Trying to understand eddy currents and laminations....

[boylesg]

From this can I take it that eddy currents only occur in that part of the core that is within the windings?

I have a MOT where there are two welds across the laminations joining the I part of the core to the E part, and one weld across the top of the E laminations.

I could not understand why these welds did not undermine insulation between the individual laminations but, if the above is true, then they are not located in that part of the transformer core where eddy current occur.

In the case of my MOT core, the centre 'prong' of the E is where the worst of the eddy currents would occur since the windings encircle this part of the core.

If you were silly enough to put a weld in this area then THIS would cause the core to glow red hot. Correct?

 

[Usama Siddiqui]

may be this can help you out,
eddy current is due to variable flux that is descending upon the core,now the area of the core determines the power dissipation of core, here R(core)=(rho)L/A the more is the area the less is its Resistance hence power dissipation and eddy current is reduced. and the flux is generally concentrated in the center ,i don think it will turn it hot .

 

[chuckey]

I came across a batch of faulty transformers at work, they were 230 V to 50V @10A, they worked but always ran really hot , say 70 degs C on their surface. The problem was that the long bolts (4 off) that ran through the stack of laminations at each corner did not have insulating bushes fitted even if they actually had sleeving down the threads, there was excessive eddy current flow. The manufacturer confessed his sins and replaced all 54 of them!!!
Frank

 

[boylesg]

and the flux is generally concentrated in the center ,i don think it will turn it hot .

I take it from this that my new understanding of why the welds on the outer edges of the laminations of a MOT do not create paths for eddy currents is accurate.

 

[mtwieg]

From this can I take it that eddy currents only occur in that part of the core that is within the windings?

I have a MOT where there are two welds across the laminations joining the I part of the core to the E part, and one weld across the top of the E laminations.

I could not understand why these welds did not undermine insulation between the individual laminations but, if the above is true, then they are not located in that part of the transformer core where eddy current occur.

In the case of my MOT core, the centre 'prong' of the E is where the worst of the eddy currents would occur since the windings encircle this part of the core.

If you were silly enough to put a weld in this area then THIS would cause the core to glow red hot. Correct?

Could you post an actual picture or drawing of this transformer? It's hard to imagine exactly what you're talking about.

In general, eddy current losses will be large whenever you have a conducting plane that is normal to magnetic flux lines. So if a weld is on the outer face of the transformer core, then it's likely not a big problem since the flux is not normal to that face. But if a large weld forms a face on the center pole then that would be a big issue.

 

[boylesg]

Sure.



Just to be clear the E I I am talking about is this:
**broken link removed**

The mounting plate at the bottom is attached to the 'I' part of the core and you can see one of the welds that join it to the 'E' part of the core.

There is another weld at the top of the 'E' part of the core.

 

[Usama Siddiqui]

may be i am not able to get into your problem bro,anyways,as you must have seen that the coil is mounted on a circular manner in the 'E' part in such a manner that both the primary and the secondary coil is circled on the middle column of that E part onto one another,which i presume would face the maximum eddy current effect,thats why it wud have got hot from the middle,secondly,since the coils are placed at the center therefore some high current (current above the rated parameter) cud make it hot as well. especially if u have attached some high load at secondary .