Methods for Resolving Fan/Motor Vibration Problems in Air-Conditioning Units: Part II - Theoretical Models for Identifying Vibration Modes Excited by Fan Impeller Imbalance

Nonoperating and operating vibration test procedures have been developed that can be used to identify the resonance frequencies and related vibration modes that are associated with fan impeller unbalance in small air-conditioning units. The test procedures are discussed in this paper, while a companion paper discusses analytical models. Operating tests on two typical air-conditioning units indicated that gyroscopic effects associated with the rotating fan impeller significantly influenced the resonance frequencies of the vibration modes of the units tested. The gyroscopic effects resulted in the resonance frequency associated with the nonrotating, rocking and swaying vibration mode to separate into two separate resonance frequencies (forward and backward whirling modes) as the fan rpm was increased. Vibration mobility tests conducted during this project indicated the forward and backward whirling modes exist simultaneously