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Resonant Speed Identification Procedure
There are several characteristic frequencies at which vibration levels may resonate with
unit structural components. These include fan speed, motor speed, bearing frequency, and
blade pass frequency. Within the overall operating speed range of a unit, it is not unusual
for one or more of these characteristic frequencies to excite the structural components
over relatively small speed ranges and create an increase in vibration levels.
If the vibration levels are excessive at these resonant speeds, they need to be locked out
to prevent the fan motors from operating at these speeds. The Nexus® Modular Hybrid
Cooler has been designed such that there are no internal resonant frequencies. However,
resonant frequencies may result from external excitation, such as vibration from adjacent
equipment.
If excessive vibration due to resonance is detected on a specific installation, the following
procedure describes how to identify the lockout speed ranges:
• Ensure the fan motor controls are off, and the power to the motor circuit is locked out.
• Attach the accelerometer (provided by others) onto the motor base.
• Close motor access panel, install all required hardware, and ensure that the panel is
secure.
• Turn on power from equipment control panel, and in user interface, change Manual
Mode to Yes.
• Adjust the fan speed for each fan motor independently and monitor the vibration
levels. If the vibration value approaches 0.2 ips (0-peak), slowly “zero in” on the
speed where the value equals 0.2 ips, and record the speed at which this occurs as
the lower end of the lockout range. Also record the vibration level at this speed.
• Adjust the fan speed for fan motors simultaneously and monitor the vibration levels. If
the vibration value approaches 0.2 ips (0-peak), slowly “zero in” on the speed where
the value equals 0.2 ips, and record the speed at which this occurs as the lower end
of the lockout range. Also record the vibration level at this speed.
• Continue to slowly increase the speed while monitoring the vibration level. If this is a
resonance, then the value should peak and eventually decrease to a level that is below
0.2 ips as the speed is increased. After the vibration level has peaked and continues
to fall, record the speed where the value equals 0.2 ips as the upper end of the
lockout range.
• Using this data, a baseline for vibration history can be developed. The vibration levels
can be monitored yearly and the trend used to indicate potential wear or the need to
replace components in the drive system.
• After the entire speed range has been checked and any resonances identified, turn off
the control panel and ensure lockout / tagout.
• Enter the unit, and carefully remove the accelerometer, along with any associated
wiring from the unit.
• Contact BAC to enter the lockout speed ranges, if any have been identified, so that the
unit will not operate at a resonant speed.
• Once it has been verified that the drive system is all clear, return the unit to its normal
operating condition.
• Keep a record of any lockout speed ranges for future reference.
NOTE:
The resonant speed
identification procedure must be
performed at start-up.
CAUTION
: Steps must be taken to
avoid operation at or near system
resonant frequency which could
result in fan failure and possible
personal injury or damage.