Environmental Engineering Reference
In-Depth Information
v
i
T1&T2
Fundamental
AC component
T1&D3
t
D2&T4
T3&T4
Figure 4.31
Voltage waveform of a quasi - sine wave inverter
Fundamental
AC component of
output waveform
v
i
t
Figure 4.32
Pulse - width - modulated (PWM) inverter output voltage
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.
Pulse - width Modulation ( PWM )
For loads that are more harmonic-critical and for grid-connection,
pulse - width modulation
(PWM) allows the transistor bridge circuit shown in Figure 4.29 to produce an almost-pure
sine wave (low order harmonics are virtually eliminated) and provides full control of its
amplitude. In a pulse-width-modulated (PWM) inverter, the transistors are switched at a much
higher frequency than that of the intended output waveform. The width (duration) of the high
frequency pulses, having short width at the edges and increasingly longer width towards the
centre of the waveform, as shown in Figure 4.32, is controlled so as to create a good approxi-
mation of a sine wave output.
In order to provide a high quality sine wave, the switching frequency needs to be as
high as possible. If it is too high, however,
switching losses
will become signifi cant,
making the inverter itself less effi cient. Transistors when at their on or off state dissipate
very little power. It is during the periods of transition between the two states that they
are most lossy. For small MOSFET PWM inverters, switching frequencies up to 20 kHz
are typical. While PWM virtually eliminates the low order harmonics, there can be sig-
nifi cant harmonics around the switching frequency and its multiples, but these can readily
be fi ltered out.
Figure 4.33 illustrates one simple method that the inverter's internal control system may
use to create the switching signals. The pulses that switch the transistors on and off are gener-
ated at the intersections of the reference sine wave with the carrier wave which is usually
triangular. In a standalone inverter, the reference sine wave would be created by the internal
control system, normally to provide a constant voltage and constant frequency output. Refer-
ence [3] provides more information on this rather complex topic.
