TRANSFORMERS

Transformer is a device that uses mutual induction between two coils to convert alternating voltage to larger or smaller alternating voltage across the other coil.

Symbols of Transformers

TYPES OF TRANSFORMERS

There are two types of transformers

1. Step-up transformers
2. Step-down transformers

STEP-UP TRANSFORMERS

Step-up transformers are transformers for increasing alternating voltage.

The number of turns in primary winding is smaller than in the secondary winding

The output voltage (in secondary) is higher than input voltage (in primary)

STEP-DOWN TRANSFORMERS

Step-down transformers are transformers for decreasing alternating voltage.

The number of turns in primary winding is larger than in the secondary winding

The output voltage (in secondary) is less than input voltage (in primary)

  

TRANSFORMER EQUATION

The transformer equation describes the relationship between the number of turns in the coil, voltage and current in primary and secondary winding

For an ideal transformer,

Also the e.m.f induced is proportional to the number of turns in the coils 

Hence the general transformer equation is given by;

NOTE:

Ideal transformer is the transformer that has the efficiency of 100%. The ideal transformer has negligible power loss

TRANSFORMER EFFICIENCY

The efficiency of a transformer is the ratio of power in secondary coils to the power in primary coils expressed as a percentage.

OR

The efficiency of a transformer is the ratio of power output to the power input expressed as a percentage.

Example 1

A transformer is used to step-down 240  V main supply to 12 V for laboratory use. If the primary coil has 600 turns, determine the number of turns in the secondary coil.

Solution

Example 2

A current of 0.6 A is passed through a step-up transformer with a primary coil of 200 turns. A current of 0.1 A is obtained in the secondary coil. Determine

a)      The number of turns in the secondary coil

b)      The voltage across the secondary coil, if the primary coil is connected to a 240 V mains.

Solution

Example 3

A step-up transformer ha 10000 turns in the secondary coil and 100 turns through the primary coil. An alternating current of 5.0 A flows in the primary circuit when connected to a 12.0 V a.c supply.

a)      Calculate the voltage across the secondary coil.

b)      If the transformer has an efficiency of 90%, what is the current in the secondary coil?

Solution