42 Frequency Converters Direct converters are characterised by: • Reduced frequency control range (approximately 25 to 30 Hz) with 50 Hz mains frequency • Common use with synchronous motors. • Suitability for applications with stringent dynamic performance requirements. 2.2 Converters with Intermediate Circuit In the vast majority of cases, the FC is equipped with an intermediate circuit. Another term for intermediate circuit is “DC bus”. Within the category of converters with an intermediate circuit, there are two subtypes: • constant intermediate circuit • variable intermediate circuit. FCs with an intermediate circuit can be broken down into four main components as shown in Fig.
2.2 Block diagram of a frequency converter with an intermediate circuit. Fig. 2.2. Block diagram of a frequency converter with an intermediate circuit Rectifi er The rectifi er is connected to a single-phase or three-phase AC mains supply and generates a pulsating DC voltage. There are four basic types of rectifi er, as shown in Fig. 2.3 Main component topologies: • controlled • semi-controlled • uncontrolled • active front-end