The primary and secondary windings of an ideal transformer would have zero resistance. However, copper wire has a certain amount of resistance.

Therefore, the primary and secondary windings have certain DC resistance values. The winding resistance of a coil can be measured with an ohmmeter.

This DC resistance is undesirable because it produces an I2R loss in transformer operation and reduces the efficiency of the transformer. The I2R power loss due to the DC winding resistance of the primary and secondary coils is called the copper loss of the transformer.

This copper loss can be reduced by winding the primary and secondary coils with larger wire. Of course, the DC resistance of the windings cannot be reduced to zero, although it might be so small that it can be neglected in practice.

Let us consider the AC resistance of the primary and secondary. When the secondary of an ideal transformer is open-circuited, the primary would have infinite reactance and would draw no current from an AC source.

However, a practical transformer does not have infinite primary reactance, although this reactance is very high in the case of an efficient transformer. Therefore, the primary of an efficient transformer draws a slight amount of AC current from the source when the secondary is open-circuited. This is called the no load current.

The no-load current is not quite 90◦ out of phase with the primary voltage because a practical transformer has copper loss and core loss. These losses consume a small amount of power from the source.

It follows from previous discussion that if a resistance load is connected across the secondary terminals of a transformer, a substantial AC current is drawn by the primary. We say that the connection of the secondary load has caused the primary to have a lower value of AC resistance.

Note that this AC resistance cannot be measured with an ohmmeter; the value of the AC resistance is simply a voltage/ current ratio. It is the ratio of primary AC voltage to in-phase primary AC current.

Even an ideal transformer would have a value of primary AC resistance that depends on the value of the load connected across the secondary terminals.

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