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Possible Motor Construction?

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LAN_403

KLH

Tue May 26 2009, 05:11PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Lately, I've been studying various types of motors and how they are constructed, in order to be better informed about what types I can use in my most recent project plan: an RC plane. I've collected quite a bit of information, and it has all been very useful.

I've tried to find inrunner stators that I can buy for making different types of brushless or contactless motors, but this has turned up no useful results, instead leading to sites where I would need to make a custom request (a.k.a. expensive minimum order). I also don't want to resort to disassembling old motors for their stators, as there is no guarantee that the stator will be matched for my design.

Instead, I thought up different methods of creating a complete magnetic circuit around the rotor. The pictures below illustrate my current idea:

The general construction for each stator piece, when viewed from the front, is shown in the picture on the left. An E Core would be put together, with the center leg on both halves trimmed down to create room for the rotor. Coils would be wound around the center legs of the two halves. When the coils are energized, they create a magnetic field in the core and a field that extends between the two center legs, thus creating a complete magnetic circuit without the need for the flux to go around a circular stator that encompasses the whole rotor. Instead it is built around a small section of the rotor without a requirement for magnetic coupling between phases. The flux for this arrangement is shown in the center picture.

All of these pictures are based on an induction motor. The rotor can simply be thought of as a short section of copper pipe, with the stator pieces spaced around the rotor. The current density in each piece of this arrangement is shown in the picture on the right. This shows that currents are induced sufficiently in the rotor ring. As for starting, I planned for the rotor to be spun up by a small separate starter motor.

It is easy to see how this arrangement could be adapted for a rotor made of permanent magnets. Another benefit of this construction is that E cores could be extracted from existing standard transformers.

There is only one concern that I have: Most magnetic steel laminations, when cut, ground, or in any way machined, lose some magnetic characteristics at the point of machining. How bad would these effects be? Would it be better to cut or to grind down the two legs to form the gap? If either of these are unacceptable in regards to magnetic properties, can ferrite cores be used and machined?

Any suggestions or comments on the construction, or advice on the machining of the E cores, would be greatly appreciated.

Thanks.

Sulaiman

Tue May 26 2009, 06:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Have a look for an old moving-coil meter
there are usually some laminations that may be ideal for your experimenting.

I don't see where the motive force comes from,
forces to the left look equal to forces to the right,
in the first diagram.

Does this mean you could use this arrangement as a "pipe launcher" ?

Since you're using eddy-currents I assume higher frequency = better?
In which case you should consider ferrite.

For the cost of P&P I can send (anyone) pairs of ferrite cores from inductors, ( PM me )
they are identical "E" pieces that when mated leave c5mm airgap in the center limb,
which is (from memory) about 18x22 mm area.
Youd need a max. 5mm rotor ... copper strip ?

KLH

Tue May 26 2009, 06:44PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Sulaiman wrote ...

Have a look for an old moving-coil meter
there are usually some laminations that may be ideal for your experimenting.

I don't see where the motive force comes from,
forces to the left look equal to forces to the right,
in the first diagram.

Does this mean you could use this arrangement as a "pipe launcher" ?

Since you're using eddy-currents I assume higher frequency = better?
In which case you should consider ferrite.

For the cost of P&P I can send (anyone) pairs of ferrite cores from inductors, ( PM me )
they are identical "E" pieces that when mated leave c5mm airgap in the center limb,
which is (from memory) about 18x22 mm area.
Youd need a max. 5mm rotor ... copper strip ?

Ferrite Usage: The plans for the motor called for the stator pieces (each E core pair and its windings) to be energized one at a time; therefore, I was thinking that instead of just one current pulse, the stator pieces could be energized by a high frequency pulse, in order to make better use of ferrite.

The motive force in the first diagram would be from the rotating magnetic field from the stator pieces. The field from each stator piece would not be rotating at first. In order for the magnetic field to rotate relative to the rotor, the starter motor that I mentioned previously would spin it up to speed. The force would then be exerted in the direction of rotation.

As for the copper strip, I am still deciding on a practical size. I'll update as soon as I can make a decision.

On another note, a pipe launcher would be interesting...

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