Sequence of Operations / Guidelines for Multi-fan Units with a VFD during Peak Load
For eco-ATW(E), see Sage
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/Sage
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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. 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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