Explain with the help of a labelled diagram the underlying principle and working of a step- up transformer.Why cannot such a device be used to step - up d.c voltage?
Draw a labelled diagram of an a.c generator. Explain briefly its principle and working.
Principle: It is a device which converts high voltage AC into low voltage AC and vice-versa. It is based upon the principle of mutual induction. When alternating current is passed through a coil, an induced emf is set up in the neighbouring coil.
Working: When an alternating current is passed through the primary, the magnetic flux through the iron core changes, which does two things. It produces emf in the primary and an induced emf is also set up in the secondary. If we assume that the resistance of primary is negligible, then the back emf will be equal to the voltage applied to the primary.
Where, N1 and N2 are number of turns in the primary and the secondary respectively and V1 and V2 are their voltages respectively.
In a step-up transformer: N2 > N1, therefore, V2 > V1
Such a device cannot be used to step up a dc because a dc voltage cannot produce a changing flux, which is very essential to the production of induced emf in the secondary.
Principle: It works on the principle of Faraday's law of electromagnetic induction. Whenever a coil is rotated in a uniform magnetic field about an axis perpendicular to the field, the magnetic flux linked with coil changes and an induced emf is set up across its ends.
The essential parts of an ac generator are shown in the figure. Initially, the armature coil ABCD is horizontal. As the coil is rotated clockwise, the arm AB moves up and CD moves down.
The magnetic flux linked with the coil at any instant is:
Induced emf will be
Let E0 be the peak value of induced emf.
E = E0 sin t
where, E0 = NBA
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