Transformer

Electricity pylons are an iconic symbol of the electricity system in the UK. They are used to transmit electricity all over the country. However, if you attempted to charge your phone from the cables running over pylons, it would explode! This is because electricity flows through the cables at extremely high voltages. To use electricity in our homes, the voltage needs to be reduced to a level suitable for household appliances. This is done by the use of transformers.

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StudySmarter Editorial Team

Team Transformer Teachers

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    Transformer definition

    Transformers are electrical devices that transfer electrical energy between alternating current circuits and can be used to either increase or decrease the voltage from one circuit to the next. They are based on the fact that an alternating current in one circuit generates a magnetic field, which can induce a voltage in a second circuit.

    Transformer diagram

    Transformers transformer diagram StudySmarterFig. 1: The green dotted line around the iron core shows how the alternating magnetic field is transported from the primary coil to the secondary coil. Wikimedia, CC-BY-SA 3.0

    There are three main parts that make up all transformers:

    • The primary coil - this is connected to the first alternating current (AC) circuit and the voltage across it is equal to the incoming voltage from the circuit. This voltage is called \(V_p\), with the subscript referring to 'primary'.
    • The secondary coil - this is connected to the second AC circuit and it provides the voltage to the second circuit. This voltage is called \(V_s\) , with the subscript referring to 'secondary'.
    • The iron core - this links the two coils, which are wrapped around opposite sides. There is no electrical connection between the two coils, they are only joined by the iron core.

    How does a transformer work?

    The circuit must have alternating current flowing through it. The reason for this can be understood by considering how a transformer functions in steps:

    1. The primary coil has the alternating current from the original circuit running through it. The coil is a metal so it becomes an electromagnet and produces an alternating magnetic field. An alternating magnetic field constantly changes direction.
    2. The changing magnetic field is carried around the iron core to the secondary coil, as can be seen in the figure above.
    3. When the alternating magnetic field reaches the secondary coil, a current is induced in it as the coil acts as a conductor.

    The above points show why direct current cannot be used in the first circuit. It will produce a magnetic field that is not alternating. As the magnetic field is not changing, no current and hence no voltage is induced in the secondary coil.

    The dotted green lines in the diagram above represent the magnetic field lines running through the iron core.

    The energy from the primary coil is transferred to the secondary coil by the magnetic field - there are no electrical connections. This means that the magnetic field must be able to be transferred very efficiently through the core.

    For this to be the case, a magnetically "soft" material should be used, such as an alloy of iron with silicon. However, some of the initial energy is always lost in real-life situations due to the resistance in the wires and the iron core resisting the changing of the magnetic field.

    Wasted energy is a big problem when working with transformers. In Electrical Power stations, massive transformers are used, and they are often contained in giant tanks so that the core and the coils are completely enclosed. A small amount of energy is lost in the form of heat, and cooling fluid is continuously pumped around the tanks to remove the heat.

    Transformer formula

    The voltage leaving the secondary coil to the external circuit is related to the ratio of the number of turns on the coils of the transformer. Using the symbols for the incoming and outgoing voltage as stated in the earlier diagram, a simple formula relates the number of turns on the two coils to the output voltage:

    VpVs=NpNS$$\frac{N_p}{N_s}=\frac{V_p}{V_s}$$

    \(N_p\) represents the number of turns on the primary coil and \(N_s\) the same for the secondary coil. The equation is very simple and is just saying that the ratio of the number of turns on the secondary coil to the number on the primary coil is equal to the ratio of the output voltage to the incoming voltage.

    The voltage of the electricity \(V_p\) running through a pylon is \(100\, \mathrm{kV}\). The voltage needs to be decreased to \(250\, \mathrm{V}\) so that it is a suitable level for the mains electricity supply in a house. This can be done by using a large transformer. The specific transformer used in this case has \(N_p = 10\,000\) turns on the primary coil. How many turns should it have on the secondary coil?

    For this question we can use the transformer equation from above:

    VpVs=NpNS

    $$\frac{N_p}{N_s}=\frac{V_p}{V_s}$$

    It can be rearranged to give:

    $$N_s=N_p\frac{V_s}{V_p}$$

    Ns=NpVsVp

    The values given in the question can then be plugged in to find the number of turns on the secondary coil:

    Ns=1000·23023000=10

    $$N_s=10\;000\frac{250\;\mathrm V}{100\;000\;\mathrm V}=25\;\mathrm{turns}$$

    So there are 25 turns on the secondary coil.

    Types of transformer

    There are two types of transformers; step-up and step-down transformers. The names give you a clue about what they do - step-up transformers increase the voltage from one alternating circuit to the next whereas step-down transformers decrease the voltage.

    Transformers Image of a Step Up Transformer StudySmarterFig. 2: A step-up transformer found inside a microwave, Wikimedia CC SA 4.0.

    For a transformer to increase (step-up) the voltage from the first circuit to the next, the number of turns on the secondary coil must be greater than that on the primary coil. Conversely, to decrease (step-down) the voltage from one circuit to the next, the turns on the secondary coil must be less than on the primary coil.

    Examples of transformers

    Transformers have many useful applications. Both step-up and step-down transformers are used in different situations.

    The national grid

    Electrical Power is supplied across the UK from power stations that send the electricity along high-voltage power cables.

    The national grid is the network of cables that connect all the places in the UK that need electricity.

    The reason that high voltages are used to transmit electrical power is that it results in transmitting the same amount of power with a lower current flow through cables, which means that the energy lost as heat will be less. Energy is lost because the resistance in the wires is fighting against the current, and they become warm resulting in a loss of thermal energy to the surroundings.

    Transformer Diagram showing electricity transmission voltages StudySmarterA diagram showing the step-up and step-down transformers used in the power transmission system in North America. The UK system is very similar, operating at slightly different voltages.

    High voltages are very dangerous and this is why the cables that transport the electrical power are high above the ground on pylons, as shown in the diagram above.

    Power stations usually generate electricity at \(25\, \mathrm{kV}\) . This is increased by step-up transformers to a voltage of up to \(400\, \mathrm{kV}\) be transmitted along the pylons, and this voltage is then decreased to around \(240\, \mathrm{V}\) once the high-voltage cables reach our local sub-station.

    Household appliances

    Radios and other electronic devices will often run off the mains voltage supply. However, their operating voltage is normally quite a lot lower than \(240\, \mathrm{V}\)! There are step-down transformers built into devices to reduce the voltage to a suitable level. On the other hand, the operating voltage of microwave ovens is higher than the mains voltage for a household so they also have a step-up transformer inside to increase the voltage.

    Transformer - Key takeaways

    • Transformers are used to increase or decrease an input voltage by a required amount.
    • The ratio of the number of turns on the primary and secondary coils is equal to the ratio of their voltages.
    • There are no electrical connections between the primary coil and the secondary coil. They are only connected by the iron core and the energy is transferred between them through alternating magnetic fields.
    • Step-up transformers increase voltage and step-down transformers decrease voltage.
    • Transformers are used in the national grid to raise the voltage and improve the efficiency of transferring electricity from power stations around the country.

    References

    1. Fig. 1: Transformer3d (https://commons.wikimedia.org/wiki/File:Transformer3d_col3.svg) by BillC (https://en.wikipedia.org/wiki/User:BillC) is licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/).
    2. Fig. 2: Caso SMG20 - MD Microwave Oven Manufacturing MD-801EMR-1-0190 (https://commons.wikimedia.org/wiki/File:Caso_SMG20_-_MD_Microwave_Oven_Manufacturing_MD-801EMR-1-0190.jpg) by Raimond Spekking (https://commons.wikimedia.org/wiki/User:Raymond) is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
    Frequently Asked Questions about Transformer

    How does a transformer work step by step?

    The current through the primary coil induces as an alternating magnetic field in the magnet, which induces a current in the secondary coil. The ratios of the turns on each coil determines the output voltage from the secondary coil.

    What is a transformer in physics?

    A transformer is an electrical device that can increase or decrease the voltage from one circuit to another.

    What are the two types of transformer?

    There are step-up transformers and step-down transformers. Step-up transformers increase the voltage, while step-down decrease it.

    What is a transformer circuit?

    A transformer circuit is a circuit in which the input voltage is increased or decreased by the use of the transformer.

    What is the principle of a transformer?

    A transformer is based on the fact that an alternating current in one circuit generates a magnetic field, which can induce a voltage in a second circuit. By varying the number of coils on each side of a transformer, the voltage can be increased or decreased between the two circuits.

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    Test your knowledge with multiple choice flashcards

    Step-up transformers ____ the input voltage.

    If you wanted to increase the voltage of a circuit by using a transformer, would you need more turns on the primary or the secondary coil?

    There is \(1\,\mathrm{V}\) across the primary coil of a transformer, the primary coil has \(50\) turns and the secondary coil has \(100\) turns. What is the voltage across the secondary coil?

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    StudySmarter Editorial Team

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