Transformer Operation

Transformer Operation Schematics

The illustrations below show typical schematic symbols for transformers. The symbol for an air-core transformer is shown in the first illustration. Parts (B) and (C) show iron-core transformers.

The bars between the coils are used to indicate an iron core. Frequently, additional connections are made to the transformer windings at points other than the ends of the windings. These additional connections are called TAPS. When a tap is connected to the center of the winding, it is called a CENTER TAP. Figure (C) shows the schematic representation of a center-tapped iron-core transformer.

schematic symbols for various transformers

Schematic symbols for various transformers.


Up to this point the section has presented the basics of the transformer operation including action, the transformer's physical characteristics, and how it is constructed. Now you have the necessary knowledge to proceed into the theory of transformer operation.


You have learned that a transformer is capable of supplying voltages which are usually higher or lower than the source voltage. This is accomplished through mutual induction, which takes place when the changing magnetic field produced by the primary voltage cuts the secondary winding.

A no-load condition is said to exist when a voltage is applied to the primary, but no load is connected to the secondary, as illustrated by the picture below.

transformer under no-load condition

Transformer under no-load condition.

Because of the open switch, there is no current flowing in the secondary winding. With the switch open and an ac voltage applied to the primary, there is, however, a very small amount of current called EXCITING CURRENT flowing in the primary.

Essentially, what the exciting current does is "excite" the coil of the primary to create a magnetic field. The amount of exciting current is determined by three factors: (1) the amount of voltage applied (Ea), (2) the resistance (R) of the primary coil's wire and core losses, and (3) the XL which is dependent on the frequency of the exciting current. These last two factors are controlled by transformer design.

This very small amount of exciting current serves two functions: Most of the exciting energy is used to maintain the magnetic field of the primary. A small amount of energy is used to overcome the resistance of the wire and core losses which are dissipated in the form of heat (power loss). Exciting current will flow in the primary winding at all times to maintain this magnetic field, but no transfer of energy will take place as long as the secondary circuit is open.

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