Six-degrees-of-freedom dynamic load identification of compressors for refrigeration system based on rigid body dynamics

Dynamic load of compressors is of great concern to designers of refrigeration products since pipes and sheet metals are easy to be excited, leading to noise or fatigue failure. The excitation is impossible to measure directly and difficult to calculate according to generation mechanism; thus, dynamic load identification which skips the mechanism is a more practicable way. However, the reported approaches have the problems of inaccuracy and clumsy. Therefore, an easy-to-implement approach is proposed, in which the accelerations and the frequency response functions from test are transformed to an arbitrarily given point based on rigid body dynamics, and then the equivalent six-degrees of freedom dynamic load of this point is calculated. This approach is validated using a motor with eccentric mass, whose centrifugal force is identified with the proposed approach and also calculated in analytical way. Comparison indicates that the error is within 5% except for the rotational speeds close to natural frequencies. Curve fitting or interpolation is recommended to correct the data in order to reduce the influence of natural frequencies. Cases studies with a rotary compressor and a scroll compressor respectively are presented. For the rotary compressor, the force magnitudes in X and Y directions are almost the same and pretty close to their quadratic polynomial fitting curves with zero intercept respectively, conforming to the characteristics of centrifugal force; the RZ component is larger than the RX and RY components as expected, but the latter two are not negligible; and there is a step change at 45 rps due to the removal of torque compensation. For the scroll compressor, the force magnitudes in X and Y directions are different, but very close to their quadratic polynomial fitting curves, respectively; the Z component has a jump in high-speed range; and there is no significant difference of dynamic force in the cooling and the heating modes.