Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 237)

4-32 Commissioning
7000L-UM300I-EN-P – June 2013 7000 “C” Frame
SGCT Anode to Cathode Resistance
Performing an Anode to Cathode resistance test not only tests the
integrity of the SGCT but also the integrity of the sharing resistor.
An abnormal device resistance measurement will indicate either a
shorted device or damaged sharing resistor.
Using an ohmmeter, measure the anode to cathode resistance each
SGCT, looking for similar resistance values across each device. Easy
access from the anode to cathode is available by going from chill
block to chill block as shown in the diagram below:
SGCT
Cathode Chill Block
Anode Chill Block
Figure 4.4 – Anode to Cathode Resistance Test Points
An SGCT when not gated on is an open circuit. A healthy device
resistance value should be close to the value-sharing resistor,
however due to parallel resistances in the firing card, the resistance
value will be slightly lower.
Example: The resistance across the anode to cathode of a 1500
amp device may be 57 kΩ even though the sharing
resistor is 80 kΩ.
A failure of an SGCT can be detected by measuring a lower than
normal resistance value; one device in the converter may read 15 kΩ
whereas the rest of the devices in the converter measured close to 60
kΩ. This indicates a partially shorted device. A fully shorted device
will read closer to 0 Ω and will be quickly identified. If the SGCT is
found to be out of tolerance, refer to Chapter 6 –
Component
Definition and Maintenance for detailed instructions on how to
replace the SGCT assembly.