SPECIAL CHARACTERISTICS AFFECTING THE SELECTION
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3
3.2
Galvanic isolation requirement
The pulse width modulation (PWM) produces common mode voltage. Because every phase (a, b and
c) can be connected only either to positive DC-bus (+U
dc
/2) or to negative DC-bus (-U
dc
/2), sum of
output voltages is always unequal to zero. The common mode voltage (CM-voltage) U
cm
can be
calculated as average of output voltages:
Table 3 presents all possible common mode voltages produced by different switching states. Used
reference point is in the middle of the DC-link.
Table 3. Common mode voltage as function of modulation sequence
Switching vector
a
b
c
U
cm
U
1
+
-
-
-U
dc
/6
U
2
+
+
-
U
dc
/6
U
3
-
+
-
-U
dc
/6
U
4
-
+
+
U
dc
/6
U
5
-
-
+
-U
dc
/6
U
6
+
-
+
U
dc
/6
U
7
+
+
+
U
dc
/2
U
8
-
-
-
-U
dc
/2
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