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Sigma F Series Cooling Tower
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Specifications: Options
25
Specification Value
Fan Motor Variable Speed Drive:
Marley All Weather ACH550 System
6.4 Add the following paragraph in the
Mechanical Equipment section when
VFD is used with customers Building
Management System: A complete UL
listed Variable Speed Drive system in
a NEMA 1 indoor, NEMA 12 indoor or
NEMA 3R outdoor enclosure shall be pro-
vided. The VFD shall use PWM technol-
ogy with IGBT switching and integrated
bypass design. VFD out put 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 position for
safety. The VFD system shall receive a
speed reference signal from the Building
Management System monitoring the
tower fluid temperature. As an option
to receiving the speed reference signal
from a building management system, the
drive must have the capability to receive
a 4-20 ma temperature signal from an
RTD transmitter. The VFD shall have an
internal PI regulator to modulate fan
speed maintaining set point temperature.
The drive's panel display 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 capabil-
ity 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 tempera-
tures while the VFD is in a by-pass mode.
Operator controls shall be mounted on
the front of the enclosure and shall con-
sist 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 lubrica-
tion the VFD system shall de energize the
motor once 25% motor speed is reached
■ Marley VFD drive systems are designed to combine absolute
temperature control with ideal energy management. The cool-
ing tower user selects a cold water temperature and the drive
system will vary the fan speed to maintain that temperature.
Precise temperature control is accomplished with far less stress
to the mechanical equipment components. The improved energy
management provides fast payback. Indeed, many utilities offer
generous rebates for users having installed VFD drives.
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