TRANSFORMER THEORY
Transformers
Turns Ratio
Each winding of a transformer contains a certain number of turns of wire. The turns ratio is
defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the
secondary winding. Turns ratio can be expressed using Equation (13-1).
(13-1)
Turns ratio
NP
NS
where
NP = number of turns on the primary coil
NS = number of turns on the secondary coil
The coil of a transformer that is energized from an AC source is called the primary winding
(coil), and the coil that delivers this AC to the load is called the secondary winding (coil) (Figure
1).
Impedance Ratio
Maximum power is transferred from one circuit to another through a transformer when the
impedances are equal, or matched. A transformer winding constructed with a definite turns ratio
can perform an impedance matching function. The turns ratio will establish the proper
relationship between the primary and secondary winding impedances. The ratio between the two
impedances is referred to as the impedance ratio and is expressed by using Equation (13-2).
(13-2)
NP
NS
2
ZP
ZS
Another way to express the impedance ratio is to take the square root of both sides of Equation
(13-2). This puts the ratio in terms of the turns ratio, which is always given for a transformer.
0
where
NP = number of turns in the primary
NS = number of turns in the secondary
ZP = impedance of primary
ZS = impedance of secondary
ES-13
Page 2
Rev. 0
