Effect of asymmetry of supply voltages on asynchronous motor operation modes

Effect of asymmetry of supply voltages on asynchronous motor operation modes was investigated using the computer-aided simulation experiments, the software complex Matlab and the package program Simulink. Publication relevance is determined by the necessity for performance of comparative analysis of the level of effect of asymmetry of supply voltages on asynchronous motors of different series and power capacities. Six motors of three different power capacities (5.5, 55, 160 kW) and of two different series (4A, AI) were modeled for investigation. The value of asymmetry of supply voltages was changed from 0 to 5%. The motor shaft load was changed from 0 to 120% of the motor rated load. Comparison of dependencies of the phase currents on the voltage unbalance ratio by reverse sequence (K2U) of asynchronous motors with short-circuited rotor series 4A and AI was made. The analysis and discussion of obtained investigation results were performed, recommendations on operation of asynchronous motors series 4A and AI in conditions of asymmetry of supply voltages were given.


Introduction
At present the three-phase AC asynchronous motors (AM) is the most common type of the electrical machines.As a rule, AM are produced by large series, of which the most widely spread are the general purpose machines of series 4A, AI.Series 4A, AI are the mass series of asynchronous motors designed for using in different industry areas.In accordance with [1,2], the asynchronous motors are designed for a life time 15 -20 years without overhaul provided that they are operated properly.Proper operation implies their running under rated parameters specified in AM passport certificate.However, under actual operating conditions of motors, depending on different factors, there is significant deviation from the rated modes of operation.
As the investigations [3][4][5][6] and many years' experience of operation demonstrate, one of the dominant causes for breakdown of motors is the asymmetry of supply voltage.AM operation under conditions of asymmetry of supply voltages results in change of the maximum rotation moment, sliding, currents in the motor windings.As a result, due to excess of the phase current rated values [7], heating of AM active parts occurs leading to winding insulation overheating and reduction of asynchronous motor lifetime.Thus, with asymmetry of voltages making 2%, the asynchronous motor lifetime, due to additional real power loss, reduces by 10 % [8].
Currently, in Russia a number of regulatory documents exists, which establish the set of the electric energy indicators and standards of quality [9], as well as requirements for reliability electrical machines [10,11,12].
The regulatory base for formation of recommendations aimed at ensuring AM operational reliability, includes: -GOST 32144-2013 "Electrical power.Electromagnetic compatibility of technical equipment.Electrical power quality standards in the general purpose power supply systems", -GOST IEC 60034-26 -2015.Part 26.Effect of unbalanced voltages on on operating characteristics of three-phase asynchronous motors, -GOST Р 52776-2007.Rotating electric machines.Rated data and features.
In accordance with GOST 32144 -2013, the following standard values of the voltage unbalance ratio by reverse sequence K 2U are established: the values of the voltage unbalance ratio by reverse sequence K 2U in the electrical power delivery point, averaged at 10 minutes time intervals shall not exceed 2% during 95% and 4% during 100% of one-week time interval.
In accordance with GOST IEC 60034-26 -2015.AM with K 2U above 1% (during continuous mode of operation) shall be unloaded "… to avoid its breakdown".And AM operation " with 5% of asymmetry excess is not recommended".
The analysis of modern publications [4,5,13] showed that these values do not always correspond to GOSTs and, in practice, can significantly exceed them.
Within the framework of this publication, it is necessary to perform investigation of synchronous motor with short-circuited rotor, their most common series 4A and AI, to make analysis of the effect of asymmetry of voltages on AM operation modes based on the motor series and power capacity.To get a more complete picture of the effect of asymmetry of voltages on AM, it is necessary to compare dependencies of AM phase currents on the voltage unbalance ratio by reverse sequence (K 2U ), under different motor shaft load values.It is obvious, that such investigation will make it possible to solve the issue of determining the level of effect of the asymmetry of voltages different series and power capacity AM, and besides, to identify the permissible rate of K 2U (under AM standard mode of operation) depending on AM load.
It is clearly understood that experimental investigations play significant part in provision of AM reliability.In their turn, they enable formulating the proposals aimed at improvement of AM operating reliability and, in addition, prevention of occurrence of situations determined by overheating of winding insulation.

Investigation materials and methods
During investigation of AM operation modes, the computer-aided simulation is taken as the basis for virtual model implemented in the visual and efficient means for interactive environment simulation modeling Simulink and programming Matlab [14,15], using Тshaped scheme for asynchronous motor replacement.
Earlier, the publication author developed the computer model enabling to make analysis of AM operation modes in conditions of asymmetry of supply voltages [16].Besides, in the obtained computer model, there is an option for investigation of transition processes in the asynchronous motor, for reading operational and imitation mechanical characteristics, of supply grid voltage, currents and frequency.
The scheme for investigation of operation modes for synchronous motor with short-circuited rotor created by means of simulation modeling in Matlab/Simulink environment is given in (Fig. 1).AM operation modes with account of below listed parameters were investigated using the developed model: the value of the motor shaft load was changed from 0 to 120 % with the pitch of 20 under different values of K 2U , the value of the voltage unbalance ratio by reverse sequencefrom 0 to 5 % with the pitch of 1 %.
The asynchronous motors series 4A, AI with the motor rated rotation frequency n=1500 rpm, with significant scatter by power capacities from 5.5 kW to 160 kW were subjected to investigation.

Investigation results and their discussion
Analisys of computer-aided simulation (Fig. 2-4) allowed revealing the following trends: 1) asynchronous motors with rated power capacity 5.5 kW: -AM series 4A Рn = 5.5 kW, i.e. the specified series motors with small rated power, are subjected to greater effect of asymmetry of supply voltage and may breakdown even under standard GOST 32144-2013 values of K 2U .the current in phase "A" exceeds the allowable over current accepted as equal to 1.1 of the rated current (this is the permissible excess value in accordance with relay protection requirement [19,20]), with K 2U = 1.3%, and for AM series AI Рn = 5.5 kW it occurs with the value K 2U = 2%.The motors series 4A overload under 80% load already working within the permissible range of K 2U , series AI under 100% load, -overcurrent of motors series 4A Рn = 5.5 kW is observed with the value K 2U above 4% with K z = 60%, and for series AI with K z = 80%, under load factor increase up to 100%, the start of winding insulation heating is observed under K 2U from 1,3% to 2%, 2) asynchronous motors with rated power capacity 55 kW: -the start of winding insulation heating is observed in the specified AM under K 2U = 0-4%, whereas this shall be outside that range limits.At that, this is observed with K z = 60-100%, -with the load of the motor series AI up to 40%, and the motor series 4A up to 60 %, AM standard operation mode will be observed with the standard value K 2U from 0 to 4%, 3) asynchronous motors with the rated power capacity 160 kW: -for the motor series AI Рn = 160 kW under K z up to 60% and the motor series 4A Рn = 160 kW under K z up to 80% the standard operation mode with K 2U from 0 to 4% will be observed, which corresponds to GOST 32144-2013.
-for both series, with K z above 80 % the start of winding insulation heating within the standard ranges of K 2U occurs, 4) the currents under motor full load are of ultimate interest.With the load value equal to 100%, the current in phase "A" for AM series 4A Рn = 5.5 kW exceeds the allowable overcurrent accepted as equal to 1.1 of the rated current under K 2U = 2% [19,20].For the motor series 4A Рn = 55 kW, excess of the allowable overcurrent occurs upon reaching K 2U = 0.9 %.This attests to the fact that the motors have not only contrasting ranges of permissible values by K 2U , but also the values, which do not always correspond to the ones standardized by the GOST.Correspondingly, during operation of motors in locations with poor quality electric power, it is necessary to adjust the supply voltage guided not by the GOST for electric power quality, but by the motor operation mode.Based on the objective set forth in the research study, the main conclusions can be represented: 1) the following indicators affect AM allowable operation mode under asymmetry of supply mode: motor series, power capacity, load factor (K z ) and voltage unbalance ratio by reverse sequence (K 2U ).
Thus, under K z within the range from 60% to 80% the start of majority of AM is observed under K 2U value below 4%.The range of AM series 4A operation modes is under K z from 0 % to 60%, the range of AM series AI is under K z from 0 % to 80% with K 2U from 0% to 4%.
Consequently, the allowable value K 2U differs not by AM series only, but also due to motor series, because of the motor power capacity immediately, 2) the allowable range of K 2U (under AM standard operation mode) with permanent asymmetry of voltages, full motor load: -for the motor series 4A Рn = 5.5 kW, the allowable range K 2U from 0 to 1.3%, for the motor series AI Рn = 5.5 kW K 2U from 0 to 2%, -for the motor series 4A Рn = 55 kW the allowable range K 2U from 0 to 0.9%, for the motor series AI Рn = 55 kW K 2U from 0 to 0.7%, -for the motor series 4A Рn = 160 kW the allowable range K 2U from 0 to 1.8%, for the motor series AI Рn = 160 kW K 2U from 0 to 1.4%.
Consequently, under excess of ultimate permissible value K 2U , overheating of AM winding insulation will be observed, 3) AM reliability significant reduction occurs under K 2U exceeding 4%.To exclude overheating of both AM series 4A, and AI breakdown, it is necessary to reduce the load.AM full load is possible only under K 2U maximum 1 % [11].Thus, after performing the complex of investigations on effect of asymmetry of voltages on asynchronous motor operation modes, the recommendations on AM series 4A and AI operation under asymmetry of supply voltages were stated: 1) AM series 4A are subjected to the largest effect of asymmetry of voltages, therefore, at the stage of AM power supply system designing and modernization, this condition should be taken into account.Consequently, AM series AI are less exposed to effect of asymmetry of voltages, and have increased operation reliability, 2) to avoid the motor overheating and premature breakdown, it is necessary to reduce its load, 3) rate setting of the voltage unbalance ratio by reverse sequence should be carried out based on the motor permissible operation modes and their models produced in lots, 4) to make responsive control during UPQI (unified power quality index) deviations, for stable operation of AM, it is necessary to control the electric power parameters at the enterprise input or at the respective motors, 5) it is proposed to use the investigation results for correction of current standards in the area of electric power quality standards, as well as in specifications for particular motor series, 6) the investigation results will be necessary for development and designing of PSP (power supply system) containing AM. Particularly, it will be relevant for regions with large amount of one-phase non-linear load, 7) the obtained values of allowable values K 2U for AM play the key role during symmetry facilities selection and implementation in the power supply system with the motor drive load, 8) it seems feasible to use the approach based on the method of computer-aided simulation in Matlab/Simulink environment for assessment of affect of voltage supply asymmetry on AM operation.

Conclusion
The effect of asymmetry of voltages on asynchronous motor operation modes was investigated using the computer-aided simulation experiments, the software complex Matlab and the package program Simulink.Following the investigation results, the significant effect of the asymmetry system of supply voltages on different series and power capacity AM operation modes was detected.Based on specific examples, change of characteristic curves, on the quantitative side, was demonstrated.Recommendations on operation of asynchronous motor series 4A and AI in conditions of asymmetry of supply voltages were presented.
During solution of issues for ensuring asynchronous motor operation reliability, the experimental investigations form the basis.Their practical implementation will allow for improving reliability of motors operated in conditions with available permanent asymmetry of voltages.

Fig. 2 .
Fig.2.Curve of current in phase "A" AM series 4A, AI with Рn = 5.5 kW from K 2U under different permanent load factors.

Fig. 4 .
Fig.4.Curve of current in phase "A" AM series 4A, AI with Рn = 160 kW from K 2U under different permanent load factors.

Table 1 .
Cataloged data of investigated three-phase motors.Characteristic curves of phase currents from K 2U for AM series 4A, AI were obtained during computer-aided simulation.Current curves in the function from K 2U , under different values of motor shaft load were plotted for each phase of considered AM.
Curve of current in phase "A" AM series 4A, AI with Рn = 55 kW from K 2U under different permanent load factors.