Watlow Series 920 Microprocessor-Based Ramping Control - Manual (Page 34)

Tuning
For optimum control performance, tune the 920 to a thermal system. The tuning
settings here are meant for a broad spectrum of applications; your system may
have somewhat different requirements. Refer to the key flow charts and dia-
grams and definitions on Pages 41,45 - 47 for prompt location and description.
NOTE:
When tuning in the Heat
mode, use
PID prompts followed by H. Set the set
point above ambient.
When tuning in the Cool mode, use PID prompts followed by C. Set the set
point below ambient.
1. Apply power to the Series 920 and enter a set point in the SYSTEM
menu. Next, go into the SETUP mode, and enter ACCESS (1), the PID
parameter Using the MODE, UP/DOWN, and ENTER keys, set the Operat-
ing Parameters initially: (Proportional Band) PB H (C) = 0, (Reset) RS H (C)
= 0.00, (Rate) RT H (C) = 0.00, (Cycle Time) CT H (C) = 5, (Rate band) RBH
(C) = 2. Under ACCESS (0) enter CAL = 0.
Allow actual process temperature to stabilize at or near Set Point (SP). The
ACTUAL display will indicate when the load is stabilized near set point.
2. Proportional Band Adjustment: Gradually increase PB until the ACTUAL
display temperature stabilizes to a constant value. The temperature will not
be right on set point because the initial reset value is 0.00 repeats per
minute. When tuning in the heat mode, the ACTUAL temperature will
stabilize below the desired set point. When tuning in the cooling mode, the
ACTUAL temperature stabilizes above set point. (When PB = 0, RS, RT,
and CT are inoperative, and the 920 functions as a simple ON/OFF control
with a 3°F or 1.7°C switching differential.)
3. Reset Adjustment: Gradually increase RS until the ACTUAL display
temperature begins to oscillate or “hunt”. Then slowly decrease RS until the
ACTUAL display stabilizes again near set point NOTE: This is a slow
procedure, taking from minutes to hours to obtain optimum value.
4. Cycle Time Adjustment: Set CT as required. Optimum system control is
always achieved with faster cycle times. However, if a mechanical contactor
or solenoid is switching power to the load, a longer cycle time may be
desirable to minimize wear on the mechanical components. Experiment until
the cycle time is consistent with the quality of control you want.
5. Rate Adjustment: Increase RT to 1.00 min. Then raise SP by 20° to
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WATLOW Series 920 User’s Manual HowToTune The Series 92O,Chapter5