Malfunctions Not Indicated by a Fault 3-25
2. Check the Anode to Cathode junction of each SCR module. With a DVM
on the 1 Megohm scale, measure the resistance across the SCR modules
as follows: (Note lead orientation is not critical). For Low kVA 1395
If a measurement results in an “infinity” reading, check F4 and F6 for
blown fuses.
3. Check the gate to cathode junction of each SCR module. With the DVM
on the 1K ohm scale, measure resistance of each junction as follows:
(lead orientation is not critical)
Loosen the four captive thumb screws on the feedback board and lift the
board to gain access to the field pulse/snubber board. Remove the gate
leads to the SCR modules. Measure the following resistances:
All resistance measurements should be between 15 to 60 ohms. If the
measurement is outside of this range or if a significant deviation among
devices is measured, module replacement may be necessary.
Field Pulse/Snubber Boards (Series A)
The field pulse/snubber boards provide isolation of the gate pulse firing
circuit and also provide DV/DT protection for the SCR modules. A
malfunction of these devices will be evident in either an overcurrent related
fault, blown or tripped incoming protection devices, or erratic motor
operation. Follow the procedure below if a board malfunction is suspected:
1. Disconnect and lock–out ALL incoming voltage sources.(See above)
Measurement Point Measurement
TB1–1 to TB2–1 greater than 500k ohms
TB1–5 to TB2–1
TB1–1 to TB2–2
TB1–5 to TB2–2
Module Measurement Points
PM7 G1 (left side of fld/snubber board) to TB2-1 G2 (left side of fld/snubber board) to
TB2-2
PM8 G1 (right side of fld/snubber board) to
TB1-5
G2 (right side of fld/snubber board) to
TB1-1
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