Transformer does not work with DC, why?

 What is transformer?


The transformer is a device which changes the electromotive force of (EMF) of an alternating current.

Transformer has different uses:

1) To increase the EMF in the power station before transporting the current.

2) To decrease the EMF at the sub-power station before supplying current for domestic use.

Transformer does not work with DC, why?

Instead of AC, if DC is supplied in the primary coil, then the magnetic field produced is of constant strength. Due to this the number of magnetic lines of force which are intersected by the secondary coil does not change. In such condition, current does not get induced from the two terminals of the secondary coil. So, transformer does not work with D.C. Transformer are used to change the EMF of Alternating current. Whereas in D.C current, it's polarity doesn't change. So transformers cannot change the EMF of D.C current.

Transformer is of two types: I) Step-up transformer

                                                       ii) Step-down transformer

Step-up transformer

The transformer which converts low voltage AC current to high voltage AC current is called Step-up transformer. It has more turns in secondary coil than in primary coil. It is used to get high voltage to accelerate the electron beam in a cathode ray tube (CRT).In microwave oven step-up transformer changes input voltage. It is also used in television sets, refrigerator etc.

Step-down transformer

The transformer that is used to converts high voltage AC to low voltage AC in several devices and purposes is called step-down transformer. It has more number turns in primary coil than number of turns in secondary coil. In distribution sub-station, a step down transformer is used to lower the high voltage AC to low voltage AC for the domestic use. The battery charger of cell phone also contain step-down transformer. It reduces high voltage current (from 220 V) to low voltage (between 3 V - 9 V). It is also used in radios, cassette player etc

The transformer has two coils fixed in a soft-iron core. One coil is called primary coil. It is connected to the source of alternating current. The number of windings in the primary coil is called primary turns and the AC voltage supplied is called primary voltage. The secondary coil is the coil through stepped-up or stepped-down AC voltage is obtained. The number of turns in secondary coil is called secondary turns and voltage is called secondary voltage. Soft iron core is a block made from the U-shaped iron strips or E and I shaped iron strips to fix the primary coil and secondary coil of the transformer. The polished U-shaped iron stripes are join together, it is called lamination of the core.

How transformers work?

Alternating current passes through the primary coil when it is connected to an AC source. The magnetic field due to AC in primary coil changes in both magnitude and direction. The changing magnetic field changes the magnetic lines of force linked with the secondary coil. According to the Faradays laws of electromagnetic induction, whenever the magnetic lines of force linked with a closed circuit changes, an EMF gets induced from two terminals of the secondary coil. Thus, a transformer is based on the principle of Mutual Induction. As per the principle of mutual induction, it states that "an EMF gets induced in a coil by passing an alternating current in the near one coil or neighboring coil."

The relation among primary voltage (V) and secondary voltage (v), and number turns in primary coil (N) and secondary coil (n) is given by the formula:

v/V = n/N

 

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