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Turns ratio

Curricula: GCSE, Sek I/II

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Aim

Find out how the voltage on the secondary winding of a transformer depends on the number of turns of the windings.

The bench

A 6 V, 50 Hz AC voltage source (sine wave) is connected to the primary winding of the transformer (500 turns, left pair of sockets). The secondary winding (250 turns, right pair) is not connected to anything. Above the transformer are AC voltmeters: the left one on the primary winding, the right one on the secondary. The number of turns is printed on the coils; the upper socket of each pair is the start of the winding.

Procedure

  1. Record the readings of both voltmeters and the number of turns of both coils.
  2. Calculate the voltage ratio \(U_2 / U_1\) and the turns ratio \(N_2 / N_1\).
  3. In the transformer's settings panel, set Secondary turns to 1000. Record the readings and compare the ratios again.
  4. Set Secondary turns to 500, then Primary turns to 250, then to 1000. How does \(U_2\) change?
Expected result

AC voltmeters show the RMS value: with an amplitude of 6 V, the primary winding shows 4.24 V. With 250 turns on the secondary winding you get 2.08 V, with 1000 turns about 8.3 V: the transformer is step-down or step-up depending on the ratio of the turns. With 500 turns on the secondary and 250 on the primary it is also about 8.3 V, with 1000 on the primary about 2.1 V: what matters is the ratio of the turns, not their number. The voltage ratio is approximately equal to the turns ratio, \(U_2 / U_1 \approx N_2 / N_1\). It is 2% lower because part of the magnetic flux of the primary winding does not pass through the secondary.

Questions

  1. How many turns are needed on the secondary winding to get 12 V from 4.24 V?
  2. What is the turns ratio in step 3? Is the transformer step-up or step-down?
  3. Why does the secondary winding give a voltage even though it is not connected to the primary by a single wire?
Answers
  1. \(N_2 = N_1 \cdot U_2 / U_1\) = 500 · 12 / 4.24 ≈ 1415 turns. The bench's transformer gives 2% less than the turns ratio, so on the bench about 1415 / 0.98 ≈ 1440 turns are needed.
  2. \(N_2 / N_1\) = 1000 / 500 = 2. The secondary winding has more turns than the primary, so the transformer is step-up: the voltage rises from 4.24 V to about 8.3 V.
  3. The alternating current in the primary winding creates an alternating magnetic flux, and the steel core carries this flux through the secondary winding. A changing magnetic flux induces a voltage in every turn of the secondary (electromagnetic induction), so the energy passes through the magnetic field without any electrical contact.