53 Frequency Converters corresponding frequency. With a constant voltage, the inverter generates both the frequency and amplitude of the voltage. Even though inverters work in diff erent ways, the basic design is always the same. The main components are controlled semiconductors, arranged in pairs in three branches, as shown in Fig. 2.3 Main component topologies. Transistors are increasingly taking the place of thyristors in the inverter stage of FCs for several good reasons. Firstly, transistors are now available for large currents, high voltages and high switching frequencies. Furthermore, unlike thyristors and diodes, transistors are not aff ected by the current zero crossing.
Transistors can enter the conducting or blocking state at any time simply by changing the polarity of the voltage applied to the control terminals. The advances made in the fi eld of semiconductor technology over recent years have increased the switching frequency of transistors signifi cantly. The upper switching limit is now several hundred kHz. Thus, magnetic interference caused by pulse magnetisation within the motor can be avoided. Another advantage of the high switching frequency is the fact that it allows variable modulation of the FC output voltage. This means that a sinusoidal motor current can be achieved, as shown in Fig. 2.12 Eff ect of switching frequency on motor current.
The control circuit of the FC merely has to switch the inverter transistors on and off in accordance with a suitable pattern. Fig. 2.12 Eff ect of switching frequency on motor current