Judgment method of the dynamic threshold value of motor protection at a dynamic load rate

To address the shortcomings of traditional motor thermal protection, which uses the motor’s temperature or temperature rise threshold operating at full load as the basis for the judgment method, a dynamic threshold judgment method based on a study of the motor’s temperature rise characteristics under a dynamic load rate is proposed. Taking a forced air-cooled, three-phase asynchronous motor with 11 kW power as the research object, a 2D/3D motor model under steady-state conditions is established. The loss distribution of asynchronous motors at different load rates is obtained through electromagnetic field simulation calculations. The finite element method is used to simulate and calculate the motor’s temperature field. Taking ambient temperature and load factors into account, the study takes place from the perspective of motor temperature rise in order to acquire the law governing the change in motor temperature increase under various working circumstances. A method of converting the temperature and warming between the motor’s stator, winding, and rotor is proposed. A dynamic threshold judgment system for accurate warning of overheating problems in motors under any load rate is set up so that the thermal protection of motors can accurately be judged under variable load conditions, which offers a crucial theoretical foundation for the design of intelligent motor temperature rise prediction systems. It contributes to a methodological underpinning for the diagnosis and early warning of thermal health in smart motor systems.


Introduction
As the main power device of diverse electric drive equipment, the safe and steady operation of the electric motor has been widely concerned.Energy losses are unavoidable during motor operation.These energy losses not only result in a loss of electrical energy but also in an increase in motor temperature, which affects the motor's stability and service life.
In recent years, many specialists and academics have conducted research into the electric motor temperature rise.Zhao and Yu [1] conducted a magneto-thermal coupling simulation analysis on PMSM using the finite element volume approach.The test results suggested that the motor's temperature rises as the degree of demagnetization increases.Khazi et al. [2] utilized the finite element method to thermally analyze a network model of a TEFC-type induction motor to determine the temperature change trends of the motor's major components under no-load and rated-load states.Wu et al. [3] used a multi-field coupling approach to investigate the distribution law of an electric motor's transient temperature field under stable and unstable heat source operating states.Zhang et al. [4] constructed a finite element analysis model to compare the thermal characteristics of two types of doubly salient machines operating at various speeds and loads.The simulation results show that the doubly salient electromagnetic machine with concentrated field magnetomotive forces has higher efficiency and lower temperature rise under water-cooled conditions.Zhang et al. [5] concluded from research that the ambient and cooling temperatures under the action of forcing factors affect the temperature rise of forced water-cooled motors.Ding et al. [6] utilized a 55-kW ship-driven asynchronous motor as an example, the three-dimensional steady-state temperature field was calculated using the finite volume element method.The temperature rise characteristics of the motor's main parts during rated load operation were analyzed.Kong et al. [7] used the heat network model and the fluid-solid coupling model to simulate the transient temperature field of a permanent magnet torque motor under rated operating conditions, respectively, and validated the accuracy of the two temperature field models.Li et al. [8] established a magneto-thermal coupled finite element 3D motor simulation model to investigate the loss and temperature change trends of permanent magnet propulsion motors.They compared and analyzed the permanent magnet propulsion motor's electromagnetic performance, loss, and temperature rise operating at different adjacent pole spacings.Chen et al. [9] studied the effect of faults on motor temperature rise characteristics under various load conditions from the perspective of motor temperature rise variation for the fault problem of induction motor stator inter-turn short circuits.Wu and Fang [10] used the thermal motor network model to figure out the actual temperature rise while operating at different loads.Then they combined the results of this calculation with experimental tests to verify the model's ability to predict the temperature rise accurately under typical loads.
A positive impact has been made on the study of motor temperature and temperature rise by the preceding research.However, a note should be made that when the characteristics of temperature rise in motors were studied, the temperature and temperature rise values of motors operating at standard ambient temperatures and at full load were primarily used as benchmarks.The influence of variables such as varying ambient temperatures and load ratios on the temperature increase of motors was not considered.Motors are not always operated at full load during actual operation.Low load rate operation is more common.If the motor continues to use the full-load state temperature and temperature increase threshold as the standard for determining the motor's state during the low load rate operation state, there will be thermal health problems of the motor at low load rates.This is because, at this time, the temperature rise does not achieve the threshold value, and the motor monitoring system is unable to detect it.This would result in the minor breakdown escalating into a major breakdown that compromises the motor's safe and stable operation.
To effectively address the shortcomings of the conventional fixed threshold judgment method, Zhou et al. [11] specifically analyzed the effects of environmental temperature and load rate on the motor temperature rise.They proposed a motor temperature rise distribution law that accounted for the multi-factor effects of temperature variation and load rate.On this basis, this paper further simulates and analyzes the motor temperature and temperature rise under different loads, obtaining the curves of allowed temperature and allowed temperature rise for the motor's stator, rotor, and winding under varying load conditions.In order to achieve true real-time monitoring of the motor's temperature and temperature rise under variable load conditions, a dynamic threshold judgment method is proposed.It is able to convert the temperature and temperature increase of motors operating under various load ratios to those operating under full load conditions and monitor them with a uniform standard of full load conditions, greatly improving the accuracy of motor warning.

Motor basic parameters and material properties
The research object of this paper is a forced-air-cooled, asynchronous motor with the model number YZ200-6.Based on the motor's structural parameters and material properties, the RMxprt module of Maxwell is used to establish a 2D/3D model by drawing point lines and surfaces, and input parameters directly generate the motor magnetic field simulation model.Table 1 shows the motor's structural parameters.This paper uses the Maxwell software to build a basic model of the motor.It assigns properties to various parts of the motor model using the materials that come with the software's material library.Table 2 lists the most important material attributes.

Motor modeling and meshing
To conveniently calculate the motor's temperature rise, the unit motor model was chosen as the object of study for the simulation of its temperature field.On the basis of the symmetry of the motor's structure and its thermal conductivity characteristics, the following assumptions are made in this paper:  The surface of the motor has a constant coefficient of heat dissipation. The change of thermal conductivity and heat transfer coefficient with temperature are neglected. The conduction medium inside the motor is uniformly distributed. The effect of the motor spindle is ignored. The loss produced by the motor is all converted into heat.
Based on the fulfillment of the previous hypotheses, taking advantage of the aforementioned structural parameters and material attributes, the 2D/3D motor model is built in Maxwell, as shown in Figures 1 and 2. After modeling the AC asynchronous motor, meshing the motor model is required before simulating the magnetic and temperature fields, which is an important step for ANSYS coupling calculations.In this paper, the model will be meshed using hexagons based on the size and shape of each area of the model.Figure 3 shows the motor meshing results.

Motor loss analysis
The various motor losses, which are the main source of heat during motor operation, should be analyzed and calculated before studying the motor's temperature increase.Motor losses are generally composed of iron losses, copper losses, stray losses, and mechanical losses.
Copper losses  in asynchronous motors consist of stator winding losses and rotor winding losses during operation, the magnitude of which depends on the size of the current in each phase winding.The formula for copper losses is: In the formula,  Indicates stator current;  indicates rotor current;  indicates stator resistance;  indicates rotor resistance.
The iron loss  in asynchronous motors consist of hysteresis loss  , eddy current loss , and stray loss  produced by the stator and rotor core in the alternating magnetic field.In the formula,  ,  , and  are the hysteresis loss factor, eddy current loss factor, and stray loss factor, respectively; ω indicates power angular frequency; "Φm" indicates the motor's air gap flux; "s" indicates the asynchronous motor's slip frequency of; "f" indicates the alternating frequency of the magnetic field; "B" indicates the magnetic flux density which varies sinusoidally; "α" indicates an empirical coefficient, generally chosen as α=2.

𝑃
The mechanical losses of asynchronous motors consist mainly of losses due to fan friction and losses due to bearing friction during operation.The stray loss of an asynchronous motor is the entire consumption of the motor during the load operation, which includes both no-load and load stray consumption.In general, mechanical and frictional losses in asynchronous motors are challenging to calculate and are typically approximated according to empirical or available motor test data.
ANSYS Maxwell software is used to conduct electromagnetic simulation on the established motor model and compute various losses of the motor calculation model.Table 3 shows the calculated results for the motor's power loss, and the motor's loss characteristics under full-load and no-load conditions are shown in Figures 4 and 5.By analyzing the distribution of lost power in the motor at full load and no load, it is revealed that changing the motor load has a significant effect on the motor's power loss, which in turn influences the motor's temperature and temperature rise.To look into the effect of variable load rate on the motor's operating conditions, this paper conducts a motor loss simulation for 0-100% load with a 10% load interval of the motor.It acquires the change curve of maximum motor loss with load rate as shown in Figure 6.

Simulation analysis of the temperature rise pattern of asynchronous motors
To research the influence of factors such as load rate on the motor's temperature and temperature rise, the losses obtained by simulation are imported in Maxwell into Fluent to simulate the motor temperature field and to summarize the changes in the motor's temperature and temperature rise under various load states.
Considering the complex structure of the motor as a whole and the difficulty of monitoring, it is impossible to measure the motor's temperature rise under different operating conditions.Thus, the sensor is placed on a specific representative part of the motor.Observation points are set for three parts of the motor: the stator, winding, and rotor, to achieve motor temperature measurement.The motor is put at a standard ambient temperature of 25°C, and the temperature rise of the stator, rotor, and windings are studied as the load rate varies.Figure 7 shows the temperature rise profiles of the rotor, windings, and stator for various load rates at an ambient temperature of 25°C.Figure 7 demonstrates that the temperature rise of the motor's rotor and winding tends to be the same, while the magnitude of the change in the stator's temperature rise is relatively small.This is because as the load rate increases, the corresponding loss will gradually increase regardless of the winding or the core, which in turn generates much heat and leads to a change in the motor's temperature rise.

Dynamic threshold variation law of motor
The difficulty in monitoring the motor's temperature and temperature rise values is that the load factor of the motor changes during actual operation, which in turn leads to changes in the motor's allowable temperature and allowable temperature rise.When thermal motor protection is judged on the basis of fixed thresholds, the differences in the motor's allowable temperature and allowable temperature rise at varied load rates are often neglected.Monitoring motor temperature and temperature rise under variable load operation according to the allowable temperature and temperature rise under full load as the standard would result in a decrease in monitoring result accuracy.
The motor simulation at 25°C ambient temperature is used as an illustration.The motor's temperature and temperature rise while the load rate is 0-100% are obtained by simulation.Combining the national standards for allowed motor temperature and temperature rise, the motor's temperature and temperature rise at a light load state are first translated into a fully loaded state's temperature and temperature rise.The motor's temperature and temperature rise under full load are converted to the motor's maximum allowable temperature and maximum allowable temperature rise under full load.The conversion coefficient that converts the motor's temperature and temperature rise at various load states to the maximum allowable temperature and temperature rise at full load is obtained.Thus, the maximum allowable values for the motor's temperature and temperature rise at full load can be divided by these conversion factors to obtain the permissible values for the motor's temperature and temperature rise at different load conditions.Figures 8 and 9 show the allowable temperature and allowable temperature rise variation curves of the motor stator, rotor, and winding with the load rate at 25°C environment temperature.Through the change curves of motor allowable temperature and allowable temperature rise thresholds in Figures 8 and 9, the temperature and temperature rise values of the stator, winding, and rotor at varied load rates can be compared, thereby achieving accurate temperature and temperature rise monitoring of motors under variable load situations.
Utilizing the variations in the motor's temperature and temperature rise at varying loads, combined with national standards for the motor's maximum allowed temperature and temperature rise, the curve of the motor's maximum permissible temperature and maximum permissible temperature rise at various load rates can be obtained.The dynamic threshold change curve corresponds to the load rate, the maximum allowable temperature, and the maximum temperature rise of the motor.The dynamic threshold method enables monitoring of the motor's temperature and temperature rise under variable loads.It compares the motor's real-time temperature and temperature rise to the values of the motor's permitted temperature and permissible temperature rise from the system's dynamic threshold curve to achieve dynamic judgment of the motor's health status at different load rates.It will greatly improve the accuracy of real-time monitoring of the motor's health conditions and compensate for the limitations of the fixed threshold judgment method.Simultaneously, based on the pattern of changes in the dynamic threshold curve, it is also possible to convert the motor's temperature and temperature rise at various load factors to the temperature and temperature rise at full load, and monitor and warn the motor's temperature and temperature rise with the threshold value of the motor under full load condition as the standard.This provides theoretical support for fault diagnosis and temperature warnings for intelligent motors.

Conclusion
Through simulation analysis, this paper obtains the dynamic threshold change law of motor temperature and temperature rise, according to which the dynamic threshold judgment method of motor thermal protection under load rate fluctuation is proposed.It makes up for the deficiency of the traditional motor fixed threshold judgment method that fails to consider the effect of load rate variation on motor temperature and temperature rise.It greatly improves the accuracy and reliability of the motor fault monitoring system.It supplies a crucial theoretical calculation foundation for the design of motor operation monitoring systems, as well as helps to validate the rationality of intelligent motor warning systems.

Figure 4 .Figure 5 .
Figure 4. Loss distribution of motor under full load

Figure 6 .
Figure 6.Motor maximum loss curve with load rate

Figure 7 .
Figure 7. Temperature rise curves of rotor, winding, and stator under various load rates at 25°C ambient temperature.

Figure 8 .Figure 9 .
Figure 8. Temperature threshold curve of rotor, winding and stator with load rate at 25°C ambient

Table 1 .
Main structural parameter of motor

Table 2 .
Material attributes of the motor

Table 3 .
Calculation results of lost power 5