Full Description

The objective of the ASHRAE Research Project No. 685-RP: Test and Analysis Methods for Resolving Fan/Motor Vibration Problems in Air-Conditioning Units, is to develop the basic analytical models and experimental procedures that can be used to find the vibration modes associated with fan impeller, motor and motor mount in airconditioning units that are excited by fan impeller unbalance, taking into account the gyroscopic effect of the rotating fan impeller, and motor torque pulsations.

Four air-conditioning units were investigated in the project. They were designated as test units #1 to #4.

For the test unit #1 and test unit #2, the vibration problems associated with fan impeller, motor and motor mount system that are excited by fan impeller unbalance taking into account the gyroscopic effect of the rotating fan impeller were investigated. It was proved, both theoretically and experimentally, that the gyroscopic effect of the rotating fan impeller did have significant effect on the forward vibration modes of the system. The classical approach and energy approaches were used in the three dimensional lumped parameter model development, in which the gyroscopic effect of rotating fan impeller was included. In the classical approach, the gyroscopic effect of the rotating fan impeller gave an additional damping matrix. In the energy approaches, additional terms were added to the stiffness matrix due to the gyroscopic effect of the rotating fan impeller. All these additional terms, either in the damping matrix or in the stiffness matrix, represent the modal coupling between the motions in the two different planes.

The model from classical approach and the model from energy approach gave very good predictions about the forward rocking mode, which is the most important vibration mode in this work. Both non-rotating and rotating tests were conducted in these first two units. The non-rotating tests were used to measure the vibration modes associated with fan impeller, motor and motor mount system and to identify the parameters used in the analytical models. The rotating tests were used to measure the vibration mode shifting due to the gyroscopic effect of the rotating fan impeller and to locate the resonance speeds corresponding to both the forward and backward rocking modes. The results from analytical models agreed very well with the experimental results both for the non-rotating and rotating tests.

For the second two test units, test unit #3 and test unit #4, the noise problems associated with torsional vibrations of motor rotor, shaft and fan impeller that are excited by motor torque pulsations were investigated. The torsional frequency responses of both the motor stator and motor mount system and the fan impeller, motor rotor and shaft assembly were studied. Different test methods were studied in the investigations for identifying the torsional resonance frequencies. The effect of the tightness of the set screw on the effective shaft length and the motor torque pulsations were investigeted too in this work.