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Sergey Bondar

Faculty of Intelligent Power Engineering and Robotics (FIER)

Department of electrical systems (ES)

Speciality Electric power systems and networks

Improvement of methods for diagnosing asynchronous motors using energy conversion quality indicators

Scientific adviser: Ph.D., Assistant professor Dmitrij Polkovnichenko

Resume Abstract

Abstract

Содержание

• Introduction
• 1. Overview of common faults of asynchronous motors
• 2. Modern methods of diagnostics of asynchronous motors
• 2.1 Overview of existing diagnostic methods for asynchronous motors
• 2.2. Diagnostics of asynchronous motors using energy conversion quality indicators
• Conclusions
• References

  Introduction

  Asynchronous electric motors (hereinafter referred to as AD) are one of the main converters of electrical energy into mechanical energy and form the basis of the electric drive of many mechanisms that are used in all sectors of the national economy [1].

  The manufacturing quality of these electric machines largely determines the technical level of the products produced in other industries of our country [2]. For many decades, AD have been necessary and irreplaceable in a wide variety of applications: in the form of pump drives, compressors, rotating furnaces and mills, also in the mining industry, chemical and petrochemical industry, oil and gas complex, in steelmaking and rolling shops of the metallurgical industry, in environmental protection technology.

  However, it should be remembered that AD is an electric machine consisting of many parts, assemblies and mechanisms [3]. A large number of different components leads to the fact that the strength characteristics of the entire machine become dependent on the reliability of each element individually. Therefore, the task of detecting malfunctions in an asynchronous motor is an integral part of its operation.

  1. Overview of common faults of asynchronous motors

  Asynchronous motors are usually designed for a service life of 15-20 years [4] without major repairs, provided they are properly operated.

  

  Figure 1 - Section of asynchronous motor

  Proper operation of the engine means its operation in accordance with the nominal parameters specified in the passport data. In practice, AD shows its endurance and simplicity at a relatively low cost. However, in real life, there is a significant deviation from the nominal operating modes, therefore, damage to engine elements may occur during the operation of the AD, which in turn leads to its premature failure. Periodic diagnostics of the equipment condition helps to identify emerging malfunctions in a timely manner, as well as accurately determine the cause of breakdowns and defects of electric motors. The mechanisms of influence of factors on the operational reliability and service life of asynchronous motors are investigated [5]. The main ones are the following:

  • quality of active and structural materials used in the manufacture of electrical machines;
  • quality of manufacture of electric machines;
  • electricity quality;
  • inconsistency of the conditions of use of machines with their execution, starting and operating characteristics;
  • lack of proper maintenance of machines and poor quality of their repair.

  The consequence of these factors are emergency modes of operation of the AD [6]. Currently, more than 70% of operated asynchronous motors are machines that have been overhauled at least once [7, 8]. Up to 30% of the electric motors used fail and are repaired annually. The vast majority of them return to the enterprise after repair and are operated until the next failure. The machine can be repaired 3-4 times, and the operating time for failure is 0.5 ... 1.5 years. Engine failure leads to serious accidents and large material damage associated with downtime of technological processes, elimination of the consequences of accidents and expensive repairs of a failed electric motor.

  Depending on the method of execution, the rotor windings of an asynchronous motor are divided into two groups: with a short-circuited winding on the rotor and with a phase winding. Motors with a short-circuited winding on the rotor are cheaper in production, reliable in operation. Most asynchronous motors are manufactured by the industry with a short-circuited rotor. In such rotors, the windings have the appearance of a squirrel cage. All malfunctions of electric motors with a short-circuited and phase rotor can be divided into two main groups: mechanical and electrical.

  Mechanical malfunctions include defects in the engine housing, fan impeller, loosening of the stator windings, deformation of the rotor shaft, bearing wear. The most frequent mechanical damage is definitely a bearing—related problem. Typical signs of bearing wear are an increase in noise during engine operation and the occurrence of vibration, as a result of which the engine begins to warm up more.

  Electrical damage can include inter-turn short circuits, winding breakage, insulation breakdown on the housing, insulation resistance reduction, insulation damage, contact and connection failure, violation of the interlayer insulation of magnetic circuits, brush wear, damage to contact rings. It is obvious that the effectiveness of diagnostics can be ensured only by the complex nature of the control results.

  Let's consider the most common malfunctions of asynchronous electric motors (Table. 1) [9].

  Table 1 - Asynchronous motor malfunctions.

  No.	Malfunctions	Photo	Statistics
  1	Overload or overheating of the stator of the electric motor	Figure 2 – Stator	31 %
  2	Inter-turn closure	Figure 3 – Closure between turns (stator)	15 %
  3	Bearing damage	Figure 4 – Dismantled damaged bearing	12 %
  4	Damage to the stator windings or insulation	Figure 5 – Burnt stator winding (insulation melted)	11 %
  5	Uneven air gap between stator and rotor	Figure 6 – Air gap	9 %
  6	Operation of the electric motor in two phases (breakage of one of the phases)	Figure 7 – Phase break	8 %
  7	Breakage or loosening of the fastening rods in the squirrel cage	Figure 8 – "Squirrel cage" rotor type	5 %
  8	Loosening the fastening of the stator windings	Figure 9 – Loosened coupling	4 %
  9	Motor rotor imbalance	Figure 10 – Rotor imbalance	3 %
  10	Shaft misalignment	Figure 11 – Types of misalignment	2 %

  The above malfunctions cause damage to the enterprise. Basically, this damage is associated with equipment downtime, disruption of the technological process or spoilage of manufactured products. In addition to the losses, a decrease in electrical and fire safety associated with possible short circuits in the stator and rotor windings of the engine can be attributed. In addition, when operating electric motors that were in unsatisfactory condition, it can lead to financial losses:

  • direct, related to the unpredictable failure of equipment and the resulting disruption of the technological process;
  • significant (up to 5-7%) indirect unproductive electricity costs due to increased power consumption. [10]

  Hence, the question of the need to diagnose the condition of the engine during its operation is acute.

  2. Modern methods of diagnostics of asynchronous motors

  2.1. Overview of existing methods of AD diagnostics

  Detection of malfunctions in a running electric motor at an early stage of their development will not only prevent a sudden stop of production as a result of an accident, but also significantly reduce the cost of repairing the electric motor and increase its service life.

  The applied motor protection devices do not ensure its safety and reduce the likelihood of occurrence of only a part of the above malfunctions.

  Modern systems and methods of diagnostics of asynchronous motors can be divided into two groups.

  The first group includes test diagnostic methods. This is the measurement of insulation resistance, leakage currents, internal resistance of the windings, the tangent of the dielectric loss angle of the windings, the high-voltage pulse method, etc. Such diagnostics contributes not only to the prevention of the development of various defects, but also to their appearance.

  The second group includes methods of functional diagnostics. They are economically the most preferable, since they do not require temporary decommissioning of electrical equipment, they allow detecting all defects affecting the resource long before failure [11-13].

  The ideal modern method of diagnosing electric motors must meet the following requirements [14]:

  • high reliability and accuracy of detecting malfunctions and damage to the electric motor;
  • the ability to detect all or a significant part of electrical and mechanical damage to the electric motor and related mechanical devices;
  • performing diagnostic measurements remotely, which is relevant in cases where access to equipment is difficult;
  • low complexity of diagnostic work (measurements) and ease of measurement;
  • the possibility of analytical processing of the obtained measurement results in a short time, using computing and software tools [15].

  To date [16] the following methods of engine condition monitoring can be distinguished:

  1. Vibration method of individual units of the unit
  2. Analysis of acoustic vibrations generated by the operating unit.
  3. Measurement and analysis of the magnetic flux in the gap of the motor and the external magnetic field.
  4. Diagnostics of the insulation condition.
  5. Measurement and analysis of temperature in individual sections of the unit.
  6. Diagnostics of bearings based on the analysis of oil for iron content.
  7. Analysis of the electrical parameters of the machine.

  2.2. Diagnosis of AD using energy conversion quality indicators

  Recently, methods based on the control of electrical parameters of motors with subsequent analysis have been widely developed. The need to develop and implement new methods for assessing both the technical condition of electromechanical converters and the energy efficiency of their operation is due to the following reasons:

  • due to the non-sinusoidal nature of currents and voltages and the heterogeneity of energy flows, the assessment of the energy efficiency of electromechanical equipment based on classical efficiency indicators and power factor is inadequate;
  • performing diagnostic and monitoring operations of electromechanical equipment both in stationary and in the field is based on simplified dependencies and mathematical models without taking into account the energy mode of operation;
  • there is a need to determine the residual resource of electromechanical equipment to improve the efficiency of energy management;
  • there is a need to regulate the quality of technological operations performed by an electric drive and assess the negative impact of poor-quality energy conversion on the supply network and maintenance personnel.

  Assessment of the state of AD by analyzing the process of energy conversion is promising, since it does not require large hardware resources to implement [17]. It is enough to use current and voltage sensors, the remaining parameters are calculated by indirect methods. This approach does not require the withdrawal of equipment from the production process, that is, it can be used as a monitoring system for the current state.

  To use this diagnostic, the concept of energy conversion quality indicators (PCPE) is introduced. When determining the PСPE, it is necessary to adhere to the following requirements [18]:

  • transparent and clear content of coefficients;
  • unambiguous connection with certain types of defects;
  • clear limit values and reasonable characteristic values;
  • observability of initial data for coefficients.

  In the works [19, 20], separate control panels were formed, which can be conditionally divided into coefficients based on the analysis of power consumption, current and electromagnetic torque of the AD.

  The indicators for the first two groups considered are easily determined due to the simplicity of measuring instantaneous values of currents and voltages. The third group of indicators is based on the analysis of the components of the electromagnetic moment, the direct measurement of which is difficult. At the same time, the determination of the instantaneous values of the electromagnetic moment allows us to obtain a number of important energy indicators.

  To analyze the energy performance of asynchronous machines, a method is proposed that is based on experimental signals of phase currents and stator voltages of the AD. The application of this method is primarily due to the fact that in most cases, in industrial conditions, the only parameters that can be simply measured, recorded and evaluated are the phase currents and stator voltages of the motor. The remaining parameters characterizing energy efficiency and the quality of energy conversion can be determined indirectly. The values of active resistances (R_A, R_B, R_C) and scattering inductances (L_A, L_B, L_C) of the stator phase windings, as well as the moment of inertia ( J ) of the electromechanical system. These parameters can be obtained using a number of standard or alternative methods for identifying AD parameters. The algorithm for determining the electromagnetic moment, shaft moment and energy parameters is shown in Fig.12 [21].

  

  Figure 12 – The algorithm of the PСPE definition.

  Analysis of the formulated values of indicators, as well as frequency analysis of the spectrum of the power consumption signal can be used for diagnostic procedures and monitoring of the technical condition of BP. Nevertheless, changes in the technical condition of BP with the development of various types of defects have different effects on the values of different indicators, that is, different indicators may be more or less informative to identify certain types of BP defects or to determine the possibility of further work (monitoring). The informative value of the indicator for identifying a certain type of damage or defect lies in the intensity of the change in its value during the development of the defect..

  Conclusions

  To date, diagnostic methods do not allow to fully diagnose equipment in all operating conditions, which means that they do not affect the reduction of costs associated with the failure of the electric motor. Therefore, the development of any new methods or the use of methods not previously used in diagnostics is relevant.

  References

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