Specifications

All interlock contacts must be rated for no less
than 10 inductive amps. The alarm circuit
provided in the control center utilizes 115-volts
AC. The alarm used must not draw more than 10-
volt amperes.
Vibration Mounting
The WMC chillers are almost vibration-free.
Consequently, floor mounted spring isolators are
not usually required. Rubber mounting pads are
shipped with each unit. It is wise to continue to
use piping flexible connectors to reduce sound
transmitted into the pipe and to allow for
expansion and contraction.
Use with On-Site Generators
WMC chillers have their total tonnage divided
between two compressors that start sequentially
and they are operated with variable frequency
drives. These features make WMC chillers
especially appropriate for use in applications
where they may be required to run with on-site
electrical generators. This is particularly true when
the generators are used for temporary power when
the utility power is lost.
System Water Volume
All chilled water systems need adequate time to
recognize a load change, respond to that load
change and stabilize, without undesirable short
cycling of the compressors or loss of control. In
air conditioning systems, the potential for short
cycling usually exists when the building load falls
below the minimum chiller plant capacity or on
close-coupled systems with very small water
volumes.
Generator Sizing: Gas and diesel generators are
sensitive to the compressor’s locked-rotor
characteristics when the chillers start up. Use the
electrical data supplied with the performance
output sheet, obtained from the McQuay sales
office, for generator sizing purposes. The chiller
data sheet will show the RLA, which is for both
compressors. Refer to Electrical Data to determine
the LRA, based on the RLA. It is important to size
the generator to handle the LRA at start up.
Some of the things the designer should consider
when looking at water volume are the minimum
cooling load, the minimum chiller plant capacity
during the low load period and the desired cycle
time for the compressors.
Assuming that there are no sudden load changes
and that the chiller plant has reasonable turndown,
a rule of thumb of “gallons of water volume equal
to two to three times the chilled water gpm flow
rate” is often used.
Starting/Stopping Procedure: The stopping of
the chiller in the event of a power failure should
be uneventful. The chiller will sense a loss of
voltage and the compressors will stop, coasting
down using power generated from their dynamic
braking to maintain the bearing magnetic field.
The stop signal will initiate a three-minute stop-to-
start timer, effectively preventing compressor
restart for three minutes. The timer is adjustable
from three to fifteen minutes; the recommended
default value is three minutes. This interval allows
the generator sufficient time to get up to speed and
stabilize. The chiller will restart automatically
when the start-to-start timer expires.
A properly designed storage tank should be added
if the system components do not provide sufficient
water volume.
System Pumps
Operation of the chilled water pump can be to 1)
cycle the pump with the compressor, 2) operate
continuously, or 3) start automatically by a remote
source.
The cooling tower pump must cycle with the
machine. The holding coil of the cooling tower
pump motor starter must be rated at 115 volts, 60
Hz, with a maximum volt-amperage rating of 100.
A control relay is required if the voltage-amperage
rating is exceeded. See the Field Wiring Diagram
on page
Transfer Back to Grid Power: Proper transfer
from stand-by generator power back to grid power
is essential to avoid compressor damage.
37 or in the cover of control panel for
proper connections.
! WARNING
Stop the chiller before transferring supply power from the generator back to the
utility power grid. Transferring power while the chiller is running can cause severe
compressor damage.
42 Catalog WMC-4