BAC 3985C - Vibration Cutout Switch (VCOS)

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SERIES 3000 COOLING TOWER
Fan Control
Variable Frequency Drive Operation
• Applicationsutilizingvariablefrequencydrives(VFDs)forfanmotorcontrolmustuse
inverterdutymotorsbuiltincompliancewithNEMAstandardMG-1,Part31.
• Operationoftheunitataspeedwhichresonateswithcomponentsofthedrivesystem
orsupportstructuremayresultinvibrationswhichcoulddamagethecomponentsor
structure,and/orcreateobjectionablenoise.Therefore,theseresonantspeedranges
shouldbeidentiedatstart-upandlockedouttopreventoperationofthemotorat
theseresonantspeeds.The“ResonantSpeedIdenticationProcedure”mustbe
conductedonpage 36.
• Pleaserefertothemanufacturer’svariablefrequencydriverecommendedstart-up
procedureforfurtherinformationorconsultwithyourlocalBACRepresentativeforany
VFDapplications.ForprojectswithBACcontrols,visitwww.BaltimoreAircoil.com.
Optional Gear Drive Units with VFDs
• Donotoperatethestandardgeardrivesbelow450RPMmotorspeed(gearinput
speed).Forspeedslessthan450RPM,alowspeedoptiongeardrivemustbe
supplied.
• Continuedoperationataspeedwhichresonateswiththegeardrivesystemmayresult
intorsionalvibrationswhichcandamagesystemcomponents.Themostcommon
indicatoroftorsionalvibrationisanunusualrumblingorgrindingnoisefromthe
geardriveatanarrowspeedrange.Thenoisewilldecreasetonormallevelswhen
thespeedisincreasedordecreasedawayfromtheresonantspeedrange.Thisnoise
isnotindicativeofadefect,butresultswhenthevibratorytorqueexceedsthedrive
torque,causingthegearteethtoseparateandclashtogetherveryrapidly.Onvariable
frequencydriveapplications,avoidoperationnearthisresonantspeedbylockingout
theresonantspeedrange.
NOTE: The minimum turndown ratio
for units with a belt drive is 10:1
(or 6 hz). The minimum turndown
ratio for units with gear drive is 4:1
(or 15 hz), unless supplied with an
oil pump or a no minimum speed
gear. Units with the oil pump do not
have a minimum speed.
ATTENTION: For a unit with a VFD,
with a switching frequency of 2.5
kHz, the line lead length cannot
exceed 100 feet. If the switching
frequency is higher than 2.5 kHz
and/or the line lead length exceeds
100 feet, a dV/dT output filter is
recommended to protect the motor.
Since the switching frequency and
maximum line length requirements
vary between VFD and motor
suppliers, contact your local BAC
Representative to determine if a
dV/dT filter is required.
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