Transformer
It is a device used to change the voltage of alternating current. A transformer, which increase the voltage is called Step Up transformer, will other transformer which decrease the voltage is called step down Transformer.Principle
It is based upon is mutual induction i.e when current is one coil change, an EMF induced in neighbouring coil.
construction
It consists of a laminates soft iron core C. Two points P1 P2 and S1 S2 of insulated copper wire are wound on it but they are well insulated from each. the coil P1 P2 is connected to the source of alternating current. It is called primary coil other coil S1 S2 is called secondary coil. current is drawn from it. If Np and Ns be the number of turns in primary coil and secondary coil, then for Step Up transformer Ns>Np and for step down Transformer. Ns<Np
Theory and working
when the primary coil is connected to the source of alternating current, magnetic flux linked with a primary coil Core and secondary coil change so EMF induced in both coil.
If is the rate of change of magnetic flux linked with each turn of the primary coil, then
Induced EMF is primary coil Ep= -Np dø/dt
If the resistance of a primary coil is small, then this induced EMF must be equal to the applied EMF.
If there is no lenkage of magnetic flux, then the rate of change of magnetic flux through each turn of secondary coil =dø/dt
Induced EMF in secondary coil Es=-Nsdø/dt
For open circuit, this induced EMF is equal to the output EMF
Divide equation 2 and 1
Es/Ep=Ns/Np=K (Transformer ratio)
For step Up Transformer K>1 and 4 step down Transformer K <1. if Ip and Is are the current in primary and secondary coil and there is no loss of energy then
output Power is equal to input power.
Or Es/Ep=Ip/Is___3
Thus when voltage is in secondary coil increase, the strength of a current decrease in the same ratio and vice versa.
Loss of energy
In practice, the output power is less than input power due to a flowing loss of energy.
copper loss:-The copper wire of primary coil and secondary coil has same resistance so some electric energy change into heat energ [P=I²R]. which is a loss of energy.
it can be reduced by using a thick copper wire.
2:- Eddy current:- As the magnetic flux linked with and iron core changes the eddy current are set up in it, which changed into heat. Which is a loss of energy.
It can be reduced by using a laminated iron core.
3:- when alternating current flow in the coil and the iron core is magnetized in one direction and demagnetize. it again magnetise in opposite direction and demagnetises. In this process of magnetization and demagnetization, some energy is wasted. it can be reduced by using a soft iron core.
4. leakage of magnetic flux:- The whole magnetic flux starting from primary coil does not pass through secondary coil. but some magnetic flux leak out into air. it can be reduced by winding both coil on same core on our other.
5. humming loss:- Due to alternating current the core vibrates and produce humming noise. thus some electrical energy is wasted in the form of a humming noise.
1. A step down Transformer is used for obtaining large current electric welding.
2. A step down Transformer is used to induction furnace for melting the metal.
3. A step up Transformer is used for the production of X rays.
4. Transformers are used in voltage regulator and stabilize power supply.
5. Small Transformer are used in radio sets, telephones, loudspeakers, sodium vapour lamps etc.
6. They are used in transmission of electric energy from generating station to consumers.
Use of Transformer for long distance transmission of electric power
The electric power is transmitted at high voltage from generating station to consumers. In doing so, the loss of energy is small.
let we want to transmit 22,000 watt.
(i) If it is transmitted at 220 volt.
I=P/V
22000/220=100A
if R is the resistance of cable then heat produce.
I²RT
Fall of potential V=IR =100R
Thus at low voltage transmission
1:- A large amount of heat is produced, which loss of energy.
2:- large fall of potential takes place. so voltage at receiving station is much smaller than generating station.
3:- The cables must be thick to carry strong current. so the cables will be e expensive and strong poles are required to support thick cable.
4:- if the electric power is transmitted at 11000 volt, then I=22000/11000=2A, which is very smaller.
The above all problem are solved because heat produced and fall of potential are very small. and thin cables are can be used. Hence the electric power at transmitting station is stepped up to High Voltage. it is done by step up Transformer. now it is the transmitted. At receiving station it is stepped down by using step down Transformer. now it is distributed to consumer.
Advantage of AC over DC
1:- The generation of AC is more economical than DC.
2:- The voltage of alternating current can be easily change by using Transformer. while other voltage of DC cannot be changed by Transformer.
3:- Alternating current can be controlled by Choke coil with a very small loss of energy. while DC cannot be controlled by choke coil. it can be controlled only by resistance with a high loss of energy.
4:- AC can be transmitted at high voltage from generation station to any other place.In this process cost of transmission is low and loss of energy is also reduce. As the voltage of DC cannot be changed by Transformer.So it transmission is costly and loss of energy is heavy.
5:- AC can be converted into DC easily by using rectifier.
6:- AC equipment such as electric more are more durable and convenient as compared to DC equipments.
Disadvantage of AC or DC
1:- The peak value of AC is high. so it is more dangerous than DC of same voltage.
2:-The shock of AC is attractive, while that DC is repulsive.
3:- In phenomena like electrolysis, electroplating etc. AC cannot be used.
4:- AC is transmitted more from the surface of conductor than inside it is called Skin effect. therefore the serverl fine wires are used to transmit high current instead of a single thick wire.
