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Elementary Alternator -
A basic AC generator consists of a loop of wire in a stationary magnetic field. When it is rotated in this magnetic field it causes an induced current in the loop. Sliding contacts are used to connect in order to an external circuit in order to use the induced current.
The pole pieces are the north & south poles of the magnet which supplies the magnetic field. The loop of wire which rotates through the field is called the armature. The ends of the armature loop are connected to rings called slip rings which rotate with the armature. Brushes ride up against the slip rings to pick up the electricity generated in the armature & carry it to the external circuit.
Operation of alternator -
Here is the way the elementary generator works. Assume that the armature loop is rotating in a clockwise direction & that its initial position is at A (zero degrees). In position ‘A’, the loop is perpendicular to the magnetic field & the black & white conductors of the loop are moving parallel to the magnetic field. If a conductor is moving parallel to a magnetic field it does not cut through any lines of force & no e. m. f. can be generated in the conductor. This applies to the conductors of the loop at the instant they go through position A - no e. m. f. is induced in them & therefore no current flows through the circuit. The current meter registers zero.
As the loop rotates from position A to position B, the conductors are cutting through more & more lines of force until at go degrees (position B) they are cutting through a maximum number of lines of force. In other words, between zero & 90 degrees the induced e.m.f. in the conductors build up from zero to a maximum value. Observe that from zero to 90 degrees the black conductor cuts down through the field while at the same time white conductor cuts up through the field. The induced e. m. f. in both conductors are therefore in series adding & the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced e. m. f. or double that of one conductor since the induced voltages are equal other. The current through the circuit will vary just as the induced e. m .f. varies being zero. At zero degrees & rising up to a maximum at 90 degrees. The current meter deflects increasingly to the right between positions A& B indicating that the current through the load is flowing in the direction shown. The direction of current flow & the polarity of the induced e. m. f. depend upon the direction of the magnetic field & the direction of rotation of the armature loop. The waveform shows how the terminal voltage of the elementary generator varies from position A to position B. The simple generator drawing on the right is shown shifted in position to illustrate the relationship between the loop position & the generated waveform.
As the loop continues rotating from position B(90 degrees) to position C (180 degrees) the conductors which are cutting through a maximum number of lines of force at position B cut through fewer lines until at position C they are moving parallel to the magnetic field & no longer cut through any lines of force. The induced e. m. f. therefore will decrease from 90 to 180 degrees in the same manner as it increased from zero to 90 degrees. The current flow will similarly follow the voltage variations. The generator action at position B & C is illustrated.
Fig. 2 A. C. Cycle From zero to 180 degrees the conductors of loop have been moving in the same direction through the magnetic field & therefore the polarity of induced e. m. f. has remained same. As the loop starts rotating beyond 180 degrees back to position A the direction of the cutting action of the conductor through the magnetic field reverses. Now the black conductor cuts up through the field & the white conductor cuts down through the field. As a result the polarity of the induced e. m .f. & the current flow will reverse. From position C through D back to position A, the current flow will be in the opposite direction that from positions A through C. The generator terminal voltage will be the same as it was from A to C except for its reversed polarity. The voltage output waveform for the complete revolution of the loop is as shown.
Working principle of alternator -
The alternator works on the same principle as of DC that is Faraday’s Laws of Electromagnetic Induction.
Construction of alternator -
Alternator consists of Rotor, Starter, Exciter.
1) Rotor -
The rotating part of alternator is known as rotor. It is made from a solid steel piece. To house the field winding slots are cut on the outer surface. The ends of winding are held by retaining ring of non- magnetic steel.
There are silent pole type rotor & cylindrical type rotor is used in alternator.
2) Stator -
The stator is built of stamping insulated on one side with paper or varnish & housed in a frame which is usually fabricated from steel plates- electrically welded. Slots to take the winding are cut round the inner surface.
3) Exciter -
The exciter is generally a DC shunt or compound generator whose voltage is up to 250V. In small alternators the exciter is mounted on the same shaft of the alternators. A variable resistance is connected in series with the shunt field of the exciter which varies the exciter voltage to vary the output voltage of the alternator. For high voltage alternators separately excited generators are used.
In Text Question – (a)
A] Fill in the blanks -
1. The alternator works on the laws of _____________.
2. There are ________& _________rotor is used in alternator.
3. Exciter has voltage up to ________.
4. The rotating part of alternator is known as __________.
5. It is easy to transmit electric power at very high voltage with small amount of current with the help of _______________.
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