A transformer is an energy coupling device that takes electrical energy at one voltage (from its source) and transforms it to another voltage. The new voltage may be higher (stepped up) or lower (stepped down), or it may remain the same as the input voltage. A good analogy is a mechanical gear train, where the shaft of the input gear rotates at a different speed than that of the output gear.


There are three major parts to a transformer: its coils, its core, and its enclosure or structural parts.

Coils. The coils are really turns of insulated copper or aluminum wire (magnet wire) that are wound around a core. There is at least one coil of wire or winding at a transformer's input (primary) and another for its output (secondary). These windings are well insulated from each other and the core to prevent shorts and grounds.

The ratio of turns in a transformer's primary winding to those in its secondary winding is known as its turns ratio. Shown in Fig. 1 is a simple transformer circuit, where the primary has six turns, and the secondary has three turns. Thus, its turns ratio is 6 divided 3, or 2:1.


A transformer's voltage ratio is the same as its turns ratio and is expressed by the following proportion.

[V.sub.P]/[V.sub.S] = [N.sub.P]/[N.sub.S] where [V.sub.P] = primary volts [V.sub.S] = secondary volts [N.sub.P] = number of turns at primary [N.sub.S] = number of turns at secondary

Thus, the voltage of the primary winding of the Fig. 1 transformer is twice that of its secondary winding.

Core. A core is made up of many thin laminations of high-grade electrical sheet steel. These laminations, which are insulated, are stacked one atop another until the desired stack height is reached. It usually takes 30 to 40 laminations to make a 1-in. high stack.

There are two kinds of losses associated with the core: hysteresis losses and eddy current losses. Hysteresis losses are directly proportional to the volume of the lamination while eddy current losses are directly proportional to the thickness of the lamination. Thus, the importance of using thin laminations of high-grade electrical steel.

For more detailed information, see "Choosing The Right Transformer, Parts 1 through 4" (February, May, and November 1992 issues; January 1993 issue).


Many times, line voltages are either higher or lower than the rated voltage of a transformer's primary. In instances such as these, the secondary voltage will be higher or lower respectively. For example, a 480V-120V single winding transformer with an input line voltage of 456V will have a secondary output voltage of 114V. This is because the transformer's voltage ratio is 4:1 (480V primary divided by 120V secondary). Thus, its secondary voltage is 456V divided by 4, or 114V. Conversely, this same transformer with an input voltage of 504V will have a secondary voltage of 126V (504V divided by 4).

Using a tap changes the voltage ratio of a transformer so that its secondary voltage stays at nominal. On large power transformers, taps on the primary are used to offset any higher or lower input voltages. These tap connections are usually set at the factory for nominal line voltage. If the voltage at the site is different, the taps are changed accordingly.

That said, what is a tap? Actually, it's a direct connection to a turn on a transformer winding at a voltage other than the normal rated voltage. Its lead is brought out to a terminal on the surface of the coil or on a terminal board of the transformer. Fig. 2 shows a transformer winding with a typical tap placement. The numbers refer to the marking of the tap. In this diagram, "1" is the highest voltage tap, "7" is the lowest voltage tap, and "3" is the normal, or nominal, voltage tap.


A whole number of turns is required between taps; otherwise, the tap lead will be on the wrong side of the transformer. For example, a winding can't be tapped at 2 3/4 or 7 1/4 turns; it must be tapped at 3 or 7 turns. Let's look at some examples.

A 480V transformer having 960 turns has 2 turns per volt. A 2 1/2% low tap on this transformer will decrease the voltage by 12V (0.025 times 480V); thus, the input voltage is 468V. Since there are 2 turns per volt, the tap would be 24 turns from the 480V normal tap.

In large transformers with few turns, taps can't be placed at exactly the right voltage. For example, a very large transformer has 5V per turn; as such, a 2 1/2% tap on the 480V primary winding (reduction of 12V) calls for 2.4 turns (12V divided 5V per turn). Because we can't tap at less than a whole turn, the tap is placed at 2 turns for 10V, and the tap becomes a 2.09% tap rather than a 2 1/2% tap.

The number of turns (and thus the turns per volt) can be varied; however, the exact voltage required may still be unattainable. For this reason, tap voltages are marked on transformer nameplates.