Making a transformer 5

[BobLWeiss]

I need to make a air-core type transformer that can produce 6 volts DC (rectified) and at least 600 ma's. What I am doing is winding the secondary coil with 27 awg magnet wire with a diameter of 1" about 150 turns. The primary will be same wire but 100 turns and about 1 1/4" diameter. The secondary coil will need to be able to spin freely in the middle of the primary for my project. I plan on sending 12 vdc at 200Khz through the primary coil to exite it. Am I on the right track or would I need more/less windings on either coil? What about heat buildup? I am not that experienced with transformers to figure it out and was hoping someone here had some knowledge? Thanks!

 

[tinfoil]

Are you using DC ('12vdc') or are you using some form of ac (200kHz)?

 

[BobLWeiss]

I am using DC 12v.

 

[tinfoil]

transformers cannot operate on DC!

 

[BobLWeiss]

If you pulse DC into a coil you can make it produce electricity on the secondary wire. The trick is pulsing it, like 200Khz wave. I've seen it done, I just need to know how many turns for both sides would produce the output I need.

 

[jghrist]

Is this a motor or a transformer? We generally do not like our transformer windings to spin around!

 

[GOTWW]

Basically you are taking 2 air-core solenoids and magnetically coupling the to make a transformer. The biggest problem will be the incomplete flux linkage due to the air gap. This site may help a little.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html#c3

and

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html#c3

There for to may an effect turns ratio of 2:1, may take a physical turns ratio of 3-4:1.

 

[GOTWW]

May be more effective to use an iron core stator with the outside wound rotor spinning around it.

 

[alehman]

Air probably won't support the magnetic flux density necessary to transform that amount of power with any reasonable efficiency.

http://www.du.edu/~jcalvert/tech/transfor.htmtml

http://faraday.fie.umich.mx/~fdeleon/Practical Aspects of Transformer Engineering.ppt

 

[GOTWW]

Not to mention at that frequency the interwinding capacitance would probably shunt out most of the primary.

 

[Skogsgurra]

What is going on in this thread?

A guy asks a simple and completely valid question.

He mentions chopped DC, which is DC - even if the "ripple contents" is high.

He also says that he wants the two halves to move (rotate) and that is being done all the time in static exciters etcetera. Rotating transformers are common.

We do not need to poke fun at him for asking these questions. Do we?

 

[Warpspeed]

Ok guys, I think I have the picture. He wants to transfer some power through a rotary transformer.

Air cored transformers do not work very well because the magnetic field does not know where to go. As soon as you try to load the secondary, the magnetic field will try to go around it instead of through it. You absolutely must have a proper magnetic path if you want to transfer any power through a transformer.

The best solution is to use two ferrite pot core halves and place a coil in each half. One side can rotate freely and the other side can be fixed. These pot cores can be purchased with a central hole, so an axle or bearing is pretty easy to make. It will behave just like any other transformer, except the two halves can rotate freely with respect to each other. Ferrite is as hard as glass, so the two halves will run for a very long time without wearing out.

This idea is not new, it is often used to couple signals and power into and out of rotating machinery where for various reasons slip rings are not appropriate.

 

[BobLWeiss]

Thank you for your input. I am posting a link where someone already has this idea working, they just don't give enough information as to the windings etc..and I need a little more power than he is producing:

example[/LINK]

 

[Warpspeed]

Bob, why not just use a pair of slip rings and some spring loaded contacts to transfer dc power direct ??

 

[BobLWeiss]

I was thinking of that but it would create alot of friction and noise.

 

[Warpspeed]

It still might be a lot simpler. Just power the slip rings direct from an ac plugpack, and fit the rectifier, filter, and voltage regulator to the rotating part.


Friction will be low if the slip rings are very small diameter. Or how about using the bearings themselves at each end of the shaft. Lots of ways to do it.

 

[mc5w]

There are also a lot of commercially made brushless selsyns a.k.a. shaft position resolvers that use a rotary transformer instead of brushes.

Most big city libraries in the U.S. have a a complete copy of the U.S. Patent Office files. Look up things like rotary transformers, brushless devices, wound rotor motors, and so forth and you will find a mountain of ideas and applications.

Mike Cole, mc5w@earthlink.net

 

[BobLWeiss]

I was kind of looking for some technical help and not "go to the library" type help. Appreciate your time.

 

[ScottyUK]

Hi Bob,

The biggest problem, other than the mechanics of making one part of the transformer rotate, may well be the size of the air gap across which you are trying to transfer power. Practically you will need some sort of transformer core, and at 200kHz it will effectively be limited to ferrite.

If you use a core, keep the airgaps as small as possible. This minimises flux leakage, making your transformer behave more like an 'ideal' transformer. The ratio of primary to secondary turns then can be closer to the theoretical V1/V2 = N1/N2. The additional turns you are considering are to compensate for the high leakage inductance of an air-cored coil.

I think Warpspeed's idea to use a pair of shell-type pot cores rotating on a common axis has some merit. If the mechanical axis is parallel to the flux lines, the windings will have no relative movement as far as the transformer is concerned, so it will behave more-or-less as it would when it was stationary.

Use a symmetrical waveform to drive the primary, rather than pulses. This will make better use of the core, maximising energy throughput.

Work on a maximum flux density of 0.3T for a general purpose ferrite like 3C8, and calculate your volts/turn for the primary winding on your chosen core. Calculate the secondary turns from the number of primary turns. You my need a few extra turns of the secondary to compensate for the effects leakage reactance.

Heating at 200kHz with tiny wire such as you are considering is pretty much according to Ohm's Law. Skin effect shouldn't be too significant, so use the DC resistance for your power calcs.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[BobLWeiss]

Thank you for the detailed suggestions. I am starting to mess around with it now and working on finding some sort of core to use on the secondary. I figured on using more turns also on the secondary to comepensate. I did a small test of 24 guage wire and used equally lengths and sent 12v through the primary and it produced about 3v on the secondary. Thats a start..