Did I get screwed on these electromagnets? - Page 1
Forum section: electronics / beginners

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 21, 2018, 02:48:49 pm
content: Hi!I need a bunch of electromagnets for a project I'm making. After some fiddling with a small set of electromagnets I realized I needed something beefier. My project will run on 12V DC and ideally I want as much electromagnetic force as I can get with those 12V at a reasonable cost. So I went on Aliexpress and ordered these:https://www.aliexpress.com/item/CL-P-25-20-Holding-Electric-Magnet-Lifting-5KG-50N-Solenoid-Sucker-Electromagnet-DC-6V-12V/32759668790.htmlThey are rated to produce 50N at 12V and 0.34A requiring ~ 4W.After a fairly lengthy shipping process they finally arrived and I started testing the magnets out. Much to my surprise it turns out that these magnets were weaker than my previous smaller magnets at 12V. I was initially very confused and checked that everything was wired correctly on my project board etc. Then I just took one of the new magnets and plugged it straight across a 12V power supply and measured, and they only draw about 0.136A at 12V. The resistance of the electromagnets are around 88 Ohms and this makes sense if you consider I = U/R = 12/88 ~ 0.136A.Have I been had here? Looking at standard formulas for electromagnetic coils the force produced is proportional to the square of the current drawn, which means that I'm getting only about (0.1364/0.34)^2 = 0.16 = 16% of the electromagnetic force that was promised.Am I missing something here? I appreciate any input.

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 21, 2018, 02:52:38 pm
content: On a side note - if I end up getting stuck with these magnets (no return or refund), would it be reasonable to have a step-up converter to 24V and run the magnets off of that instead in order to get a beefier current/force? I'm going to pulse the magnets and they will only operate at about 2-3 seconds max at a time, do you think that might produce any thermal issues?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2018, 03:17:53 pm
content: Wouldn't surprise me if they are actually 24V.They're a ball of wire, if they get too hot at 24V you'll know. Can probably run 48V if it's for a few seconds....Tim

author: Nusa
author_rank: Super Contributor
timestamp: July 21, 2018, 04:14:19 pm
content: With Aliexpress, confirming delivery is basically the same as saying "I'm happy with this purchase." You have to look at how they use it, not just the words themselves. Once you confirm, aliexpress releases the funds and are pretty much out of the picture so far as helping you goes. If you haven't confirmed, you can start a conversation about having received the wrong product or a product that doesn't meet specs. If you don't like the results of that conversation, you can open a dispute.I never confirm delivery until I've verified I got what I ordered, or at least what I expected to get.

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 21, 2018, 04:31:59 pm
content: Quote from: T3sl4co1l on July 21, 2018, 03:17:53 pmFair enough, I might give it a shot.Quote from: Nusa on July 21, 2018, 04:14:19 pmSolid tip, I'll keep that in mind. I don't think I ever confirmed the delivery. Either way I've opened up a dispute now.

author: amyk
author_rank: Super Contributor
timestamp: July 21, 2018, 05:08:46 pm
content: Yes, it sounds like they might actually be 24V.

author: SL4P
author_rank: Super Contributor
timestamp: July 22, 2018, 06:00:27 am
content: Keep in mind that whatever voltage you choose...You can use PWM to adjust the pull-in vs holding power needed.I typically pull in at 100%, and release to 30% after - say 50mS...

author: IconicPCB
author_rank: Super Contributor
timestamp: July 22, 2018, 06:46:11 am
content: The heading identifies the coils as either 6, 12 or 24V coils.Your choice to specify what You need.(B in Cyrillic alphabet is V in in latinic ALPHABET)

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 23, 2018, 09:24:01 am
content: Quote from: amyk on July 21, 2018, 05:08:46 pmQuote from: IconicPCB on July 22, 2018, 06:46:11 amYes it's likely a 24 V magnet, but it specifically says 12 V on it so I did what I could to get the right type.Quote from: SL4P on July 22, 2018, 06:00:27 amThat's a smart idea, however I'll be using the magnets to send on/off pulses in the 100-500 Hz range.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 23, 2018, 10:38:22 am
content: Can you measure the pull of the magnets at various voltages and check against the specs from the seller?That would tell you what you got for sure.

author: Jwillis
author_rank: Super Contributor
timestamp: July 23, 2018, 03:04:32 pm
content: Should say right on it what voltage it is .Is it possible that your current is to low? See attached picture.oops sorry wrong one.my bad

author: SL4P
author_rank: Super Contributor
timestamp: July 23, 2018, 05:40:33 pm
content: QuoteInteresting to see how they work in that role.You’ll want a substantial snubber to quench the back EMF operating like that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2018, 07:22:08 pm
content: More to the point, you'll need a lot of drive voltage, both turning on and turning off. Probably quite a lot more than rated voltage (regardless of whether that rating is correct or not).Example, stepper motors typically being rated for, say, 3 or 5V, but driven at 30V or more, to get the required current slew rate (dI/dt) to run at reasonable frequencies (~kHz). The driver is a switched constant current (usually hysteretic control), so excessive real power is not delivered to the motor (it never delivers, e.g., (30V)^2 / (DCR) watts, continuously, to a winding), but enough reactive power is delivered to deal with stray inductance of the windings.Tim

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 24, 2018, 09:33:18 am
content: Quote from: TERRA Operative on July 23, 2018, 10:38:22 amI don't really have the equipment to do proper measurements of force, but it would be cool to do.Quote from: Jwillis on July 23, 2018, 03:04:32 pmThe sticker on the physical magnets I received specifically say 12V.Quote from: SL4P on July 23, 2018, 05:40:33 pmMy plan is to drive them with a standard L293D H bridge which has built-in protection if I'm not mistaken. On the few tests I've done they seem to work quite well.Quote from: T3sl4co1l on July 23, 2018, 07:22:08 pmThat's interesting and is actually relevant to my project. I'm using the magnets to vibrate strings (hence the 100-500Hz range) and ideally I want the strings to start vibrating as quickly as possible when pulsed, so I might have to look into spiking the voltage for the first few hundred milliseconds or so. I'm controlling the magnets with an ardunio and L293D H bridges that are fed a constant 12V from a fairly hefty power supply (so no noticeable voltage drop under load). Do you have any suggestions for how I would go about spiking the driving voltage for the initial pulses?

author: IconicPCB
author_rank: Super Contributor
timestamp: July 24, 2018, 11:28:44 am
content: PWM is a possibility however dual spike and pedestal driver is probably the way to go.With PWM use a higher voltage ( say double the rated voltage ) and modify the PWM to alter average DC to energise the coil in a two step fashion.EDIT:The alternative is to operate the coil switching at a highvoltage but include series resistance to ensure coil current is safe. This will also alter the electrical time constant of coil favourably.

author: stj
author_rank: Super Contributor
timestamp: July 24, 2018, 01:16:53 pm
content: they are sold as "holding magnets",these are a specific type of magnet inteded to do things like keeping doors open,they often have no noticeable pull but are very strong once a steel plate is against them.i have worked with rectangular ones that had so much hold that you couldnt seperate the plate - used for security doors that unlock in a fire situation., yet the "pull" was un-noticable to the point i actually wondered if the coil power was routed via the casing so it only energised when it was in contact.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2018, 05:10:52 pm
content: Quote from: TakeItUpANotch on July 24, 2018, 09:33:18 amOh, so not even 12V, but 10V...L293 is pretty shit, it's old and best forgotten. A MOSFET bridge (with logic level inputs) is available almost as cheaply, while capable of more voltage and current, faster, and with less voltage drop. (I don't have a number handy.)That said, the contemporary solution would be L297.You might as well get an integrated coil driver -- yes, there are ready-made ICs for this!So, that said, to "pluck" the string as fast as possible: consider this is a linear system, from coil voltage/current, to string wobbling. You're fine, it's fairly instantaneous even at a tiny voltage. As long as you don't mind having a small deflection. If this is an electric guitar, just turn up the gain. SNR may be worse, feedback may be worse, but there's probably enough dB inbetween to get away with it.Now, if you know how much deflection you need, then you simply need that much coil drive times the mechanical gain of the magnet.Note that mechanical gain depends wholly on how strong the field is. Is this an air cored bobbin, to be fit onto something? Does this have pole pieces? Surely you need to extend the pole pieces with a nose, so that they pluck just one string, not "most" of them? If there are no pole pieces, it's just about hopeless, the field from an air core coil is very weak indeed. You will gain far more from adding pole pieces, than going from a AA battery to anything that has been discussed here!Tim

author: TakeItUpANotch
author_rank: Newbie
timestamp: July 30, 2018, 05:30:49 pm
content: Quote from: T3sl4co1l on July 24, 2018, 05:10:52 pmDo you have any recommendations on what MOSFET bridge I should get? The good thing about the L293D is it allows me to drive four magnets off of a single H-bridge making it a fairly inexpensive solution. But the voltage drop is a concern for sure.I was thinking I could just simply drive the magnets with a simple transistor and put a flyback diode to protect the rest of the circuitry. Any reason why I shouldn't?What I mean by having an instantaneous string reaction is overcoming the initial inertia of the string, so I get a quick sonic response out of it. The strings will vibrate an acoustic box and I don't plan on amplifying the strings at all by using guitar pickups or something like that. So the more they vibrate/deflect, the better really (within reason of course).For reference, I'm building my own version of this instrument:http://aaron-sherwood.com/works/magnetophone/I'm admittedly not that familiar with magnetic field theory, but is it possible to focus the magnetic field of by adding a contraption that narrows the flux? A quick google gives me thisIf a certain number of lines of magnetic flux is flowing in a piece of iron, and the shape of the iron changes so that its cross sectional area becomes smaller but the same number of lines remains the same, the flux density will increase. This is a way to increase B without having to increase Φ.I was thinking of achieving this by simply adding a chopped of screw and nut and attaching it to the core piece (making sure the nut and screw sits flush against the core piece) like this:Do you think that could work? I'm hoping that this area reduction in the magnetic field combined with a doubling of driving voltage (24V instead of 12V) will give me plenty magnetic strength to vibrate the strings effectively.I appreciate the help and feedback btw. I'm new here but this forum really is a great place to ask these questions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2018, 10:54:52 pm
content: Why not rip open a small (say 10-50W) loudspeaker, and arrange a linkage from voice coil to wire support, the support being flexible so it can be pushed, while eliminating tensile strain from the loudspeaker?It's ready made, the pole pieces are as good as they can get, and it's magnet based so you don't have to worry about attracting a string (square law, it attracts from either polarity), it's linear (pulls one way, pushes the other). You'll also get enough efficiency that you can drive the strings with whatever waveform you like, you can "pluck" them very hard, drive fundamental and harmonics, whatever you like. It's a loudspeaker with a weird notchy filter on it (the wire and attachments resonate at whatever frequencies).Tim