How a Transformer Works

Up to this point the chapter has presented the basics of the transformer including transformer action, the transformer's physical characteristics and how the transformer is constructed. This knowledge and understanding is sufficient to proceed into the theory of operation of a transformer.

No-Load Condition

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 in the following figure. 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 (V_a)
2. The resistance (R) of the primary coil's wire and core losses
3. The X_l which is dependent on the frequency of the exciting current.

These last two factors are controlled by transformer design.

FIGURE

This very small amount of exciting current serves two functions:

1. Most of the exciting energy is used to maintain the magnetic field of the primary.
2. 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.

Producing a Back-EMF

When an alternating current flows through a primary winding, a magnetic field is established around the winding. As the lines of flux expand outward, relative motion is present, and a Back-EMF is induced in the winding. This is the same Back-EMF that you learned about in the chapter on inductors. Flux leaves the primary at the North Pole and enters the primary at the South Pole. The Back-EMF induced in the primary has a polarity that opposes the applied voltage, thus opposing the flow of current in the primary. It is the Back-EMF that limits exciting current to a very low value.

Inducing a Voltage in the Secondary

As the exciting current flows through the primary, magnetic lines of force are generated. During the time current is increasing in the primary, magnetic lines of force expand outward from the primary and cut the secondary. As you remember, a voltage is induced into a coil when magnetic lines cut across it. Therefore, the voltage across the primary causes a voltage to be induced across the secondary.