Applications
Aside from the obvious application of converting bulk amounts of power from one distribution standard to another, frequency changers are also used to control the speed and the torque of AC motors. In this application, the most typical frequency converter topology is the three-phase two-level voltage source inverter. The phase voltages are controlled using the power semiconductor switches and pulse width modulation (PWM). Semiconductor switching devices and anti-parallel connected freewheeling diodes form a bridge, which can connect each motor phase to the positive or negative dc-link potential. The PWM changes the connections of the phases between the positive and the negative dc-link potentials so that the fundamental wave voltage has the desired frequency and amplitude. The motor reacts primarily to the fundamental frequency and filters out the effects of harmonic frequencies.
Another application is in the aerospace and airline industries. Often airplanes use 400 Hz power so 50 Hz or 60 Hz to 400 Hz frequency converter is needed for use in the ground power unit used to power the airplane while it is on the ground. Airlines might also utilize the converters to provide in-air wall current to passengers for use with laptops and the like.
In renewable energy systems, frequency converters are an essential component of doubly fed induction generators (DFIGs) as used in modern multi-megawatt class wind turbines.
Frequency changers are typically used to control the speed of motors, primarily pumps and fans. In many applications significant energy savings are achieved. The most demanding application areas are found on the industrial processing lines, where the control accuracy requirements can be very high. This is particularly true in the nuclear power and weapons industry, where these devices regulate the operation of refinement centrifuges. The recently disclosed STUXNET worm targets specific Siemens brand frequency converters, causing erratic operation of Iran's centrifuge plant.
An HVDC-system can serve as frequency converter for large loads.
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