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Fan System - Capacity Control
There are several methods for capacity control of the evaporative cooling unit. Methods include: Fan motor cycling, the use of two speed
motors, and the use of variable frequency drives (VFD's).
Note: for the eco-ATW with Sage
Fan Motor Cycling
Fan Motor Cycling requires the use of a single stage thermostat which senses the fluid temperature (closed circuit coolers) or condensing
temperature (evaporative condensers). The contacts of the thermostat are wired in series with the fan motor's starter holding coil. For
individual fan motor cyclings interlock fan motors before fan start to prevent issues with reversing idle fans.
Sequence of Operation for Fan Motor Cycling
Fan Motor Cycling is often found to be inadequate where the load has a wide fluctuation. In this method, there are only two stable levels
of performance: 100% of capacity when the fan is on, and approximately 10% of capacity when the fan is off. Please note that rapid
cycling of the fan motors can cause the fan motor to overheat. Controls should be set to only allow a maximum of six start/stop cycles
per hour. The recirculation pump may not be used as a means of capacity control and should not be cycled frequently. Excessive cycling
can lead to scale build-up, resulting in reduced wet and dry performance.
Variable Frequency Drives
The use of a variable frequency drive (VFD) provides the most precise method of capacity control. A VFD is a device that converts a fixed
AC voltage and frequency into an adjustable AC voltage and frequency used to control the speed of an AC motor. By adjusting the
voltage and frequency, the AC induction motor can operate at many different speeds.
The use of VFD technology can benefit the life of the mechanical components with fewer and smoother motor starts and built-in motor
diagnostics. VFD technology has particular benefit on evaporative cooling units operating in cold climates where airflow can be
modulated to minimize icing and reversed at low speed for de-icing cycles. Applications using a VFD for capacity control must also use
an inverter capable motor built in compliance with NEMA standard MG-1.
NOTE: VFD's should not be used on pump motors. The pumps are designed to be operated at full speed and are not
intended to be used as capacity control.
The type of motor, manufacturer of the VFD, motor lead lengths (between the motor and the VFD), conduit runs and grounding can
dramatically affect the response and life of the motor. Select a high quality VFD that is compatible with the fan motor(s) in the EVAPCO
unit(s). Many variables in the VFD configuration and installation can affect motor and VFD performance. Two particularly important
parameters to consider when choosing and installing a VFD are switching frequency and the distance between the motor and VFD often
referred to as lead length. Consult the VFD manufacturer's recommendations for proper installation and configuration. The motor lead
length restrictions can vary with the vendor. Regardless of motor supplier, minimizing lead length between the motor and drive is good
practice.
VFD Lockout Notification
Sequence of Operations / Guidelines for Multi-fan Units with a VFD during Peak Load
For eco-ATWE, see Sage
1.
Both fan motors off – Pump running on one cell.
2.
Both fan motors off – Pump running on both cells.
3.
Both VFDs turn on at the manufacturer's recommended minimum operating speed (25%) – Pump running on both cells.
4.
Both VFDs speed up uniformly (they should be synchronized on start-up) – Pump running on both cells.
5.
Both VFDs are on full speed – Pump running on both cells.
NOTE: The VFDs need to have a pre-set shutoff to prevent water temperatures from becoming too cold and to prevent
the drive from trying to turn the fan at near zero speed. Operating below 25% of motor speed achieves very little return
in fan energy savings and capacity control. Check with your VFD supplier if operating below 25% is possible.
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and eco-ATWE with Sage
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control Panel O&M
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consult the manual.
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