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Operation and Maintenance Instructions
Sequence of Operation for Two Fan Units with Two Speed Motors During Peak Load
1. Both fan motors on full speed – full water flow over both cells
2. One fan motor on high speed, one fan motor on low speed – full water flow over both cells
3. Both fan motors on low speed – full flow over both cells
4. One fan motor on low speed, one fan motor off – full water flow over both cells
5. Both fan motors off – full water flow over both cells
6. Both fan motors off – full single cell flow through one cell
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 also 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 NEMA standard MG-1. This is an available option from EVAPCO. The
standard fan motors supplied by EVAPCO are not intended for use with VFDʼs.
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. The motor lead length restrictions vary with the motor vendor. Regardless of
motor supplier, minimizing motor lead length between the motor and the drive is good practice.
Sequence of Operation for Multi-fan Units with a VFD During Peak Load
1. The VFDs should all be synchronized to speed up and slow down uniformly.
2. 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.
3. 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.
For more details on the use of variable frequency drives, please request copies of EVAPCOʼs Engineering Bulletins 39 and 43.
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