Contents
Physics Topics can also be used to explain the behavior of complex systems, such as the stock market or the dynamics of traffic flow.
How do Generators create Electricity?
The electric generator is a machine for producing electric current or electricity. The electric generator converts mechanical energy into electrical energy. A small generator is called a dynamo. For example, the small generator used on bicycles for lighting purposes is called a bicycle dynamo.
Principle of Electric Generator
The electric generator is an application of electromagnetic induction. The electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor. In an electric generator, a rectangular coil (having straight sides) is made to rotate rapidly in the magnetic field between the poles of a horseshoe-type magnet. When the coil rotates, it cuts the magnetic field lines due to which a current is produced in the coil.
Electric generators are of two types :
- Alternating Current generator (A.C. generator), and
- Direct Current generator (or D.C. generator).
Please note that A.C. generator is also written as a.c. generator and D.C. generator is also written as d.c. generator. We will now discuss both the types of electric generators, one by one. Let us start with the A.C. generator.
A.C. GENERATOR
“A.C. generator” means “Alternating Current generator”. That is, an A.C. generator produces alternating current, which reverses its direction continuously. A.C. generator is also known as an alternator. We will now describe the construction and working of an A.C. generator.
Construction of an A.C. Generator
A simple A.C. generator consists of a rectangular coil ABCD which can be rotated rapidly between the poles N and S of a strong horseshoe-type permanent’ magnet M (see Figure). The coil is made of a large number of turns of insulated copper wire. The two ends A and D of the rectangular coil are connected to two circular pieces of copper metal called slip rings R1 and R2.
As the slip rings R1 and R2 rotate with the coil, the two fixed pieces of carbon called carbon brushes, B1 and B2/ keep contact with them. So, the current produced in the rotating coil can be tapped out through slip rings into the carbon brushes. The outer ends of carbon brushes are connected to a galvanometer to show the flow of current in the external circuit (which is produced by the generator).
Working of an A.C. generator
Suppose that the generator coil ABCD is initially in the horizontal position (as shown in Figure). Again suppose that the coil ABCD is being rotated in the anticlockwise direction between the poles N and S of a horseshoe-type magnet by rotating its shaft.
(i) As the coil rotates in the anticlockwise direction, the side AB of the coil moves down cutting the magnetic field lines near the N-pole of the magnet, and side CD moves up, cutting the magnetic field lines near the S-pole of the magnet (see Figure). Due to this, induced current is produced in the sides AB and CD of the coil.
On applying Fleming’s right-hand rule to the sides AB and CD of the coil, we find that the currents are in 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 BADC (see Figure). Thus, in the first half revolution (or rotation) of coil, the current in the external circuit flows from brush B1 to B2.
(ii) After half revolution, the sides AB and CD of the coil will interchange their positions. The side AB will come on the right hand side and side CD will come on the left side. So, after half a revolution, side AB starts moving up and side CD starts moving down. As a result of this, the direction of induced current in each side of the coil is reversed after half a revolution giving rise to the net induced current in the direction CDAB (of the reversed coil). The current in the external circuit now flows from brush B2 to B1.
Since the direction of induced current in the coil is reversed after half revolution so the polarity (positive and negative) of the two ends of the coil also changes after half revolution. The end of coil which was positive in the first half of revolution becomes negative in the second half. And the end which was negative in the first half revolution becomes positive in the second half of revolution. Thus, in 1 revolution of the coil, the current reverses its direction 2 times. In this way alternating current is produced in this generator.
The alternating current (A.C.) 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 reverses its direction 2 times, so in 50 revolutions of coil, the current reverses its direction 2 × 50 = 100 times.
Thus, the A.C. supply in India reverses its direction 100 times in 1 second. Another way of saying this is that the alternating current produced in India reverses its direction every second. That is, each terminal of the coil is positive (+) for \(\frac{1}{100}\) of a second and negative (-) for the next \(\frac{1}{100}\) of a second. This process is repeated again and again with the result that there is actually no positive and negative in an A.C. generator.
A.C. generators are used in power stations to generate electricity which is supplied to our homes. These days most of the cars are fitted with small A.C. generators commonly known as alternators. The bicycle dynamos are very small A.C. generators.
We have just described a simple A.C. generator. In practical generators, the voltage (and the current) produced can be increased :
(a) by using a powerful electromagnet to make the magnetic field stronger in place of a permanent magnet.
(b) by winding the coil round a soft iron core to increase the strength of magnetic field.
(c) by using a coil with more turns.
(d) by rotating the coil faster.
(e) by using a coil with a larger area.
In power stations, huge A.C. generators (or alternators) are used to generate current for the A.C. mains which is supplied to homes, transport and industry. The power house A.C. generators have a fixed set of coils arranged around a rotating electromagnet (see Figure). Thus, in large power house generators, the coils are stationary and the electromagnet rotates.
The big coils of a power house generator are kept stationary because they are very heavy and hence difficult to rotate. The electromagnet can, however, be rotated more easily. The shaft of electromagnet of a generator is connected to a turbine. When the turbine is turned by fast flowing water (or pressure of steam), then the electromagnet turns inside the coils and generator produces electricity.
At Hydroelectric Power House, the generator is driven by the power of fast flowing water released from a dam across a river. In Thermal Power House, the generator is driven by the power of high pressure steam. The heat energy for making steam from water comes from burning coal, natural gas or oil.
At Nuclear Power House, the heat energy for making steam comes from nuclear reactions taking place inside the nuclear reactor. The high pressure steam turns a turbine. The turbine turns the generator. And the generator converts mechanical energy (or kinetic energy) into electrical energy (or electricity). This electricity is then supplied to our homes.
D.C. GENERATOR
We have just studied an A.C. generator which produces alternating current. In order to obtain direct current (which flows in one direction only), a D.C. generator is used. Actually, if we replace the slip rings of an A.C. generator by a commutator, then it will become a D.C. generator. Thus, in a D.C. generator, a split ring type commutator is used (like the one used in an electric motor).
When the two half rings of commutator are connected to the two ends of the generator coil, then one carbon brush is at all times in contact with the coil arm moving down in the magnetic field while the other carbon brush always remains in contact with the coil arm moving up in the magnetic field. Due to this, the current in outer circuit always flows in one direction. So, it is direct current. A diagram of D.C. generator is given in Figure.
We can see from Figure that the only difference between a D.C. generator and an A.C. generator is in the way the two ends of the generator coil are linked to the outer circuit. In a D.C. 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 A.C. generator, we connect the two ends of the coil to two full rings of copper called slip rings. There is no commutator in an A.C. generator.
Electromagnetic Induction Using Two Coils
So far we have learnt that electromagnetic induction can be brought about by moving a straight wire between the poles of a U-shaped magnet or by moving a bar magnet in a circular coil of wire. We will now study that electromagnetic induction can also be produced by using two coils. This is because if current is changed in one coil, then current is induced in the other coil kept near it. No relative motion of the coils is needed in this case. This will become more clear from the following discussion.
Two circular coils A and B are placed side by side, close to each other (see Figure). Coil A is connected to a battery and a switch S whereas coil B is connected to a galvanometer G.
(i) Let us pass current in coil A by pressing the switch. As soon as we pass current in coil A, the pointer of galvanometer attached to coil B shows a deflection, but quickly returns to zero position. This means that on switching on the current in coil A, an electric current is induced in coil B momentarily. If the current is now kept ‘on’ in coil A, nothing happens in the galvanometer of coil B.
(ii) Let us now switch off the current in coil A. As soon as we switch off the current in coil A, the pointer of galvanometer attached to coil B again shows a momentary deflection, but on the opposite side. This means that on switching off current in coil A, an electric current is induced in coil B but in a direction opposite to that when the current was switched on.
(iii) If we keep on switching the current ‘on and off’ in coil A rapidly, then the galvanometer pointer will keep moving on both the sides of zero mark continuously, showing that a continuous current is induced in coil B. Since the current induced in coil B changes direction continuously, so it is an alternating current (or a.c.).
From this discussion we conclude that whenever the current in coil A is changing (starting or stopping) then an electric current is induced in the nearby coil B. Coil A which causes induction is called primary coil whereas coil B in which current is induced is known as secondary coil. A current is induced here even though the coils are not moving relative to each other. We will now explain why a change in current in coil A induces current in coil B.
(i) When we switch on current in coil A, it becomes an electromagnet and produces a magnetic field around coil B. The effect is just the same as pushing a magnet into coil B. So, an induced current flows in coil B for a moment. When the current in coil A becomes steady, its magnetic field also becomes steady and the current in coil B stops.
(ii) When we switch off the current in coil A, its magnetic field in coil B stops quickly. This effect is just the same as pulling a magnet quickly out of coil B. So, an induced current flows in coil B in the opposite direction.
Thus, the current is induced in coil B by the changing magnetic field in it when the current in coil A is ‘switched on’ or ‘switched off’.
If the coil A is connected to alternating current (which keeps on changing), then a constant current will be induced in coil B whose magnitude will depend on the relative number of turns of wire in coil A and coil B. This fact is used in making transformers for stepping up (increasing) or stepping down (decreasing) the voltage of alternating current. These transformers are used at power stations and in a variety of electronic appliances such as radio sets and T.V. sets, etc. Let us solve some problems now.
Example Problem 1.
A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is :
- pushed into the coil ?
- held stationary inside the coil ?
- withdrawn from inside the coil ?
Solution:
- As a bar magnet is pushed into the coil, a momentary deflection is observed in the galvanometer indicating the production of a momentary current in the coil.
- When the bar magnet is held stationary inside the coil, there is no deflection in galvanometer indicating that no current is produced in the coil.
- When the bar magnet is withdrawn (or pulled out) from the coil, the deflection of galvanometer is in opposite direction showing the production of an opposite current.
Example Problem 2.
Explain why, the direction of induced current in the coil of an A.C. generator changes after every half revolution of the coil.
Solution:
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 downwards in the magnetic field, after half revolution, it starts moving in opposite direction – upwards.
Similarly, the side of 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 sides of the coil in the magnetic field after every half revolution, the direction of current produced in them also changes after every half revolution.