Study On Speed Detection By Rotor Slot Harmonic Based On Improved CZT Algorithm

With electrical submersible pumps are widely used in oil fields, it is particularly important to monitor working conditions of electric submersible pump wells and diagnose their faults. Motor speed is a vital parameter and it can reflect working conditions and faults of electrical submersible motors. However, due to the complicated working environment of electrical submersible pumps, it is inconvenient to install speed sensors for speed measurement. The main content of this paper is how to calculate an accurate speed value without speed sensors.In order to calculate motor speed, rotor slot harmonic method and spectral analysis algorithm are adopted in this paper. The main work and achievements are as follows:(1) Frequency spectrums with different signal parameters are calculated by Chirp-Z Transform(CZT, for short). According to the spectrum characteristic analysis results, two improved CZT algorithms are proposed in this paper. One is Orthometric Average CZT(OACZT, for short) algorithm, and the other one is Shortest-sampling CZT(SCZT, for short) algorithm. Simulation results show that OACZT accuracy for spectrum analysis is quite good, especially fundamental frequency accuracy, which can be controlled within 0.0001~0.001Hz(0.0002%~0.002%). Besides that, fundamental frequency can be calculated by SCZT method only in one signal cycle. SCZT method has good dynamic and real-time performance and its accuracy can be improved after combining with iteration theory. Overall, OACZT method is selected to calculate the motor speed and SCZT method is used to analyze the fluctuation of power signal frequency.(2) Due to complex components of actual electrical signals, Multi-frequency Component Elimination theory is proposed in this paper. Its main idea is that firstly eliminate fundamental and harmonic components before calculating the parameters of relatively smaller rotor slot harmonic. Experiment results show that the accuracy of rotor slot harmonic frequency calculated by this scheme is maintained at 10-2Hz(0.001%).(3) After improving the accuracy of fundamental and rotor slot harmonic frequency, the motor speed accuracy is increased correspondingly. Motor speed verification experiments are implemented based on motor experimental platform, and the results are compared with photoelectric tachometer results in this paper. The comparison results demonstrate the feasibility of the proposed method and the achieved speed accuracy is 0.6 rpm(0.04%) or even less.Finally, signal acquisition for electric submersible pumps are carried out at the oilfield site, and then motor speed is calculated by the scheme proposed in this paper.