GENERATOR (AC and DC):
The principle of rotating a conductor in a magnetic field is used in electricity generators. A generator converts mechanical energy (rotational) into electrical energy by using the principle of electromagnetic induction.
AC Generator or Dynamo:
When a coil (conductor) is rotated in a magnetic field, the magnetic flux linked with it changes and therefore an alternating e.m.f. is induced in the coil.
Construction: The main parts of dynamo are:-
(i) Field magnet: It is a strong horse shoe permanent magnet. An electromagnet run by a DC source can also be used for high power generators.
(ii) Armature: It is a soft iron core on which a coil ABCD having a large number of turns of insulated copper wire is wound. This armature (or coil) is rotated rapidly in the magnetic field between the poles of the magnet. The soft iron core has not been shown in figure to make things simple.
(iii) Slip rings: The ends of the armature (or the coil) are connected to two coaxial metallic slip rings R1 and R2 which rotate along with the coil.
(iv) Brushes: Two brushes B1 and B2 made of carbon press against the slip rings R1 and R2 respectively. The external circuit (i.e. load) is connected between the other ends of brushed. The brushes B1 and B2 do not rotate along with the coil.
Working of an AC generator:
Suppose that the coil ABCD is being rotated in the clockwise direction between the poles N and S of a permanent magnet. Again suppose that the generator coil ABCD is initially in the vertical position (CD at top and AB at bottom) of the magnetic field produced by the permanent magnet. As there is no flux cut, therefore induced current at this position is zero.
(i) As the coil rotates in the clockwise direction, the side AB of the coil moves up cutting the magnetic lines of force near the N- pole of the magnet and side CD moves down, cutting the lines of force near the S-pole of the magnet. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions A to B and C to D. Thus, the induced currents in the two sides of the coil are in the same direction and we get an effective induced current in the direction ABCD. If there are larger numbers of turns in the coil (as shown in figure), the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1.
(ii) After half revolution, the sides AB and DC of the coil will interchange their positions. The side AB of the coil moves down cutting the magnetic lines of force near the S- pole of the magnet and side CD moves up, cutting the lines of force near the N-pole of the magnet. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions B to A and D to C. Thus, the induced currents in the two sides of the coil are in the same direction and we get an effective induced current in the direction DCBA. This means that the current in the external circuit now flows from B1 to B2.
Thus, the direction of induced current in each side of the coil is reversed after half revolution. Since the direction of induced current in the coil is reversed after half revolution so that polarity (positive and negative) of the two ends of the coil also changes after half revolution, is called an alternating current (abbreviated as AC). After every half revolution, each side of the generator coil starts moving in the opposite direction in the magnetic field. The side of the coil which was initially moving upwards, after half revolution, it starts moving downwards. Due to the change in the direction of motion of the two sided of the coil in the magnetic field every half revolution, the direction of current produced in them also changes after every half revolution. In 1 revolution of the coil, the current changes its direction 2 times. This device is called an AC generator.
The alternating current (AC) produced in India has a frequency of 50 Hz. That is, the coil is rotated at the rate of 50 revolutions per second. Since in 1 revolution of coil, the current changes its direction 2 times, so in 50 revolutions of coil, the current changes its direction 2 × 50 = 100 times. Thus, the AC supply in India changes its direction 100 times in 1 second. Another way of saying this is the alternating current produced in India changes its direction every 1/100 second. That is, each terminal of the coil is positive (+) for 1/100 of a second and negative (-) for the next 1/100 of a second.
DC Generator (or DC Dynamo):
“DC generator” means “Direct Current generator”. That is, a DC generator produces direct current.
Construction of a DC Generator:
The construction of a DC generator is identical to an AC generator except one difference. The coil is mounted between the curved poles of a permanent magnet is such a way that it can rotate between the poles N and S.
The two ends of the coil are soldered (or welded connect) permanently to the two copper half rings (or split rings) R1 and R2 are called as commutator. A commutator is a copper ring split into two parts R1 and R2; these two parts are insulated from one another and mounted on the shaft. The end A of the coil ABCD is welded to part R1 of the commutator and end D of the coil is welded to part R2 of the commutator. The commutator rings are mounted on the shaft of the coil but insulated and they also rotate when the coil rotates.
There are two stationary carbon brushes B1 and B2 are kept pressed separately the two half rings. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. When the coil is rotated, the two half rings R1 and R2 touch the two carbon brushes B2 and B2 one by one. So, the current produced in the rotating coil can be tapped out through the commulator half rings into the carbon brushes.
The function of commutator rings is to reverse the direction of current following through the coil every time the coil just passes the vertical position during a revolution. We cannot join the battery wire directly to the two commutator’s half rings to pass current into the coil because if we do so, then the connecting wires will get twisted when the coil rotates. So, to pass the electric current to the coil, we use two carbon strips B1 and B2 known as brushes. The carbon brushes are fixed to the base of the Generator and they press lightly against the two half rings of the commutator. The function of carbon brushes is to make contact with the rotating rings of the commutator and through them to supply current to the coil. It should be noted that any one brush touches only one ring at a time, so that when the coil rotates, the two brushes will touch both the rings one by one.
Working of a DC generator:
Suppose that the generator coil ABCD is initially in the horizontal position and the coil ABCD is being rotated in the clockwise direction between the poles N and S of a horseshoe type magnet.
(i) As the coil rotates in the clockwise direction, the side
AB of the coil move up cutting the magnetic lines of force near the N-pole of
the magnet and side DC moves down, cutting the lines of force near the S-pole
of the magnet in figure. Due to this, induced current is produces in the sides AB
and DC of the coil. On applying Fleming’s right-hand rule to the sides AB and
DC of the coil we find that the currents in them are in the directions B to A
and D to C respectively. Thus, we get an effective induced current in the
direction DCBA. Due to this the brush B1 becomes a positive (+) pole and brush B2 becomes negative (-)
pole of the generator.
(ii) After half revolution the sides AB and DC of the coil will interchange their positions. The side AB will come on the right hand side and start moving down whereas side DC will come on the left-hand side and start moving up. But when sides of the coil interchange their position, then the two commutator half rings R1 and R2 automatically change their contacts from one carbon brush to the other. Due to this change, the current keeps flowing in the same direction in the circuit. Te brush B1 will always remain positive terminal and brush B2 will always remain negative terminal of the generator. Thus, a DC generator supplies a current in one direction by the use of a commutator consisting of two half-rings of copper.
Difference between a DC generator and an AC generator:
In a DC generator we connect the two ends of the coil to a commutator consisting of two, half rings of copper. On the other hand, in an AC generator, we connect the two ends of the coil to two full rings of copper called slip rings.
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