Transformers - Practice Questions & MCQ

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Quick Facts

Transformers is considered one the most difficult concept.
21 Questions around this concept.

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The core of any transformer is laminated so as to

reduce the energy loss due to eddy currents

make it light weight

make it robust & strong

increase the secondary voltage.

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A coil is suspended in a uniform magnetic field, with the plane of the coil parallel to the magnetic lines of force. When a current is passed through the coil it starts oscillating; it is very difficult to stop. But if an aluminium plate is placed near to the coil, it stops. This is due to :

development of air current when the plate is placed.

induction of electrical charge on the plate

shielding of magnetic lines of force as aluminium is a paramagnetic material.

electromagnetic induction in the aluminium plate giving rise to electromagnetic damping.

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Concepts Covered - 1

Transformers

Transformers

It is a device that raises or lowers the voltage in ac circuits through mutual induction. It consists of two coils wound on the same core. The alternating current passing through the primary creates a continuously changing flux through the core. This changing flux induces an alternating emf in the secondary.

Step-up Transformer: A transformer in which the output (secondary) voltage is greater than its input (primary) voltage is called a step-up transformer.

Step-up Transformer: A transformer in which the output (secondary) voltage is less than its input (primary) voltage is called a step-down transformer

The transformer works on ac only and never on dc.
It can increase or decrease either voltage or current but not both simultaneously.
Transformer does not change the frequency of input ac.
There is no electrical connection between the winding but they are linked magnetically.
Effective resistance between the primary and secondary winding is infinite.
The flux per turn of each coil must be same i.e.
$\phi _S=\phi_P \\$ so, $-\frac{d\phi _P}{dt}=-\frac{d\phi _S}{dt}$

If $N_P$ =number of turns in primary, $N_S$ = number of turns in secondary,

$V_P$ = applied (input) voltage to primary,

$V_S$ = Voltage across secondary (load voltage or output),

$e_P$ = induced emf in primary ;

$e_S$ = induced emf in secondary,

$\phi$ = flux linked with primary as well as secondary, current in primary;

$i_S$ = current in secondary (or load current)

As in an ideal transformer, there is no loss of power i.e. $P_{out}=P_{in}$ so, $V_Si_S=V_Pi_P$ and $V_{P} \approx e_{P}, V_{S} \approx e_{S}$ .

Hence,

$\frac{e_{S}}{e_{P}}=\frac{N_{S}}{N_{P}}=\frac{V_{S}}{V_{P}}=\frac{i_{P}}{i_{S}}=k$ , k = Transformation ratio.

Efficiency of transformer (η): Efficiency is defined as the ratio of output power and input power i.e. η.

For an ideal transformer, $P_{out}=P_{in}$ so η=100.

For practical transformer, $P_{in}=P_{out}+P_{losses}$ . Efficiency of a practical transformer lies between 70-90 %.

So $\eta = \frac{P_{\text {out}}}{\left(P_{\text {out}}+P_{L}\right)} \times 100=\frac{\left(P_{\text {in}}-P_{L}\right)}{P_{\text {in}}} \times 100$ .