sarto alternators
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SARTO, Jomar C. Energy Conversion
BSECE 4-4
ALTERNATORS
I. DEFINITION
An alternator is an electromechanical device that converts mechanical
energy to electrical energy in the form of alternating current. Most alternators use
a rotating magnetic field. In principle, any AC electrical generator can be called
an alternator, but usually the word refers to small rotating machines driven by
automotive and other internal combustion engines. Alternators in power stationsdriven by steam turbines are called turbo-alternators.
In an automobile, alternator is considered as one of the major automotive
charging systems together with the battery and voltage regulator. It charges the
battery and powers up the electrical devices of the vehicle. An alternator
produces alternating current (AC).
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II. PARTS
Rotor Assembly – The rotating part of the alternator that holds the main
field coils, field poles, slip rings, and the shaft. It is supplied by a DC
voltage through the split rings. It acts as a rotating electromagnet that
provides the magnetic field for the alternator.
Stator
–
The stationary part of the alternator that consists of the fixedarmature poles and armature windings. It receives the magnetic field
propagated by the rotor assembly and then produces a multi-phase AC
output.
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Slip Rings – It serves as rotating terminals for the DC input of the rotor
assembly. The two end points of the main field coil are connected to them.
It has a constant contact with the brushes.
Brush Assembly – It holds the brush holder. It is usually made of
insulating material.
Brush Holder – It serves as the holder of the brush as well as the
external terminal wires for the DC power input.
Brush – It is a conducting material usually made of carbon which has a
constant and direct contact with the split rings. It delivers the DC power
input to the rotor assembly.
Brush Cover
–
The plate that covers and protects the brush assembly.
Diode Set – The main solid state rectifier of the alternator which converts
the multi-phase AC power to DC power output.
Shaft – The rod that holds the rotor assembly and provides mechanical
contact to the prime mover.
Bearings – Highly smooth rotating parts that holds the shaft.
Cover –
It serves as the holder and casing for the stator assembly.
Front Cover – The front end cover of the alternator. It holds the bearing
for the front end of the alternator. Serves as a protection.
Bearing Cover – Covers the bearings which serve as protection from
minute particles that may clog the rotation of the rotor.
Pulley/Cooling Fan – Serves as the mechanical contact for the prime
mover as well as the ventilation system for the generator to prevent it from
overheating.
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III. PRINCIPLE OF OPERATION
Rotating Field Alternator
When a magnetic field cuts across a wire (typically a coil of copper wire)
an electrical current is induced, this is called electromagnetic induction. The
amount of electrical current and voltage will depend on the strength of the
magnetic field (the flux), how quickly the magnetic field moves, how many winds
in the coil of wire and the thickness of the copper wire.
Alternators generate electricity using the same principle as DC generators,
namely, when the magnetic field around a conductor changes, a current is
induced in the conductor. Typically, a rotating magnet, called the rotor turns
within a stationary set of conductors wound in coils on an iron core, called the
stator. The field cuts across the conductors, generating an induced EMF
(electromotive force), as the mechanical input causes the rotor to turn.
The rotating magnetic field induces an AC voltage in the stator windings.
Often there are three sets of stator windings, physically offset so that the rotating
magnetic field produces a three phase current, displaced by one-third of a period
with respect to each other.
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The rotor's magnetic field may be
produced by induction (as in a
"brushless" alternator), by permanent
magnets (as in very small machines), or
by a rotor winding energized with direct
current through slip rings and brushes.
The rotor's magnetic field may even be
provided by stationary field winding, with
moving poles in the rotor. Automotive
alternators invariably use a rotor winding,
which allows control of the alternator's
generated voltage by varying the current in the rotor field winding. Permanent
magnet machines avoid the loss due to magnetizing current in the rotor, but are
restricted in size, due to the cost of the magnet material. Since the permanentmagnet field is constant, the terminal voltage varies directly with the speed of the
generator. Brushless AC generators are usually larger machines than those used
in automotive applications.
An automatic voltage control device controls the field current to keep
output voltage constant. If the output voltage from the stationary armature coils
drops due to an increase in demand, more current is fed into the rotating field
coils through the voltage regulator (VR). This increases the magnetic field around
the field coils which induces a greater voltage in the armature coils. Thus, the
output voltage is brought back up to its original value.
Alternators used in central power stations may also control the field
current to regulate reactive power and to help stabilize the power system against
the effects of momentary faults.
IV. ADVANTAGES OVER DYNAMOS
Alternator can give higher charge rate at slow speeds due to multiple
phases
Alternators approach maximum output much earlier
Alternators are much more efficient
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Alternators are more reliable
Easier to diagnose
V. ANALYSIS AND CONCLUSION
Alternators are AC generators that uses DC input voltage as its field
exciter. Most of alternators are rotating field alternators where the main fields are
the ones that rotate instead of the armature. Alternators usually come in multi-
phase packages which gives a much smoother output levels. In automobiles, the
alternator output voltage is rectified to produce DC voltage. This is done by the
solid-state rectifying devices such as the diodes. Alternator can give higher
charge rate at slow speeds due to multiple phases and thus approach maximum
output much earlier. Alternators are much more efficient because it can produce
higher output at slower rotating speed. They have smaller construction than the
dynamo and tend to be more reliable. Alternators are easier to maintain because
it uses slip rings rather than commutators. A commutator tends to wear the
brushes sooner because of its rough surface. Commutator also introduces spark
arcs when it uses higher voltages. Alternators are easier to diagnose because
they use simpler topology of voltage regulation circuit.
VI. REFERENCES
http://auto.howstuffworks.com/alternator2.htm
http://blogcarparts.blogspot.com/2011/09/what-does-alternator-do.html
http://automotiveservices.blogspot.com/2011/02/alternator.html
http://www.microgreen.co.uk/alternator-basics.html
http://en.wikipedia.org/wiki/Alternator
http://www.autoshop101.com/trainmodules/alternator/alt102.html
http://www.rowand.net/Shop/Tech/AlternatorGeneratorTheory.htm
http://www.ehow.com/facts_5455888_alternator-basics.html