I haven't given transformer laminations much thought but I would like to know why a laminated transformer creates less eddy currents than a solid iron core. The prompt for this question is that I noticed some small transformers heat badly and they are laminated but some bright spark thought it would be a good idea to weld the laminations in place. The laminations still significantly reduce the losses but not as well as if the plates were left isolated. So why are the welded laminations not acting like a solid lump since they are electrically joined?

 

The laminations are supposed to be electrically isolated - http://www.allaboutcircuits.com/vol_2/chpt_9/8.html

 

Hi BT
Yes that is my understanding too but a major transformer manufacturer has decided to weld the plates of 'E' type transformers along the centre of both of the sides perhaps beacause it is a cheap and easily automated way of fixing the plates to stop vibrations. They do heat more than units that have isolated plates but they are orders of temperature cooler than a solid core. The explanation on this site is the established view of the eddy currents and maybe because the current has to flow up the plate and through the weld is the reason that significanlty less current flows so gives less heat.

 

Interesting little story here. When I worked in a broadcast station we had a 10KW standby transmitter that just wouldn't modulate right. The low frequency response was just terrible. This was a conventional "big iron" transformer using huge modulation transformer and reactor.

We carefully checked the inductance of the tranformer....well within specs. Checked coil for shorts and resistance....all perfectly normal. Did a hipot test to check for shorts and leaks...passed with flying colors.


After about a year of this, we learned that the varnish used in this transformer was known to break down and become non-insulating after about thirty years...result....no laminations! Fortunately, they were able to pressur impregnate the core and bring it back up to like-new!

Oddest problem I'd ever encountered in all those years in the biz!

Eric