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NEW METHODS FOR CALCULATING OF COOLING SYSTEMS OF DIRECT CURRENT ELECTRIC MACHINES
06.10.2025 13:25
[3. Technical sciences]
Author: Oleksii Tretiak, Doctor of Technical Sciences, Associate Professor, National Aerospace University "Kharkiv Aviation Institute", Kharkiv; Viacheslav Nazarenko, PhD Student, National Aerospace University "Kharkiv Aviation Institute", Kharkiv; Serhii Serhiienko, PhD Student, National Aerospace University "Kharkiv Aviation Institute", Kharkiv Anton Zhukov, PhD Student National Aerospace University "Kharkiv Aviation Institute", Kharkiv
ORCID: 0000-0002-7295-5784 Oleksii Tretiak
ORCID: 0009-0000-8154-9131 Viacheslav Nazarenko
ORCID: 0009-0000-6377-209X Serhii Serhiienko
ORCID: 0009-0001-5229-1685 Anton Zhukov
One of the trends of general optimization of DC electric machines is to improve their cooling conditions. This mainly concerns the cooling conditions of the main active parts, in particular the frame with poles and the armature together with the commutator and brush-holders device.
According to statistical data on the causes of typical emergency situations in DC electric machines, a fairly high proportion of failures is due to malfunctions of the collector assembly and its brush-holders device, including: switching malfunctions of various degrees, collector heating above the maximum permissible value, uneven brush wearing out, local collector beating, vibration and chipping of brushes, and a decrease in insulation resistance below the maximum permissible value. The root cause of some of these malfunctions may be insufficient cooling of the collector assembly.
An important feature of the thermal calculation of the collector unit is the need to solve the thermal problem with three types of heat releases: electrical losses in the classical formulation, losses due to friction of the brushes against the collector surface and additional losses caused by the action of parasitic currents. In the work [1, p. 2] the main methods of calculating and designing collectors, based mainly on experimental observations on existing machines are considered. On the basis of such calculations, it is possible to determine the expected maximum temperature of the assembly, but they do not allow to establish an accurate temperature distribution between the elements of the collector and the brush-holders device. Accordingly, to meet the requirements of reliability during operation, there is a need to revise existing methods with the implementation of solutions in three-dimensional formulation in software complexes using CFD methods [2, p. 2].
The thermal state of the collector of a double-armature DC motor was analyzed in a three-dimensional setting using the SolidWorks Flow Simulation computational package [3, 4, p. 2]. For this purpose, a three-dimensional model of the armature was built, including a protective casing around the collector. The heat transfer coefficients were determined automatically using the cooling air flow simulation.
A rectangular grid was used as the basic components. To ensure the required accuracy of the calculation, automatic mesh densification was used in places where the dimensions of the structural elements are significantly smaller than the cell size of the main grid.
According to the obtained distribution of flow speeds (see Fig. 1), there are "dead zones" with practically stationary air in the working zone, and for approximately half of the cooling circuit the air speed does not exceed 0.1 m/s. Thus, in this case, air blowing of the upper and lateral surfaces of the brushes is practically absent. In the immediate vicinity of the collector, the calculated air speeds are about 2 m/s, which should provide somewhat better cooling conditions for the collector itself and the internal parts of the brushes.
Figure 1 – Air flow speed lines
Fig. 2 – Air speed diagram in the horizontal plane
Fig. 3 – Temperature distribution diagram
The central part of the collector is located in the area with low speeds (see Fig. 2). The air passes along the side walls, and near the working zone with maximum heat release, instead of the required maximum speeds, on the contrary, their decrease occurs. These factors indicate a complicated cooling of the structure.
According to the results of the three-dimensional ventilation calculation, the heat transfer coefficient of the collector surface is 25 W/(m2·K), and the heat losses are 8700 W. The expected range of collector surface temperatures is within 261…264 °C (see Fig. 3), which far exceeds the maximum permissible temperatures both from the point of view of the operation of the brushes and from the point of view of the heat resistance of the existing electrical insulation of the collector relative to the housing and between its individual copper plates [5, p. 4].
The study found that the self-ventilation system of the collector through convection is extremely inefficient. The collector temperature in the worst-case scenario can reach 264 °C, which far exceeds the permissible operating temperatures of the insulating materials used in the collector's design, as set by the heat resistance classes.
References
1. Design of Rotating Electrical Machines Juha Pyrhönen, Tapani Jokinen and Valéria Hrabovcová © 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-69516-6
2. Tretiak, O., Arefieva, M., Makarov, P., Serhiienko, S., Zhukov. A. et al., "Study of Different Types of Ventilation and Cooling Systems of Bulb Hydrogenerators in a Three-Dimensional Setting," SAE Int. J. Mater. Manf. 18(3), 2025, https://doi.org/10.4271/05-18-03-0020
3. Akin J.E. Finite Element Analysis Concepts via SolidWorks. New Jersey; London; Singapore; Beijing; Shanghai; Hong Kong; Taipei; Chennai: World Scientific, 2009. – 303 p.
4. Tretiak, O., Smyk, S., Kravchenko, S., Smakhtin, S., Brega, D., Zhukov, A., Serhiienko, S., & Don, Y. (2024). Devising a calculation method for modern structures of current-conducting elements in large electric machines in a three-dimensional statement. Eastern-European Journal of Enterprise Technologies, 4(1 (130), 87–96, https://doi.org/10.15587/1729-4061.2024.310049
5. DSTU ІЕС 60085:2015 Electrical insulation. Thermal evaluation and designation. https://online.budstandart.com/ua/catalog/doc-page?id_doc=64354
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