A frequency converter is a power electronics device for steplessly adjusting the rotation speed of an alternating current (AC) motor, such as a short circuit motor. By using frequency converters, the rotation speed of a controlled motor can be increased considerably above the nominal frequency of the motor. The rotation speed of a short circuit motor is proportional to frequency, so by steplessly changing the frequency of a motor its rotation speed changes correspondingly. The frequency converter is typically composed of two converters, a rectifier and an inverter, between which is provided a direct voltage or a direct current intermediate circuit. The rectifier and the inverter may also be located physically separately, and one rectifier may supply a plurality of inverters via a common intermediate circuit. A frequency converter also include an intermediate circuit for filtering a direct voltage received from the rectifier and a control unit for controlling the operation of the inverter on the basis of received measurement information to enable a correct frequency to be provided for the voltage to be fed to a motor. Frequency converters for supplying power to three-phase asynchronous motors essentially comprise a rectifier (AC to DC converter) which supplies a DC voltage to a smoothing or filter capacitor and an inverter (DC to AC converter) using power transistors or thyristors and energized by said filtered DC voltage. Frequency converters for electric motors include two types of configurations, including the pulse-width-modulated frequency converter (PWM frequency converter) and the pulse-amplitude-modulated frequency converter (PAM frequency converter). Both not only change the frequency of the voltage lying at the motor but also the voltage itself to control the rotational speed of the motor with low losses and to be able to operate the motor with an essentially constant torque.
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