AC GENERATORS
Compiled and presented by
Doren Nedrick
What is a Generator?
The
phrase to generate means "to
produce." An electric generator is a
machine that produces a voltage by
means of electromagnetic induction.
This is done by rotating coils of wire
through a magnetic field or by rotating
a magnetic field-past coils of wire.
Construction of the
Generator
Construction of the Generator
Contd
A
continuous alternating voltage can be
produced by rotating a coil of wire between
the poles of a permanent magnet (Fig. 1).
This is a simple generator.
The coil is called the armature. The ends of
the armature coil are connected to slip rings
(Fig. 1) that are insulated from each other
and from the armature shaft on which they
are mounted.
The brushes press against the slip rings.
They make it possible to connect the
rotating armature to an external circuit. The
armature must be driven by mechanical
force.
Factors Affecting the Magnitude
of Induced Voltage
The
value of the voltage induced by
generator action at any instant depends on
three things:
(1) The flux density of the magnetic field
(the greater the flux density, the greater the
induced voltage);
(2) The velocity of the conductor motion
(induced voltage increases as the velocity of
the conductor increases); and
(3) The angle at which a conductor cuts
across flux lines (the greatest voltage is
induced when the conductor cuts across flux
lines at a 90-degree angle).
How does the angle affect the
amount of voltage?
What cause the rotor to
rotate
The
rotating parts of large ac generators are
called rotors. They are turned by steam
turbines, hydro-(water-driven) turbines, or
diesel engines. These generators produce
the electric energy used in our homes and in
industry.
Small ac generators are usually driven by
gasoline engines. Such generators are
commonly used to provide emergency
power (back up generator). Alternating
current generators are also called
alternators.
Methods of Exciting an a.c.
generator
In
the small ac generator, the
excitation current needed to energize
the field windings is first obtained from
a battery or the output of the
generator itself.
The excitation current must be direct,
but the output current is alternating.
Thus, the output current must first
pass through a rectifier to be changed
into direct current and used as
Methods of Exciting an a.c.
generator Contd
In
some alternators, the rectifier circuit
is located within the generator
housing. In large ac generators, dc
exciter generators produce the
excitation current. This is mounted
either on the shaft of the main
generator or located nearby.
Types of AC Generator
Therearetwotypesof AC
generators:thestationary field, rotating
armature; andtherotatingfield, stationary
armature. SmallAC generatorsusually have
a stationary field and a rotating armature.
Stationary Field Rotating
Armature Disadvantage
One
important disadvantage to this
arrangement is that the slip ring and
brush assembly is in series with the
load circuits and, because of worn or
dirty components, may interrupt the
flow of current.
In
Rotating-Armature
Generator
small ac generators, the armatures are
usually the rotors, or rotating parts. The
rotors turn within magnetic fields produced
by stationary field windings called stators.
Rotors have collectors, or slip rings, in
contact with carbon brushes.
SLIP RINGS
Rotating-Field Generator
In
rotating field generators, armatures
are stationary. They are made of
winding conductors placed in the slots
of the frame assembly.
FIELD
WINDINGS
FIELD
POLES
COLLECTOR
(SLIP) RINGS
Principal parts of a rotating field
generator: Rotor
The
field windings are wound around
pole pieces on the rotor assembly and
they are connected to slip rings.
FIELD
WINDINGS
COLLECTOR (SLIP)
RINGS
FIELD
POLES
(A) Armature (stator) Contd
The
excitation current passes to the
field windings through the carbon
brushes that are in contact with the
slip rings. Generators of this kind are
used in most large power-generating
plants.
The Basic Circuit Of A Single
phase A.C. Generator
In this instance, the rotating electromagnet is supplied
from an outside source with direct current (exciting
current), forming a unidirectional electromagnet in the
rotor.
When the rotor is rotated, this magnetic field cuts the
conductor of the field and induces an alternating e.m.f.
into them.
This is termed a revolving-field a.c. generator.
3-phase Outputs
Shows
a similar arrangement but the single pair of
coils have been replaced with three sets of coils,
thus producing three sets of phases displaced at
120o to one another.
Frequency
The
frequency of the output depends
on two factors: (a) number of pairs of
poles; and (b) the speed of rotation in
revolutions per second.
F (Hz) = p (pairs of poles) x N (rev/s)
Output Voltage
The
output voltage of a generator
depends on the speed of the rotor, the
number of armature coils, and the
strength of the magnetic field
produced by the field windings.
Power-station generators usually have
output voltages of 10,000 V or more.
