If we apply 120-volt AC, 60-hertz power to the primary of a power transformer with its secondary open, a very small current will flow through the primary winding. Only a small quantity primary current will flow because the inductive reactance of the primary winding is very high. However, if a load is then placed on the secondary winding
the load impedance of the secondary will be reflected back to the primary winding. This effect is normally referred to as reflected impedance.
The Reflected impedance is the effect that makes the primary current to automatically increase when the secondary current increases. Without it, a transformer could not come into function. Value of Reflected impedance is calculated by multiplying the square of the turns ratio, by the value secondary load. For example, consider that we have a transformer with a 10:1 turns ratio. In mathematical terms, a 10:1 ratio is the same as dividing 10 by 1, or simply 10. Therefore, the square of the turns ratio is 10 times 10, or 100. If load of a 5-ohm is placed on the secondary of this transformer, the reflected impedance seen at the primary will be 5 x 100, or 500 ohms.
This value of 500-ohm reflected impedance, seen at the primary, will be much less than the inductive reactance provided by the primary with the secondary open. As a result, the primary flow of current will automatically increase in order to compensate for the increased current flow in the secondary. In our preceding example, a “step-down transformer” was discussed.
The Reflected impedance works the same way in a “step-up transformer.” For example, consider that we have a step-up transformer having turns ratio of 1:10. In Mathematical terms, this is the same as 1 divided by 10, or 0.1. The square of 0.1 is 0.01. If a load of 5-ohm is placed on the secondary of this hypothetical transformer, the value reflected impedance seen at the primary will be 0.01 times 5, or 0.5 ohm.