sarto alternators

8/13/2019 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).



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.



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.



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.



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



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



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