Fan System - Capacity Control; Fan Motor Cycling; Sequence Of Operation For Fan Motor Cycling; Two-Speed Motors - EVAPCO ATC-E Operation And Maintenance Instruction Manual

Operation and Maintenance Instructions
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) .
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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 .

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 .
IMPORTANT:
THE RECIRCULATION PUMP MAY NOT BE USED AS A MEANS OF CAPACTY CONTROL AND SHOULD NOT BE
CYCLED FREQUENTLY . EXCESSIVE CYCLING CAN LEAD TO SCALE BUILD-UP AND REDUCES WET & DRY
PERFORMANCE . FREQUENT CYCLING OF THE SPRAY PUMP, WITHOUT THE FANS IN OPERATION, WILL PROVOKE
DRIFT AND SPRAY WATER MIGRATION OVER THE AIR INLET LOUVERS, WHICH IS PROHIBITED IN MOST
COUNTRIES . PLEASE CONSULT YOUR LOCAL LEGISLATION .
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 .
It is important to note that 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 a 30 second delay when switching from high speed to low speed .
Sequence of Operation for Two Cell Units with Two Speed Motors during Peak Load
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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 .

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 and changes it into an AC adjustable 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 duty motor built in compliance with IEC standard . This is an available option from EVAPCO .
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 Evapco fan motors . 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 .
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/Sage control Panel O&M
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3 consult the manual .
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