Transformers

Transformers

by Omar Jordan

Poll

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​Pictures of Transformers

​How did we get transformers?

​In one of his experiments, Faraday used a coil connected to a battery, in place of a permanent magnet, to induce an e.m.f in a second coil. Both coils were wrapped around an iron ring. The coil on the battery circuit was called the primary coil and the other was the secondary coil. This coil was connected to a centre-zero galvanometer. When the switch was closed a current appeared momentarily in the second coil, indicating the presence of an e.m.f. When the switch was opened a momentary current flowed in the opposite direction. But when the current remained steady in the primary coil, there was no current in the secondary coil

​How did we get transformers? (Con't)

​Thus, a changing magnetic flux from the primary coil had caused an induced e.m.f in the secondary coil.

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The phenomenon in which an e.m.f is induced in a coil by a flux change from another coil is called mutual induction.

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The presence of the iron core increased the magnetic flux, as the magnetic field produced by the current-carrying primary coil caused the domains in the iron to become aligned producing a magnet.

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The ring shape ensured that most of the flux produced by the primary coil linked the secondary coil.

​​How did we get transformers? (Con't)

Multiple Choice

From the above, it is deduce that a transformer must used which type of supply?

• A.C.

• D.C.

• Both

• None of them

​Circuit symbol for a transformer

​The following represents the circuit symbol for a transformer

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​​**An a.c. supply can be used to keep the flux changing continuously since the alternating current ( which in turn produces an alternating voltage) changes constantly in magnitude and direction. A d.c. can NEVER be used for a transformer.

​What is a Transformer?

​A transformer is an electromagnetic device that changes (transforms) voltage and current levels in an AC circuit.​

It is a static device that transfers electrical energy from one circuit to another by electromagnetic induction between two coils i.e. without any direct electrical connection between them.

​The working principle is very simple. It depends on Faraday's law of electromagnetic induction: the rate of change of flux in a conductor or coil is directly proportional to the induced EMF in it. If the circuit of the second winding is closed then a current flows through it.

​Types of Transformer Cores

​Shell Type Core

​Core Type Core

​Transformer Windings

​The winding which takes electrical power from the source, is generally known

as primary winding of transformer. The winding which gives the desired output

voltage due to mutual induction in the transformer, is commonly known as

secondary winding of transformer.

​Operation of a transformer

If a continuously changing e.m.f is applied to the primary coil, the current in the coil, and therefore the magnetic flux it produces, will also change continuously. An induced e.m.f will be produced in the adjacent secondary coil, through the process of mutual induction, as the continually changing flux from the primary coil cuts across the secondary coil. If there is no energy loss between the primary and the secondary coil, the output voltage appearing across the secondary coil is proportional to the number of turns in the coil.

• ​Vs is the voltage induced in the secondary coil in volts

• Vp is the voltage applied to the primary coil in volts

• Ns is the number of turns on the secondary coil

• Np is the number of turns on the primary coil

​Turn ratio and voltage ratio for transformers

Transformers are all about "ratios". The ratio of the primary to the secondary, the ratio of the input to the output, and the turns ratio of any given transformer will be the same as its voltage ratio. In other words for a transformer: "turns ratio = voltage ratio". The actual number of turns of wire on any winding is generally not important, just the turns ratio and this relationship are given as:​

​Main function of transformers

​The main purpose of a transformer is to transform voltages at preset ratios and we can see that the primary winding has a set amount or number of windings (coils of wire) on it to suit the input voltage.

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If the secondary output voltage is to be the same value as the input voltage on the primary winding, then the same number of coil turns must be wound onto the secondary core as ere are on the primary core giving an even turns ratio of 1:1 (1-to-1).

​Step-up transformers

​A transformer can change a given alternating input voltage to a higher or a lower output voltage. In a step-up transformer, the output voltage in higher than the input voltage; thus, the number of turns in the secondary coil is greater than the number in the primary coil. A step-up transformer is used to increase the alternating voltage that comes from the power plant for transmission in the cables.

​Step-down transformer

​In a step-down transformer, the output voltage is lower than the input voltage;

thus, the number of turns in the secondary coil is less than the number of turns in the primary coil. There is usually a step-down transformer which comes just before the supply reaches the consumer to lower the voltage.

Multiple Choice

What type of transformer is shown?

• Step-up transformer

• Step-down transformer

• 1:1 transformer

• None of the above

​Utilization of step-up and step-down transformers

Multiple Choice

What type of transformer is mounted on the pole outside of your house?

• Step-up transformer

• Step-down transformer

• 1:1 transformer

• None of the above

​Autotransformer

​An Auto Transformer is a transformer with only one winding wound on a laminated core. An autotransformer is similar to a two-winding transformer but differs in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides.

​Variable autotransformer

Multiple Choice

An autotransformer is more effiecient than a double wound transformer.

• True

• False

​Current from a transformer

​Transformer Ratios

Next Transformer Efficien​cy

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