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I have a project where I would like to use an electromagnet to lift an item for a longer period of time. I was looking at a magnet like this one:

http://www.ebay.com/itm/110880874455?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649

Problem is that seller states that it should not be used for more than 5-10 minutes at a time (without severe reduced lifespan). I would like a magnet that could stay on for an hour at a time at least and lift/hold maybe 20kg. Is this possible?

I was thinking that one might be able to do this by getting a significantly larger magnet than required and powering this on a lower voltage than specified to avoid it burning out (less current going through the coil), but this is just a guess on my part.

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    \$\begingroup\$ The ebay link doesn't say anything much about the electromagnet but i suspect it will overheat - it consumes 10W of power. Have you contacted the seller to ask about you need to hold for 1 hour? Can it be on for 5 mins and off for 5mins repeatedly? Could you therefore use two alternately? \$\endgroup\$ – Andy aka May 30 '13 at 9:06
  • \$\begingroup\$ I agree with Andy - heating of the coil is the problem. Is there some way you could mechanically hold the item using the electro-magnet to lock (latch) and release it? Have look at stone lifters. ggrgroup.com/stone-lifters/boulder-stone-scissor-grab \$\endgroup\$ – JIm Dearden May 30 '13 at 9:26
  • \$\begingroup\$ I asked for the duty cycle / time it could be on for this item: ebay.com/itm/40mm-12V-Holding-Electromagnet-Lift-20kg-Solenoid-/… The response I got was: "Reasonable working hours are 5-10 minutes, Could work for an hour but it will affect the life and temperature will be high , may burned coil, suggest not use too long time , Thank you " \$\endgroup\$ – falense May 30 '13 at 10:24
  • \$\begingroup\$ Having two might just do the trick, otherwise there is (as mentioned) always the option of having some sort of locking mechanism. However that increases the build complexity a lot. Is there anyone who have done something similar before? Like a electromagnet for pulling the object in, then maybe a solenoid (stroke) for locking? \$\endgroup\$ – falense May 30 '13 at 10:28
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    \$\begingroup\$ This may be relevant - you may be able to back-off the supply once the object is lifted and physically at rest/stable. This comes from solenoid valves - you need the full supply to switch but you can back-off the supply to about a third once the valve has operated. \$\endgroup\$ – Andy aka May 30 '13 at 13:10
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Old reply, but thought it may help others Tricks for getting solonoid continuous duty (stay on) and redusing energy! (use resister(still now resister gets warm(think cooling). also create duty cycle see here: http://www.electronics-tutorials.ws/io/io_6.html

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    \$\begingroup\$ It's worth including some of the details from that page relating to duty cycle so the answer remains useful if the link ever dies. \$\endgroup\$ – PeterJ Mar 17 '14 at 7:05
  • \$\begingroup\$ Interesting method! \$\endgroup\$ – Kamil Jun 15 '14 at 15:20
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You are guessing right.

You can use larger electromagnet and feed it with same power as smaller magnet. You will get similar lift.

Thing that reduces lifespan is temperature. Bigger electromagnet has higher surface area and can disspate more heat. So if it cools itself more than small electromagnet - it can take more power without overheating.

You can look at electromagnets with diffrent shapes (more surface), or find electromagnet that can be screwed up to some aluminum that will help to disspate heat and keep temperature below 85'C (thats safe temperature for typical wire insulation types).

Feeding DC electromagnet with PWM (variable duty cycle) sometimes may be bad idea, because:

  1. Electromagnets like this shown on Ebay are not built for alternating current. Solid core will produce additional heat due to Eddy currents. Thats why AC transformers and coils are made of thin plates instead of solid metal.

  2. Alternating current (PWM), depending on PWM frequency may cause little windings movement, that may finally damage insulation after long time.

You can prevent these 2 things by adding some filter after your PWM regulator (big capacitor?).

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Since the magnet can only stay energized, at most, for 10 minutes out of every hour, that's about a 17% duty cycle.

Assuming that the winding dissipates 120 watts with 12 volts across it means that its winding has a resistance of 1.2 ohms and the current through it will be 10 amperes.

Then, for continuous operation, (100% duty cycle) the coil must dissipate no more than 17% of 120 watts, which is 20.4 watts.

Since P = E²/R, E = sqrt(PR) and, for a coil dissipation of 20.4 watts,

E = sqrt (20.4W * 1.2R) ~ 4.95 volts.

With the magnet rated to hold 20kg - or whatever - with 12 volts across it, there's absolutely no way that it'll be able to hold that weight with only 5 volts across it.

The obvious solution is either to find an electromagnet which will provide the hold you need and is rated for 100% duty cycle or, as you say, find a bigger one with duty cycle spec'ed at less than 100% for a higher load, but which can go forever on a lower voltage supply and will give you the hold you need.

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