters, rotating and static exciters.
The exciter maintains an output voltage by directly varying the
amount of excitation as a function of the generator load. There are
a further two types of rotating and static exciters, namely brushed
and brushless excitation systems and shunt and series type
excitation systems respectively. The main difference between the
two rotating systems is the brushed system requires a slip ring,
commutators and brushes and the brushless does not. This
negatively affects the lifetime of the system as the DC voltage
eventually wears out the slip ring. However, the brushless system
are more complicated to design and, as a result, are more
expensive. The difference between the static exciters is the shunt
type operates with power from generator voltage only while the
series type operates from generator voltage and current.
REFERENCE:
1. Kehlhofer, Rolf. Combined-cycle Gas & Steam Turbine Power
Plants. Lilburn, GA: Fairmont, 1991. Print.

5. CHARLES ISIADINSO
2. "Generators - Air-Cooled Generators | GE Energy." GE
Energy. N.p., n.d. Web. [Accessed17 June 2014].
3. Petchers, Neil. Combined Heating, Cooling & Power
Handbook: Technologies & Applications. Lilburn, GA:
Fairmont, 2003. Print.