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Marley
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Sigma 1000/1200 Cooling Tower
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Specifications: Options
24
Specification Value
to modulate fan speed maintaining set
point temperature. The drive's panel dis-
play shall be able to display the set-point
temperature and cold-fluid temperature
on two separate lines. The bypass shall
include a complete magnetic bypass
circuit and with capability to isolate the
VFD when in the bypass mode. Transfer
to the bypass mode shall be manual in
the event of VFD failure. Once the motor
is transferred to the by-pass circuit the
fan motor will run at constant full speed.
The bypass circuit will not modulate ON
and OFF based on fluid temperature. The
application must be able to handle very
cold fluid temperatures while the VFD
is in a by-pass mode. Operator controls
shall be mounted on the front of the
enclosure and shall consist of start and
stop control, bypass/VFD selection, Auto/
Manual selections, manual speed control.
To prevent heating problems in the fluid
cooloer fan motor and to assure proper
gear reducer lubrication the VFD system
shall de energize the motor once 25%
motor speed is reached and cooling is no
longer required. The fluid cooler manu-
facturer shall supply VFD start-up assis-
tance. Tower vibration testing throughout
the speed range is required to identify
and lockout any natural frequency vibra-
tion levels which may exceed CTI guide-
lines.
Marley Premium VFD System
6.4 Add the following paragraph in the
Mechanical Equipment section when
VFD is used as a stand alone system:
A complete UL listed Variable Speed
Drive system in a NEMA 12 indoor
or NEMA 3R outdoor enclosure shall
be provided. The VFD shall use PWM
technology with IGBT switching and
integrated bypass design. VFD output
switching shall not cause mechanical
issues with gearbox teeth or drive shafts.
The VFD shall catch a fan spinning in the
reverse direction without tripping. The
panel shall include a main disconnect
with short circuit protection and external
operating handle, lockable in the off posi-
tion for safety. The system shall include
a solid state, PI temperature controller
to adjust frequency output of the drive
in response to the tower fluid tempera-
ture. The temperature of the fluid and