Rapidly Reversing Polarity Electromagets

What do you mean? Please explain? Are you talking about some sort of tester or testing device or testing procedure? I would know the CT polarity test whether reverse or not? :)

I must apologise. I cannot think of any way to express myself clearer.

Imagine two bar shaped electromagnets. ANY size. Imagine they are activated by any device. All I wat to know is how to rapidly change the polarity of the magnets through the activating device.

Let's back up a bit.

Can you tell us what it is you are trying to build and why? [p[] Or, maybe it would be easier to tell us what type of problem you are trying solve.

Thanks for response but I do not know what more to explain.

Two magnets ANY length. "ON" by a device.

How do I rapidly change the polarity on and off?

Sorry I can't think of any way to be more clear.

I can.

You are telling me what you want to do to solve a problem, but you are not telling us what the problem is.

In your case you may be right, but you know nothing of basic electronics, which means I could be here all day trying to answer your questions only to find that the proposed solution is totally wrong for the problem at hand.

You don't go into the doctor's office and tell them you need your spleen removed, please schedule a surgery for me. You tell him I have pain in my abdominal region, which is the first step in describing the problem, not the solution.

Does that help?

change your negative to a positive voltage. And of course your positive input to a negative input.

Are you trying to build a device to disable electronic equipment? Because that is what you are describing....or a degaussing coil?

http://www.wikihow.com/Make-an-Electromagnetic-Pulse

http://www.ofrei.com/page_220.html#2909

http://en.wikipedia.org/wiki/Magnetic_hysteresis

Maybe one of these?

http://cr4.globalspec.com/thread/79369/Safe-Way-to-the-Reverse-Polarity-of-a-Electromagnet

Thanks for your response but, as I said, I have ZERO knowldeg of electronics and the link proved too difficult for me to understand.

No.

ALL I want to do is rapidly chnage the polarity of two elctromagnets about four to twenty times a second.

I honestly can't see how to make this any more simple.

Good answer.

This is the simplest solution for the OP.

Go to an online tone generator site like:-

http://onlinetonegenerator.com/

Or find a downloadable offline frequency generator.

(Listen to it on your headphones to make sure it works at a higher frequency)

Key in the frequency you want and start the generator.

Plug in a PC speaker with base capability. The coil in the middle of the speaker will be doing exactly what you want.

Either build your own solenoid, or remove the speaker cone hold it still and stick your bar of iron in the middle of the coil.

Total genius! I couldn't have put it better myself.

#24

I do not contest your very clever solution BUT it will not work as you suggest and for several reasons.

In a loudspeaker the coil moves due to the interaction between its magnetic field and a field generated by a PM.

If you place in the coil free of all the rest an soft iron bar the impedance of the system will not allow any more the same behavior. In fact the magnetic field generated by the coil is very weak and with high inductance it will not be possible to reach for the magnetic field same dynamics as for the original device.

The solution is NOT what the OP asked for.

Absolutely correct.

The requirement is for two electromagnets. It did not say they were going to have an air core or an iron core.

The requirements state a variable frequency from 3 to 20 Hz, but it doesn't tell us the wave shape (square, sine, complex), nor the amplitude.

Using a stereo amp to drive these coils may or may not work as we do not know the impedance of each coil, how they are wired, nor the current required (just the length). The amp may be able to drive these coils, but the impedance and any variation may prohibit the amp from accomplishing the task if they fall outside the rating of the amp.

However, the idea may help demonstrate concept, but the actual requirements, which are much too loose, will probably not be met. We don't know for sure and I don't think anyone can provide the real end solution except by dumb luck.

What I do not understand is all the effort to suggest solutions when there are already available of the shelf: there are different types of solenoids either single or double acting which could be reworked for the requested goal. One has to buy those and make simple modifications, the coils are already designed for the usual voltages and accept as it is needed for their function soft iron cores. This would be exactly what the Op asked for.

It is enough to block the EM core and the function is given.

How is the Shakespeare play "Much ado for..?

If I had to guess (and with the dearth of info from OP that's all anyone can do), OP is trying to see if he can make his Watthour meter turn at half speed or maybe even in reverse.

OP please tell me I'm wrong by providing more details of your design.

No isea what a watthour meter is. Sorry.

Just two straight-forward, simple electromagents (probably bought from a toy or DIY store?) perhaps bar or horse shoe shape, powered by a "device" which activates them.

How do I rapidly reverse the polarity of the magnets about ten times second?

Still not quite sure what you're up to so here's what you need:

A variable speed motor or drill to drive a commutator. You should look at these pictures and find something that matches what you have in mind, no electronics involved, and you can jury-rig something from them that you can build from tin cans and scraps of wood. Just DO NOT attach anything to an AC outlet!!! This is for small batteries only...

Many thanks for your patience and assistance.

I shall try to follow your advice.

Lookup mosfet (or IGBT with integrated diodes) FULL bridge topology, with some kind of current control. Depending on how fast you need the state change (not the frequency) to happen and the required overall efficiency, it can get complicated and expensive. If you want a "plug and play" device for this, you will probably not find something cheap so, pay someone competent to build one for you. For this app, I would for example use a mild cost digital amplifier on square input signal and feedback from output current compared to the wanted current scheme to control it. S.M.

The answer is YES but the changing frequency depends on several factors as for instance how big the magnets can be. If they are only d=1x1 then the limit is very high if they are d=10x10 it will be less and the bigger they are the lower the limit. The limit depends also on how big a force you want to generate, which supply is available (3V or 250V) AC or DC

As you see your question as you put cannot be answered the right way.

Try to describe what you want to achieve and then may be you can obtain an answer with a good level of seriousness.

I really appreciate all the comments from you experts but I cannot pretend to know something which I do not.

In my efforts to keep things simple, I appear to have erroneously made things worse.

One more attempt to explain. . . Let's just say I wish to conduct an experiment which demonstrates that two electromagnets can be made to reverse polarity a number of times a second.

In other words, if anyone knows how ANY two electromagnets magnets of "S" size may have their polarity reversed "T" times a second by "D" device I would appreciate those details.

In my ignorance I see that as simple logic and apologise if it is not.

Thanks anyway for your tolerance.

Just hook up a speaker to an iPhone or stereo and play one of your favorite tunes.

That's about as simple as it gets. The voice coil is an electromagnet.

Just use a switch. Turn it on and off as many times as you want. The electromagnet will create its own negative voltage, assuming you are switching a positive voltage on and off. Do you know how to use a voltmeter? Or an oscilloscope?

Let's back up a little. If you are in the UK, and you took a wire and wrapped it around a steel bar and plugged the two ends of the wire into a wall socket, the magnetic field of the electromagnet that you have created will change polarity 100 times per second, because you have 50Hz power. 50Hz means 50 cycles per second, and a cycle is a change from zero, to 100% positive voltage, back to zero, then to to 100% negative voltage, then back to zero. So you have changed polarity twice in a single cycle of AC power.

Are you then asking how you can effectively slow that change rate down to 10 times per second, or 5Hz? (actually you have been all over the map on the rate in your posts, so I'm just going by the last one). If so, you can either get an AC alternator that is separately excited and connect it to some form of variable speed device that can run that alternator at 5Hz, or you can get a solid state frequency generator, for example a PWM generator, capable of a 5Hz output.

There is of course MUCH more to this, especially when it comes to avoiding having the entire thing catch on fire, hence the warnings saying, in effect, DON'T TRY THIS AT HOME.

Most likely you have several electromagnets right in front of you that change thousands of times a second, the speakers for your computer. If your computer has WiFi then there is an electromagnet that changes millions of times a second.

Unfortunately you grasp so little knowledge of fundamental electricity and you are so timid about explaining your goal that nobody can help you.

Have a nice day.

Power them with AC?

Download this free software program. Online Tone Generator - Free, Simple and Easy to Use. Listen to it with ear buds to get a feel for low frequency sounds.

Then plug in a small speaker of the type shown by Anonymous Hero.

You should be able to see the speaker cone's movement.

Gently touch the dust dome to feel the vibrations.

It doesn't get any simpler.

Good luck.

The Famous Cash:

Yes you can do that. The name of the circuit is H-Bridge. These are manufactured for different levels of operating current, voltage and power ratings. They may use BJT, MOSFET or IGBT. MOSFET H-Bridge turn around faster.

An H bridge, exactly. If the electromagnets are doing any serious work, their coils will have considerable inductance, and there'll be lots of wasted E = ½ L I² energy in the process. That was the subject of a CR4 thread about two years ago, where I posted a scheme to recover the energy.

It's just an H-bridge with a diode and a big capacitor. Depending on the stored energy and the size of the capacitor, the H-bridge transistor voltages may soar to well above the supply voltages, and I discuss suitable MOSFET driver ICs.

After struggling through the acronyms and researching a little, I believe this may also have possibilities.

Thank you.

The Famous Cash:
No problem. Here is a reference board from great company so try their demo board if it is still available to buy.
http://www.analog.com/en/circuits-from-the-lab/cn0196/vc.html

http://www.analog.com/static/imported-files/circuit_notes/CN0196.pdf

Thank you.

I am pursuing a number of avenues, including this.

A lot to take on board. This may take some time.

A DPDT telegraph key could do this pretty well, if your fingers are fast enough.

Nice one!

What you want to do is easily done, though you may want to keep an eye on the temperature of the coil/magnet as incorrectly designed/used, they can get really hot!!!!

Enough even to catch fire in some cases.......

I do not get the impression that you are aware of such possible problems, nor did anyone else here mention it either......be careful.....

...nor did anyone else here mention it either...

I don't recall anybody ever warning me to not try the most diabolically stupid things with electrical apparatus when I was in my larval stage.....release the smoke and jump and dance a little I say.

These lessons are never forgotten.

You weren't on CR4 then. Neither was I!!

I did a few jumps and dances too from time to time.....

My earliest with 240VAC I was about 2 years old.....

Worst was 3 Phase 440VAC at around 19 years old in the RN!!

...did you notice that the more shocks you received the less they seemed to bother you? Like an immune response.....

Don't try this at home kids.

I thought it was just me! You could be right!!!

Lowered sensitivity?

I wonder if the body somehow senses it and it makes you less likely to be injured ????

...or maybe getting bitten, smacked and belted raises one's respect...

Mine was a ballast voltage exceeding 500 volts which caused me to spasm and move from the back of the aircraft into the flight deck in what seemed like no time at all.

As far as safety with coils, I found putting a light bulb in series with the experiment minimized smoking. (Though a home made selsun caused a HUGE amount of smoke. Didn't have enough light bulbs I guess!)

You can do this with a ' Picaxe' motor driver board. It uses a microprocessor linked to an LM293D motor driver chip and can deliver about 1 amp to a 6 V supply.

http://www.picaxe.com/Hardware/Project-Boards/PICAXE-08-Motor-Driver-Board/

You will need programming software free from Picaxe and a usb lead which they sell. Its easy to program.

NOW I think this is what I require!

Many thanks. Investing this now.

According to the PDF it reverses motor direction which tells me the polarity is reversed so I should be able to do something to control it.

Took a long time but thanks to all to all kind enough to assist.

The Famous Cash writes:
>NOW I think this is what I require!
Motors aren't my field of interest but reading through many comments, perhaps the following link might help some? It shows as to how polarity is reversed and else. It has a nice explanation and a figure too.
https://www.facebook.com/photo.php?fbid=578619228869849&set=a.153511201380656.36107.138747849523658&type=1&theater

reverses motor direction which tells me the polarity is reversed

You think? Think again. As well as reading the PDF for a VFD read a text book on AC motors.

what about a VFD? it's ready to run, off the shelf, with many internal protections. Not sure you can get one allowing operation at 4 Hz.

You can salvage a bar type electromagnet from a small 2 stroke motor ( ex lawnmower etc. ) In there it is used to generate the spark. Hook it up to your mains outlet and it will cycle through lots of reversals per second.

Before you do this please tell a friend all about what you are doing so that s/he can nominate you for a Darwin award.

Apart from de-gaussing ( google it ) I don't see what you are trying to achieve. You state that you want to demonstrate the effect of rapid on/off magnetism. This infers that you want to SEE something; you won't. Can you see lights flicker on and off at just 50 times per second? IMHO it would be a better demonstration to just switch on and then off manually.

Electric motors run by switching electromagnets on and off rapidly and sequentialy. Are you trying to invent an electric motor? It's been done.

What do you need to buy? A CD player, a small speaker to plug in to it and a test cd that has several frequencies recorded on it. Expose the speaker and play the cd, if you are lucky you may see the speaker move at 20 Hz ( cycles per second ) and feel it at 50 Hz you will certainly start to hear it at higher frequencies. This is of course only ONE electromagnet you specified TWO. Do you want them to alternately attract and repel each other?

Jim

For a slow change of polarity (e.g. one change per second), you could use just an electromechanical relay (with two contacts) combined with an R & C circuit (time delay) and a couple of diodes (for back EMF suppression). (The R & C values will determine the frequency of the polarity changes. No actual electronics are involved but still you need to do some soldering.)

However, this is not your case as you need 3 to 20 changes per second. A way to do this, is to use a small signal generator (or use a virtual one form the Internet as Lyn suggested), produce the proper signal and drive your electromagnets with this signal through a power amplifier (e.g. probably even your home audio amplifier could do the job, considering that it can supply the power that is needed and it can reproduce low frequency signals, as low as 4Hz). By using a generator, you can set the frequency or the shape of the signal (i.e. the way that the polarity changes) accurately.

Use a Direct Current source. Connect a DPDT micro switch wired as a polarity changer and connect the sensor wire up to a metronome. Adjust metronome to the frequency you want.

Attach a "cat's whisker" DPDT micro switch to the skin of a bass drum. Use the switch to reverse polarity when the drum is beat. As long as you beat the drum hard enough and frequently enough you will be changing the polarity at the rate of the drum vibrations.

If you have quick wrist, forget the other stuff and just repeatedly throw the DPDT reversing switch with your fastest hand.

Good Luck, Old Salt

I confess that I did not read all the answers so if I am repeating what someone else already said, my apologies. Instead of advise on creating a such device, let me give you a common application where one is used and you may be able to us it to help find a solution.

After magnefluxing (strong elctro magnet (DC) putting a field through the piece while the operator coats the piece with iron filings suspended in a solution) a crankshaft it must be demagnetized. The appliance used for the process has a 60 hz reversing electro magnet built into the machine so when the crankshaft is done being tested it must be demagnetized if it is to be placed back in use. This reversing magnetic field is at the other end of the appliance. The crank is simply rolled to the other end and moved back an forth under the reversing coil. These machines are found in full service machine shops.

You might also search for tools that demagnetize.

That is a nice information on demagnetization. I like CR4 for this reason that something new pops up from some mind.

From the sounds of your lacking basic fundamental knowledge of electricity, you must be a Human Resource Manager that just landed a new job at an electrical or electronics manufacture. And trying to figure out what kind of questions to ask a *new job applicant that's applying for an Electrical Engineer position.

You should find a friend close by you that understands basic electricity to assist you in your experiment. Basic electricity can cause some big problems if not handled properly, like **(1) start an Unwanted Fire or (2) Kill You!

Also, keep in mind, your assistance needs to have a wooden stick or broom handle readily at hand and properly instructed on how to use it, either to disengage you from the power source or knock you unconscious, so you don't suffer too much while your frying your self to death!

*See CR4 link, http://cr4.globalspec.com/thread/88667#newcomments

** As the saying goes, "Once the magic smoke leaves the can, there's no putting it back in."

Thank you for your advice.

Unfortunately, you do not appear to have been too accurate in your calculations.

I retired at the age of 31 which is 37 years ago having run a casino and owning a number of businesses.

My IQ is 127 so I should be able to instruct a friend how to knock me unconscious as I twitch on the ground.

Joking apart, I believe the solution will be found following earlier kindly advice to explore picaxe. This is what I am currently doing.

Thanks to all for your input. Greatly appreciated.

You may be a good candidate for locating an electronics club. Even a robotics club or Amateur Radio club would be a good place to find a mentor to help you with your project.

While there are lots of people online, nothing beats a face-to-face mentor that can help you learn what you need to know.

The picaxe, as it stands, is a wimp solution. From your description (magnets two or three inches long) it appeared that some real power control was called for. That's why I added a link, #37, to my inductive-power-recovery reversing circuit. High-voltage, high-power H bridges, with serious MOSFETs, including power-recovery versions, like I described, can be run from the logic output of microcontrollers, if you prefer. But take care, power electronics is not all about firing up your favorite microprocessor.

I take your comments on board and am exploring this amongst many other possibilities.

Thank you for your assistance.

http://www.automationstock.com/siemens_MM440_variable_frequency_drives.htm

just an example of a VFD (frequency converter) made in the UK.

it's ready to run with inductive loads. choose the current you need, the smaller models work with 1 phase 230 V. but watch the minimum operating frequency.

analog input(s) can be used to change frequency, e. g. with a potentiometer.

The magnetic field is generated by moving charges. In electromagnets, this is done by wrapping wire around iron or steel. In fact, you can do this at home with a battery, some wire and a nail. Wrap the wire around the nail, and connect one end of the wire to the positive battery terminal and the other to the negative. You'll find that one end of the nail is the "north" and the other the "south" pole of the magnet. If you now reverse the leads (which means the electrons will 'flow' in the other direction), you'll find the magnet's polarity is reversed. By the way, when I say battery, I mean a small one, like a 9V. A car battery might be a pretty bad idea :-)with the help of some relays and timers you can easly switch flow direction and so is the polarity if i understand you well.

Again, only if the desired wave shape is to be square.

The magnetic field magnitude won't be a square wave shape. I know you know this AH but others here may not.

Fair enough, but the point was that the excitation signal is undefined in both power and wave shape.

We know the range of frequency, but the prevailing assumption is that the excitation signal is a square wave and there is nothing to support that.

Once again we are given a question that would be better served if the original poster simply explained what he wanted to do and not necessarily how he wanted to do it with incomplete information.

Echo

Square wave through an inductor...

Nothing very square happening on the inductor.

OK, since we're strolling down EM theory 101, the OP should be be given some more information. The waveform of the magnetic field magnitude produced by the inductor (coil, electro-magnet) will follow the waveform of the voltage across the resistor, not the voltage across the inductor.

A mechanical switch, like your relay contacts, does not produce a nice square wave (if nice square wave is what you want)...

rings like a bell when it turns on an then off. An inductor hanging off it complicates things further.

I greatly appreciate all the contributions and apologise for my inability to express things more clearly. (I feel the question has raised some interest amongst you experts so, hopefully, that is some good news.)

I have been thinking the best way to advance the situation and am making this last attempt to move things forward. If it makes things worse, I will back off also.

1) IF I buy two of these magnets. EL120405 from TIMSTAR, to what do I connect both of them to place them in the "on" state? They would need to able to operate for long periods of time.

2) Once having achieved the magnets to be "on", to what device (Picaxe?) would I connect to silultaneously reverse the polarity of the magnets twice a second? Again over long periods.

3) Once having achieved being able to reverse their polarity twice a second, is it just a question of adjusting the device to select other speeds of change up to (say) 50 times a second?

I have some programming skills and would probably be able to send code to a device such as picaxe. (I have a commercial program (for snooker halls) which switches lights on and off through the main computer program.)

I trust this helps and not create further confusion. I am trying to be as precise as I can.

I looked at the Timstar website and found this magnetics demonstration magnet but the only information I found about this device was the price. When I looked closer at the image I did see the maximum voltage rating of 12V but nothing was obvious about power, current, breakdown voltage, core material hysterysis or inductance value. There was not even any information about the field polarities of the two coils in this rig.

I suspect a simple DC coupled audio amplifier kit between your signal generator and this magnet could work. Getting sub-sonic frequencies (less than 60Hz) into the magnet will require the amplifier to be DC coupled.

Good Luck

Hi

The Picaxe motor board ( wimp solution) couldn`t be easier to use. Download the pdf file on the link, it gives full instructions. Download the Picaxe Basic programming language which is free. Connect a battery pack max 6V ( don't get polarity wrong may damage microprocessor),connect the electromagnets to the connecting blocks as shown and program it. If I remember a simple loop as follows will do what you want. But please read from the instructions with BASIC and see examples.

Main: 'label'

forward a 'electromagnet 1 gives N - S'

forward b ' electromagnet 2 gives N - S'

pause 100 ' time delay 100ms'

backward a

backward b

pause 100

goto main

ignore comments in inverted commas

See how it goes as said cycling electromagnets may generate heat but give it a go.

Many thanks.

Exploring this now.

Are you making a fountain of ball bearings?