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Marley
/
NC Class Stainless Cooling Tower
/
Specifications: Options
17
cold-water temperature. The application
must be able to handle very cold water
while 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 cooling tower 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
cooling tower manufacturer shall supply
VFD start-up assistance. Tower vibration
testing throughout the speed range is
required to identify and lockout any natu-
ral frequency vibration levels which may
exceed CTI guidelines.
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 IP52 indoor or
IP14 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 pro-
tection and external operating handle,
lockable in the off position for safety.
The system shall include a solid state,
PI temperature controller to adjust fre-
quency output of the drive in response
to the tower cold-water temperature.
The temperature of the cold water and
set point shall be displayed on the door
of the control panel. The bypass shall
include a complete magnetic bypass
circuit with capability to isolate the VFD
when in the bypass mode. Transfer to
the bypass mode shall be automatic in
the event of VFD failure or for specific
trip conditions allowing safe transfer of
utility voltage to the motor. Automatic
bypass with an earth ground condition is
not allowed. The bypass contactor shall
be cycled on and off while operating in
bypass, to maintain the set-point temper-
ature of the cold water. The drive design