Notes On Transformers - ICSE Class 10 Physics
A transformer is a device used to increase or decrease the voltage of an alternating current. It is used in power transmission to reduce power losses and to increase the efficiency of power supply. A transformer changes voltage, but not the frequency of the alternating current.  There are two types of transformers. They are step-up transformer and step-down transformer. A step-up transformer is used to increase a low voltage alternating EMF to a high voltage alternating EMF. A step-down transformer is used to decrease a high voltage alternating EMF to a low voltage alternating EMF.

Principle

When magnetic flux changes in a coil, an alternating induced current is generated in it and the magnitude of e.m.f. of current induced depends upon the number of turns in the coil. This is called electromagnetic induction. Transformers work on the principle of mutual induction. According to the principle of mutual induction, when two coils are placed close to each other and if one of them carries a time varying current, then a current is induced in the other coil. 


Construction

A transformer consists of a laminated core made from thin rectangular frames of soft iron. The frames are placed one over another and insulated from each other by lacquer, a kind of varnish. On one arm of the core, a coil, ‘P,’ of insulated copper wire is wound. This coil is called the primary coil and is connected to an alternating voltage source, whose voltage is to be stepped up or stepped down. On the other arm, another coil, S, of insulated copper wire is wound. This is the secondary coil. The induced alternating EMF is obtained as an output from the terminals of this coil.

If the secondary coil has a greater number of turns than the primary, then the voltage gets stepped up, that is, the output voltage is higher than the input voltage. This type of a transformer is called a ‘step-up’ transformer. In this type of a transformer, there is less current in the secondary coil than in the primary coil. Conversely, if the secondary coil has less turns than in the primary coil, the transformer is called a ‘step-down’ transformer. In such a case, the output voltage is less than the input voltage. That is, the voltage is stepped down, or reduced, and the current increases.


Factors on Which Energy Loss in a Transformer Depends

In an ideal transformer there should be no loss of energy. However, in reality, all transformers incur small energy losses due to various factors, such as:


• Heat in the coils
• Eddy currents in the core
• Hysteresis loss in the core
• Magnetic field link loss


Uses

A major use of transformers is in power transmission lines. Transformers are used in networks that transmit electricity over long distances. we use a step-up transformer to increase the voltage and consequently decrease the current in the transmission line. This reduces the loss of energy as heat. The voltage is then stepped down at various substations and utility poles, before it reaches your home in the range of 220 to 240 volt.


Typically, when electricity is transmitted at high voltages, the voltage is in a multiple of 11. Thus, we have electricity being transmitted at 66, 33, 22 and 11 kilovolt. When electricity reaches city from the generating stations, it is typically stepped down to 6.6, 3.3 or 1.1 kilovolt. At the sub-station in your area, it is stepped down to 440 volt for three-phase circuits, and 220 to 240 volt for single-phase circuits. In India, domestic electricity supply is at 220 to 240 volt, but in the US, it is at 110 volt.

A transformer cannot be used with direct current since its working is based on changing magnetic flux produced by a time-varying current of a given frequency in one coil inducing a varying current of the same frequency in the other coil. Since the magnitude of direct current does not vary with time, a transformer cannot be used with direct current.

Summary

A transformer is a device used to increase or decrease the voltage of an alternating current. It is used in power transmission to reduce power losses and to increase the efficiency of power supply. A transformer changes voltage, but not the frequency of the alternating current.  There are two types of transformers. They are step-up transformer and step-down transformer. A step-up transformer is used to increase a low voltage alternating EMF to a high voltage alternating EMF. A step-down transformer is used to decrease a high voltage alternating EMF to a low voltage alternating EMF.

Principle

When magnetic flux changes in a coil, an alternating induced current is generated in it and the magnitude of e.m.f. of current induced depends upon the number of turns in the coil. This is called electromagnetic induction. Transformers work on the principle of mutual induction. According to the principle of mutual induction, when two coils are placed close to each other and if one of them carries a time varying current, then a current is induced in the other coil. 


Construction

A transformer consists of a laminated core made from thin rectangular frames of soft iron. The frames are placed one over another and insulated from each other by lacquer, a kind of varnish. On one arm of the core, a coil, ‘P,’ of insulated copper wire is wound. This coil is called the primary coil and is connected to an alternating voltage source, whose voltage is to be stepped up or stepped down. On the other arm, another coil, S, of insulated copper wire is wound. This is the secondary coil. The induced alternating EMF is obtained as an output from the terminals of this coil.

If the secondary coil has a greater number of turns than the primary, then the voltage gets stepped up, that is, the output voltage is higher than the input voltage. This type of a transformer is called a ‘step-up’ transformer. In this type of a transformer, there is less current in the secondary coil than in the primary coil. Conversely, if the secondary coil has less turns than in the primary coil, the transformer is called a ‘step-down’ transformer. In such a case, the output voltage is less than the input voltage. That is, the voltage is stepped down, or reduced, and the current increases.


Factors on Which Energy Loss in a Transformer Depends

In an ideal transformer there should be no loss of energy. However, in reality, all transformers incur small energy losses due to various factors, such as:


• Heat in the coils
• Eddy currents in the core
• Hysteresis loss in the core
• Magnetic field link loss


Uses

A major use of transformers is in power transmission lines. Transformers are used in networks that transmit electricity over long distances. we use a step-up transformer to increase the voltage and consequently decrease the current in the transmission line. This reduces the loss of energy as heat. The voltage is then stepped down at various substations and utility poles, before it reaches your home in the range of 220 to 240 volt.


Typically, when electricity is transmitted at high voltages, the voltage is in a multiple of 11. Thus, we have electricity being transmitted at 66, 33, 22 and 11 kilovolt. When electricity reaches city from the generating stations, it is typically stepped down to 6.6, 3.3 or 1.1 kilovolt. At the sub-station in your area, it is stepped down to 440 volt for three-phase circuits, and 220 to 240 volt for single-phase circuits. In India, domestic electricity supply is at 220 to 240 volt, but in the US, it is at 110 volt.

A transformer cannot be used with direct current since its working is based on changing magnetic flux produced by a time-varying current of a given frequency in one coil inducing a varying current of the same frequency in the other coil. Since the magnitude of direct current does not vary with time, a transformer cannot be used with direct current.

Videos

Activities

Activity 1

Phet.colorado has created a java applet to visualsise the the magentic field and the direction of the magnetic compass in the magnetic field. Using this java applet we can understand the electro magnetic induction, mutual induction and hence the working of electro magnet, transformer and generator.

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Activity 2

Micro.magnet.fsu.edu has created a java applet to understand the working of a transformer. Using this we can understand how a transformers steps up and steps down the voltage.

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References

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