Rockwell Automation WebPak 3000 DC Drives Hardware Reference, Installation and Troubleshooting - Manual (Page 53)

Troubleshooting/Diagnostics 4-11
4.9 SCR Diagnostics and Adjusting Open SCR Sensitivity
Under normal drive operation, the load carried by each SCR is relatively equal, as shown in
figure 4.1.
Figure 4.1: SCR Armature Current During Normal Operation
If one or more SCRs fail to turn to on, a unique pattern of insufficient or missing current pulses
results, as shown in figure 4.2.
Open SCR diagnostics detects SCRs that are not conducting by analyzing the level of current
produced by each SCR pair firing. If one or more SCR consistently fails to produce current at a
level approximately equal to other SCRs that fired, the drive concludes that an open SCR has
occurred.
The open SCR diagnostic calculates the percentage deviation of current feedback for each pair
of SCRs from the average current feedback. The percent deviation must exceed the value set
for OPEN SCR SENSITIVITY (P.600) before the diagnostic proceeds to the next part of the
diagnostic.
In the next part of the diagnostic, deviations from the average current are integrated over time
to eliminate transient effects from the calculation. When the integration reaches the value of
OPEN SCR TRIP THRESH (P.601), an open SCR fault is generated.
If you are experiencing nuisance SCR faults, you can use these parameters to increase the
tolerance to conditions that could trigger an open SCR fault.
For a description of parameters OPEN SCR SENSITIVITY (P.600) and OPEN SCR TRIP
THRESH (P.601), refer to the WebPak Software Reference manual.