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1.9.8Electricity, Energy and Waves

Investigating the Output of a Transformer

5 min read 6 parts Lesson 54 of 54 Unit quiz

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Section 1 of 6Aim and Variables

1.9.8 Investigating the Output of a Transformer

The investigation tests whether the output voltage is proportional to the number of secondary turns.

Independent variable
  • The number of turns on the secondary coil, changed by winding on more or fewer turns.
Dependent variable
  • The secondary voltage, measured across the secondary coil with an a.c. voltmeter.
Control variables
  • Keep the number of primary turns, the input voltage and the frequency of the supply the same.
Temperature
  • Switch off between readings so the wires and core do not warm, which would change the results.

Section 2 of 6Method

1
Wind a fixed number of turns of insulated wire, for example 100, on one limb of a C-shaped core as the primary coil.
2
Wind a chosen number of turns on the other C-core as the secondary coil, and clamp the two cores together.
3
Connect the primary coil to a low-voltage a.c. supply and an a.c. voltmeter, and the secondary coil to another a.c. voltmeter.
4
Switch on, record the primary and secondary voltages, then switch off at once.
5
Repeat with different numbers of secondary turns, and take repeat readings of each to find a mean.

Section 3 of 6Analysing the Results

Analysing the Results
Graph
  • Plot secondary voltage on the y-axis against secondary turns on the x-axis;
    • a straight line through the origin is expected.
Gradient
  • The gradient is Vp ÷ Np, the voltage per turn, so it should be the same as the primary voltage divided by primary turns.
Prediction
  • With 4.0 V across 100 primary turns, 350 secondary turns should give 14 V.
Real results
  • Measured values are slightly lower because of energy losses, and a poor core connection makes this worse.

Section 4 of 6Safety and Improvements

Low voltages
  • Use a low-voltage supply only, because mains voltage is dangerous and the coils must be well insulated.
Hot components
  • Wires can get warm when currents flow, so switch off between readings and let the equipment cool.
Better core contact
  • Clamp the C-cores tightly together so that the field passes through the core with little loss.
More readings
  • Using more values of secondary turns, and repeating each, makes the graph more reliable.

Section 5 of 6Worked Example

Question

  • In a transformer investigation the primary has 100 turns and the input is 4.0 V a.c.
  • Calculate the expected secondary voltage for 350 turns, and the gradient of a graph of Vs against Ns.

  1. Gradient of the graph = Vp ÷ Np = 4.0 ÷ 100 = 0.040 V per turn.
  2. Expected secondary voltage = gradient × Ns = 0.040 × 350.
  3. 0.040 × 350 = 14 V.

Answer Vs = 14 V; gradient = 0.040 V per turn

Section 6 of 6Exam Checklist & Tips

1.9.8 – Key Points for the Exam

  • a.c. voltmeter A meter designed to measure the effective value of an alternating voltage.
  • C-core
    • A C-shaped piece of iron;
      • two together form a closed magnetic loop.
  • Proportional Two quantities are proportional if one doubles when the other doubles.
  • Independent variable The quantity that the experimenter changes deliberately.

Common Mistakes

  • Using a d.c. supply, which gives no output because there is no changing magnetic field.
  • Changing the primary turns at the same time as the secondary turns so the test is not fair.
  • Joining the cores loosely, which reduces the output and makes the results unreliable.
  • Forgetting that the line of best fit should pass through the origin because zero turns gives zero voltage.
Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.