Sequence Of Operations / Guidelines For Multi-Fan Units With A Vfd During Peak Load; Identify And Lock-Out Harmful Resonant Frequencies - EVAPCO ATC-E Operation And Maintenance Instruction Manual

Sequence of Operations / Guidelines for Multi-fan Units with a VFD 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 . Both VFDs turn on at the manufacturer's recommended minimum operating speed (20-25%) – 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 .

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% (13Hz)
and 100% (50Hz) speeds, which creates an opportunity for operation where resonant frequencies exist . Sustained operation 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 accessories 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 . Relevant resonant
frequencies (if they occur) can only be identified 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 .
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