Transformer

1. Learning Objectives

show an understanding of the principle of operation of
a simple laminated iron-cored transformer and recall
and solve problems using N1/N2=V1/V2=I2/I1 for an
ideal transformer
1 of 43
© Boardworks Ltd 2010

2. Transformers

A transformer changes the peak value of an alternating
voltage.
Transformers consist of two coils: the primary coil and a
secondary coil.
primary coil
secondary coil
When an alternating voltage passes through the primary coil,
an alternating magnetic field is created, which induces an
alternating e.m.f. in the secondary coil.
2 of 43
© Boardworks Ltd 2010

3. The transformer rule

primary coil
secondary coil
Np
Ns
Np
Ns
no. coils
flux linkage
induced e.m.f.
Vp =
-Np
t
Vs =
Dividing the equation for the e.m.f. of the
secondary coil by the one for the primary
coil gives the transformer rule:
-Ns
t
Vs
Vp
=
In a step-up transformer: Ns > Np so Vs > Vp.
In a step-down transformer: Ns < Np so Vs < Vp.
3 of 43
Ns
Np
© Boardworks Ltd 2010

4. Transformer efficiency

The efficiency is very high for a typical transformer. A laminated
core reduces eddy currents, the wires have very low resistance
and a soft iron core allows easy magnetizing/demagnetizing.
power supplied by secondary coil
transformer efficiency =
power supplied to primary coil
VsIs
transformer efficiency =
VpIp
In a step-up transformer, voltage is stepped up and
current is stepped down.
In a step-down transformer, voltage is stepped down
and current is stepped up.
4 of 43
© Boardworks Ltd 2010

5.

5 of 43
© Boardworks Ltd 2010

6. Glossary

6 of 43
© Boardworks Ltd 2010

7. What’s the keyword?

7 of 43
© Boardworks Ltd 2010

8. Magnetic fields: equations summary

8 of 43
© Boardworks Ltd 2010

9. Multiple-choice quiz

9 of 43
© Boardworks Ltd 2010