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Also see current, voltage, and direct current. In electricity, alternating current (AC) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement. Household utility current in most countries is AC with a frequency of 60 hertz (60 complete cycles per second), although in some countries it is 50 Hz. The radio-frequency (RF) current in antennas and transmission lines is another example of AC. An AC waveform can be sinusoidal, square, or sawtooth-shaped. Some AC waveforms are irregular or complicated. An example of sine-wave AC is common household utility current (in the ideal case).
If, however, an application or production process requires an adjustable AC voltage (that is, a controllable speed), frequency inverters are used. These frequency inverters can generate an AC voltage that is variable in amplitude (the output voltage level) and frequency from a constant AC voltage. How does a frequency inverter work? A frequency inverter is therefore connected upstream of a motor to generate an AC voltage that can be adjusted to meet customer requirements.