McQuay WMC Service Manual

www.DaikinApplied.com 37 IOM 1210-1 • MAGNITUDE
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MODEL WMC CHILLERS
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Compressor Capacity Control
Compressor capacity is determined by the status of the leaving
chilled water temperature (LWT), which is a direct indicator
of whether the chiller is producing enough cooling to satisfy
the cooling load. The LWT is compared to the active chilled
water setpoint, and compressor loading or unloading ensues,
considering any capacity overrides that may be in effect.
Capacity Overrides
The conditions described in the following subparagraphs
override normal capacity control when the chiller is in the
COOL mode. Of the following limits, the one creating the
lowest amp limit is in effect. The resulting present limit value
for compressor current is stored in the Active Demand Limit
variable.
Low Evaporator Pressure
If the evaporator pressure drops below the Low Evaporator
Pressure – Inhibit setpoint, the unit will inhibit capacity
increases. If the evaporator pressure drops below the Low
Evaporator Pressure - Unload setpoint, the unit will begin
capacity decreases.
High Discharge Temperature - Load
If the discharge temperature rises above the High Discharge
Temperature - Load setpoint and the Suction SuperHeat is <
15.0 °F, the unit will begin capacity increases.
Soft Load
Soft Loading is a congurable function used at compressor
startup to limit the maximum current draw on the compressor in
a ramp-up type manner. It is only active on the rst compressor
to start. The setpoints that control this function are:
• Soft Load – (ON/OFF)
• Begin Amp Limit – (%RLA)
• Maximum Amps – (%RLA)
• Soft Load Ramp – (seconds)
The active soft load limit value (in % RLA) increases linearly
from the Begin Amp Limit setpoint to the Maximum Amps
setpoint over the amount of time specied by the Soft Load
Ramp setpoint. If the amp draw rises above the currently active
soft load limit value, the unit will inhibit capacity increases. If
the amp draw rises to 3% or more above this value, the unit will
begin capacity decreases.
Maximum LWT Rate
The maximum rate at which the leaving water temperature
can drop (chiller mode = COOL) is limited at all times by the
Maximum Rate setpoint. If the rate exceeds this setpoint,
capacity increases are inhibited.
Demand Limit
The maximum amp draw of the compressor can be limited by
a 4 to 20 mA signal on the Demand Limit analog input. This
function is only enabled if the Demand Limit setpoint is set to
ON. The amp limit decreases linearly from the Maximum Amp
Limit setpoint (at 4 mA) to the Minimum Amp Limit setpoint (at
20mA). If the amp draw rises above the limit value, the unit will
inhibit capacity increases. If the amp draw rises to 3% or more
above this value, the unit will begin capacity decreases.
Network Limit
The maximum amp draw of the compressor can be limited by
a value sent through a BAS network connection and stored
in the Network Limit variable. If the amp draw rises above the
limit value, the unit will inhibit capacity increases. If the amp
draw rises to 3% or more above this value, the unit will begin
capacity decreases.
Minimum Amp Limit
The minimum amp draw of the compressor can be limited by
the Minimum Amps setpoint. If the amp draw drops below the
limit value, the unit will load capacity to maintain minimum
amps.
Maximum Amp Limit
The maximum amp draw of the compressor is always limited
by the Maximum Amps setpoint. This limit has priority over all
other functions including manual capacity control. If the amp
draw rises above the limit value, the unit will inhibit capacity
increases. If the amp draw rises to > 3% or more above this
value, the unit will begin capacity decreases.

McQuay WMC Specifications

General

Brand	McQuay
Model	WMC
Category	Chiller
Language	English