CFD Measurements of the Cooling Air in a DC-Motor

Latest Mechanical Engineering Seminar Topic on CFD Measurements of the Cooling Air in a DC-Motor

 Abstract :

The cooling system of a DC-motor is examined in this thesis. A change of direction of the
cooling air is desired to prevent the generated coal dust from entering into the windings of the
machine. Ultimately this will have a negative effect on the cooling in the machine and the loss
of cooling needs to be compensated through other ways. The purpose of this thesis is to work
for an improved operational safety and performance of the DC-motor and to make it more
competitive in the market. By modelling the interior geometry of the machine and defining
the boundaries in the software programs Gambit and FLUENT respectively, the motion and
the heat transfer of the airflow could be simulated. The simulation results would give us an
understanding of the flow pattern which later could be used to develop design modifications
on the cooling system of a DC-motor. In this thesis the main focus lies on creating a
simulation model with a sufficiently fine mesh size.

Summary :

The cooling system in a certain rotating machine of the type direct current (DC-) motor is
examined in this thesis. In a DC-motor there is coal dust generated at the connecting surfaces
between a stationary part where the electric current is inserted to the rotor (the Brushes) and
the adjoining rotating surface (the Commutator). This dust is sucked in through the windings
of the machine when the airflow is going from the non-drive side of the machine, which is the
most common nowadays, and this dust is causing a lot of damage. The idea investigated in
this thesis is to drive the airflow through the machine from the opposite side, the drive side,
and by doing so also avoid getting the dust into the machine. From a thermodynamic point of
view though this is less efficient and the deterioration of the cooling should be counteracted
by improving the cooling in other ways. The purpose is to work for an improvement of the
operational safety and the performance of the DC-motor and to make it more competitive in
the market.

The first and main goal with this thesis work was to create a simplified model of the DCmotor
that simulates the flow and heat transfer of the cooling air through the system. This
model could then be used to analyze the airflow through the machine. By examining different
properties of the cooling air it would eventually lead to a better understanding of the general
motion and heat-transfer pattern of the air in the system. The final objective would have been
to use the simulation model and the knowledge of the patterns in the fluid to develop design
modifications in the machine that would improve the cooling. Unfortunately this goal was
never reached because of the complexity of the task and time limitation of the thesis.
For the building up and the simulation of the created models the software programs Gambit
and FLUENT were used. First of all a simple 2D model version was made to give us an
overview of what kind of problems that could occur during modelling and simulation. A cross
section segment of the Commutator was modelled and simulation results of the velocity and
temperature in the airflow were generated from it. From those results it was noted that there
was a short cutting airflow along the outer wall where cold air went directly from the inlet to
the outlet passage, and another flow closer to the Commutator wall that was circulating in a
vortex formation distributing the heat from the surface to a larger area.

The main modelling work was put on a 3D-model of the DC-motor with sizes of geometry
and mesh parametrisized. Since the modelling was rather complex it was delimited to include
just the Commutator part of the machine. Further it was necessary to systemize the geometry
building to get a higher level of control and this was accomplished by defining the whole
geometry in Python script which would generate a Gambit journal file when compiled. In the
Python script the geometry was divided into three separate sections, each projected onto a
general radial plane, and there after extruded stepwise in the axial direction. The formed
volumes could then be activated or deactivated from its mesh according to the conditions in
the machine which is reproduced. Because of the structure in the axial direction it is possible
to use prism-shaped mesh cells in each section and thereby decrease the number of mesh cells needed. All the volumes containing air were meshed and simulated as well as one solid
volume, the copper layer on the Commutator surface where axial heat conduction occurs.
Further the turbulence model used here was the k-omega model. It was chosen because it was
more convenient since you are able to use both a near wall function and a function for the
wall roughness for the same surface.

For the 3D-simulations made the boundary conditions are based on very roughly set or
assumed values and can therefore only be used for studies of relative effects. The influence of
the mesh size on the results was studied by generating and simulating meshes with different
sizes and then comparing the results. Data used for this comparison were the average heat
transfer coefficient on the Commutator surface and the net heat transfer rate of the cooling air
through the boundaries. The data fluctuation within each mesh was studied as well to examine
their time dependence. It was found that all the simulated mesh sizes gave rather similar
results when compared to each other and when compared throughout the fluctuation in time
within each mesh. Therefore the differences in mesh size could be neglected. Visual
comparisons were also made of the simulation results between the meshes, and the monitored
results were the temperature and the velocity patterns of the cooling air. The fluctuation of the
flow of each mesh was visually examined as well, by creating a movie out of a bunch of
velocity pictures.
A steady state solution could not be reached for the simulations because the quick oscillation
in the flow the solutions are prevented from being stabilized. On the other hand it would have
been too time-consuming to look for a transient solution at once, since the oscillating flow
needs small time steps to be resolved while the solid heat conduction needs a long time to
stabilize. So therefore it was necessary to divide the whole simulation process into two steps,
first a transient simulation where the heat transfer was neglected and thereafter a steady state
simulation of the heat transfer with the flow fixed at its current position. Through this way the
simulation could be made within reasonable time and the result would be accurate enough
since the heat transfer in the model is not likely to influence the fluid motion noticeably.



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