Watlow QPAC Modular SCR Power Control Service - Manual (Page 28)

28 WATLOW QPAC Service Manual
Troubleshooting, Chapter 5
SCRs
Testing SCRs in the Unit
A shorted SCR will show full uncontrollable line voltage at the T terminals. To test,
turn main power OFF, remove the L and T wires. On a QPAC-01, measure resis-
tance from L1 to T1. On a QPAC-32, measure resistance between L1 to T1 and
L3 to T3. On a QPAC-33, measure resistance between L1 to T1, L2 to T2, and L3
to T3. A reading of 5 ohms resistance or less indicates that the SCR is shorted. A
high resistance reading indicates that the SCR is good.
On a Q33 unit, a low reading will occur in one direction and a high reading in the
other direction. This is normal.
Installing an SCR Package
After removing the old SCR package, thoroughly clean the heat sink and the base
of the new SCR package to be installed. Add a thin coat of DOW-DC-4 thermal
joint compound, or equivalent, to the base of the heat sink or bottom of SCR pack-
age. Put in screws and tighten. See Chapter 1, page 5, for replacement parts and
torque settings.
Troubleshooting the Base and Control Cards
QPAC-01
The QPAC-01 main board provides power and FWZ (to detect zero crossover) to
the control board. The control board then outputs a pulse for each half cycle to
turn ON the instrument SCRs which in turn fire the power SCRs. Two windings of
the power transformer provide power for the instrument SCRs. See schematic 02-
0655 on Page 33.
QPAC-32
The master portion of the QPAC-32 operates the same as the QPAC-01. Once
SCR #2 fires, transformer T2 receives load voltage and T2B then turns on SCR #4
approximately 60° before zero crossing on the L3 phase. As soon as L3 goes neg-
ative, SCR #4 is ON. The next half cycle, SCR #1 is fired, and T2C turns on SCR
#3. Pin 5 on the Q32 base is connected to Pin 1. All phase angle cards have Pin
5 grounded, therefore if a phase angle card is used on a Q32 base, it will stay in
permanent emergency shut down. See schematic 02-0682 on Page 34.
QPAC-33
The QPAC-33 is a lot more complicated since it controls all three legs and can be
used for zero cross, or phase control. U6, U3A, and U3B make up a noise filter
and the zero cross detector (FWZ). This produces a negative going pulse at each
zero crossing for phase angle, and slightly after zero for zero cross firing. U1 and
U2 make up a phase locked loop (PLL) which runs at 256 times the full wave fre-
quency (30.72KH at 60 Hz or 25.6KH at 50 Hz). This divides each half cycle into
256 parts. When the control card sends out a pulse, U7A picks the correct half
cycle, and sends the pulse to U3D which starts counter #1 (U9) and counts to one
for a short delay. When U1 counts down, it sends a pulse to the output board to
fire SCR 1, and resets U3C, and sets U4A. This starts counter #2 (U10) which
counts to 85 (to give a 2.77 millisecond delay), and sends an output pulse to fire
SCR #2, resets U4B, sets U4D, and starts counter #3. Counter #3 U11) counts to
85, sends an output pulse to fire SCR #3, and resets U4C to finish the cycle.