Transformers
TRANSFORMER THEORY
Current Ratio
The current in the windings of a transformer is inversely proportional to the voltage in the
windings. This relationship is expressed in Equation (13-5).
(13-5)
VP
VS
IS
IP
where
IP
= primary coil current
IS
= secondary coil current
Since the voltage ratio is equal to the turns ratio, we can express the current ratio in terms of the
turns ratio, as in Equation (13-6).
(13-6)
NP
NS
IS
IP
Example 1:
When operated at 120 V in the primary of an iron core transformer, the current
in the primary is 4 amps. Find the current in the secondary if the voltage is
stepped up to 500 V.
Solution:
VP
VS
IS
IP
Next, we solve for IS.
IS
VP
VS
IP
IS
120
500
4 amps
IS
0.96 amps
Example 2:
A transformer with 480 turns on the primary and 60 turns on the secondary draws
0.6 amps from a 120 V line. Find IS.
Solution:
NP
NS
IS
IP
Rev. 0
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