When a transformer is energized with no load, the secondary voltage will be exactly the primary voltage divided by the turns ratio (NP/NS). When the transformer is loaded, the secondary voltage will be diminished by an amount determined by the transformer impedance and the power factor of the load.

This change in voltage is called regulation and is actually defined as the rise in voltage when the load is removed. One result of the definition of regulation is that it is always a positive number.

The primary voltage is assumed to be held constant at the rated value during this process. The exact calculation of percent regulation is given in Equation

where cos is the power factor of the load and L is per unit load on the transformer.

The most significant portion of this equation is the cross products, and since %X predominates over %R in the transformer impedance and cos predominates over sin for most loads, the percent regulation is usually less than the impedance (at L = 1).

When the power factor of the load is unity, then sin is zero and regulation is much less than the transformer impedance.

A much simpler form of the regulation calculation is given in Equation

For typical values, the result is the same as the exact calculation out to the fourth significant digit or so.

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