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Old Nov 26, 2012, 12:04 AM
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How does a coreless motor work?

Hi there,

I'm just wondering how a coreless motor works... any ideas? I've looked around the web, but couldn't find anything with my search terms. I'm kinda looking into how the following are done:

1) Switching polarities on the rotor
2) how the coreless rotor is winded up (ie. do they alternate between wires for every 'wind')

Just trying to fully understand how servos work so I can have a deeper understanding of coreless motor servos
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Old Nov 26, 2012, 12:13 AM
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It's similar to the metal core, except the part that switches polarities (comutator) is at the top. This is the coreless motor from a micro heli that i opened up.



It's like an outrunner. The permanent magnet is in the center, and the electomagnet is a very thin spinning cylinder made up of copper wires that slide in between the magnet and the steel wall. The part that switches polarities is in the lower right pic. Brushes are in the lower left. In a toy motor, there are three contact points, on the coreless, there are five.

On second thought, it's nothing like a metal core
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Old Nov 26, 2012, 12:53 AM
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Do you know how the magnetic fields look like on rotor when the copper wires are energized? Unlike the metal cored motors, the coreless ones don't loop into a coil. I'm having trouble visualizing how the rotor would rotate.

Thanks for the pictures! it helps!
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Old Nov 26, 2012, 01:23 AM
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I haven't actually unwound it, so i can't verify, but the coiling may actually be there. If you notice, the cylinder at the top (open end is on the left), the directions of the wires on the outside is opposite of the one inside as seen in the middle-right pic. They're overlapping coils shaped into a cylinder.



Not sure what the magnetic field looks like but there are dozens of them close together. Maybe the reason there are five contacts instead of the usual three is so they can simultaneously polarize a small group and form a legitimate circle for a magnetic coil.
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Old Nov 26, 2012, 12:28 PM
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As far as I know, a coreless motor is not functioning on the principle of equal polarity magnets rejecting and opposite polarity magnets attracting, but on "Lorenz's Law" that says that a straight conductor in a magnetic field, carrying a current, experiences a force perpendicular to that conductor and perpendicular to the magnetic field lines.

You might notice that all wires are parallel, and not really forming a coil.

In servo's coreless motors are used because they accelerate faster, but I believe they are slightly less efficient compared to normal brushed motors with Iron core.

Brgds, Bert
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Last edited by Brutus1967; Nov 26, 2012 at 01:55 PM. Reason: Euh... I think it was Lorenz, not Lenz....
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Old Nov 26, 2012, 12:52 PM
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Quote:
Originally Posted by Brutus1967 View Post
As far as I know, a coreless motor is not functioning on the principle of equal polarity magnets rejecting and opposite polarity magnets attracting, but on "Lenz's Law" that says that a straight conductor in a magnetic field, carrying a current, experiences a force perpendicular to that conductor and perpendicular to the magnetic field lines.
You're thinking of the induction motor I think.

A coreless motor is the same as any other brushed motor. Remember making a motor with a nail and winding wire around it? If you pulled out the nail and left only the windings, that would be a coreless motor. It is more efficient because of lower rotating mass.
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Old Nov 26, 2012, 01:22 PM
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Originally Posted by jasmine2501 View Post
You're thinking of the induction motor I think.

A coreless motor is the same as any other brushed motor. Remember making a motor with a nail and winding wire around it? If you pulled out the nail and left only the windings, that would be a coreless motor. It is more efficient because of lower rotating mass.
No.... I am definitely not thinking of an induction motor.... that is a completely different thing that needs 2 or 3 phase AC to run, the rotating magnetic field in the stator windings create inducted eddy currents in the weak-iron rotor, and indeed, the rotor then forms a conductor carrying a current and subject to a magnetic field.

If you coil a wire (core or not) you are creating an electromagnet. A straight wire however is not a coil, and does not create a traditional bipolar magnetic field when carrying a current, but still a magnetic field does exert a force on that conductor if a current passes through it.

Of course a coreless motor is a brushed motor, because the static field magnet has North and South magnetic poles. Without brushes the wire being forced to the left when passing one side of the fieldmagnet, would be experiencing a counterforce when passing the other side of the magnet. So you still need a form of commutation.

In the pictures in post #2 you can clearly see that the copper wires are all straightened and parallel in the area where they are subject to the magnetic field of the stator magnet.

Brgds, Bert
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Old Nov 26, 2012, 01:58 PM
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http://www.freepatentsonline.com/4103196.pdf

http://www.freepatentsonline.com/4103196.html
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Old Nov 26, 2012, 02:15 PM
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There is a distinct and clearly visible difference between the arrangement of the rotor windings in the above mentioned patents, and the rotor shown in post #2.....

Brgds, Bert
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Old Nov 26, 2012, 02:54 PM
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In the rotor shown in post 2 I am only able to see the outermost layer of windings.

The text for the patent says:

FIG. 1 is an axial section showing a motor which is typical of the prior art.
FIG. 2 is a perspective view of the rotor used in the construction of FIG. 1.
FIGS. 3 and 4 show respectively, in a "developed" view and in a schematically perspective view, the nature of the winding used in the present invention.
FIGS. 5 and 6 are corresponding views showing an alternate form of winding.
FIG. 7 illustrates, in perspective, a rotor and associated brushes constructed in accordance with the invention.
FIG. 8 shows a greatly enlarged fragment of the end edge engaged by the brushes.
FIG. 9 is a fragmentary section looking along line 9--9 in FIG. 8 and showing the use of wear-resistant plating.
FIG. 10 is an axial section taken through a motor constructed in accordance with the invention.

To me at least, image 6 in the patent (the alternate form of winding) looks a lot like this -




And that looks a lot like the motor used in the V911and even the MSR, except I can't see the underlying layers in the MSR image.



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Old Nov 27, 2012, 11:24 PM
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Quote:
Originally Posted by Brutus1967 View Post
As far as I know, a coreless motor is not functioning on the principle of equal polarity magnets rejecting and opposite polarity magnets attracting, but on "Lorenz's Law" that says that a straight conductor in a magnetic field, carrying a current, experiences a force perpendicular to that conductor and perpendicular to the magnetic field lines.

You might notice that all wires are parallel, and not really forming a coil.

In servo's coreless motors are used because they accelerate faster, but I believe they are slightly less efficient compared to normal brushed motors with Iron core.

Brgds, Bert
Thanks for pointing me in the right direction. I think I understand what's going on, forgot about the Lorentz force.

Here's a cross section diagram of how I think a coreless motor works after interpreting what you said:



We know F = I x B

F - force
I - current
B - magnetic field

TL;DR: We have 2/3 variables, when I and B are 90 degs from each other, it produces a force which turns the motor.

We have the current and we have the magnetic field, so it produces a force orthogonal to to the mag field and current when the direction of the magnetic field is perpendicular to the current. There has to be some type of logic as to when a particular wire should be powered, but (if I'm not wrong) that's the theory behind a coreless motor.

Corrections are more than welcomed!
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Old Nov 28, 2012, 12:30 PM
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You can look at this stuff in many different ways.

On the lowest level, you have the fundamental Lorentz Force.

However you can look at it also as the interaction of two magnetic fields that are trying to align themselves in an anti-parallel (e.g N-S, S-N) fashion.

Any moving current generates a magnetic field around the wire carrying the current. The vector sum of all these little current carrying segments produce the equivalent of a larger magnetic field.

The advantage of a coreless motor is less inertia to movement---it can accelerate faster. Also since there is no core (=iron type material) that is being magnetized by the current, you have no hysteresis or eddy current losses. However a core is like a magnifier of the field generated by the windings. In other words you get a stronger magnetic field with less current. So a coreless motor is likely to have more resistive losses than an equivalent core motor that is making the same torque.

So you win and you lose some!
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Old Nov 29, 2012, 11:27 PM
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Quote:
Originally Posted by Alan Hahn View Post
You can look at this stuff in many different ways.

On the lowest level, you have the fundamental Lorentz Force.

However you can look at it also as the interaction of two magnetic fields that are trying to align themselves in an anti-parallel (e.g N-S, S-N) fashion.

Any moving current generates a magnetic field around the wire carrying the current. The vector sum of all these little current carrying segments produce the equivalent of a larger magnetic field.

The advantage of a coreless motor is less inertia to movement---it can accelerate faster. Also since there is no core (=iron type material) that is being magnetized by the current, you have no hysteresis or eddy current losses. However a core is like a magnifier of the field generated by the windings. In other words you get a stronger magnetic field with less current. So a coreless motor is likely to have more resistive losses than an equivalent core motor that is making the same torque.

So you win and you lose some!
Thats a good way of thinking about it!
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