52 Frequency Converters In the last few years manufacturers have devised intermediate circuits without capacitors and inductors (chokes). This has been generally termed “capacitor less” or “slim” intermediate circuit. The control circuit controls the rectifi cation of the supply voltage in a way that lower inrush currents can be achieved and so that mains interference can be limited to values of less than 40% (fi fth harmonic). This results in the following characteristics: • Lower building cost • No charging circuit required • More compact and lower weight construction • Susceptibility to supply system voltage dips.
That is, the FC is more likely to trip in the event of voltage dips, due to transients in the supply system • Mains interference can occur in the high frequency spectrum • The high ripple associated with the intermediate circuit reduces the output voltage by approximately 10% and results in higher motor power consumption • The restart time for operation may be longer, due to three processes occurring: – Re-initialisation of the FC – Magnetisation of the motor – Ramping up to the required reference for the application 2.5 Inverter The inverter is the last of the main elements making up the FC.
The inverter processes represent the fi nal stage in terms of generating the output voltage and frequency. When the motor is connected directly to the mains, the ideal operating conditions apply at the rated operating point. The FC guarantees good operating conditions throughout the whole speed range by adapting the output voltage to the load conditions. It is thus possible to maintain the magnetisation of the motor at the optimal value. From the intermediate circuit, the inverter obtains one of the following: • Variable direct current • Variable DC voltage • Constant DC voltage In each case, the inverter must ensure that the supply to the motor is an AC voltage.
In other words, the frequency of the motor voltage must be generated in the inverter. The inverter control method depends on whether it receives a variable or a constant value. With a variable current or voltage, the inverter only needs to generate the