Condition Monitoring of VSD-fed Induction Motors

Ralikalakala, Lebohang

Condition Monitoring of VSD-fed Induction Motors

Thesis / Dissertation

2023

Abstract

Induction motor drives are presently the most fully fledged technology amidst the various motor drives not utilising commutators [1]. They consist of an induction motor (IM) fed from a variable frequency AC inverter. They are mostly used in applications which need good dynamic and steady state performance over a large range of speeds [2]. The induction motor is often referred to as the workhorse of the industry due to its predominance in the industry – constituting around 90% of all the motors in the industry [3], [4]. Its admired features include relatively low cost, high efficiency, robustness, and ease of maintenance [5], [6]. These motors are of two types, the wound-rotor and the squirrel cage [1], [5], with the three-phase squirrel cage induction motor being the most common, constituting over 80% [7], [8]. On the other hand, variable frequency AC inverters are also of two types, the voltage source inverter (VSI) and the current source inverter (CSI). VSI is used for most induction motor drive applications due to its simpler control and efficient operation in addition to its low weight, cost, and volume. Despite their high reliability and robustness, induction motors are still prone to faults [9] due several factors such as being subjected to diverse, continuous, and harsh conditions [10], being subjected to frequent start-stop cycles leading to wear and cracking of machine elements, or being subjected to continued motor overload for long periods required by some industrial processes, resulting in increased thermal stress [11]. Studies have shown that one category of faults that an induction motor may suffer from is the rotor faults [12]. Of these, the broken rotor bar fault in squirrel cage induction motors is the most common [13], constituting 5-10% of all induction motor faults [14]. Besides the induction motor, the state of health of the driving inverter is also important to the overall reliability, integrity, and availability of the induction motor drive system. The power electronic converter (inverter) constitutes about 82.5% of all the faults that occur in the inverter-fed motor drives [15] and of this, 38% occur due to failures of the power electronic switching transistors [16], [17]. These faults include short circuit faults, open circuit faults and intermittent gate-misfiring faults [18]. Open switch faults and short circuit faults are very common in inverters. Consequently, the continuous monitoring of the induction motor drive components (e.g., the induction motor or inverter) is crucial for an early and timely detection of faults to avoid the propagation of the faults and the total breakage of the drive or part of the drive. This will in turn reduce the risk of reduced output, increased emergency maintenance costs and out-of-service problems [10]. The process of continuously monitoring the condition or state of health of a system is called condition monitoring. It is very indispensable in electric drives. One condition monitoring technique commonly used for the detection of broken rotor bar fault is the motor current signature analysis (MCSA), which is focused on the application of the Fourier transform to machine's stator current under steady state operating conditions [7]. The MCSA assumes that the motor is sufficiently loaded and operating under steady state conditions [19]. Despite its robustness, this fault diagnostic tool suffers nonnegligible drawbacks, especially when applied in the industry, where there are many real-time factors such as speed variation or load changes which affect the operation of the induction motor. Similarly, diagnostic variables technique (DVT) is a condition monitoring technique commonly used in the detection and diagnosis of inverter open switch faults. Contrary to MCSA, it uses diagnostic variables and current average values to detect and localise open switch faults [17], [20], [21]. This method can detect both single open switch and double open switch faults [21]. The objective of this study is to design and develop an open-loop volts-per-hertz controlled induction motor drive for study of faults across the various parts of the drive. In particular, the study focuses on the broken rotor bar fault on the motor-end of the drive and the inverter open switch faults on the converter side of the drive. It commences with investigation of the impacts of the various inverter open switch faults (single switch open circuit, single phasing, and double switch open circuit faults) on the performance of the induction motor, before performing fault detection and diagnosis of these faults to validate the diagnostic variables technique. Then it experimentally establishes some of the shortcomings of MCSA in the detection of the broken rotor bar fault of a Class B inverter-fed 250 W, 190 V, 50 Hz and 2-pole, star-connected threephase squirrel cage induction motor. Finally, it proposes a complementary broken rotor bar fault detection technique, for inverter-fed induction motor, that is premised on the variation of impedance with rotor position at standstill to supplement MCSA. The results for investigation of the impact of open switch faults show that both simulation and experimentation produced results with high concordance. They show that double switch open switch fault results in the largest increase in distortion and in DC offset voltage and current than the other two faults, while relatively less changes on these parameters are observed for single phasing fault. A similar trend is observed in speed and torque pulsations of the motor under the respective faults. Similarly, the method of diagnostic variables has proven valid for both simulation and experimental results and is able to detect and localise inverter open switch fault for single and double switch fault. Furthermore, the MCSA is shown to be incapable of detecting broken rotor bar fault under no-load conditions because the sideband harmonics overlap the fundamental component. Likewise, inverter switching harmonics mask and interfere with higher ordered sideband harmonics in the case of inverter-fed motors, thus decreasing the confidence level in this technique. Finally, the proposed standstill impedance variation test shows large change, to the value of 9.59% of average impedance, in impedance as function of rotor position, and this is used to characterize the broken rotor bar fault. The inverter harmonics and switching noise do not impact on this variation in impedance. Further exploration of the standstill impedance variation approach in inverter-fed induction motors showed that it is possible to detect anomalies within the motor due to broken rotor bar fault by utilizing the geometrical orientation of the three-phase stator windings instead of changing the rotor position. That is, by manipulating the inverter switching such that the pulsating flux is aligned with each of the three magnetic axis of the three-phase stator windings, three impedances (one along each magnetic axis of the windings) are obtainable. The average of the absolute differences of these three impedances is large enough to characterize the broken rotor bar fault except when the rotor is positioned at 30°, 70° and 150° relative to the chosen reference position. These positions are referred to as critical rotor positions since the difference in impedance at these positions is very small and can lead to the possible misdiagnosis of the fault (false negative indication of the fault). It is noteworthy that this extension of the standstill impedance variation test is only applicable to 2-pole machines only since their mechanical degrees is equal to their electrical degrees.

Keywords

Engineering

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