Question

In a transformer, the number of turns of primary coil and secondary coil are 5 and 4 respectively. If 220 V is applied on the primary coil, then the ratio of primary current to the secondary current is :

• A. 4 : 5
• B. 5 : 4
• C. 5 : 9
• D. 9 : 5

Answer 1

Answer: (A)

Answer is D.

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows us to change (transform) the voltage from the primary to the secondary coil.
The Turns Rule is:
Ns/Np=Vs/Vp
Where,
Ns = number of turns on the secondary coil
Np = number of turns on the primary coil
Vs = voltage across the secondary coil
Vp = voltage across the primary coil
So if number of turns on the primary coil is more than on the secondary coil, the output voltage will be lesser than the input voltage. This is called a step down transformer.
In this case, the turns ratio is 5:4.
Np/Ns=Vp/Vs;
That is, 5/4 = 220/x.
$x=\frac{220\times 4}{5}=176V$. The ratio of the voltages is 5:4.
It is a known fact that voltage and current are inversely proportional and so the ratio of the primary current to the secondary current is 4:5.
Hence, the the ratio of the primary current to the secondary current is 4:5.