WWW.BALTIMOREAIRCOIL.COM
35
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 identified at start-up and locked out to prevent operation of the motor at
these resonant speeds. The “Resonant Speed Identification 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.
7