36
HXV HYBRID COOLER
Fan Control
7
Independent Fan Control
BAC recommends controlling all fans together. However, follow the instructions below if
independent fan control is performed.
In an operating induced draft unit with independent fan capabilities and no partitions,
idle fans tend to windmill in the reverse direction. A windmilling fan poses no threat to
the system while turning freely, but can create a large shock load when the fan motor is
suddenly powered up. Proper staging of fans when starting from a windmilling condition
will prevent excessive stresses on the drive system.
•
VFD:
The recommended control option is to use a variable frequency drive to control
all of the motors. VFDs regulate motor speed electronically and start motors with
reduced voltage and frequency. The result is a gentle motor start, and therefore
reduced stress.
• No VFD
–– Staging Up: Turn all motors off for 15 seconds. Following the 15 second delay,
bring all required fans online. Allow for a 1 second time delay between fan stages to
reduce staging current.
–– Staging Down: Turn off the fan motor. No need for delays.
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
.
• Please refer to the manufacturer’s variable frequency drive recommended start-up
procedure for further information or
consult with BAC for any VFD applications.
NOTE:
With evaporative cooling, a
15 second fan motor delay will not
be noticed when staging up.
NOTE:
The minimum turndown ratio
for units with a belt drive is 10:1
(or 6 hz).
NOTICE:
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 that 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.
