First of all, the power
of a single alternating current phase fluctuates. For domestic use, this does not truly present a problem. However, for the operation of electric motors
it is advisable to use a current that produces more constant power. This is obtained by using three separate power lines with alternating current, running in parallel but with each current phase shifted by 1/3 of a cycle in relation to the other phases.
Three-phase alternating current is produced at the power station in a generator with three separate windings. A single phase application can be connected between the phase and zero. Three-phase applications can be connected using all three phases in two ways, in a star (Y) or delta (Δ) configuration.
With the star connection, a phase voltage lies between the outlets. With a delta connection, a main voltage lies between the outlets. Industrial compressors were among the first industrial machines to be equipped with Variable Speed Drives (VSD
), also called Variable Frequency Drives, to control the rotational speed and torque of AC induction motors by controlling the frequency of the electric power lines to the motor. The most common design converts the three phases of the AC input power to DC power using a rectifier bridge. This DC power is converted into quasi-sinusoidal AC power by using an inverter switching circuit (now IGBT-type power semiconductor switches) and pulse width modulation (PWM) techniques.