Electro Magnet

[Oxbo Rene]

Good evening,
This is a little project I put together today and want to run it by you guys for your purusal.
Years ago I came by an Electro magnet used on "Mag Drills" , which are industrial drills used to drill holes in steel beams in the steel fabrication buisness. These drills have a very strong electro magnet on their base in order for them to be set in any position to drill holes, very usefull devices.
This particular one was laying in a pile of junk in the welding shop where I worked, the drill was completely destroyed and discarded into the junk pile, etc.
Well, I saw the magnet and knew that I'd like to play with it one day.
Finally having all my honey-do's caught up (and finally finished that scoreboard project (tx's to Mr Gibbs)), I decided to hook it up and have a really nice (strong) magnet to play with.
Only thing I knew about it was that it's resistance was 120 ohms, had no idea what the voltage requirement was.
I did a search on the internet and luckily found a schematic for one.
Apparently, the one I found (see attachment) just requires full wave rectification.
So, I conclude, perhaps this one is similar.
I devised a circuit and have implimented it, and, it works great, but, after about 10-15 minutes the magnet starts to get warm, and warmer.
This tells me that I am somewhat exceeding it's amperage rating.
Measuring the voltage I get 146VDC which corrolates to 1.2 amp.
Am thinking it probably uses only 1 amp, so am proposing adding a 26 ohm 50 Watt resistor
in series with the coil, that should drop the current down to 1 amp.
Then again, I'm thinking that's not a lot of drop, it'll probably require a bit more resistance to drop it down further, so that, I could let it run for considerable time without it heating up (as it does in real life situations using a retail drill, etc)
Any observations and opinions would be appreciated.
Am fully aware of the danger messing with the "mains" .........
I used heavy duty FBR and Cap just because I had them, etc......
Retail drill schematic didn't come out too clear but, is readable, neutral line seems to be missing on pic.......
Tx's
Oxbo

 

[crutschow]

The reason the magnet is overheating is because you added a 1000µF filter capacitor on the rectifier output that the other circuit does not have. That raises the voltage from 120Vrms rectified pulsating DC to the peak AC voltage of 146V steady-state DC. Why did you add the capacitor? Just remove it and it should operate as originally designed without overheating.

 

[Hero999]

It's also possible that it's designed to work from AC not DC, although if that's the case I would've expected it to overheat more quickly than that.

 

[Oxbo Rene]

Hmmmmm, true enough.
I wanted purer DC for better bang for the buck.
But, now that you point it out, is easily recognizable.
Guess I just needed someone to point out the obvious to me.
And, when I did check it out before adding the cap, it did have
awesome holding power.
OK, thanks a lot, I'm on it ...........
Oxbo

 

[crutschow]

I wanted purer DC for better bang for the buck.

The bang would be when the electromagnet coil shorts from overheating.

 

[Oxbo Rene]

To add;
I hadn't noticed before but, the electrolytic cap I used was only rated for 100V instead of 400V.
I went out there (back porch) in the morning when the thing was cold, and decided to time it to see how long it really took till warm up.
After five minutes to the second, there was a crackle, pop, boom, with beautiful white smoke coming out of the enclosure, fuse gone.
Inspection results in smoking/leaking capacitor, so was a no brainer, cut the cap out, new fuse, everything works fine, takes 15 minutes to achieve a mild warming, which I would expect being it is a coil, etc.
I can hold a 1.5 " (X) 1" (X) 6" steel flat bar on it, barley lifting one side and can feel the 120cps pulses.
If I lay the flat bar across the top of it = no way I can pull it off.
So, works good enough ..................................