A transformer consists of two coils, a primary winding and secondary winding. An alternating current (AC) applied to the primary winding induces a voltage in the secondary winding through electromagnetic induction. The turns ratio between the windings determines whether the transformer steps up or steps down the voltage. Transformers are used for applications such as voltage regulation, impedance matching, phase shifting, isolation, and producing multiple voltage levels.

1. Transformers

2. Electromagnetic Induction
Transformer
Consists of two coils, a primary winding and a
secondary winding
Rated in volt-amperes (VA)
Primary winding
Coil containing the AC voltage
Secondary winding
Coil in which the voltage is induced

3. Mutual Inductance
Expanding magnetic field in loaded secondary causes
current increase in primary
Figure 18-4.Transformer with a loaded secondary.

4. Turns Ratio
Turns ratio
Determines whether the transformer is used to step up,
step down, or pass voltage unchanged
Can be expressed as:
turns ratio = NS/NP
where: N = number of turns

5. Turns Ratio (cont’d.)
Step-up transformer
Produces a secondary voltage greater than its primary
voltage
Turns ratio is always greater than one

6. Turns Ratio (cont’d.)
Step-down transformer
Produces secondary voltage less than its primary
voltage
Turns ratio is always less than one

7. Applications
Transformer applications include:
Stepping up/down voltage and current
Impedance matching
Phase shifting
Isolation
Blocking DC while passing AC
Producing several signals at various voltage levels

8. Applications (cont’d.)
Figure 18-5. A transformer can be used to generate a phase shift.

9. Applications (cont’d.)
Figure 18-6. A transformer can be used to block DC voltage.

10. Applications (cont’d.)
Figure 18-7. An isolation transformer prevents electrical
shock by isolating the equipment from ground.

11. EMF equation for transformer

16. Open circuit test
To find no load losses
Watt meter reading represents core losses

17. Short circuit test
Used for finding equivalent impedence and leakage
reactance
Cu losses at full load(for efficiency)
Regulation of transformer
Z01=Vsc/I1
W=I1xI1xR01 =>R01=W/R1xR1
X01=(Z01^2-R01^2)^1/2

18. Losses in transformer
Core loss or iron loss
Hysteresis loss

19. Applications (cont’d.)
Figure 18-8. An autotransformer is a special type of transformer
used to step up or step down the voltage.

20. Applications (cont’d.)
Figure 18-9. A variable autotransformer.

21. Summary
A transformer consists of two coils, a primary
winding and a secondary winding
 An AC voltage is put across the primary winding,
inducing a voltage in the secondary winding
Transformers allow an AC signal to be transferred
from one circuit to another

22. Summary (cont’d.)
Transformers are rated in volt-amperes(VA)
The turns ratio determines whether a transformer is
used to step up, step down, or pass voltage
unchanged
Transformer applications: impedance matching,
phase shifting, isolation, blocking DC while passing
AC, etc.