8
Operation and Maintenance Instructions
Variable Frequency Drive Lock-out Notification
WARNING: Qualified personnel should use proper care,
procedures and tools when maintaining
the Fan/Drive System in order to prevent
personal injury and/or property damage.
WARNING: Identify and Lock-out Harmful Resonant
Frequencies
A Variable Frequency Drive (VFD) fan system, unlike traditional
fixed-speed systems, is designed to operate between 25%
(15Hz) and 100% (60Hz) speeds, which creates an opportunity
for operation where resonant frequencies exist. Sustained op-
eration at resonant frequencies may lead to excessive vibration,
fatigue of structural components and/or drive system noise and
failure. Owners and operators must anticipate the existence of
resonant frequencies and lock out frequencies during start-up
and commissioning in order to prevent drive system operational
problems and structural damage. As a part of the normal start-
up and commission processes, resonant frequencies should be
identified and locked-out in the VFD’s software.
The unit’s supporting structure, external piping, and accesso-
ries contribute to the overall harmonic make-up and stiffness
of the system. The choice of VFD will also have a significant
influence on how the system behaves. Consequently, not all
resonant frequencies can be determined in advance at the
manufacturer’s factory during final inspection and testing. Rel
-
evant resonant frequencies (if they occur) can only be identi-
fied accurately after the installation in the system.
To check for resonant frequencies in the field, a run-up and
run-down test must be performed. Additionally, VFD carrier
frequencies should be adjusted to best align the VFD with the
electrical system. Refer to your drive’s start-up procedures for
additional information and instruction.
The procedure of checking for resonant frequencies requires
stepping through the VFD’s operating range at (2) Hz intervals
from the lowest operating frequency to full speed. At each step,
pause long enough for the fan to reach steady-state. Note
changes in unit vibration during this time. Repeat from full speed
to minimum speed. Should vibration-inducing frequencies exist,
the run-up and run-down test will isolate the resonant frequencies
which then must then be locked-out in the VFD programming.
Two-Speed Motors
The use of a two-speed motor provides an additional step of
capacity control when used with the fan cycling method. The
low speed of the motor will provide approximately 60% of full
speed capacity.
Two-speed capacity control systems require not only a two-
speed motor, but also a two-stage thermostat and the proper
two-speed motor starter. The most common two-speed motor
is a single winding type. This is also known as a consequent
pole design. Two-speed two-winding motors are also
available. All multi-speed motors used in evaporative cooling
units should be variable torque design.
NOTE: When two-speed motors are to be used, the motor
starter controls must be equipped with a decelerating time
delay relay. The time delay should be a minimum of 30
seconds when switching from high speed to low speed.
Sequence of Operations / Guidelines for Multi-fan Units
with a VFD during Peak Load
For eco-ATWE, see Sage
2
/Sage
3
control Panel O&M:
1. Both fan motors off – Pump running on one cell.
2. Both fan motors off – Pump running on both cells.
3. One fan motor on low speed, one fan motor off – Pump
running on both cells.
4. Both fan motors on low speed – Pump running on both
cells.
5. One fan motor on high speed, one fan motor on low speed
– Pump running on both cells.
6. Both fan motors on full speed – Pump running on both
cells.
Recirculated Water System -
Routine Maintenance
Suction Strainer in Cold Water Basin
The pan strainer should be removed and cleaned monthly
or as often as necessary. Make certain that the strainer is
properly located over the pump suction, alongside the anti-
vortexing hood.
Cold Water Basin
The cold water basin should be flushed out quarterly, and
checked monthly or more often if necessary, to remove any
accumulation of dirt or sediment which normally collects
in the basin. Sediment can become corrosive and cause
deterioration of basin materials. When flushing the basin, it is
important to keep the suction strainers in place to prevent any
sediment from entering the system. After the basin has been
cleaned, the strainers should be removed and cleaned before
refilling the basin with fresh water.
