C
CraftExam
medium difficulty10 min

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.

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

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Understand it

  • 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.

Example

In power transmission, voltage is stepped up before long-distance transmission to reduce current for the same power, which reduces I2R loss in transmission lines.

Formula and rule

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A transformer has Np = 500 turns and Ns = 2500 turns.

  1. 1It is connected to a 220 V AC supply.
  2. 2Vs/Vp = Ns/Np = 2500/500 = 5, so Vs = 5 × 220 = 1100 V.
  3. 3Since secondary turns are greater, it is a step-up transformer.
  4. 4In an ideal case, if output current is 2 A, output power is 1100 × 2 = 2200 W, so primary current is 2200/220 = 10 A.

Method you can reuse

  1. 1Write the turns ratio relation.
  2. 2Substitute known turns and voltage values with units.
  3. 3Decide step-up or step-down from Ns compared with Np.
  4. 4If current is asked, use approximate power equality for an ideal transformer.
  5. 5For theory questions, mention mutual induction and changing magnetic flux.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

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.

Don't mix this up

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.

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    Example: In power transmission, voltage is stepped up before long-distance transmission to reduce current for the same power, which reduces I2R loss in transmission lines.

  3. 3

    Add for 3 marks

    A transformer has a primary coil connected to an AC source and a secondary coil connected to the load.

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