How does the inverter convert DC to three-phase AC with varying frequency and voltage?
The mechanism for converting DC to AC using an inverter is explained below. As shown in Fig. 13, when switches S1 and S4 are ON, and S2 and S3 are OFF, +E (V) is applied to the motor windings. On the other hand, when S1 and S4 are OFF and S2 and S3 are ON, –E (V) is applied. No voltage is applied when all the switches are off. Each switch can be turned on or off at any timing. For example, on/off switching at the timing shown in Fig. 13 (a), produces a voltage waveform that can be called AC. The average voltages are shown by the dotted line. The voltage is decreased by controlling the on/off switching as shown in (b), and the frequency is increased by controlling the on/off switching as shown in (c). For simplicity, the figure illustrates a single phase, but a three-phase circuit is actually used. The switches have evolved first from a thyristor to a smaller and lighter gate turn-off thyristor (GTO) that can be turned off without external voltage, and then to the insulated gate bipolar
