Question

A step up transformer connected to a 220 V AC line is to supply 22kV for a neon sign in the secondary circuit. In the primary circuit a fuse wire is connected which is to blow when the current in the secondary circuit exceeds 10 mA. What is the turn ratio of the transformer?
A. 50
B. 100
C. 150
D. 200

Answer 1

Hint: Step up transformer is that transformer which increases the input voltage from primary windings to output voltage in secondary windings. It is based on the principle of induction.
Voltage transformation ratio i.e. ratio of induced emf, to the number of turns is constant.

Formula used:
Voltage transformation ratio, $k=\dfrac{{{E}_{s}}}{{{N}_{s}}}=\dfrac{{{E}_{p}}}{{{N}_{p}}}$

Complete step-by-step answer:
A transformer is an electrical device which is used in electricity transmissions. It is used to increase or decrease the supply of voltage without any change in the frequency of AC between circuits. Transformer is based on the principle of electromagnetic induction.
A transformer consists of two coils: primary and secondary having high mutual inductance. When an AC current is passed through the primary coil, due to mutual induction, current is induced in the secondary coil.
The transformer in which secondary output voltage is greater than primary input voltage is known as step up transformer. In this type of transformer, the number of windings in secondary coil is more as compared to number of windings in primary coil.
In a transformer, the ratio of induced emf and number of windings in a coil is constant and is known as voltage transformation ratio.
$k=\dfrac{{{E}_{s}}}{{{N}_{s}}}=\dfrac{{{E}_{p}}}{{{N}_{p}}}$
Where $E$ denotes induced emf $N$denotes number of windings and the letter in subscript denotes the coil.
In this question, 220 Voltage is primary coil input voltage and 22 kV is output secondary coil voltage. We substitute these values in above expression and get
\[\dfrac{220\,V}{{{N}_{p}}}=\dfrac{22\,kV}{{{N}_{s}}}\]
$\Rightarrow \dfrac{{{N}_{s}}}{{{N}_{p}}}=\dfrac{22000}{220}=100$
Turn ratio of the transformer is 100.

So, the correct answer is “Option B”.

Note: A transformer is used to change to increase or decrease the supply of voltage without any change in the frequency of AC between circuits and is based on the principle of electromagnetic induction.
Transformers cannot produce power. The current and voltage are stepped in opposite directions i.e. one increase than other decrease.