# Transformer

VP  :  V= NP :  NS

• VP   voltage across the primary coil in V
• VS    voltage induced in the secondary coil in V
• NP    amount of turns in the primary coil
• NS    amount of turns in the secondary coil

PP = PS      (Assuming 100 % energy transfer, no energy dissipated in heating)

VP IP = VS IS

• PP    power of the primary coil W (of VA)
• IP     current in the primary coil in A
• PS      power of the secondary coil  in W (of VA)
•  IS       current  in the secondary coil  in A

Examples

1.   The primary coil of a transformer consists of 400 turns. The secondary coil

80 turns.

The voltage across the primary coil is 230 V

Find the voltage across the secondary coil

VP  :  VS  = NP :  NS

230 : VS = 400 : 80

230 : VS = 5 : 1

5 Vs = (230)(1)

VS = 230/5 = 46 V

2.    The primary coil is connected with an AC source

A bulb is connected with the secondary coil.

The voltage across the bulb is 6.0 V and

the current through the bulb is  0.4 A

The turns ratio is 5:1

(NP : NS  = 5: 1)

Calculate the current in the primary coil

PS = VS IS = (6.0)( 0.4) = 2.4 W

VP = 5 VS = (5 )(6.0) = 30 V

PP = PS = 2.4 W

PP = VP IP

2.4 = 30 IP

IP = 2.4/30 = 0.8 A