Rockwell Automation 23P PowerFlex DC Stand Alone Regulator and Gate Amplifier - Manual (Page 65)

Rockwell Automation Publication 23P-UM001D-EN-P - July 2012
65
Chapter
5
Power Interface and SCR Firing Order
Parallel Power Modules
and Load Share Reactors
When power modules are placed in parallel, load share reactors must be in place
on the AC side of the power modules on each of the phases (see
Figure 20 on
page 53 ). The purpose of the load share reactors is to ensure that any one of the
group of parallel power modules does not take up more than “its share” of the
load. When one of the parallel legs of the power module circuit has an increase in
current, an increased voltage drop will appear across the load share reactor in that
leg of the circuit. The increased voltage drop of the reactor will result in a reduced
current in that leg of the circuit. The reactors provide a self regulating passive
means to ensuring balance of current between the parallel power modules. The
inductance of the load share reactors is typically 3.5
µH. For retrofit situations,
the site review must cover this issue. For new installations, the reactors need to be
included in the order.
If reactors are not used, then current sharing cannot be expected to work
effectively. As Thyristors (SCRs) increase in temperature from additional
current, their voltage drop may actually decrease due to their construction as two
bipolar transistors (parallel bipolar transistors do not share load very well unless
they are “matched”). This causes less voltage drop to occur across the SCR and
thus more current will result in that leg of the circuit and the SCR could
ultimately fail due to a condition termed “thermal runaway”.
Phase Sequence and
Armature SCR Firing Order
Table 25 - Phase Sequence and Armature SCR Firing Order, Sequence L1, L2, L3
Table 26 - Phase Sequence and Armature SCR Firing Order, Sequence L3, L2, L1
AC Line Pairs Gated Pairs
+ -
L1 L3 4THY & 3THY
L2 L3 5THY & 3THY
L2 L1 5THY & 1THY
L3 L1 6THY & 1THY
L3 L2 6THY & 2THY
L1 L2 4THY & 2THY
AC Line Pairs Gated Pairs
+ -
L1 L2 4 THY & 2 THY
L3 L2 6 THY & 2 THY
L3 L1 6 THY & 1 THY
L2 L1 5 THY & 1 THY
L2 L3 5 THY & 3 THY
L1 L3 4 THY & 3 THY