Transformers
TRANSFORMER THEORY
Voltage Ratio
The voltage of the windings in a transformer is directly proportional to the number of turns on
the coils. This relationship is expressed in Equation (13-4).
(13-4)
VP
VS
NP
NS
where
VP = voltage on primary coil
VS = voltage on secondary coil
NP = number of turns on the primary coil
NS = number of turns on the secondary coil
The ratio of primary voltage to secondary voltage is known as the voltage ratio (VR). As
mentioned previously, the ratio of primary turns of wire to secondary turns of wire is known as
the turns ratio (TR). By substituting into the Equation (13-4), we find that the voltage ratio is
equal to the turns ratio.
VR = TR
A voltage ratio of 1:5 means that for each volt on the primary, there will be 5 volts on the
secondary. If the secondary voltage of a transformer is greater than the primary voltage, the
transformer is referred to as a "step-up" transformer. A ratio of 5:1 means that for every 5 volts
on the primary, there will only be 1 volt on the secondary. When secondary voltage is less than
primary voltage, the transformer is referred to as a "step-down" transformer.
Figure 2 Example 1 Transformer
Example 1:
A transformer (Figure 2) reduces
voltage from 120 volts in the primary
to 6 volts in the secondary. If the
primary winding has 300 turns and the
secondary has 15 turns, find the
voltage and turns ratio.
Solution:
VR
VP
VS
120
60
20
1
20:1
TR
NP
NS
300
15
20
1
20:1
Rev. 0
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