Environmental Engineering Reference
In-Depth Information
T1
T3
D1
D3
v
x
+
i
o
DC
power
source
v
dc
v
i
v
o
T4
T2
D4
D2
Figure 4.29
Transistor bridge
v
i
T1&T2
Fundamental
AC component
t
T3&T4
Figure 4.30
Voltage waveform of a basic square wave inverter
The diodes, shown next to each of the transistors, make the circuit appear more complicated
but, in practice, this is not an issue. They are necessary in order to provide a path for the
current when the transistors are turned off.
Basic Square Wave
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 (Modifi ed Square Wave)
A minor modifi cation 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 modifi ed square wave,
but it is a much better approximation to a sine wave. Setting the switching angles to 60-120-
