Carrier 48AJ Operating And Service Manual

Also see for 48AJ: Installation, Start-Up and Service Instructions Installation instructions Operation and service guide Started guide

44Table 56 — Run Status Cool DisplayNext Stage EDT Increase (SUB.R) — This variable repre-sents (if subtracting a stage of cooling) how much the tempera-ture should rise in degrees depending on the R.PCT calculationand exactly how much capacity is to be subtracted.SUB.R = R.PCT * (C.CAP — capacity after subtracting acooling stage)For Example: If R.PCT = 0.2 and the control would be sub-tracting 30% capacity by taking the next step down, 0.2 times–30 = –6 F (SUB.R)Rise Per Percent Capacity (R.PCT) — This is a real time cal-culation that represents the amount of degrees of drop/riseacross the evaporator coil versus percent of current runningcapacity.R.PCT = (MAT – EDT)/ C.CAPCap Deadband Subtracting (Y.MIN) — This is a control vari-able used for Low Temp Override (L.TMP) and Slow ChangeOverride (SLOW).Y.MIN = -SUB.R*0.4375Cap Deadband Adding (Y.PLU) — This is a control variableused for High Temp Override (H.TMP) and Slow ChangeOverride (SLOW).Y.PLU = -ADD.R*0.4375Cap Threshold Subtracting (Z.MIN) — This parameter isused in the calculation of SumZ and is calculated as follows:Z.MIN = Configuration→COOL→Z.GN * (–10 + (4*(–SUB.R))) * 0.6Cap Threshold Adding (Z.PLU) — This parameter is used inthe calculation of SumZ and is calculated as follows:Z.PLU = Configuration→COOL→Z.GN * (10 + (4*(–ADD.R))) * 0.6High Temp Cap Override (H.TMP) — If stages of mechani-cal cooling are on and the error is greater than twice Y.PLU,and the rate of change of error is greater than 0.5° F per minute,then a stage of mechanical cooling will be added every 30 sec-onds. This override is intended to react to situations where theload rapidly increases.Low Temp Cap Override (L.TMP) — If the error is less thantwice Y.MIN, and the rate of change of error is less than–0.5° F per minute, then a mechanical stage will be removedevery 30 seconds. This override is intended to quickly react tosituations where the load is rapidly reduced.Pull Down Cap Override (PULL) — If the error from setpoint is above 4° F, and the rate of change is less than –1° F perminute, then pulldown is in effect, and “SUM” is set to 0. Thiskeeps mechanical cooling stages from being added when theerror is very large, but there is no load in the space. Pulldownfor units is expected to rarely occur, but is included for the raresituation when it is needed. Most likely pulldown will occurwhen mechanical cooling first becomes available shortly afterthe control goes into an occupied mode (after a warm unoccu-pied mode).Slow Change Cap Override (SLOW) — With a rooftop unit,the design rise at 100% total unit capacity is generally around30° F. For a unit with 4 stages, each stage represents about7.5° F of change to EDT. If stages could reliably be cycled atvery fast rates, the set point could be maintained very precisely.Since it is not desirable to cycle compressors more than 6 cy-cles per hour, slow change override takes care of keeping thePID under control when “relatively” close to set point.SumZ Operation — The SumZ algorithm is an adaptive PIDstyle of control. The PID is programmed within the control andthe relative speed of staging can only be influenced by the userthrough the adjustment of the Z.GN configuration, described inthe reference section. The capacity control algorithm uses amodified PID algorithm, with a self adjusting gain whichcompensates for varying conditions, including changing flowrates across the evaporator coil.Previous implementations of SumZ made static assump-tions about the actual size of the next capacity jump up ordown. This control uses a “rise per percent capacity” techniquein the calculation of SumZ, instead of the previous “rise perstage” method. For each jump, up or down in capacity, thecontrol will know beforehand the exact capacity changebrought on. Better overall staging control can be realized withthis technique.SUM Calculation — The PID calculation of the “SUM” isevaluated once every 80 seconds.SUM = Error + “SUM last time through” + (3 * Error Rate)Where:SUM = the PID calculationError = EDT – Cooling Control PointError Rate = Error – “Error last time through”NOTE: “Error” is clamped between –50 and +50 and “Errorrate” is clamped between –20 and +20.This “SUM” will be compared against the “Z” calculationsin determining whether cooling stages should be added orsubtracted.Z Calculation — For the “Z” calculation, the control attemptsto determine the entering and the leaving-air temperature of theevaporator coil and based upon the difference between the twoduring mechanical cooling, determines whether to add orsubtract a stage of cooling. This is the adaptive element.The entering-air temperature is referred to as MAT(mixed-air temperature) and the leaving-air temperature of theevaporator coil is referred to as EDT (evaporator dischargetemperature). They are found at the local display under theTemperatures→CTRL sub-menu.ITEM EXPANSION RANGE UNITS CCN POINT WRITE STATUSCOOL COOLING INFORMATIONC.CAP Current Running Capacity % CAPTOTALCUR.S Current Cool Stage COOL_STGMAX.S Maximum Cool Stages CLMAXSTGDEM.L Active Demand Limit % DEM_LIM forcibleSUMZ COOL CAP. STAGE CONTROLSMZ Capacity Load Factor -100 – +100 SMZADD.R Next Stage EDT Decrease ^F ADDRISESUB.R Next Stage EDT Increase ^F SUBRISER.PCT Rise Per Percent Capacity RISE_PCTY.MIN Cap Deadband Subtracting Y_MINUSY.PLU Cap Deadband Adding Y_PLUSZ.MIN Cap Threshold Subtracting Z_MINUSZ.PLU Cap Threshold Adding Z_PLUSH.TMP High Temp Cap Override HI_TEMPL.TMP Low Temp Cap Override LOW_TEMPPULL Pull Down Cap Override PULLDOWNSLOW Slow Change Cap Override SLO_CHNG