Method and apparatus for determining electric motor speed using

Abstract

A programmable apparatus is used to determine the running speed of an ac induction motor based on vibration and flux measurements. A first analysis is performed on at least one of the vibration and flux signals according to a first criterion to determine candidate speeds and whether speed will be determined based on the first analysis or whether a second analysis is required. A second analysis is selectively performed if required by the first analysis. In the first analysis, vibration signals (for example) are transformed to the frequency domain, such as by Fast Fourier Transform, and vibration amplitude peaks contained in the resulting spectrum are analyzed to identify candidate peaks. If only one candidate peak is identified, the frequency at which the candidate peak occurs is used to determine motor running speed. If two or more candidate peaks are identified and the motor is a two-pole motor, the candidate peak having the maximum amplitude is used to determine running speed. If two or more candidate peaks are identified and the motor has more than two poles, a second analysis is performed by producing a flux spectrum. Peaks at various frequencies within the flux spectrum are compared to the candidate peaks and each candidate peak is scored based on the comparison. The scores are compared to a predetermined score threshold to determine which candidate peak will be used to determine motor running speed. Preferably, each candidate peak is also subjected to testing to determine whether the candidate peak accurately represents the frequency at which the true peak occurs.