As we saw in the discussion about generating direct current, when an armature rotates through a fixed magnetic field, current moves through the armature coil in different directions at different points of its rotation.
If we were to continually draw power off the same end for an entire rotation of the armature, we would see current increase for the first 90° of its rotation and decrease for the second 90° of rotation. It would then repeat the exact same pattern for the next half rotation, with the current flowing in the opposite direction.
The result is alternating current - current that changes the direction of its flow every half turn (or half cycle) of the armature. If we graph AC current, we use both sides of the horizontal zero line to show that each time the current rises to its maximum level and returns to zero, it does so in the opposite direction. A matched pair of up and down curves reflects one full rotation of the armature coil, and is called one cycle.
During these cyclic changes in the current, the voltage is changing as well, and at the same frequency as that of the current. Sometimes, these cyclic changes are not smooth sine waves, but are distorted. The number of complete cycles, per second, however, is still the same for both voltage and current. When a wave is not sinusoidal, it is said to have "harmonics" or noise. These are usually higher frequency or totally random components to the signal. Harmonics in current are created, for instance, when a magnetic core saturates or when there is a semiconductor in the circuit. A distorted waveform for the no-load (exciting) current of a transformer is shown here.
The frequency can be expressed as follows:
