Specifications
24 Centrifugal Chillers IMM WMC-2
8. Optional customer supplied 115 VAC, 25-VA maximum coil rated, chilled water pump relay (ep1 & 2) may be wired as shown.
This option will cycle the chilled water pump in response to chiller demand.
9. The condenser water pump must cycle with the unit. A customer supplied 115 VAC 25 VA maximum coil rated, condenser water
pump relay (CP1 & 2) is to be wired as shown. Units with free-cooling must have condenser water above 60°F before starting.
10. Optional customer supplied 115 VAC 25 VA maximum coil rated cooling tower fan relays (C1 - C2 standard, C3-C4 optional)
may be wired as shown. This option will cycle the cooling tower fans in order to maintain unit head pressure.
11. Auxiliary 24 VAC rated contacts in both the chilled water and condenser water pump starters must be wired as shown.
12. 4-20mA external signal for chilled water reset are wired to terminals 71 and 51 on the unit controller; load limit is wired to
terminals 71 and 58 on the unit controller.
13. Optional Control Inputs. The following 4-20 ma optional inputs are connected as shown:
• Demand Limit; Terminals 58 and 71 common
• Chilled Water Reset; Terminals 51 and 71 common
• Evaporator Water Flow; Terminals 59 and 71 common
• Condenser Water Flow; Terminals 60 and 71 common
14. Optional Control Power Source. 115 volt control power can be supplied from a separate circuit and fused at 20 amps inductive
load. Connection is to terminals 85 and 86 common.
15. 4-20 mA external signal for chilled water reset are wired to terminals 71 and 51 on the unit controller; load limit is wired to
terminals 71 and 58 on the unit controller.
Care must be taken when attaching leads to compressor terminals to assure proper sequencing and connection
torque.
Control Wiring
The control circuit on the McQuay centrifugal packaged chiller is designed for 115-volts. Control power is
supplied from a factory-wired transformer located in the electrical box.
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.
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 the electrical data on page 19 to determine the LRA, based on the RLA. It is important to
size the generator to handle the LRA at start up.
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, but 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.
Transfer Back to Grid Power: Proper transfer from stand-by generator power back to grid power is essential to
avoid compressor damage.
!
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..
The necessary procedure for reconnecting power from the generator back to the utility grid is show below.
These procedures are not peculiar to McQuay units only, but should be observed for any chiller manufacturer.