Mechanical Engineering

# Design and estimation of a BLDC and its parameters from Scratch

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Hello, everyone,
Although BLDC motors are not my specialty, I find electric motors a very exciting topic.

I would like to build one of my own one day. I know that the design is a science in itself and the connections between the parameters are very complex and they also influence each other.

Nevertheless I would like to know if someone can give me a help how to roughly estimate the first parameters of a motor.

Since I will print the stator and rotor, the motor will not withstand high thermal stress, it is simply a matter of understanding the relationships and finally comparing the result with the assumption.

In most books you can only read how the voltage from the speed and the torque is related to the coil current.
I would be interested in the relationship between the following parameters:

- Number of poles...
- Rotor diameter
- Number of windings
- Wire diameter
- Speed
- Torque
- Voltage

I would try to deduce it from a very simple case.
Actually, I have a coil with direct current flowing through it, which is attached to a lever arm. On the other side, I have a permanent magnet with a static field like in this little skecht i made:

The force generated by such a coil depends on the current, the length of the wire and its magnetic flux density.
F_coil=I⋅L⋅B
This would mean that the torque depends on the current, the number of windings ( length ) and the diameter of the stator (lever arm r).

If I now increase the number "n" of coils and thus the number of poles, e.g. to 10, the forces would add up and thus also the torque.
That means, turning momentum could be described by:
M_motor=F⋅r⋅n

Can you say that about it? So if I neglect all the material properties like relative permeability and so on?

For me, that would mean that the torque depends only on the current. It follows that the thickness of the wire must be chosen so that its resistance is high enough to allow the current required for the torque to flow at the available voltage.
If the wire is too thin, the resistance might be too high, resulting in a lower current, which would not produce enough force?

I would conclude that a high voltage requires more turns and a thinner wire, because with a smaller current, the higher number of turns produces the same force?

Conversely, I would conclude that at low voltage a thicker wire with fewer turns could be used because the high current would produce the same force with fewer turns as with less current and many thin turns?

If the magnetic field turns fast, also the motor moves faster, how fast the motor reaches this speed, depends only on the acceleration, this depends on the torque, force, current ?

My goal is to think about some parameters, i.e. speed / torque and to develop the necessary parameters, i.e. the number of coils, windings, diameters etc.

Of course, I won't get exactly what I have calculated before, but I think it would be a very interesting project.

These would be my thoughts as a completely clueless person.
If somebody has a good source where the whole thing is explained in an understandable way or even if somebody of you could give me a "big picture" about these connections, I would be very grateful.

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Your drawing seems to imply the magnet on the stationary part and the winding on the moving part. Do yourself a favor and reverse those two items. Then you will not have to pass winding current onto a rotating element.

Better yet, rather than re-invent the wheel, buy a book on motor design and discover what is already well known. Might not be as much fun, but if you are serious about this, it will lead to much better results.

DrD

Edited by DrD
correct spelling

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