50 Frequency Converters This makes it possible to vary the eff ective value of the DC voltage depending on how long the transistor conducts. This can be expressed as: U V = U x t off t on + t off When the chopper transistor interrupts the current, the fi lter inductor (or “choke”) attempts to produce an infi nitely high voltage across the transistor. To prevent this from happening, the chopper is protected by a freewheeling diode, as shown in Fig. 2.9 Variable DC voltage intermediate circuit. Fig.
2.10 Chopper transistor regulates the intermediate circuit voltage with corresponding eff ective value The fi lter in the intermediate circuit smooths the square-wave voltage after the chopper , while keeping the voltage constant at a given frequency. The frequency associated with the voltage is generated in the inverter.