Carrier WEATHERMAKER 48/50AJ User Manual

Page 47

Table 62 — Run Status Cool Display

Slow Change Cap Override (SLOW) — With a rooftop unit,

the design rise at 100% total unit capacity is generally around

 F. For a unit with 4 stages, each stage represents about

7.5

F of change to EDT. If stages could reliably be cycled at

very 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 the

PID under control when “relatively” close to set point.
SumZ Operation — The SumZ algorithm is an adaptive PID

style of control. The PID is programmed within the control and

the relative speed of staging can only be influenced by the user

through the adjustment of the Z.GN configuration. The capaci-

ty control algorithm uses a modified PID algorithm, with a self

adjusting gain which compensates for varying conditions, in-

cluding changing flow rates across the evaporator coil.

Previous implementations of SumZ made static assump-

tions about the actual size of the next capacity jump up or

down. This control uses a “rise per percent capacity” technique

in the calculation of SumZ, instead of the previous “rise per

stage” method. For each jump, up or down in capacity, the

control will know beforehand the exact capacity change

brought on. Better overall staging control can be realized with

this technique.
SUM Calculation — The PID calculation of the “SUM” is

evaluated once every 80 seconds.
SUM = Error + “SUM last time through” + (3 * Error Rate)

Where:

SUM = the PID calculation, Error = EDT – Cooling Control

Point, Error Rate = Error – “Error last time through”
NOTE: “Error” is limited to between –50 and +50 and “Error

rate” is limited to between –20 and +20.

This “SUM” will be compared against the “Z” calculations

in determining whether cooling stages should be added or

subtracted.
Z Calculation — For the “Z” calculation, the control attempts

to determine the entering and the leaving-air temperature of the

evaporator coil and based upon the difference between the two

during mechanical cooling, and then determines whether to

add or subtract 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 the

evaporator coil is referred to as EDT (evaporator discharge

temperature). They are found at the local display under the

Temperatures



CTRL sub-menu.

The main elements to be calculated and used in the calcula-

tion of SumZ are:
1) the rise per percent capacity (R.PCT)

2) the amount of expected rise for the next cooling stage

addition

3) the amount of expected rise for the next cooling stage

subtraction

The calculation of “Z” requires two variables, Z.PLU used

when adding a stage and Z.MIN used when subtracting a stage.

They are calculated with the following formulas:
Z.PLU = Z.GN * (10 + (4*(–ADD.R))) * 0.6
Z.MIN = Z.GN * (–10 + (4*(–SUB.R))) * 0.6

Where:

Z.GN = configuration used to modify the threshold levels used

for staging (Configuration



COOL



Z.GN)

ADD.R = R.PCT * (C.CAP – capacity after adding a cooling

stage)
SUB.R = R.PCT * (C.CAP – capacity after subtracting a cool-

ing stage)

Both of these terms, Z.PLU and Z.MIN, represent a thresh-

old both positive and negative upon which the “SUM”

calculation must build up to in order to cause the compressor to

stage up or down.
Comparing SUM and Z — The “SUM” calculation is com-

pared against Z.PLU and Z.MIN.

• If “SUM” rises above Z.PLU, a cooling stage is added.

• If “SUM” falls below Z.MIN, a cooling stage is subtracted.

There is a variable called SMZ which is described in the

reference section and which can simplify the task of watching

the demand build up or down over time. It is calculated as

follows:

If SUM is positive: SMZ = 100*(SUM/Z.PLU)
If SUM is negative: SMZ = –100*(SUM/Z.MIN)

Mixed Air Temperature Calculation (MAT) — The mixed-

air temperature is calculated and is a function of the economiz-

er position. Additionally there are some calculations in the con-

trol which can zero in over time on the relationship of return

and outside air as a function of economizer position. There are

two configurations which relate to the calculation of “MAT.”

These configurations can be located at the local display under

Configuration



UNIT.

ITEM

EXPANSION

RANGE

UNITS

CCN POINT

WRITE STATUS

COOL

COOLING INFORMATION

C.CAP

Current Running Capacity

CAPTOTAL

CUR.S

Current Cool Stage

COOL_STG

REQ.S

Requested Cool Stage

CL_STAGE

MAX.S

Maximum Cool Stages

CLMAXSTG

DEM.L

Active Demand Limit

DEM_LIM

forcible

SUMZ

COOL CAP. STAGE CONTROL

SMZ

Capacity Load Factor

-100 – +100

SMZ

ADD.R

Next Stage EDT Decrease

ADDRISE

SUB.R

Next Stage EDT Increase

SUBRISE

R.PCT

Rise Per Percent Capacity

RISE_PCT

Y.MIN

Cap Deadband Subtracting

Y_MINUS

Y.PLU

Cap Deadband Adding

Y_PLUS

Z.MIN

Cap Threshold Subtracting

Z_MINUS

Z.PLU

Cap Threshold Adding

Z_PLUS

H.TMP

High Temp Cap Override

HI_TEMP

L.TMP

Low Temp Cap Override

LOW_TEMP

PULL

Pull Down Cap Override

PULLDOWN

SLOW

Slow Change Cap Override

SLO_CHNG

ITEM

EXPANSION

RANGE

CCN

POINT

DEFAULTS

UNIT

UNIT CONFIGURATION

MAT.S

MAT Calc Config

0 - 2

MAT_SEL

MAT.R

Reset MAT Table
Entries?

Yes/No

MATRESET