YORK
5587001-YSG-A-520 | Start-Up & Operation Guide
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As cooling demand increases/decreases based on unit
configuration, the main control board communicates
the cooling demand via Modbus™ to condenser board
1. Condenser board 1 sends a 0–10 VDC signal to the
compressor VFD to increase/decrease compressor
speed based on cooling demand.
Envelope Control
Envelope control is a sequence that keeps the
variable speed drive compressor within the al-
lowable range of the operating conditions at the
superheat and sub-cooling values defined in the
envelope.
Operating the compressor at evaporating tem-
peratures that are higher than the specified tem-
peratures results in a higher oil circulation rate. A
higher then specified oil circulation rate can re-
duce heat exchanger efficiency and possibly re-
sult in oil pump-out.
To ensure the variable speed drive compressor
operates within the envelope, an offset (+ or -) is
added to the desired compressor speed whenev-
er the boundary conditions are crossed.
When compressor operation does not move back
inside the operating envelope within 5 minutes,
or if operating conditions continue to move fur-
ther outside the operating envelope, the variable
speed drive compressor shuts down.
To allow for pressure stabilization, envelope con-
trol is ignored for the first 60 seconds after initial
compressor start-up. When operating conditions
move back inside the envelope, the compressor
speed offset is removed after a delay, and the
compressor is allowed to run at the desired speed.
Oil Management
Oil management is designed to keep the oil level
above the minimum level required for variable
speed drive compressors.
Occasionally during low load conditions, the vari-
able speed drive compressor speed is reduced,
and this can cause oil to become trapped in the
refrigeration system. Temporarily increasing the
speed of the variable speed drive compressor
helps bring the trapped oil back to the variable
speed drive compressor.
The temporary speed boost can be triggered in
one of two ways:
Field changes to the VFD programming should not be
required. Any field changes to the factory programming
must be approved through the product technical sup-
port team.
Compressor Staging Sequences
The compressors are staged ON/OFF to maintain the
active DAT cooling setpoint in normal cooling operation.
NOTE:
During dehumidification operation, the com-
pressors are staged ON/OFF to maintain the
active evaporator air temperature (EAT) set-
point. This information is covered later in the
section
Both single speed and variable speed drive compres-
sors utilize the same staging sequences with a few mi-
nor differences.
Compressor Staging Algorithm
The unit controller utilizes the patented algorithms to
sequence compressor stages as a method of quickly
meeting the cooling demand while preventing over-
shoot.
When more than three compressors are installed, the
unit controller excludes staging combinations that re-
sult in one circuit having two or more compressors op-
erating than the other circuit.
Rapid Start
To reduce the time it takes to get compressors opera-
tion, the unit controller utilizes the optimum stage up
algorithm. This algorithm determines the number of
compressor steps required to meet the initial cooling
demand.
Rapid start is only utilized when:
• Power to the unit is turned OFF, then back ON
• When transitioning from UNOCC mode to OCC
mode
Rapid start is active for the first 5 minutes of cooling
operation. During rapid start, the inter-stage time delay
is reduced to 15 seconds.
Rapid start cannot be user disabled.
Variable Speed Drive Compressor Control
The variable speed drive compressor is always on cir-
cuit 1. If part of a tandem set, it is compressor 1A.
The variable speed drive compressor is controlled by a
VFD. The compressor manufacturer supplies the VFD.
