How to provide power to a spinning circuit ?

Hi Tauno,

>>How about transmitting a little power on a license free band such as ISM
>>(maybe 13.65MHz?), picking it up via small coils on the shaft and
>>rectifying it there.
>>
>>Of course you'd have to check the legal situation in the countries where
>>this is going to be marketed.
>>
>>    
>>
>We tried it - the shaft is prone to get most of the power and
>convert it to heat due to iron losses. The coupling link geometry
>has to be designed so that the magnetic field hits the secondary
>coil but not the shaft and surrounding metal parts.
>  
>
The coils would have to be perpendicular to the shaft, not wound around 
it. For example, you could have several oblong coils wound on flex or 
baked into a molding and the winding would never loop around the shaft. 
You could even have only one coil but that is difficult to mount and 
clean unless you use a flex circuit that can be wrapped around and held 
at a distance from the shaft's surface. The thing is to create an 
antenna that has a reasonable effectiveness.

>The solution in our case was a ferrite transformer at 20 kHz. The
>power transferred was small, some hundreds of mW. It showed
>that the transformer air-gap variation due to mechanical tolerances of
>shaft mounting had a pronounced effect on the transfer efficiency.
>  
>
Yes, the air gap can be a problem. But it may be ok if you only need 
little power and there is no hardcore limit on primary power (not 
battery powered etc.).

Regards, Joerg

http://www.analogconsultants.com

Joerg wrote:
> 
>>> How about transmitting a little power on a license free band
>>> such as ISM (maybe 13.65MHz?), picking it up via small coils
>>> on the shaft andrectifying it there.
>>>
>>> Of course you'd have to check the legal situation in the
>>> countries wherethis is going to be marketed.
>>>
>> We tried it - the shaft is prone to get most of the power and
>> convert it to heat due to iron losses. The coupling link
>> geometry has to be designed so that the magnetic field hits
>> the secondarycoil but not the shaft and surrounding metal parts.
>
> The coils would have to be perpendicular to the shaft, not wound
> around it. For example, you could have several oblong coils
> wound on flex or baked into a molding and the winding would
> never loop around the shaft. You could even have only one coil
> but that is difficult to mount and clean unless you use a flex
> circuit that can be wrapped around and held at a distance from
> the shaft's surface. The thing is to create an antenna that has
> a reasonable effectiveness.
> 
>> The solution in our case was a ferrite transformer at 20 kHz.
>> The power transferred was small, some hundreds of mW. It showed
>> that the transformer air-gap variation due to mechanical
>> tolerances of shaft mounting had a pronounced effect on the
>> transfer efficiency.
>
> Yes, the air gap can be a problem. But it may be ok if you only
> need little power and there is no hardcore limit on primary
> power (not battery powered etc.).

If you already have something rotating, why worry about any funny
transmissions.  Just create a magnetic field with a permanent or
electromagnet, mount some sort of coil on the rotating device, and
presto, you have an alternator.  The real power source is whatever
is doing the spinning.

I am coming in late, so I may well have missed something mentioned
earlier.

-- 
"The most amazing achievement of the computer software industry
 is its continuing cancellation of the steady and staggering
 gains made by the computer hardware industry..." - Petroski

CBFalconer <cbfalconer@yahoo.com> says...

>If you already have something rotating, why worry about any funny
>transmissions.  Just create a magnetic field with a permanent or
>electromagnet, mount some sort of coil on the rotating device, and
>presto, you have an alternator.  The real power source is whatever
>is doing the spinning.
>
>I am coming in late, so I may well have missed something mentioned
>earlier.

In this particular case, IIRC it spins too slow and needs too much
power for that the be practical.

A rotating transformer is the best answer in this case.  It's just
like any other transformer, except the primary and secondary can 
rotate relative to each other.  Standard, off-the-shelf technology.