Evaluation of the influence of rotor magnetic anisotropy on condition monitoring of 2 pole induction motors

Abstract

Non grain-oriented silicon steel laminations used in electric machines show a small degree of non-ideal magnetic anisotropy, which is introduced during the rolling process. If the rotor laminations are punched and stacked in a uniform direction, magnetic asymmetry is present in the rotor core. In this paper, it is shown that rotor core anisotropy can produce twice slipfrequency, 2sf s , modulation that can be misinterpreted as rotor faults in 2 pole induction motors when performing motor current signature analysis (MCSA). Unnecessary motor inspection due to false rotor fault alarms is a common on-going problem in the field, and it is shown for the first time in this work that rotor anisotropy is a root cause of false MCSA alarms in 2 pole induction motors. The influence of rotor core anisotropy is analyzed in detail, and it is shown that 2sf s components produced by the rotor fault and rotor anisotropy interact, making reliable fault detection difficult with MCSA. It is also shown that off-line testing is currently the only available means of detecting the fault, and on-line monitoring of the space harmonics-induced current components is proposed as a viable solution for providing reliable rotor fault detection for 2 pole motors with rotor anisotropy. The analysis and conclusions presented in this paper are verified through testing on custom built lab motors and on 3.3 kV motors.

Identification of false rotor fault indications produced by on-line MCSA for medium voltage induction machines

“…These components can be produced due to magnetic asymmetry in the rotor due to [4], [7]- [13], [26]- [29]:…”

“…Its remote monitoring capability based on stator current measurement available in the motor control center makes it an attractive and convenient technology to use in an industrial setting. There has been active research performed over the last 20+ years on applying MCSA for detecting faults in MV induction motors, and it has been most successful in the field for detecting damage in rotor bars or end rings [1]- [13]. When the motor is operating with asymmetry due to rotor cage damage under steady state, the rotor fault frequency sidebands, given by (1) can be observed in the stator current, where, f s is the supply frequency, and s is the rotor slip.…”

“…Although MCSA technology has been successful in the field, false fault indications are common and a number of cases have been reported [4]- [13]. False indications can be classified depending on the actual motor condition and the diagnosis of MCSA, as summarized in Table I.…”

Time-frequency vibration analysis for the detection of motor damages caused by bearing currents

“…Most of them rely on the inherent constraints of the underlying signal processing tool, the Fast Fourier Transform (FFT); the application of this technique implies that time information is lost, i.e., FFT is capable to extract the frequency components present in the signal but not how they evolve over time (a fact that limits the amount of information obtained with this tool, since it becomes difficult to analyse how the amplitude of a certain fault component increases over time, unless repeated FFT analyses are carried out). In addition, faultrelated components may fall near other components that are not related to the failure, a fact that complicates the reliable diagnosis of the failure or may even lead to a wrong diagnostic [11][12][13][14]. Finally, the tool is not suitable in variable speed applications, since the slip-dependant fault frequencies may move (as the speed varies) spreading through the spectrum, becoming hence impossible to evaluate their real amplitude (unless complex algorithms are applied) [15].…”

A reliable indicator to detect non-adjacent broken rotor bars severity in induction motors

“…One of these drawbacks is the fact that it can lead to eventual false indications (either positive or negative) in many situations that, on the other hand, may be rather common in industry. In this regard, recent works have reported possible false indications of MCSA in the case of presence of pulsating load torques [8], [9], existence of rotor cooling ducts [4] or magnetic core anisotropy [10], diagnosis under low-slip conditions [9] or presence of broken outer bars in double cage rotors [11], among others. In these situations, alternative approaches such as the analysis of the power signature [12], [13], motor startup current [4], [9], [11], the application of the modified Prony method [14], or off-line methods [10], [15] have been proposed to overcome the MCSA problems, showing very high potential.…”