Two-Speed Motors; Speed Motors During Peak Load - EVAPCO ESW Series Operation And Maintenance Instruction

Variable Frequency Drive Lock-out Notification
Qualified personnel should use proper care, procedures and tools when maintaining the Fan/Drive System in order
to prevent personal injury and/or property damage.
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% (15Hz) and
100% (60Hz) 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 unit 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.
For more details on the use of variable frequency drives, please download a copy of EVAPCO's Engineering Bulletin 39 from
evapco.com.
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.
NOTE: 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 30 seconds when switching from high speed to low speed.
Sequence of Operation for Two Cell Units with Two Speed Motors 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. 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.
2 /Sage 3 control Panel O&M
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