Rotordynamic Evaluation of Centrifugal Compressor Using Electromagnetic Exciter

Abstract

Since heavier gases exert larger effects on rotordynamic stability, stability evaluation is important in developing or designing high-pressure compressors. To evaluate the rotor stability during operation, an excitation test using a magnetic bearing is the most practical method. In stability analysis, labyrinth seals can produce significant cross coupling forces, which particularly reduce the damping ratio of the first forward mode. Therefore, forward modes should be distinguished from backward modes in the excitation test. One method that excites only the forward modes, not the backward modes (and vice versa), is the use of a rotating excitation. In this method, the force is simultaneously applied to two axes to excite the rotor in circular orbits. Two trigonometric functions, i.e., cosine and sine functions, are used to generate this rotation force. Another method is the use of a unidirectional excitation and a mathematical operation to distinguish the forward whirl from the backward whirl. In this method, a directional frequency response function that separates the two modes in the frequency domain is obtained from four frequency response functions by using a complex number expression for the rotor motion. In this study, the latter method was employed to evaluate the rotor stability of a high-pressure compressor. To obtain the frequencies and damping ratios of the eigenvalues, the curve fitting based on system identification methods, such as the prediction error method, was introduced for the derived frequency response functions. Firstly, these methods were applied to a base evaluation under a low-pressure gas operation, in which the stability mainly depends on the bearing property. Using the obtained results, the bearing coefficients were estimated. Next, the same methods were applied to stability evaluations under high-pressure gas operations. The destabilizing forces were also estimated from the test results and compared with the calculation results.

Reviews of Magnetic Bearing Development in Japan

“…For centrifugal compressors, a test in which the stability of shaft-bearing systems is measured by attaching a magnetic bearing to the end of a shaft as an exciter has generally been carried out. Hitachi Plant Technologies, Ltd., (Japan) reported test results for a high-pressure centrifugal compressor (42) .…”

Frequency response function measurement of a rotor system utilizing electromagnetic excitation by a built-in motor

“…In FRF measurements, a few types of exciters are used. Conventionally, impact hammering and electromagnetic actuators (Aenis et al, 2002;Takahashi et al, 2012;Tsunoda et al, 2016Tsunoda et al, , 2017a are generally used, but are undesirable. Hammering a rotating body is not simple and is unrealistic because of safety.…”

“…Hammering a rotating body is not simple and is unrealistic because of safety. Electromagnetic exciters are able to generate the required sweep excitation force to measure the FRF from the displacement of the rotor, enabling faults to be detected Frequency response function measurement of a rotor system utilizing electromagnetic excitation by a built-in motor (Aenis et al, 2002), estimates of the stability margin to be made (Tsunoda et al, 2016(Tsunoda et al, , 2017a, and eigenvalues to be measured (Takahashi et al, 2012). However, installation of electromagnetic actuators is inconvenient and expensive, increasing the production costs.…”