Environmental Engineering Reference
In-Depth Information
This concept has the disadvantage that the output voltage level is directly pro-
portional to the fluctuations of the battery voltage between, for instance, a final
discharge voltage of 11 V and a gassing voltage of 15.5 V. For a 12 V lead accu-
mulator and assuming a constant transmission ratio for the transformer, the output
voltage covers a range from 210 to 297 V.
The so-called trapezoid or "quasi-sine" inverter, based on the same circuit prin-
ciple, avoids this disadvantage. However, its output voltage contains a blanking
interval. An appropriate controlling circuit ensures that the width of the blanking
interval is adjusted, so that even for different input voltages an almost constant
actual output voltage can be achieved.
S1
S2
S1
S2
S1
S2
S3
S3
S3
S1
S1
S1
230 V
50 Hz
230 V
50 Hz
230 V
50 Hz
t
t
t
S3
S4
S3
S4
S3
S4
S4
S4
S4
S2
S2
S2
t
t
t
Fig. 6.18
Circuit principle of a rectangular inverter
During operation of such inverters, it always has to be verified whether the in-
tended consumers, generally designed for sine-shaped voltages, can perform re-
liably under the given voltage conditions. Generally, the supply of electrical
power with such properties for light bulbs, irons or other simple consumers, such
as drilling machines, does not cause any problems. However, if such consumers
are equipped with transformers or capacitive voltage dividers at the power entry,
noise, considerable additional losses or even deterioration of the consumers may
occur. Some electronic appliances (e.g. washing machines) should not be directly
connected to these inverters, since they require the defined zero-crossing of sine-
shaped voltages for their internal control system.
The output voltage of the sine inverter, by contrast, is identical to that of the
public supply system, so that all consumers can be provided with power useable
without any problems. Out of the great variety of topologies or circuits Fig. 6.19
shows the principle of a pulse width modulated (PWM) inverter.
Unlike rectangular inverters the input voltage is "chopped" at much higher fre-
quencies (some 10 kHz to a few 100 kHz). For the positive half-wave of the sine
oscillation, shown in Fig. 19 switch S1 is continuously closed while switch S2 is
activated and deactivated at high frequencies with a variable pulse/pause ratio
(pulse width modulation, PWM). An appropriate pulse pattern ensures a rectangu-
lar voltage at the bridge circuit exit, whose timely average curve has a sine shape.
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