I've been looking at some electromagnet door locks to use for other purposes than a door lock and I've found that they seem to be exceptionally strong for their size and power usage. The 12vdc models consume between 3-5w and have a 600lb holding force. I've seen other electromagnets that are made from old transformers that people claim 150-250 lbs that use much more energy (like 4-8 times as much).

I'm wondering if there is some "bad" marketing on these locks or if there is something inherently bad with using an old transformer (like a MOT with the secondary removed). The size of the MOT magnet seems to be much larger than the door magent.

here is a link to a couple door magnets that make the 600lb claim.

http://www.ebay.com/itm/Single-Door...ck-280KG-600LB-12V-Door-Lock-US-/141563107309
http://www.ebay.com/itm/Single-Door...ck-280KG-600LB-12V-Door-Lock-US-/141563107309
http://www.homedepot.com/p/1-3-8-in...Lock-with-LED-Indicator-Light-80004/203229893

 

The holding force will depend strongly on the full closure of the magnetic circuit. Transformers have a laminated core and so it would be difficult/impossible to get close mating of the core to the armature whereas the purpose built units both surfaces are very flat and so close mating is assured.

 

Those are similar to two things used in metal working. The first is an electromagnetic chuck to hold parts down. usually for surface grinding. The second is for sheet metal bending, called in generic terms a magnabend.

They use a solid core not laminated core like a transformer. But a transformer core could be used, but just the "E" parts of the core. The part that gets held makes the other part. These two types mentioned earlier have more strength, magnetically, than say a solenoid because they use both poles of the magnet, N and S. The part that is held makes up the "keeper"(like on an old style horseshoe magnet).

Some information on a DIY "magnabend", by the guy that developed them, he explains it better and gives a plan that works. You would need to scale it down for a door lock.
http://aaybee.com.au/Magnabend/Building Your Own Magnabend.html