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ABSTRACT
This presentation showcases a simple dynamo as one of the electric machines that works with Michael Faraday’s principle of electromagnetic induction. This is used in the generation of electricity.
INTRODUCTION
Dynamo can also be referred to as generator.
Dynamo(generator) is a machine that converts mechanical energy into electrical energy for transmission and distribution over power lines to domestic, commercial, and industrial customers. Generators also produce the electrical power required for automobiles, aircraft, ships, and trains.
Principle involved in a Generator
In 1831 British scientist, Michael Faraday, announced his discovery that moving an electrical conductor, such as a length of copper wire, through a magnetic field caused electric current to flow in the conductor. This effect came to be known as electromagnetic induction. Faraday’s experiments showed that electric current could be produced mechanically with electromagnetic induction, not only with batteries.
Electromagnetic induction
Electromagnetic induction is the production of electric current or voltage in a conductor whenever there is a relative motion between a conductor and a magnetic field (or a magnet).
Faraday’s Experiments
First Experiment:
In one of Faraday’s experiments, he wound two coils on an iron ring, as shown in the figure below, and observed that when the switch was closed, a deflection was obtained on the galvanometer; but when the switch was opened, the galvanometer showed a deflection in the opposite direction.
A diagram of Faraday's iron ring-coil apparatus
Second Experiment:
• Produce a winding with several coils of insulated copper wire .
• Connect the winding to a sensitive galvanometer as shown.
• Move the N-pole of a bar magnet horizontally towards the end of the coil. Note the direction of deflection of the galvanometer.
• Move the magnet away from the coil and note the new direction of deflection of the needle of the galvanometer.
Diagram of 2nd experiment:
The Results of above Experiments :
• Current will flow in the galvanometer whenever there is relative motion between the coil and the magnet.
• The induced e.m.f or current depends on:
(a)the speed of motion of the magnet or the coil. The faster the speed of motion, the larger the induced current.
ELECTROMAGNET
§ An electromagnet consists of a soft iron core in a current carrying solenoid.
§ An electromagnet is a temporary magnet; magnetization and demagnetization occur rapidly when the current is switched ON and OFF respectively.
§ The magnetic field strength due to such a solenoid is increased by the presence of the soft iron core or rod within the solenoid.
Arrangement of an electromagnet
An electromagnet is made by winding the coils or solenoids in opposite directions round the arm of a soft-iron core or bar.
When current flows from a battery round the solenoid , it is observed that one end acts as a S-pole and the other a N-pole.
Such an arrangement produces a STRONG magnetic field .
Classes of Generator
There are two classes of generator:
• Alternating current (A.C) generator
2. Direct current (D.C) generator
Simple Alternating current (A.C) generator:
The A.C generator consists of:
• An armature–a rectangular coil consisting of a large number of turns of insulated wire wound on a laminated soft-iron core.
• A magnetic field created by the curved poles of a horse-shoe magnet or an electromagnet.
• Two copper slip rings to which the ends of the rectangular coil are connected and which rotate with the armature.
• Two stationary carbon brushes which are made to press lightly against the slip rings.
Principle of operation for A.C generators
• The armature is initially at the vertical position. No magnetic flux is cut and hence no induced current exists.
• When the armature rotates, the change in magnetic flux increases and the induced current increases until its maximum value at the horizontal position.
• As the armature continues on its rotation, the change in magnetic flux decreases until at the vertical position, no induced current exists.
• Subsequently upon reaching the horizontal position again, the induced current is maximum, but the direction of the induced current flowing through the external circuit is reversed.
• The direction of the induced current (which flows through the external circuit) keeps on changing depending on the orientation of the armature.
• This induced current is also known as alternating current. The current is positive (+) in one direction and negative in the other (-). The slip rings play a critical role in the generation of alternating current.
A.C waveform
Simple Direct current (D.C) generator:
The D.C generator consists of:
• An armature–a rectangular coil consisting of a large number of turns of insulated wire wound on a laminated soft-iron core.
• A magnetic field created by the curved poles of a horse-shoe magnet or an electromagnet.
• A split-ring commutator to which the ends of the rectangular coil are connected.
• Two stationary carbon brushes which are made to press lightly against the slip rings.
Principle of operation for D.C generator
• Initially the armature is vertical. No cutting of magnetic flux occurs and hence induced current does not exist.
• When the armature rotates, the change in flux increases and the induced current correspondingly increases in magnitude.
• After rotating by 90°, the armature is in the horizontal position. The change in magnetic flux is maximum and hence the maximum induced e.m.f is produced.
• When the armature continues to rotate, the change in flux decreases. At the 180° position, there is no change in flux hence no induced current exists. The induced current achieves its maximum value again when the armature is at 270°. After rotating 360°, the armature returns to its original position.
D.C waveform
Difference between A.C and D.C generator
The principles of operation for both A.C and D.C generator are the same; but the only difference is that A.C generator works with two slip rings connected at the ends of armature coil while D.C generator works with split-rings commutator which are connected at the ends of armature coil as well.
Function of the split-ring in D.C generator
The split-ring or commutator is a current reverser. When the armature coil is rotated, the commutator automatically switches each end of the coil from one brush to the other each time the coil completes one-half a revolution. As the current reverses in the coil after each half revolution, the connections between the coil and the brushes are reversed through this action of the commutator. This reversal occurs at the moment when the coil is vertical (i.e. when current is zero).
Practical Generator
To produce larger electromotive force in A.C or D.C generators,
• The armature is constructed with a large number of turns in the coil.
• The coil is wound on a soft iron so as to increase the magnetic flux through the coil.
• The strength of the magnetic field is made as high as possible.
• The armature is made to rotate at a faster rate.
Typical AC Generator
A typical AC generator consists of
1. Stationary stator: The stator may be either a permanent magnet or an electromagnet. Where the stator is an electromagnet, it contains a specific number of coils, each with a specific number of windings.
2. Rotor: The rotor is mounted within the stator. The rotor consists of a specific number of field poles, each with a specific number of windings. The rotor contains magnetic fields which are established and fed by the exciter. When the rotor is rotated, AC is induced in the stator. The changing polarity of the rotor produces the alternating characteristics of the current. The generated voltage is proportional to the strength of the magnetic field, the number of coils (and number of windings of each coil), and the speed at which the rotor turns
3. Collector assembly: it usually consists of collector slip rings, brushes, and brush holders.
Slip rings are usually made of nonferrous metal (brass, bronze or copper); iron or steel is sometimes used. Slip rings usually do not require much servicing. The wearing of grooves or ridges in the slip rings is retarded by designing the machine with limited end-play and by staggering the brushes. Surfaces of the slip rings should be bright and smooth, polishing can be performed with fine sandpaper and honing stone.
Brushes are electrical conductors that make sliding contact between a stationary and a moving part of a generator while completing a circuit and conveying a current.
Hence; in AC generator,
• Alternating current is advantageous for electric power transmission.
• Most large electric generators are of the AC type.
Typical DC Generator
• DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage. The highest potential commonly developed by such generators is 1500 V.
• In some newer machines this reversal is accomplished using power electronic devices, for example, diode rectifiers.
• Modern DC generators use a commutator of many segments while a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant.
Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes, smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.
Generator rating
The capacity of a generator is equal to the product of the voltage per phase, the current per phase, and the number of phases. It is normally stated in megavolt-amperes (MVA) for large generators or kilovolt-amperes (kVA) for small generators.
Both the voltage and the current are the effective, or rms, values (equal to the peak value divided by √2).
conclusion
The generation, transmission and distribution of electricity over power lines to domestic,commercial,and industrial customers is done with dynamos(generators) of different classes and capacity which work with the principle of electromagnetism.
This principle has helped so much in mechanism of turbines, and also in producing the electrical power required for automobiles, aircrafts, ships, trains and bicycles.
REFERENCES
Simon, Andrew L. (1998). Made in Hungary:
Hungarian contributions to universal culture. Simon Publications. p. 207. ISBN 0-9665734-2-0.
"Ányos Jedlik biography".
Hungarian Patent Office. Retrieved 10 May 2009.
Augustus Heller (April 2, 1896). "Anianus Jedlik". Nature (Norman
Lockyer) 53 (1379): 516. Bibcode:1896Natur..53..516H. doi:10.1038/053516a0.
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