Static frequency changers

The static frequency changers can be broadly classified as Direct and Indirect static frequency changers. The direct frequency changers are some times called as Cycloconverters. These convert the AC supply source frequency to a variable frequency. The input frequency ratio typically ranges from 0 to 0.5 and for the better
waveform control of the output voltage, the frequency ratio is limited to 0.33 . The smaller range of frequency variation is suitable for low-speed and large-power applications.

For a majority of applications, a wide frequency range is desirable due to the requirements over the desired speed range. In such applications, the indirect frequency conversion methods are employed. An indirect frequency changer consists of two power conversion stages; first stage is Rectification (AC to DC) and the second stage is Inversion (DC to AC). The indirect frequency changers are broadly classified depending on the source that supplies the input power to them and that can either be a voltage source or a current source. In both cases, the power input is kept to a specified constant. The output frequency becomes independent of the input supply frequency by means of the DC link. Varies configuration of indirect frequency changers have evolved keeping in view the diversity of application. However, these only differ in the way the two power conversion stages are incorporated. The more common configurations are the so called PWM inverter fed induction motor drive, Variable-Voltage Variable-frequency (VVVF) induction motor drive and Variable-Current Variable-frequency (VCVF) induction motor drive.