51 Frequency Converters 2.4.2 Constant Intermediate Circuit Fig. 2.11 Constant DC intermediate circuit The intermediate circuit can consist of a fi lter comprising a capacitor and/or an inductor (choke). Typically, electrolytic capacitors are used due to their high energy density. Although capacitors have a limited service life, they off er the following benefi ts:
• Smoothing of pulsating DC voltage (U Z1 ) • Availability as an energy reserve for supply voltage drops • Availability for energy storage for load surges and generative operation of the motor DC inductors off er the following advantages • The FC is protected against mains transients • Smoothing of current ripple, which in turn increases the service life of the intermediate circuit components, especially the capacitors • Reduction of mains interference and the option of smaller supply conductor cross sections. This function can also be implemented by means of line chokes upstream of the FC When planning an installation it is important to note that the inductors are heavy and can get hot.
Hot spots can arise. This form of intermediate circuit can be combined with various types of rectifi er. In the case of fully controlled rectifi ers, the voltage is kept constant at a given frequency. Thus, the voltage that is supplied to the inverter is a pure DC voltage (U Z2 ) with variable amplitude. With semi-controlled or uncontrolled rectifi ers, the voltage at the inverter input is a DC voltage with constant amplitude (approximately 2 times the mains voltage). The anticipated voltage and frequency are both generated in the inverter.