Watlow 12LS Controller - Manual (Page 93)

5-2
5.1.5 DISTRIBUTED ZERO CROSSING
Distributed Zero Crossing [DZC] output is the other time proportioning output available from the
12LS. This output is primarily for very fast acting electrical heating loads using SSR's. The open
air heater coil is an example of a fast acting load. It should never be used with electro-mechanical
relays.
The combination of Distributed Zero Crossing and a solid state relay can approach the effect of
analog phase angle fired control at a reduced cost.
The DZC output is a TIME PROPORTIONING output where the controller decides for each
cycle of the AC line if the power should be on or off. There is no fixed cycle time since the
on/off decision is made for each AC cycle.
For example if the control output is 25% the power would be turned on for 1 AC cycle and off
for the next 3 AC cycles. This pattern would repeat until the output level changed, for example to
28%. The power would then be on for 1 AC cycle and off for 3 cycles, then after repeating 1 on
and 3 off several times the power would be set on for 2 cycles and off for 2 cycles. The result is
after 100 cycles the power will have been on for 28 cycles and off for 72 cycles.
Since the time period for 60Hz power is 16.6ms the switching interval is very short and the
power is applied very uniformly. Switching is still only done at the zero crossing of the AC
power reducing the generated electrical noise.
5.2 TUNING PID LOOPS
The control loop to be tuned should be placed in auto after tuning. If, the loop is in auto and
controlling, to avoid upsetting the process place the control in manual. After the loop is placed
in manual, the PID values may be changed without upsetting the output. After tuning, placed the
loop back into auto. If, a small upset of the output is not important, than the PID constants may
be tuned while in auto.
When tuning, remember that time is a factor in most processes and especially in temperature
processes. Along with the dynamics of the process, the results may be slow to see. Allow time
between adjustments, before making new ones. 20 minutes is highly recommended for most
processes.
5.2.1 PROPORTIONAL CONTROL:
When using single mode PB only for control, set the TI and TD to OFF. The initial PB setting
may be obtained from the PID TUNING CONSTANTS Table. Normally using P only for control
will require a smaller PB than using PI and PID. A range of 3% to 10% of the SP is a useful
range for the PB with P only control.