Watlow QPAC Modular SCR Power Control Service - Manual (Page 10)

10
WATLOW QPAC Service Manual
Operation Theory, Chapter 3
Q32
Three Phase, Two Leg - Q32
Power Base Operation
The Q32 uses two pair of SCRs connected in inverse parallel to control two lines of
a three phase ungrounded, resistive load. The main board contains the power
supply, the output drivers, connections for the transformer and any of the zero
cross control cards. It is interlocked so that it will not allow any output if a phase
angle card is installed. Q32's will accept 208 to 480 volt transformers as standard.
600 volt units need to be special ordered.
The master portion of the Q32 operates the same as the Q01. Once SCR #2 fires,
transformer T2 receives load voltage and T2B then turns on SCR #4 approximately
60 degrees before zero crossing on L3 phase. As soon as L3 goes negative, SCR
#4 is ON. The next half cycle, SCR #1 is fired, and T2C turns on SCR #3. See
schematic 02-0682 on Page 34.
Power
Supply
FWZ
Control Card
Opto
Opto
T1
L1
Slave
Transformer
T3
L3
L2
VAC
3Ø
Heater
L3
L2
Load Circuit
SCR1
SCR2
SCR3
SCR4
T2
Figure 5 -
Q32 Board Layout
Figure 6 -
Q32 Block Diagram
and Typical Load
Circuit