Compressor Starting - Johnson Controls York YD Manual

You can performs a lockout on a compressor to prevent it from running. Only one compressor at a
time can be locked out.
Two CM-2 current boards, one for each compressor motor, provides motor Overload and Power
Fault protection for units with E-M starters. Units with SSS or VSDs have motor Overload and Power
Fault protection built in to the starter.
The oil system consists of a common oil sump for both compressors. There are two oil heaters
in the sump. These maintain proper sump temperature when both compressors are shut down.
This temperature is monitored by the oil temperature thermistor. There are two oil pumps, one
for each compressor. Each is driven by a variable speed oil pump (VSOP) drive. The oil pressure
is monitored by a common sump oil pressure transducer monitoring the low side pressure. Two
pump oil pressure transducers, one for each compressor, monitor the high side oil pressure for
each compressor.
The Chiller Current Limit setpoints are used to specify the motor current limit for the chiller as a
whole. There are two setpoints: the Chiller Pulldown Demand Limit setpoint (in effect during the
chiller pulldown period) and the Chiller Current Limit setpoint (in effect after the chiller pulldown
period). These setpoints are programmable over the range of 30% to 100% FLA. The setpoints
applied to the Chiller Full Load Amp (FLA) rating, which is the combined total of the full load amp
ratings of both motors. Each motor provides one-half of the Chiller Full Load Amps. For example,
if the Chiller Full Load Amps is 1600 A, the Full Load Amps of each motor is 800 A. If the Chiller
Current Limit setpoint is 70%, the total chiller motor current is limited to 1120 A. If both motors
are running, each would be limited to 560 A, providing a chiller total of 1120 A. If only one motor
is running, it is allowed to operate at twice the Chiller Current Limit, up to 100%. In this example,
with the total chiller current limited to 1120 A, one running motor would be allowed to operate at its
100% FLA rating of 800 A.

Compressor starting

Both compressors are not started at the same time. The lead compressor is started first. If it does
not meet the load demand during the chiller pulldown period and later during the chiller steady
state period, the lag compressor is brought on line.
There are three features that can be used to prevent the second compressor from being brought
online.
• Use the Compressor Lockout setpoint to lock out either compressor. The compressor
designated for lockout canot run.
• The Compressor Mode setpoint selects Staged, Dual, or Single Compressor mode of
operation. In Normal Mode, the lag compressor is brought online, if necessary, to meet the
load requirement as described below. In Single Mode, the lag compressor is not brought
online, regardless of the load requirement.
• Use the Lead Compressor Pulldown Demand Limit setpoint to load the chiller slowly. It
prevents the Lag compressor from coming on line for a specified period of time while limiting
the motor current of the Lead compressor to a specified value during that time period. These
features are detailed separately below.
If equipped with the compressor variable geometry diffuser (VGD), the chiller is started with the
VGD in the fully open position.
Upon receipt of a chiller start signal, the lead compressor is selected. The compressor with the
least amount of run time that is ready to run is selected as the lead compressor, unless it is locked
out. The lead compressor enters the prelube period (prelube duration determined by program
switch SW1-3) and the lead compressor discharge valve is opened. All normal prelube functions
are performed. If the lead compressor discharge valve does not fully open, as indicated by the
discharge valve limit switch, within 40 seconds, a Safety shutdown is performed and the message
10 Model YD Mod D with OptiView Control Center