Percent Capacity Vs. 4 To 20 Ma Signal; Space Temperature Control Mode Response For C.typ=7 - Carrier 38APS025 Manual

Sequence of Operation — Capacity control is determined by
the signal provided by the Building Management System. The
field-supplied 4 to 20 mA signal is converted to a linear scale
for percent capacity vs. 4 to 20 mA input as shown in Fig. 86.
100
90
80
70
60
50
40
30
20
10
0
2 5
mA SIGNAL
Fig. 86 — Percent Capacity vs. 4 to 20 mA Signal
While using this control method, the control algorithm
sending the 4 to 20 mA signal must be properly tuned to avoid
rapid cycling or hunting. The machine controls will select the
capacity stage with the least amount of error relative to the
Building Management System's requested percent capacity.
This may result in a capacity stage greater than the requested
percent capacity. Circuit loading methods are illustrated in
Fig. 87-139.
If the unit has a digital compressor and the Building Man-
agement System is requesting a capacity percentage less than
the capacity that the first compressor can provide, the MBB
waits until the error between the requested capacity and the
unit's capacity with the first compressor is less than the re-
quested capacity and zero percent capacity before the control
will start the digital compressor.
Error is calculated by the following formula:
([% Capacity – Requested % Capacity]/Requested Percent
Capacity) x 100
For simplicity to illustrate this point, the 38APD030 is used.
This unit uses two compressors of the same capacity. In order
to get the digital compressor to start, the requested percent ca-
pacity signal must be above 8 mA corresponding to 25% ca-
pacity. At 8 mA, the error between 0% and 25% is the same as
the error between 25% and 50% (the compressor's percent ca-
pacity contribution is the same). Once the signal exceeds the
8 mA value, the first compressor is allowed to start. See
Fig. 88. The digital compressor will start and operate at full ca-
pacity for 90 seconds before reducing the compressor capacity
to meet the 25% unit capacity (8 mA) request.
The digital compressor will run at full capacity for 90 sec-
onds before adjusting the capacity to the requested stage, for oil
return. Following the 90-second period the control will begin
to duty cycle the digital compressor to better match the load.
Once the digital compressor reaches full load, the control will
begin adding or subtracting compressors every 90 seconds to
reach the desired capacity stage. If the 4 to 20 mA signal is
lost, the MBB will reduce the capacity stage to 0, and generate
an alarm.
For mechanical cooling, the unit's Control Method (Config-
uration OPT2 CTRL) and inputs must allow the machine
to run. See Control Methods on page 60 for additional infor-
mation.
On power up or changing from Off to Enabled, the machine
will remain off until Minutes Off Time (Configura-
tion OPT2 DELY) timer has expired. See Minutes Off
Time on page 128 for additional information. Liquid Line So-
lenoid Valve operation is as described in Liquid Line Solenoid
Valves on page 40. Time Guard is honored for all compressors.
10 15 20
For specific information on Time Guard, see MDTG - Time
Guard Active on page 135.
In this Control Type, Space Temp Control Mode, Run
Status VIEW SPT.M=0 (Off Cool)
Status VIEW STAT=1 (Off Local), 2 (Off CCN), 3 (Off
Time) or 4 (Off Emrgcy). SPT.M=3 (Cool On) as long as
Control Mode, STAT=5 (On Local), 6 (On CCN), or 7 (On
Time). In this Control Type, if the Indoor Fan Status Switch
opens (Inputs GEN.I ID.F.A=OFF),
(STAT) will change to 8 (IDFS Not On) as long as there is no
call for cooling (CL.MA < 4 mA). If there is a call for cooling
(CL.MA > 4 mA), the unit will alarm and STAT switch to 4
(Off Emrgcy). Table 34 shows the space temperature control
mode response for C.TYP=7.
Table 34 — Space Temperature Control Mode
Response for C.TYP=7
22
CONTROL MODE
STAT
1 (Off Local)
2 (Off CCN)
3 (Off Time)
4 (Off Emrgcy)
5 (On Local)
6 (On CCN)
7 (On Time)
8 (IDFS Not On)
9 (SPT Satisfied)
38APD Units — On a call for cooling, the Indoor Fan Status
Cir A (Inputs GEN.I ID.F.A) is checked. The switch must
be closed before the capacity routine will start. The lead circuit
is determined. See Lead/Lag Determination on page 112. The
lead compressor will be determined and started. See Circuit
Compressor Staging on page 112. If the lead compressor is a
digital compressor and is enabled, the compressor will start ful-
ly loaded for 90 seconds prior to starting to cycle between load-
ed and unloaded.
The unit will stage compressors every 90 seconds to meet
the requested capacity. With every change in capacity that oc-
curs, a 90-second time delay is initiated and the capacity stage
is held during this time delay. If the capacity change is a change
of the digital compressor only, the compressor will be modulat-
ed to the requested capacity without moving in increments.
For example, if the requested capacity requires a change to the
compressor unload time from 8 to 2 seconds, that change will
occur without the intermediate steps of 7, 6, 5, 4, and 3 sec-
onds. If additional capacity requires the lag circuit to start, the
lag circuit's lead compressor will be determined and started.
During operation, the controls will stage the condenser fans to
maintain head pressure. See Head Pressure Control on
page 117.
As part of normal operation, if a single compressor in a mul-
tiple compressor circuit (38APD040-130) operates for the cu-
mulative time of 60 minutes without an increase in circuit ca-
pacity, an oil recovery routine is initiated. When this routine is
initiated, a second compressor in the circuit is started. Once
started, the normal capacity control routine takes priority and
will turn off the compressor if needed.
As the capacity requirement begins to decrease, the ma-
chine controls will reduce the unit's capacity in response to the
requested capacity signal. If the unit is equipped with Mini-
mum Load Control, it will not be active until the unit is on its
last stage of capacity.
77
as long as Run
Control
INDOOR FAN SPACE TEMPER-
STATUS CIR A ATURE CONTROL MODE
ID.F.A SPT.M
Off or On 0 (Cool Off)
Off or On 0 (Cool Off)
Off or On 0 (Cool Off)
Off or On 0 (Cool Off)
On 3 (Cool On)
On 3 (Cool On)
On 3 (Cool On)
Off 3 (Cool On)
Not Applicable
Mode