Applying a very simple switching pattern to the transistors in the bridge circuit shown in Figure 4.29 allows it to produce a basic square wave as in Figure 4.30. A basic square wave can be used to supply some non-critical loads such as incandescent light bulbs and simple universal motors, but it is not suitable for the majority of AC loads, which are designed to operate from a sinusoidal supply.
The basic square wave has a sinusoidal component at the fundamental frequency, as shown in Figure 4.30, but it also has a very high harmonic content. These harmonics will typically cause excessive heating in induction motors and transformers. For the same reason, a basic square wave is not acceptable for grid connections.
Quasi-Sine Wave (Modified Square Wave)
A minor modification to the control system allows the bridge circuit to produce a quasi-sine wave. As shown in Figure 4.31, the quasi-sine wave is really just a modified square wave, but it is a much better approximation to a sine wave. Setting the switching angles to 60-120-
Fundamental AC component
60-120° eliminates the third harmonic completely and makes the waveform good enough for many practical loads. The switching angles may be adjusted slightly to control the RMS voltage, bearing in mind that this adjustment increases harmonics.