10
OMM 1008-1
Use with On-Site Generators
Magnitude chillers have their total tonnage divided between two compressors (all but single compressor Model
WMC 145S) that start sequentially and they are operated with variable frequency drives. These features make
Magnitude 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.
Starting/Stopping Procedure:
The stopping of the chiller in the event of a power failure is typically 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 and 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.
1.
Set the generator to always run five minutes longer than the unit start-to-start timer, which could be set
from 15 to 60 minutes. The actual setting can be viewed on the operator interface panel on the
Setpoint/Timer screen.
2.
Configure the transfer switch, provided with the generator, to automatically shut down the chiller before
transfer is made. The automatic shut-off function can be accomplished through a BAS interface or with
the “remote on/off” wiring connection shown in
Figure 8
on page 29. A start signal can be given anytime
after the stop signal since the three-minute start-to-start timer will be in effect.
Chiller Control Power:
For proper operation on standby power, the chiller control power must remain as
factory-wired from a unit-mounted transformer. Do not supply chiller control power from an external power
source because the chiller may not sense a loss of power and do a normal shutdown sequence.
Multi-Chiller Setup
Component Description
Communication Setup
The communication wiring and setup required for dual compressor operation is performed in the
factory and should be reviewed when the chiller is initially started after installation or if there is any
change made in the chiller control hardware.
RS485 communication wiring between chillers should be field wired before start-up and installed as
a NEC Class 1 wiring system.
