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Transformer: Step-Up, Step-Down, Efficiency and Losses

A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core.

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Student-friendly explanation

A transformer has a primary coil connected to an AC source and a secondary coil connected to the load. Alternating current in the primary produces a changing magnetic flux in the core. This changing flux links the secondary coil and induces an emf in it. For an ideal transformer, Vs/Vp = Ns/Np and input power equals output power approximately. If Ns is greater than Np, the transformer is step-up; if Ns is less than Np, it is step-down. Real transformers have losses due to coil resistance, eddy currents, hysteresis, and flux leakage.

How to write this in exams

  1. 1

    Start with the exact idea

    A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core.

  2. 2

    Then show how to use it

    Write the turns ratio relation. Substitute known turns and voltage values with units. Decide step-up or step-down from Ns compared with Np. If current is asked, use approximate power equality for an ideal transformer. For theory questions, mention mutual induction and changing magnetic flux.

  3. 3

    Add one concrete example

    In power transmission, voltage is stepped up before long-distance transmission to reduce current for the same power, which reduces I^2R loss in transmission lines.

  4. 4

    Avoid this incomplete answer

    Assuming step-up voltage also steps up current is wrong. In an ideal transformer, increasing voltage decreases current so that power is approximately conserved.

Definition

A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core.

Example

In power transmission, voltage is stepped up before long-distance transmission to reduce current for the same power, which reduces I^2R loss in transmission lines.

Rule to remember

Ideal transformer relation: Vs/Vp = Ns/Np. Approximate power relation for ideal transformer: Vp Ip = Vs Is. Efficiency eta = output power/input power x 100 percent. Vp and Vs are in volt, Ip and Is in ampere, and Np and Ns are numbers of turns. These relations assume negligible losses and strong flux linkage between coils.

Memory hook

More secondary turns means more secondary volts, but current moves the opposite way in an ideal transformer.

Examples and method

Worked example

A transformer has Np = 500 turns and Ns = 2500 turns. It is connected to a 220 V AC supply. Vs/Vp = Ns/Np = 2500/500 = 5, so Vs = 5 x 220 = 1100 V. Since secondary turns are greater, it is a step-up transformer. In an ideal case, if output current is 2 A, output power is 1100 x 2 = 2200 W, so primary current is 2200/220 = 10 A.

Method to apply

Write the turns ratio relation. Substitute known turns and voltage values with units. Decide step-up or step-down from Ns compared with Np. If current is asked, use approximate power equality for an ideal transformer. For theory questions, mention mutual induction and changing magnetic flux.

Diagram support

The diagram should show primary coil, secondary coil, laminated soft iron core, AC input, output/load, magnetic flux path, and turns Np and Ns. For step-up, Ns > Np; for step-down, Ns < Np.

How CBSE asks it

Exams ask students to identify step-up or step-down action, calculate secondary voltage/current, explain why AC is needed, list losses, or state methods to reduce energy losses.

Avoid common mistakes

Common confusion

Students often say a transformer works with DC. A transformer requires changing magnetic flux, so it works with AC or changing current, not steady DC.

Common wrong answer

Assuming step-up voltage also steps up current is wrong. In an ideal transformer, increasing voltage decreases current so that power is approximately conserved.

Exam tip

In transformer numericals, compare turns first. More turns on the secondary means higher secondary voltage; fewer turns means lower secondary voltage.

Quick check

Why is a transformer not used with a steady DC supply?

A transformer is not used with steady DC because steady current produces nearly constant magnetic flux after switching. Without changing flux, no continuous emf is induced in the secondary coil.

Answer writing and exam use

1-mark answer

A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core.

2-mark answer

A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core. Ideal transformer relation: Vs/Vp = Ns/Np. Approximate power relation for ideal transformer: Vp Ip = Vs Is. Efficiency eta = output power/input power x 100 percent. Vp and Vs are in volt, Ip and Is in ampere, and Np and Ns are numbers of turns. These relations assume negligible losses and strong flux linkage between coils. In power transmission, voltage is stepped up before long-distance transmission to reduce current for the same power, which reduces I^2R loss in transmission lines.

3-mark answer

A transformer has a primary coil connected to an AC source and a secondary coil connected to the load. Alternating current in the primary produces a changing magnetic flux in the core. This changing flux links the secondary coil and induces an emf in it. For an ideal transformer, Vs/Vp = Ns/Np and input power equals output power approximately. If Ns is greater than Np, the transformer is step-up; if Ns is less than Np, it is step-down. Real transformers have losses due to coil resistance, eddy currents, hysteresis, and flux leakage. Ideal transformer relation: Vs/Vp = Ns/Np. Approximate power relation for ideal transformer: Vp Ip = Vs Is. Efficiency eta = output power/input power x 100 percent. Vp and Vs are in volt, Ip and Is in ampere, and Np and Ns are numbers of turns. These relations assume negligible losses and strong flux linkage between coils. A transformer has Np = 500 turns and Ns = 2500 turns. It is connected to a 220 V AC supply. Vs/Vp = Ns/Np = 2500/500 = 5, so Vs = 5 x 220 = 1100 V. Since secondary turns are greater, it is a step-up transformer. In an ideal case, if output current is 2 A, output power is 1100 x 2 = 2200 W, so primary current is 2200/220 = 10 A. Exams ask students to identify step-up or step-down action, calculate secondary voltage/current, explain why AC is needed, list losses, or state methods to reduce energy losses. Assuming step-up voltage also steps up current is wrong. In an ideal transformer, increasing voltage decreases current so that power is approximately conserved.
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