Operational reliability of low-voltage asynchronous motors at asymmetrical supply voltage

Issues of operating modes of 4A series asynchronous squirrel-cage motors and AI serried motors at asymmetrical supply voltage for different values of motor shaft load are considered on the basis of simulation using Matlab software package and Simulink package. In the simulation, currents in asynchronous motor phases were evaluated as a function of K2U, KZ, and based on these results, permissible operation parameters were evaluated for induction motors. The results obtained are the basis for technical solutions aimed at improving operational reliability of asynchronous motors, they will allow taking into account permissible operating parameters of asynchronous motors not only in the design, but also in operation in power supply systems of various facilities.


Introduction
Reliability is the most important technical and economic indicator of asynchronous motors (AM) quality which defines ability to operate reliably with unchangeable technical characteristics during a given period of time under certain operating conditions [1,2]. However, in actual electric motor operation conditions, due to various factors, there is a significant deviation from normal operating modes. The vast majority of the reason for this is poor voltage quality in the networks, in particular, asymmetry of supply voltage. Supply voltage asymmetry has a negative impact on AM, reduces operational reliability and increases risk of motor failures. Thus, service life of asynchronous motors operating under full load is reduced by 2 times at a voltage asymmetry of 4%. When the voltage is unbalanced by 5%, available power of the motor is reduced by 5-10% [3,4].
In this paper, operation modes of 4A series squirrelcage AM and AI series motors at asymmetrical supply voltage with different values of the motor shaft load were studied to identify AM operating modes.
Long-term trouble-free operation of AМ is achieved as a result of implementation of measures aimed at maintaining technical and operational characteristics within the limits necessary for their trouble-free operation.

Materials and methods of the study
The study of AM operating modes can be effective if computer simulation with virtual model implemented in a visual and effective simulation tool Simulink of interactive programming environment Matlab for Tshaped induction motor equivalent circuit [5,6] are used.
Simulation scheme for studying operation modes of asynchronous squirrel-cage motor, created by means of simulation in the Matlab/Simulink environment, is shown in (Fig.1). Using developed computer model, modes of squirrel-cage AМ were studied taking into account the following: -value of the motor shaft load varied from 0 to 120 % with 20 % increments at different K2U values, value of reverse sequence voltage asymmetry coefficient was from 0 to 5 % with 1% increments.

Results of the study and discussion
On the basis of the results obtained in the course of computer simulation and using linear approximation of the data, analytical dependence of the AМ current in the most loaded phase depending on the factors -reverse sequence voltage asymmetry coefficient (K2U), load factor (Kz), type of series and the electric motor powerwas derived.
The AМ current dependence was defined by formula: -for 4A series motor: I % of I nom = ((−0.0002P + 0.163) K 2U + (0.8126 Kz + 0.081))*100, % -for AI series motor: I % of I nom = ((−0.0003Р + 0.1535) K 2U + (0.8126 Kz + 0.081))*100, % The conducted study allowed establishing the following regularities: 1) at supply voltage asymmetry, following indicators influence permissible AМ parameters: power, load factor (Kz) and coefficient of negative sequence voltage asymmetry (K2U), 2) permissible AМ operation parameters (at normal AМ operation mode) are identified based on overload capacity. 3) the permissible value of K2U is determined, when the engine is fully loaded Thus, permissible values of K2U are different not only for different AМ series, but they also vary within the motor series, and they directly dependent on the motor power.
In accordance with standards GOST 32144-2013, permissible AМ operating parameters are: а) at K2U from 0% to 2%, range of normal operating modes of 4A series AМ is Kz from 0 % to 70%, range of normal operating modes of AI series AМ is Kz from 0 % to 80%, b) at K2U from 2% to 4%, range of normal operating modes of 4A series AМ is Kz from 0 % to 31%, the range of normal operating modes of AI series AМ is Kz from 0 % to 35%.
In the case of long-term operation, normal AМ operation at K2U = 2% is possible with reduced load: -for 4A series motor with Рn = 7.5 kW -to 70%, for AI series motor with Рn = 7.5 kW -to 73%, -for 4A series motor with Рn = 75 kW -to 74%, for AI series motor with Рn = 75 kW -to 77%, -for 4A series motor with Рn = 200 kW -to 80%, for AI series motor with Рn = 200 kW -to 82%. 4) permissible K2U range (in the case of AМ normal operation) at full load of the motor is defined as follows: -for 4A series motor with Рn = 7.5 kW -allowable K2U range is from 0 to 1.28%, for AI series motor with Рn = 7.5 kW -allowable K2U range is from 0 to 1.36%, -for 4A series motor with Рn = 75 kW -allowable K2U range is from 0 to 1.39%, for AI series motor with Рn = 75 кВт -allowable K2U range is from 0 to 1.58%, -for 4A series motor with Рn = 200 kW -allowable K2U range is from 0 to 1.68%, for AI series motor with Рn = 200 kW -allowable K2U range is from 0 to 2.21%.
As a result of the study on influence of main factors on AМ operation mode, it should be noted: 1) regulation of K2U should be performed based on the permissible operating modes of motors and their commercially available models. It was found that, at long-term or repetitive voltage asymmetry, technical balancing means with output asymmetry of not more than 1.3% should be provided for AM power supply sources even if K2U value complies with requirements of GOST 32144 -2013 [11], 2) necessity of optimal solution to change AМ load or application of input technical balancing means in power supply system is defined primarily by following factors: technical and economic analysis of the solution feasibility; nature of asymmetry in the network; features of the production process in the enterprise. Based on the obtained data on permissible Kz value, technical balancing means should be applied for AМ when K2U = 1.3% is reached; if K2U exceeds this value, it is necessary to reduce the AМ load. So, for example for 4A series AМ with Рn = 7.5 kW at K2U = 3%, it is necessary to reduce the load by 50%; for 4A series AМ with Рn = 200 kW at K2U = 3% -by 35%. When the motor load is reduced, range of normal operating mode is doubled. If the technological process of the enterprise and drive mechanisms connected to the AМ care able reduce the load, this method can be more cost-effective in the short production term than installation of technical balancing means or purchasing of more powerful AМ, 3) in the AМ operating conditions, when power supply network containing electric motor equipment is designing, it is necessary to take into account influence of permissible K2U, Kz values. This will increase reliability and efficiency of asynchronous motors and power supply systems in general.

Conclusion
By means of computer simulation experiments, using Matlab software package and Simulink package, permissible operating parameters of asynchronous motors were defined. Quantitative evaluation of main factors influencing AМ operation mode is shown with examples. This study results can be used to solve problems of power quality management optimization at enterprises.
The work was executed within the framework of FSBEI HPE Trans-Baikal State University Council for Research and Innovative Activities Scientific Grant No. 276-GR of 01/02/2019 implementation.