Transformer

• • Transformer is used

to increase or decrease AC Voltage

(potential difference).

• Transformer is consisted of a

soft iron core

, a

primary coil

, and a

secondary coil

.

Primary coil is connected to AC supply

, while the

secondary coil is connected to the electrical

component (lamp, computer, TV, etc) Soft iron Primary coil secondary coil Symbol of transformer

Principle of Transformers

• A transformer works only with alternating current supply (AC supply).

• Mutual induction: When the primary coil is connected to a source of AC voltage, the changing current creates a changing magnetic field, which is carried through the core to the secondary coil. In the secondary coil the changing field induces a varying voltage.

• The voltage induced in the secondary coil is called the secondary voltage and voltage induced in the primary coil is called the primary voltage.

• In a transformer,

the ratio of input voltage to output voltage

depends on the

ratio

of the number of

turn

on the

primary coil and secondary coil

, as shown by formula: • V S = secondary voltage or output voltage (Volt) • V P = primary voltage or input voltage (Volt) • N S = number of turns on secondary coil • N P = number of turns on primary coil

• In the above photograph the school’s transformer was used. 10.05V (rounded by the meter to 10.1V) across the primary coil of 2,000 turns resulted in an output of 100.5V across the secondary coil of 20,000 turns.

Step-up and step-down transformer

• Step-up transformer used

to increase

the AC voltage (V P < V S ) -

secondary voltage is larger

than the primary voltage (V P < V S ) -

number of turn

on secondary coil is

larger

than on primary coil (N P < N S ) -

Output current is smaller

than

input current

(I P > I S ) Step-down transformer used

to decrease

the AC voltage (V P > V S ) -

secondary voltage is smaller

than the primary voltage (V P > V S ) -

number of turn

on secondary coil is

smaller

than on primary coil (N P > N S ) -

Output current is the larger

than

input current

(I P < I S )

• In an

ideal transformer (100% efficient),

the electric power delivered to the secondary coil equals the power supplied to the primary coil. It can be represented by the following equations: P P = P S V P .I

P = V S .I

S P P = Power in primary coil or power input (Watt) P S = Power in secondary coil or power output (Watt) I P = Current in primary coil or current input (Ampere) I S = Current in secondary coil or current output (Ampere)

Energy Losses in a Transformer are caused by:

1) Resistance of winding The windings of copper wire – have some

resistance

-

heat is produced

2) Eddy Currents.

Iron core – changing

magnetic field of the primary

– induced –

current

(eddy currents)- caused

heating

.

Reduced by laminating core made of sheets 3) Leakage of field lines

Field lines

produced –

primary

may not cut the

secondary

– has

air gap

or

badly designed

Application of Eddy Currents: Speedometer

Application of Eddy Currents: Metal Detector

Diagram of Power Transmission

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