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Electromagnetic Induction

The transformer turns-ratio equation

  • 3 things to know
  • Paper 2

Overview

The turns ratio equation, calculating an output voltage or the number of turns on a coil, and telling a step-up from a step-down transformer by its turns.

What you need to know

Answer each one before you reveal it.

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Key term

State the turns ratio equation used for a transformer.

Numbers

A transformer changes 230 V into 11.5 V and has 2000 turns on its primary coil. Calculate the number of turns on its secondary coil.

Numbers

A transformer with 1200 turns on its secondary coil raises 100 V to 400 V. Calculate the number of turns on its primary coil.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the turns ratio equation used for a transformer.

    Model answer

    Primary voltage ÷ secondary voltage = number of primary turns ÷ number of secondary turns.

  2. 2

    A transformer has 200 turns on its primary coil and 1000 turns on its secondary coil. Calculate the output voltage for an input of 12 V.

    Model answer

    Output voltage = 12 × 1000 ÷ 200 = 60 V.

  3. 3

    A transformer changes 230 V into 11.5 V and has 2000 turns on its primary coil. Calculate the number of turns on its secondary coil.

    Model answer

    Secondary turns = 2000 × 11.5 ÷ 230 = 100 turns.

  4. 4

    A transformer has 800 turns on its primary coil and 200 turns on its secondary coil. Calculate the output voltage for an input of 240 V.

    Model answer

    Output voltage = 240 × 200 ÷ 800 = 60 V.

  5. 5

    A transformer with 1200 turns on its secondary coil raises 100 V to 400 V. Calculate the number of turns on its primary coil.

    Model answer

    Primary turns = 1200 × 100 ÷ 400 = 300 turns.

  6. 6

    A transformer has 500 turns on its primary coil and 2500 turns on its secondary coil. State whether it steps the voltage up or down, and by what factor.

    Model answer

    It steps the voltage up by a factor of five.

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