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8.4 Transformer

Function of Transformer

A transformer is an electrical device which increases or decreases an alternating voltage.

A transformer works based on the principle of electromagnetic induction.

The structure and the symbol of a Transformer

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Types of Transformer

Type

Number of turns

Voltage

Current

Example

Step-up

Ns > Np

Vs > Vp

Ip > Is

EHT power supply, transformer

for power transmission

Step-down

Np > Ns

Vp > Vs

Is > Ip

Normally used in lab rotary, use in radio

Operating Principle of a Transformer

When an a.c. voltage is supplied to the primary coil, an alternating current flows through coil.

The soft -iron core is magnetized.

The magnet produced varies in magnitude and direction.

This causes a changing magnetic flux to pass through the secondary coil.

As a result ,an induced e.m.f across the secondary coil is produced.

What happens if a d.c. power voltage is applied to the primary coil?

If a d.c. voltage is applied to the primary coil of the transformer, the output voltage at the secondary coil will be zero. This is because the magnet produced in the soft-iron core is constant magnitude and direction. Hence the magnet does not create a changing magnetic flux in the secondary coil.

When the switch is opened an closed frequently in a high speed, the d.c. voltage is applied to the primary coil of the transformer can produces the induced e.m.f across the secondary coil .

Relationship between the number of coils , N and the voltage ,V in a Transformer.

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Efficiency of a Transformer

image

Relationship between Output Power and Input Power of an Ideal Transformer

For an ideal transformer, the output power is equal to the input power since there is no energy loss during the process of transforming voltage , or the efficiency becomes 100%.

Therefore,

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Example 1

The number of turns in the primary coil and the secondary coil of a transformer are 50 and 250 respectively.

What is the output voltage when the transformer used the electricity supply voltage 12V?

Solution

Example 2

The figure shows a 12V, 36W bulb lights up with normal brightness when it is connected to a 240V mains supply through a transformer. The number of turns of the primary coil is 500.

clip_image008

What is the number of turns of the secondary coil?

Solution

Example 3

The figure shows a 12V, 48W bulb lights up with normal brightness when it is connected to a 240V mains supply through a transformer.

clip_image010

Calculate

(a) The number of turns of the primary coil.

(b) The current in the secondary coil.

(c) The efficiency of the transformer.

Solution

Example 4

A step-down transformer connected to 240 V mains power supply delivers 90 W of power at 30 V at a notebook computer . The number of turns of the secondary coil is 800.

[ Assume the transformer is ideal]

Calculate

(a) The number of turns of the secondary coil?

(b) The current in the secondary circuit?

(c) The current in the primary coil?

Solution

Factors that affect the efficiency((energy losses) of a transformer and ways to improve the efficiency ( minimize energy losses of a transformer.


Cause energy losses

Way to minimize energy losses

1. Resistance of coil.

Energy is lost as heat in the coil

because E= I2R

Thick copper wire is used to reduce the resistance of the coil

2. Eddy current in iron core.

The changing magnetic field

causes the electrons in the

soft iron core to circulate

around as eddy current.

The eddy current produce

heating in core and causes

loss of power .

Using a laminated iron core to make the iron core as insulation.

Eddy current are not able to flow through the layers of insulation.

3. The hysteresis loss.

The loss of energy to

magnetized and

demagnetized the core

by the alternating current in

the primary coil.

Using soft iron for the core because soft iron core can be magnetized and demagnetized easily . So the core requires little energy to be magnetized and demagnetized .

4. Leakage of Magnetic Flux..

Electrical energy is lost when

a fraction of the magnetic flux

produced by the primary

coil does not link with the

secondary coil.

Winding the secondary and primary coils on top each other.

The experiment to investigate the relationship between the number of turns of the secondary coil and output voltage of a transformer.

Inference:

The output voltage of the transformer depends on the number of turns of the secondary coil.

Hypothesis:

The output voltage of the transformer increases as the number of turns of the secondary coil increases.

Hypothesis:

The output voltage of the transformer depends on the number of turns of the secondary coil.

Aim of the experiment :

To investigate the relationship between the number of turns of the secondary coil and output voltage of a transformer.

Variables in the experiment:

Manipulated variable: the number of turns of the secondary coil

Responding variable: The output voltage

Fixed variable: number of turns of the primary coil and the input voltage.

List of apparatus and materials:

Low power voltage, primary and secondary coils, iron-core, voltmeter and connection wire.

Arrangement of the apparatus:

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The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The number of turns of the secondary coil is recorded = N

The low voltage power supply is switched on.

The reading of the voltmeter is recorded = V

The experiment is repeated 5 times with different number of turns of the secondary coil.

Tabulate the data:

   N

           

   V

           

Analysis the data:

Plot the graph V against N

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