49 Frequency Converters 2.4.1 Variable Intermediate Circuit Fig. 2.8 Variable DC intermediate circuit This type of intermediate circuit consists of a very large inductor, also known as a “choke”, and is combined with a fully controlled rectifi er as shown in Fig. 2.3 Main component topologies part 5, and Fig. 2.8 Variable DC intermediate circuit. The inductor converts the variable voltage from the fully controlled rectifi er into a variable direct current. The load determines the size of the motor voltage. The advantage of this kind of intermediate circuit is that braking energy from the motor can be fed back into the supply network without the need for additional components.
The inductor is used in current-source FCs (I-converters). Fig. 2.9 Variable DC voltage intermediate circuit Finally, a chopper can be inserted in front of a fi lter, as shown in Fig. 2.9 Variable DC voltage intermediate circuit. The chopper contains a transistor which acts as a switch for turning the rectifi ed voltage on and off . The control circuit regulates the chopper by comparing the variable voltage after the fi lter (U V ) with the input signal. If there is a diff erence between these values, then the ratio of the time t on (when the transistor is conducting) to the time t off (when the transistor is blocking) is adjusted.