MEE Tihany 2007: Diagnosing Asynchronous Electric Motor Faults
Rahne Eric, B.Sc. in Electrical Engineering, founder of PIM Professional Industrial Measurement Technology Ltd., vibration diagnostic expert
Introduction
As a mechanical system, electric rotating machines can be examined using the same methods as any other rotating machine. However, due to the construction and operation of electric rotating machines, not only mechanical forces and resulting vibrations occurring in the driven rotating machines are present. The electromechanical energy conversion in electric rotating machines takes place through electromagnetic fields. The resulting forces not only produce the desired torque but also cause time-varying and directional stresses - and mechanical deformation - of individual components. If an electrical element of an electric rotating machine is damaged from an electrical point of view, this leads to the formation of an unevenly distributed electromagnetic field. Consequently, larger, asymmetric, or time-varying mechanical loads on individual components must be expected. In such cases, in motors, higher electrical energy consumption can result in lower mechanical power output, while in generators, the supplied electrical energy decreases while driving mechanical energy at the same time. The decrease in efficiency is accompanied by greater losses that are converted into heat, thus increasing the thermal load on the components.
Electrical and Mechanical Phenomena Related to Electric Motors
The poles of the stator of asynchronous motors are always arranged in pairs so that the magnetic field lines pass through the rotor. Otherwise, there would be no induction, hence no force, and ultimately no torque. Incidentally, thanks to the paired arrangement - assuming a cylindrical stator and rotor, and perfect positioning of the rotor in the center of the stator - the radial (vibration-inducing) forces balance each other out. Since the magnetic interaction strongly depends on the air gap between the rotor and stator, it is obvious that uneven radial forces are generated when the rotor is not positioned in the center of the magnetic field. The rotor of an electric motor is usually not a homogeneous body - for example, induction motors (asynchronous motors) are equipped with embedded conductors (rotor bars). These are crucial for the desired electromagnetic interaction to occur: as the rotor rotates, the rotor bars pass in front of the stator poles. In the process, voltage is induced in the bars, causing current to flow in them, creating an electromagnetic field around them. The interaction between the fields around the individual rotor bars and the rotating electromagnetic field of the stator results in the torque of the electric motor.
Diagnosing Electric Motor Faults Based on Electrical Parameters and Vibration Measurements
Based on the above, it is clear that in the case of any electrical fault, asymmetric (unbalanced) or periodic forces occur, which can be perceived as vibrations on the motor. By understanding the relationship of vibration phenomena caused by faults, not only mechanical faults of the electric motor but also electrical problems can be diagnosed. Additionally, electrical parameter measurements during operation can be used for fault diagnosis, with one of the most practical applications being the detection of rotor bar breakage. The most commonly used method for detecting the breakage of rotor bars, rings, or end rings is the phase-by-phase examination of current in the low-frequency range. For this, the current of each phase must be measured separately using current clamps. The amplitude ratio observed between the network frequency and its pole modulation frequency sidebands in the obtained current spectra provides information for fault detection.
Bearing Damage Due to Shaft Currents (e.g., Static Charging)
Primarily, the rotors of electric motors are prone to electrostatic charging during normal operation. Despite this phenomenon being long known, it is often not given enough attention. Bearing damage due to electrical discharge or transient or shaft currents is known as smudging, electrical pitting, grooving, or arcing surface damage. For current flow to occur, the presence of a voltage potential difference is necessary (e.g., induced voltage and ground). Due to the nature of current, it flows through the smallest resistance, which is usually through the bearings of the electric motor. However, it is also possible for current flow to pass through a connected machine or equipment. For example, in electric motors equipped with directly driven tachometer generators, it is very common for the flowing current to first damage the bearings of the tachometer generator, as they are smaller in size and therefore offer less transient resistance. Bearing lubrication also plays a crucial role. Due to its insulating effect, a static charge voltage potential can develop on the shaft. When this voltage exceeds the breakdown voltage of the bearing lubrication, sparking occurs, damaging the surfaces of the metallic components and oxidizing the lubricant. This process repeats periodically in the form of new arc discharges, continuously wearing down the bearing raceway surface. PIM Professional Industrial Measurement Technology Ltd. H-1221 Budapest, Tanító u. 19/A Tel.: (1) 424-00-99 Fax: (1) 424-00-97 e-mail: pim@pim-kft.hu web: www.pim-kft.hu www.termokamera.hu www.gepszakerto.hu
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