Watlow QPAC Modular SCR Power Control Service - Manual (Page 11)

WATLOW QPAC Service Manual
11 Operation Theory, Chapter 3
Q33
Three Phase, Three Leg - Q33
Power Base Operation
The Q33 uses three SCR/diode pairs
(hybrid) in inverse parallel to control a
resistive or inductive ungrounded load.
The Q33 has two boards. The main board
contains the power supply, phase locked
loop (PLL), phase counters, zero cross fil-
ter/detector, phase loss, and phase rota-
tion circuitry. The output board contains
the SCR protection, artificial neutral, and
output drivers. The Q33 accepts 208 to
480 volt transformers as standard.
600VAC must be special ordered.
The Q33 is intended for use on phase control although it can be used on zero
cross (with some loss of output), by moving one jumper to shift the zero cross
notch.
The Q33 is much more complicated than the Q32, since it controls all three leg's
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 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
frequency. (30.72KH at 60 Hz or 25.6KH at 50Hz). This divides each half cycle
Figure 7 -
Q33 Board Layout
and Power Board
zero cross jumper