Carrier 33ZCVAVTRM Specifications download pdf (Page 240)

After making these adjustments, the output should be stable (not

oscillating) and the controlling sensor should be at or approaching

setpoint.

6. Adjust the Derivative Term.

Determine if the application requires a derivative term.

Normally, this would only be required when, after careful

adjustment of the submasters proportional gain and the

masters proportional and integral gains, excessive overshoot/

undershoot is observed. The purpose of the derivative term is

to reduce or eliminate overshoot in systems which have a very

rapid rate of change at the controlling sensor. Most HVAC

applications that use a master/submaster approach do not

respond that quickly, therefore the derivative is normally not

necessary or used. As such, the default value for the deriva-

tive gain is zero. The actual mathematical function of the

derivative term is two-fold; it will subtract from the value of

the proportional and integral calculation (thus reducing

overall output) when the controlling sensor is approaching

setpoint, and conversely, it will add to the proportional and

integral calculation (thus increasing the overall output) in

cases where the controlling sensor is drifting away from

setpoint. In cases where there is no change in the value of the

controlling sensor, (that is to say the change in loop error is

zero), the derivative will have no effect (the calculated deriva-

tive term is zero).

If after careful adjustment of the proportional and integral

gains, your application does require a derivative term (indi-

cated by excessive overshoot), set it to a value approximately

25% of the proportional gain, and re-test and re-adjust (by +/-

50% intervals) until overshoot is reduced to a satisfactory

level.

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Carrier 33ZCVAVTRM Specifications Page 240