TRANSFORMER

                   Transformer

It is a device which is used to convert AC of high voltage into AC of low voltage or vice versa. 

       Types of tranformer

There are two types of transformers

(1) step up transformer

No. Of turns on primary coil is less than no. Of turns in secondary coil. 

It is used to convert low voltage AC to high voltage AC

(2). Step down transformer

It is used to convert high voltage AC to low voltage AC. 

No. Of turns in primary coil is greater than no. Of turns in secondary coil. 

Principle - it works on the principle of mutual induction, I. E., when a changing current is passed through one of the two inductively coupled coils, an induced emf is set up in the other coil. 

Theory and working 

As wkt when a time varying current flows through primary coil a magnetic field sets up inside it which is also flows through secondary coil. Therefore the flux linked with primary coil is equal to the flux linked with secondary coil. 

If N1 and N2 be the no. Of turns in primary and secondary coil and E1 and E2 be the induced emf produced in primary and secondary coil. 

Then, acc. to faraday's law

E1 = - N1[d(phie) /dt]    .... (1) 

E2 = - N2[d(phie) /dt]    .... (2) 

From dividing eq. (2) by (1) 

We get, 

E2/E1 = N2/N1

                     Note

(1). E1 taken as input emf and E2 taken as output emf

E2/E1 = output emf/input emf = N2/N1

(2). The ratio (transformation ratio) N2/N1 is turn ratio or transformation ratio, denoted by k. 

If N1 is greater then N2

N2/N1 is smaller than 1

It means, 

E1 is greater than E2

Or

Input emf is greater than output emf

It is a step down transformer. 

(3). If N2 is greater than N1

N2/N1 is greater than 1

It means, 

E2 is greater than E1

Or

Output emf  is greater than input emf

It is a step up transformer. 

(4). For an ideal transformer input power  = output power. 

Current in primary and secondary coil. 

If I1 and I2 be the current in primary and secondary coil

E1I1 = E2I2

E1/E2 = I2/I1   or E2/E1 = I1/I2

Efficiency of transformer

It is defined by percentage ratio of output power to the input power. It is denoted by n and given as

n = (output power /input power) × 100℅ 

n = (E2I2/E1I1)  ×  100℅

The efficiency of real transformers is fairly high(90 - 99℅) though not 100℅

Energy losses in transformers

The  main causes for energy loss in transformers are as follows:

(1). Copper loss 

        Some energy is lost due to heating of copper wires used in the primary and secondary windings. This power loss ( = I × I × R) can be minimised by using thick copper wires of low resistance. 

(2). Eddy current loss

        The alternating magnetic flux induces eddy currents in the iron core which leads to some energy loss in the form of heat. This loss can be reduced by using laminated iron core.

(3). Hysteresis loss

        The alternating current carries the iron core through cycles of magnetisation and demagnetisation. Work is done in each of these cycles and is lost as heat. This is called hysteresis loss and can be minimised by using core material having narrow hysteresis loop. 

(4). Flux leakage

        The magnetic flux produced by the primary may not fully pass through the secondary. Some of the flux may leak into air. This loss can be minimised by winding the primary and secondary coils over one another. 

(5). Humming loss

        As the transformer works, it's core lengthens and shortens during each cycle of the alternating voltage due to a phenomenon called magnetostruction. This gives rise to a humming sound. So some of the electrical energy is lost in the form of humming sound. 

Some uses of transformer

1. Small transformers are used in radio recievers, telephones, loud speakers, etc.
2. In voltage regulators for TV, refrigerators, air conditioners, computer, etc.
3. In stablised power supplies.
4. A step down transformer is used for obtaining large current for electric welding.
5. A step down transformer is used in induction furnace for melting metals.
6. A step up transformer is used for the production of X - rays.
7. In the transmission of electric energy from the generating stations to the consumers.

Disadvantages of transmitting the electrical power at low voltage

1. Large lengths of transmission cables have appreciable resistance. Hence a large amount of energy(IRTI) will be lost as heat during transmission. 

2. Large voltage drop (IR) occurs along the line wire. Hence the voltage at the receiving station will be much smaller than that at the generating Station. 

3. To carry large currents and to keep the resistance of transmission wires low, thick wires have to be used. The cost of installing thick wires will be extremely high. 

Use of transformers is long distance power 

The long distance power transmission at low voltage and high current is neither efficient nor economical. If I is the current in the cable, and R it's resistance, the power wasted in the cable is IRI. The power loss can be reduced by reducing I or R. The power supplied by the generator is given by P = VI, where V is the voltage across it's terminals. Since I = PV, for a given amount of power P, the power loss is less if I is less or  V is high. 

In actual practice, a typical power station generates 1000 kw at 6600 volts. This voltage is first stepped up to 132000 volts before transmission. Transmission lines from different power stations in a region deliver power to a common regional pool, called the grid. From the grid, the power is fed to the cities at 33000 V, the stepping down is done outside the city. Then again at a substation, the supply is stepped down to 6600V. For domestic purposes, the voltage is again stepped down to 220 V. 

Advantages of AC over DC

1. The generation of AC is more economical than DC. 

2. The alternating voltage can be easily stepped up or stepped down by using a transformer. 

3. The alternating currents can be reduced by using a choke cool without any significant wastage of energy. 

4. The alternating currents can be transmitted to distant places without any significant line loss. 

5. Also AC van be easily converted into DC by using rectifiers. 

6. AC machines are simple and robust and do not require much attention during their use. 

Disadvantages of AC over DC

1. Peak value of AC is high (Io = 2 of power of 1/2 × Irms). It is dangerous to work with AC. 

2. In phenomena like electroplating, electro refining, electrotyping, etc AC cannot be used. 

3. AC is transmitted more from the surface of conductor than from inside. This is called skin effect. Therefore, several fine insulated wires ( and not a single thick wire) are required for the transmission of AC.