Best choice of material for induction charging unit. 1

[paulintheocean]

Dear All

I'm designing an induction charging unit for use in the marine environment and would appreciate some advice.

Its construction is that of a secondary core docking within a primary (much like your electric toothbrush).

Sizes are approx. Primary : ext. diameter 250mm
int. diameter 220mm
length 200mm

Secondary : ext. diameter 185mm
int. diameter 100mm
length 200mm

Ignoring corrosion issues (each wound core is completely potted/isolated from the salt water environment) what would you recommend as the material of choice for these cores. Due to the material dimensions and a intended manufacturing qty of one unit I have provisionally chosen EN1A carbon steel billets to be machined.

Literature would indicate cast steel as being a favourable choice but for a one-off would be to expensive a process.

Thanks in anticipation of a response.

Paul Coughlin
Mech. Design Engineer

 

[sreid]

I may be missing something here but I think you are basically building a transformer. I think you need to use laminated electrical steel or you'll have large hysistersis and eddy current losses in the core.

 

[paulintheocean]

Hi Sreid,

Your right in that me describing this as an induction charging unit is just another name for a transformer, when charging submersible units in a marine environment this is the term given to the form of charging.

Look at the dimensions you'll see that the construction is essentially one cylinder docking into a 'donut' style cradle on a very large scale.

The sheers size of the units alone does not allow me to use laminated electrical steel. Additionally some machining operations need to be performed. You cannot machine laminated steel without splintering/shattering of the glass/metal layers, EDM is not suitable either, which leaves laser cutting as the best method).

The only option is to use a material that is readily machinable and available as billets in the sizes required.

Large hysisteris losses and poor efficiency are acceptable and the risk of eddy currents has been removed by applying a split along the axis of each core.

 

[UKpete]

The split along the axis of each core will not stop the eddy currents, they will flow within each half. Assuming you are operating at 50/60Hz you effectively have an induction heater. It will get hot and possibly burn out the windings.

Laminated electrical steel is used in the very largest electrical machines, it is also machined and ground. I'm not too sure what you mean about glass/metal layers, typically laminations might be 0.5 - 1.0mm thick with a few microns of inorganic coating to prevent conduction of eddy currents between laminations.

Alternatives are powder iron assemblies - these have high electrical resistivity but reasonable magnetic properties. There are also soft ferrites, powder technology but work at lower flux density than steel.

 

[RobGer]

Hi Paul,
I'm designing an electro magnet requiring a machinable core of suitable material.
Have you found a suitable core material for your project ?

Regards.

Rob Ger

 

[nbucska]

Can you use higher frequency?
RobGer: you may use powdered iron in e.g. epoxy
as Pete suggested . If you need, you can line itup by
magnetizing it during curing.

You may use a bundle of iron wires.
If you need more help, we need more info.



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Warpspeed]

Agree with UKpete, while cast iron and mild steel have fairly reasonable magnetic properties at low flux densities, they will also act as one giant shorted turn. The core absolutely must be either laminated or tape wound to minimise eddy current flow.

I assume you are planning to use the standard 50/60Hz mains frequency ?

Another problem with this is that the total air-gap between core halves must be kept very small. That might be difficult to do, and also maintain a robust long lasting protective anti corrosion layer. You will also need to ensure the primary cannot be energised until the secondary half is full engaged, otherwise the primary magnetizing current might be destructively high. A shunt magnetic air-gap might go some way to solving that problem.

All quite difficult to do I am afraid.