Electro magnets anyone?

Status
Not open for further replies.
M

milemaker13

Senior Member
Location
Suburbs of Chicago
We just got a large lamination die in house that has several , about 18 or 20 I think, electromagnets mounted in the die face. These would supposedly retain the part until a part catcher is in position and then they would release to drop the part.

I have no info on this, the die looks like someones dropped project, and any wiring they had is all cut up. Just as well because they had 5 prong 277/480 and 4 prong 240 twist locks suppling power, and 4 flat trailer wiring to connect prox switch sensors.

These are 1.5 " diameter, 24 vdc magnets. Their cut sheet says they should draw about 6 watts each (.25 amps) with a holding force of 88 lbs each (our part may weigh about 10-15 lbs tops).

One question is how should I switch power? Would a contactor handle this, or would I be looking at SCR type devices? The switching cycle should be less than 20 strokes per minute.

Another question is about polarity... If I apply DC one direction the magnet should attract steel.. If we apply DC in the other direction (reverse polarity) would this EM now repel steel? One of my co-workers thinks this could aid in stripping the part off the die face. Do DC electromagnets act in this way?

I am about to start some mad scientist type testing, but figured I'd drag you all into it as well. Thanks!!
 
A relay/contactor should be fine to switch, but switching DC is much harder on contacts than AC because there is no zero crossing. SCR's work only for DC, so if you have any AC controls feeding recitfiers and you want to switch the input, SCR's won't work. For that you'd need triacs. As far as polarity, it doesn't matter, as long as all the coils are the same. Repulsion only applies if you have two magnets, not ferrous metal.
 
What's your DC source? It will be easier to switch the AC into a rectifier than the DC itself. Also, make sure the supply can tolerate the back-spike from the coils when you turn them off. With the right supply you can even ramp up/down the DC :).
 
meternerd said:
As far as polarity, it doesn't matter, as long as all the coils are the same. Repulsion only applies if you have two magnets, not ferrous metal.
Click to expand...
But it would be great if it worked like that, wouldn't it? When I was a kid I read a Tom Swift book where he invented a repelling magnet he called a repelatron that could be tuned to repel any substance. :)
 
electro Mags pull both polarities.

Best to switch would be FETs, but you need to design the gate drivers. The inherent body diode will snub the turnoff.
 
If there is induced magnetism in the workpiece or residual magnetism in the magnets because of hysteresis (iron instead of air core), then a short reverse pulse may speed release.
 
I worked on scrap handling magnets at a foundry. They could lift five tons. Simply turning the magnet off and the load would drop off a bit at a time, apply the short weak negative pulse and the load dropped immediately.

If the OP is using these magnets in a process that requires speed then residual magnetism is going to be a problem.
Some more information would help.
 
Great info so far. Let me try to answer in order...
I don't have a power supply yet, I don't know what may have been used in the past. I like the idea of switching the AC vs DC for contact life, also switching between voltages and polarities could present issues.

When you say reverse the polarity for a moment at 1/3 power... Do you mean 1/3 voltage? that's about 8 volts.

What qualities would I need to look for to ensure my power supplies can handle the "back spike" you mention? I mean, is there a term or rating I should look for?

How about using two separate power supplies (24vdc & 9 vdc) with outputs wired for reverse polarity and switching the AC input to these?

Ram comes down, mags come on at say 180*. Ram comes up, straight polarity contactor drops out @ 350, reverse contactor comes in @ 352 til 360. Part falls perfectly into outstretched tray everytime;)

They've done studies ya know... 70% of the time, it works every time :thumbsup:
 
Last edited:
DC supplies tend to have polarity-sensitive components (electrolytic caps, surge protection, etc.) on their output, so I wouldn't connect two in parallel with opposite polarity. Better to switch the outputs.

/mike
 
Mike-
Thank you. Nearing the edge of my operational envelope, I'd have figured this out eventually... But probably not before wasting precious smoke. :lol:

Say there is 20 magnets, at a 1/4 amp apiece that's about five amps. If I use the appropriate contactors, surely this would not be a problem? Also, this may help with any kind of start up lag the power supplies may have... Like was mentioned earlier, speed is a major factor because at 20 SPM each revolution takes only 3 seconds.

OK, so we still use the two separate power supplies but switch the outputs instead of the inputs. I should be able to use the 24v supply for controls at the same time then, since it will now be powered up at all times.
 
Jraef said:
If it were me, I wouldn't waste a lot of time trying to invent something that is already made and can be bought, installed and operating in a short time. It's not like this has never been done before.

http://www.magnetechcorp.com/reversing_current.html
Click to expand...

But ... that takes all the fun out of it. :D



Seriously for anything but my own home I am a firm believer in using purpose built parts and do not try and cobble things together. I think that just makes repairs more likely and more costly in the future.
 
Yeah, I've come across electromag controlers at McMaster carr that can do the flip auto matically for me. I need to go out and count the actuall number of EMs in the die, but this unit outputs 100 watts (based on 20 mags I would need about 120 watts). So I would need to use two ($750 apiece), which may also provide some advaltage since this is a 2 out die, each part could be controlled seperatly. This may come in handy when we are trying to time this all out on the floor.

I am still researching online. I will check the website you mentioned.
 
Hey, terrific news! This here magnetechcorp has a 250 watt model for the bargan price of only $600. Includes pole swapping for dropping parts... Hmmmm, it IS like they have done this before!!

How about a machine that will slice white bread into uniform thickness? Anyone on that yet?
 
150725-0856 EDT

Note: the three second cycle time. At 8766 hours per year this is 8766*60*60/3 = 10,519,200 cycles per year. The relays in the proposed power supply are very unlikely to last anything close to a year. If only 8 hours per day divide by 3.

Two factors are of importance --- 1 mechanical life, and 2 metal transfer from one contact to the other from uni-directional current flow. The metal flow can be reduced by inserting a polarity reversing relay leading to the current switching contacts that is toggled when current flow is not present.

This application requires solid-state switching and appropriate snubbers for good switching life.

.
 
Status
Not open for further replies.
Top