BENSHAW M2L 3000, User Manual

Page: 35 / 140

4 – Theory of Operation
35
Converter Details
The standard transformer consists of a single 3-phase primary winding, with three 3-
phase secondary windings that are phase displaced to produce an 18-pulse current
waveform in the primary winding. The primary winding includes taps to adjust for slight
variations in the incoming line voltage, to accommodate nominal, low, or high line con-
ditions. Variations of ±5% can be accommodated by jumpering the incoming AC cables
to the to +5% tap for high line conditions and the -5% tap for low line conditions.
Three secondary windings are physically shifted on the transformer core to yield volt-
ages that are phase displaced by ±20 electrical degrees with respect to the primary
winding. Figure 4 shows the schematic diagram of a typical transformer.
Each secondary winding is connected to a dedicated 6-pulse, full-wave bridge rectifier
that converts AC to DC, as shown schematically in Figure 7.
The DC terminals of all three (3) rectifier bridges are connected in series, such that the
appropriate output voltage is generated at the output of the VFD.
X9
X7
X8
X3
X1
X2
X4
X5 X6
+20o
0o
-20o
X4
X6
X5
X3
X2
X1
+5%
-5%
H2
H1
H2
H3
+5%
-5%
H3
+5%
-5%
H1
X7
X9
X8
Figure 7: Main Transformer
Figure 8: 6-Pulse Bridge Rectifier
12, 24 and 36 pulse converters are optionally avalilable