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Physics 3 Marks Question

Alternating Current · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 2 questions.

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Question 13 Marks
Explain the use of motor starter.
Answer
Motor Starter : In a motor, the resistance of the armature is kept very small to reduce copper losses. Therefore, when the full supply emf is applied to the armature in the beginning (when the back emf is zero), a very large current will flow through the armature causing damage to the insulation of the windings. Further, if electric supply fails by chance, while the motor is running, the large induced emf produced at break can damage the armature.
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These possible damages to the armature are prevented by introducing a large variable resistance R, called 'starter resistance' in series with the armature and the mains.
The motor is started with the handle H on the first stud S so that whole of the resistance R is in the circuit. The current through the armature is thus small. As the armature gains speed, the back emf is progressively developed. The resistance R is now gradually withdrawn by moving the handle to the stud T. The handle is held in this position by the electromagnet E, which is in series with the field coil. If the main electric supply fails, the spring S' brings the handle back to the off position.
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Question 23 Marks
What is transformer? What is its transformation ratio ?
Answer
Transformer : It is the most important electrical appliance used to convert electric power at low voltage and high current to electric power at high voltage and low current and vice versa. It works on the principle of mutual induction, hence it works for alternating current only and not for direct current. It was first devised by Josheph Henry.
Transformers are of two types: (i) Step-up transformer. (ii) Step-down transformer.
Step-up transformer convert power at low voltage and high current to power at high voltage and low current.
Step-down transformer convert power a high voltage and low current to power at low voltage and high current.
Transformation Ratio : In a transformer the ratio of number of turns in its secondary coil to the number of turns in its primary coil is called its transformation ratio and is denoted by r.
Hence.
$r = \frac{\text{Number of turns in secondary coil}}{\text{Number of turns in primary coil}}$
$r=\frac{N_{s}}{N_{p}}$
Since step-up transformer Ns > Np and in step-down transformer NS < NP hence the value of r is greater than 1 for step-up transformer and is less than 1 for step-down transformer.
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