Computational methods for studying the thermal state of frequency-controlled asynchronous traction motors

. The article discusses methods for calculating the thermal state of a frequency-controlled asynchronous traction motor, and provides calculations of the thermal state of an asynchronous motor powered by a semiconductor frequency converter with an autonomous inverter. Methods for assessing the thermal state of traction asynchronous electric motors of locomotives under various operating modes are considered. Computational methods for assessing the thermal state of asynchronous motor units allow you to continuously monitor the thermal state of an asynchronous traction motor and prevent possible emergencies in a timely manner.


Introduction
With frequency regulation of the traction asynchronous motor, additional losses occur in the stator winding, this occurs from the influence of higher harmonics, from magnetic losses in the motor stator, as well as the magnetic flux created due to the stepped form of the supply voltage. All these factors lead to a significant distortion of temperature fields. At the same time, due to additional fields, the ratio of power losses of losses of individual structural elements changes.
Increased heating due to power losses when powered by voltage which is different from the sinusoidal one leads to an increase in losses by 29% [1,2].
In traction mode, locomotives with asynchronous drive increase the likelihood of overheating of the traction engine structural elements. The consequences of overheating are: breakdowns of the insulation of the windings, inter-turn short circuits in the stator windings, accelerated aging of the insulation, which leads to a reduction in service life [1][2][3]. Asynchronous drive electric motor (ADEM) overheating also occurs when the engine is running in idle, traction and braking modes.

Methods
The task is to improve the traction characteristics of asynchronous electric drives, increase energy performance by optimally controlling the motor currents according to the energy saving criterion when creating the required electromagnetic torque by optimally controlling the motor currents according to the energy saving criterion when creating the required electromagnetic torque. Development of methods for assessing the thermal state of a frequency-controlled asynchronous traction motor powered by a semiconductor frequency converter with an autonomous inverter Development of methods for improving the energy performance of mainline electric locomotives by controlling a traction asynchronous motor according to the criterion of minimum power loss with power supply voltage asymmetry. Calculation of the modes of occurrence of subharmonic components in traction power supply networks of railway transport containing traction converters, controlled and uncontrolled rectifiers.
To assess the parameters of overheating of asynchronous traction motors, methods of constructing thermal protection systems are used, as well as assessment of the thermal state of an asynchronous traction motor in starting and traction modes. When assessing the thermal state of the electric motor, it is taken into account that the main factor of insulation destruction is not the temperature itself, but the processes in the insulation that occur during sudden heating and cooling of the insulation, therefore, an urgent problem is the development of methods for assessing the thermal state of the machine, taking into account the influence of these processes on the thermal resource of insulation. to predict the reliability of the engine in operation, a thermal model is presented that allows taking into account the influence of the operating modes of the traction engine and its cooling system on the temperature distribution in it. The creation of thermal models of the engine allows you to solve auxiliary tasks. In particular, monitoring the temperature of the engine components makes it possible to regulate the traction of the locomotive, ensuring the movement of heavy trains without overheating the windings, control the power of the power circuit in case of a malfunction of the cooling system of electric machines. Methods of studying the thermal state of an asynchronous traction motor allow us to obtain a reasonable assessment of the thermal state of the engine, which means to ensure higher efficiency. Considering that the process of heating the engine when powered by a traction converter acquires the character of "self-heating". the calculation uses a two-mass model of a traction asynchronous motor with the separation of the stator winding into a separate node. This model makes it possible to reproduce the temperature dynamics of the parts of the asynchronous traction motor that are most critical to heating. The traction transmission includes an asynchronous traction motor, therefore, a model of a thermal node was developed to calculate the temperatures of its windings. The model takes into account the conditions of forced convection from the main cooling surfaces of the engine and free convection from the bed and bearing shields to the ambient air. For each elementary node, a heat balance equation was compiled, which has the form: The model takes into account the conditions of forced convection from the main cooling surfaces of the engine and free convection from the bed and bearing shields to the ambient air. For each elementary node, a heat balance equation was compiled, having the form [1,3,4].

Results and discussion
As a design element, half of the stator is selected in the longitudinal direction, the nodes of the thermal circuit simulate the main elements of the engine design, taking into account distributed heat sources, an equivalent circuit is drawn up. The power of heat sources Pe1 is distributed between bodies 1 and 2 in proportion to the length of the grooved and frontal (lf) parts of the stator winding.
The power of P3 is the sum of the main losses in steel Pst and half of the losses of additional Radd power. The coefficient of additional kadd losses takes into account the influence of technological factors.
The power of P5 is the sum of the electrical losses in the rotor winding of Pe2 and half of the additional losses.
Power P4, represents a part of the mechanical Pmech losses spent on internal ventilation. Power P6, represents a part of mechanical losses. The power of Pmech spent on friction in bearings. Part of the mechanical losses is spent in the external fan and does not participate in increasing the temperature of the internal parts of the engine.
The temperature increase in the stator winding is due to electrical losses the grooved part e.g1 ′ and losses in the frontal parts e.f1 ′ of the coils and is determined by the formula: Exceeding the temperature of the stator core over the temperature of the air inside the machine, °C, -a coefficient that takes into account that part of the losses in the stator core and in the grooved part of the winding, When calculating the thermal state of an asynchronous motor powered by a semiconductor frequency converter with an autonomous control inverter at rated load, the range of voltage and frequency changes is taken into account [2,3,5]. The transition from traction mode to idle mode is characterized by additional electrical and magnetic losses, which are somewhat reduced. This reduces the ambient air temperature in the cooling circuit [6][7][8][9][10]. This leads to a change in the heat dissipation power. In the main nodes of the engine, the heat dissipation power remains unchanged. During the transition from the load interval to the idling interval and from the idling interval to the load interval, the initial temperature values at intervals and for all nodes are determined by the formula: The current time t at each interval of each cycle starts from zero. The determination of the actual heating time is counted from the heating time of the reference unit, which selected the stator core of the electric motor [11][12][13][14][15][16]. In the calculations, the assumption is made: the thermal conductivity of the body material is large enough, the internal temperature differences of heating on the surface are determined by the time of actual heating of the support unit: where T is the heating constant of the stator core steel; 0 -stator core temperature; steady-st -steady-state temperature of the stator core; en -temperature of the environment. Actual temperature of traction engine components: -constant heating of the node. The excess of air temperatures when purging it through the traction engine is determined taking into account the obtained values of the actual heating temperatures of the engine components from the system of heat balance equations where is the actual air heating temperature; ∑ ⋅ =1 -the sum of heat emissions in the heated area; −1 ⋅ -air heating in the current area; ∑ =1 -the sum of the conductivities of a given area of air.

Conclusion
It follows from the calculation results that the heat exchange in the engine nodes in the initial section differs from the heat exchange in steady-state mode for an engine operating from a semiconductor frequency converter with an autonomous inverter. For the rotor winding in this mode, there is a reversal of the heat flow through the air gap in comparison with its direction in the steady-state mode. This is due to the current temperature values of the stator teeth and the rotor winding. The temperature in idle mode is reduced due to the heat dissipation power. Temperature fluctuations are limited by additional electrical and magnetic losses from higher harmonics of the current. Thermal calculation of traction asynchronous motors makes it possible to analyze the thermal state of all elements of the ADEM structure during its operation on an electric locomotive in traction mode.