Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 314)

6-24 Component Definition and Maintenance
7000-UM150I-EN-P – June 2013 7000 “B” Frame
In addition to the snubber circuit, a sharing resistor is connected in
parallel with the SGCT. The function of the sharing resistor is to
ensure the voltage is shared equally among the SGCTs when
connected in series. SGCTs are connected in series to increase the
total reverse voltage blocking (PIV) capacity as seen by the electrical
circuit. A single SGCT has a PIV rating of 6.5 kV. This single
device will provide sufficient design margin for electrical systems
with 2.3 kV medium voltage supply. At 4.16 kV, 2 SGCTs must be
connected in series to provide a net PIV of 13 kV to achieve the
necessary design margin. Similarly, three SGCTs must be connected
in series at 6.6 kV.
The cooling requirements of the SGCT are achieved by placing the
SGCT between two forced air-cooled heatsinks, one heatsink on the
anode and the other heatsink on the cathode. The force placed on the
SGCTs differs with the size of the device. The clamp assembly on
the right hand side of the inverter module generates these forces.
Pressure on the SGCTs must be uniform to prevent damage and to
ensure low thermal resistance. Uniform pressure can be achieved by
loosening the heatsink mounting bolts, tightening the clamp and then
tightening the heatsink bolts. See section “Uniform Clamping
Pressure” for instructions.
External filtered air will be directed through the slots of the heatsinks
to carry away the generated heat from the SGCTs. The door filter is
necessary to ensure the slots on the heatsinks do not get plugged.
Uniform Clamping Pressure
It is very important to maintain proper pressure on the thyristors.
Follow this procedure whenever changing devices, or whenever the
clamp is loosened completely.
1. Apply a thin layer of Electrical Joint Compound (Alcoa EJC No. 2
or approved equivalent) to the clamp head pressure pad face.
2. Torque the heatsink bolts to 13.5 N-m (10 ft-lb.) and then loosen
each bolt two complete turns.
3. Tighten the clamp to the proper force until the indicating
washers can just be turned by the fingers with some resistance.
4. Torque the heatsink bolts to 13.5 N-m (10 ft-lb.) starting with the
center heatsink and moving outward alternating left to right.
5. Check clamp indicating washer.