Electromagnet polarity switching....

[Bo Cash]

I regret I know nothing about subject but wonder if anyone could advise where I could obtain a device (including magnets) which will change the polarity of two electro magnets from once per second to hundreds of times per second?

 

[berkeman]

I regret I know nothing about subject but wonder if anyone could advise where I could obtain a device (including magnets) which will change the polarity of two electro magnets from once per second to hundreds of times per second?

Welcome to the PF, Bo.

To give you a good answer, we need to know how big the electromagnets need to be. It is simple to use a desktop signal generator to make the AC currents to switch the polarity of the drive of small electromagnetic coils. But it takes large power amplifiers to switch the currents for large electromagnets. The larger the electromagnet, the slower it will switch...

http://media.mlive.com/mid-michigan-business_impact/photo/omnisource02jpg-a8c4704f510ca828.jpg

 

[Bo Cash]

Many thanks for your response.
As I mentioned, I have no real knowledge about the subject but, to answer as best I can, I would say the electro magnets would be rectangular and about two inches long.
Ideally, I see the device to have a dial which could be rotated to increase the number of times the polarity would be switched.
I trust this is sufficient to enable you to provide a solution. Should the experiment prove of interest, I may try with larger magnets but that is a long way off, I believe.

 

[berkeman]

Many thanks for your response.
As I mentioned, I have no real knowledge about the subject but, to answer as best I can, I would say the electro magnets would be rectangular and about two inches long.
Ideally, I see the device to have a dial which could be rotated to increase the number of times the polarity would be switched.
I trust this is sufficient to enable you to provide a solution. Should the experiment prove of interest, I may try with larger magnets but that is a long way off, I believe.

Yes, that helps. So it sounds like you can just use a bench-top signal generator to make the AC current that you drive through the electromagnet coil. What equipment do you have available to you for this?

http://www.lead-man.cn/anli/xhfsq.jpg

 

[Bo Cash]

I have no equipment whatsoever and zero knowledge about how to make one. I suspect I will have to find someone locally (Edinburgh, Scotland who could provide such.

 

[berkeman]

Okie dokie. Look for somebody with simple EE lab resources, like the signal generator shown above. If you can say more about your application, we may be able to offer more guidance.

 

[Bo Cash]

Many thanks for your kind input.
I will search for what you suggest.

 

[CWatters]

Find two old loudspeakers?

 

[Bo Cash]

I'm assuming you mean to use the loudspeakers for the electro magnets. I still do not know how I would be able to rapidly reverse the polarity.
As to further information. If ANY two magnets could have their polarities rapidly changed by some device from once a second to a few hundred times a second, that would be sufficient to get me on the right road.
Preferably rectangular magnets about two inches long with sufficient power to attract iron object weighing an ounce or two.
IE. ANY means to achieve this polarity changing to ANY two electro magnets would be helpful.. . . I simply do not know at this stage what current, et cetera will ultimately be involved.
Thanks.

 

[CWatters]

When you play music the amplifier reverses the polarity to the speaker causing the speaker cone to move in and out.

 

[davenn]

I still do not know how I would be able to rapidly reverse the polarity.

berkeman told you how in post #4

 

[Bo Cash]

Thanks to all for advice.
I simply am unable to add any more which might help.
I cannot trace anyone who could supply a device which would definitely work but my ignorance on the subject is highly detrimental.
Thanks again.

 

[Tom.G]

This is a long post. It gives suggested sources, connection instructions, and detailed guidance on making an electromagnet for your project, from the beginning.

use a bench-top signal generator to make the AC current

There are many different ones available with prices from USD$10 to USD<many thousand>. A Google search for SIGNAL GENERATOR LOW FREQUENCY finds 195 000 000 items! Look for one that is in your price range and frequency range. There is one at https://www.alibaba.com/product-det...?spm=a2700.7724857.main07.1.96159dbbNJ0Ys&s=p in the USD$10 - $50 price range, and one at https://www.amazon.com/dp/B00PQ05Z9A/?tag=pfamazon01-20 for USD$33... and many more.

You would connect this to an audio amplifier (HiFi amplifier) and then connect your electromagnets to the amplifier output, instead of connecting the amplifier to loudspeakers.

For making electromagnets, find some iron at your local hardware store of the shape you want. If you can not find anything suitable, another source is "Rebar", the metal rods used in the construction industry to reinforce concrete. Try a builders supply outfit. After cutting the iron to size, file off any sharp or rough edges. Then wrap a layer or two of electrical tape around the iron to protect the wire insulation from any rough spots and supply an even base for winding, this is rather important.

For wire to make the electromagnets for the first experiments, probably between 20 gauge to 24 gauge would be a good start. If you can not find a supplier for your location, you could find an old transformer (or even a new one) and take it apart for the wire. Here in the U.S., "doorbell transformers" for electric doorbells are readily available in hardware stores.

Now you need to wind the wire around the iron core. To get the highest number of turns in the smallest space the wire must be wound in even, smooth layers, leaving some extra wire length to make connections to later. Tape a few inches (5cm to 10cm) to one end of the core for one connection. Then start smoothly winding wire around the iron core, keep light tension on the wire so it is tight enough to not slide around. Each turn should be touching but not overlapping the prior turn.

This step is highly recommended but optional if the wire insulation is in good condition, there are no kinks in the wire, and you are being very careful. When you get to the end of a layer, wrap a thin sheet of paper around to cover the layer. Hold it in place with a bit of tape. This provides a stable surface for the next layer and helps avoid short circuits between the layers.

Now start the next winding layer. Make sure the wire winds around the core in the same direction as the previous layer did. Since you are starting at the other end of the core, it is easy to get this wrong if you turn the core around to get a better grip on it. Getting the winding direction the same is needed so that the magnetic field of the layers add to each other. If the winding direction changes between layers the magnetic fields will subtract from each other.

The layers do not have to be the same number of turns. When winding by hand, If you are putting on many layers it is often easiest to have fewer turns on the later layers so the windings do not fall off the ends of the magnet.

When you decide you have enough windings, leave some extra wire for connections and cover the finished coil with a couple layers of tape. This is to provide mechanical protection to the wire and to hold it in place during handling.

Just for your information, the strength of an electromagnet changes directly with the number of turns and the amount of current (Amps) flowing thru it; until it reaches an upper limit defined by the size and material of the core.

For a "First Prototype" this should be enough for you to get a feel of what is needed. Magnetic strength can be calculated based on the core size and material, the number of turns, and the amount of current thru it. If you get to a point where such calculations are needed, others here are much better at that than I am! Specifically, one @jim hardy comes to mind.

I know, spelled out in this amount of detail makes the whole thing sound boring and tedious. But if you are even slightly mechanically inclined it's not bad. Making an electromagnet is often a Grade school or Junior High school project!

Have Fun... and keep us updated.

Tom

 

[jim hardy]

Are you handy with computers ? An Arduino or Raspberry guy perhaps?
if so
Take a look at this product
http://www.mouser.com/new/Infineon-Technologies/infineon-h-bridge-kit-2go/

it is a DC motor driver , built around
an H-Bridge that supplies voltage at either polarity to control direction of the motor, and at variable voltage by PWM
and a programmable microcomputer to control it

you'll have to add a power supply probably around fifteen volts seventy five watts

It should drive an electromagnet.
but you'll have to climb its learning curve . Fortunately it plugs into a PC's USB .
Here's the user manual
http://www.mouser.com/pdfdocs/Infineon-H-Bridge_Kit_2Go_UM-UM-v01_01-EN.pdf

look it over and see if it is something you feel comfortable tackling.

 

[jim hardy]

There's a variety of electromagnets on EBAY
http://www.ebay.com/bhp/electromagnet-12v

be aware though that it's the nature of inductance to resist changing polarity very often. As Berkeman pointed out , the bigger the slower.

You might be physically unable to make hundreds of reversals per second.

 

[f95toli]

I'm assuming you mean to use the loudspeakers for the electro magnets. I still do not know how I would be able to rapidly reverse the polarity.
As to further information. If ANY two magnets could have their polarities rapidly changed by some device from once a second to a few hundred times a second, that would be sufficient to get me on the right road.

No, what you are describing is pretty much exactly just a loudspeaker; albeit with a large (and heavy) voicecoil and no diaphragm. A few hundred Hertz means you are in the frequency range of woofer and you might even be able to drive it using a normal stereo amplifiier.
You should be able to find plenty of information online about people building things out of loudspeaker elements; something like a DIY "buttkicker" would perhaps fit the bill (although they are generally made for lower frequencies).

 

[NascentOxygen]

a device (including magnets) which will change the polarity of two electro magnets from once per second to hundreds of times per second?

Have you indicated what you plan to have the magnets doing?

 

[sophiecentaur]

Have you indicated what you plan to have the magnets doing?

To go any further with this, we have to know about the application. A well designed electromagnet will give a 'good field strength' for its design current but that will mean its Inductance will be high - catch 22. But, from the size you give, it will not be a massively high powered device so there should be a solution.

No, what you are describing is pretty much exactly just a loudspeaker;

It could do - but that is not an electromagnet - it's a Linear Motor, which may be what you need if you are after an Actuator.

 

[Bo Cash]

This is a long post. It gives suggested sources, connection instructions, and detailed guidance on making an electromagnet for your project, from the beginning.There are many different ones available with prices from USD$10 to USD<many thousand>. A Google search for SIGNAL GENERATOR LOW FREQUENCY finds 195 000 000 items! Look for one that is in your price range and frequency range. There is one at https://www.alibaba.com/product-det...?spm=a2700.7724857.main07.1.96159dbbNJ0Ys&s=p in the USD$10 - $50 price range, and one at https://www.amazon.com/dp/B00PQ05Z9A/?tag=pfamazon01-20 for USD$33... and many more.

You would connect this to an audio amplifier (HiFi amplifier) and then connect your electromagnets to the amplifier output, instead of connecting the amplifier to loudspeakers.

For making electromagnets, find some iron at your local hardware store of the shape you want. If you can not find anything suitable, another source is "Rebar", the metal rods used in the construction industry to reinforce concrete. Try a builders supply outfit. After cutting the iron to size, file off any sharp or rough edges. Then wrap a layer or two of electrical tape around the iron to protect the wire insulation from any rough spots and supply an even base for winding, this is rather important.

For wire to make the electromagnets for the first experiments, probably between 20 gauge to 24 gauge would be a good start. If you can not find a supplier for your location, you could find an old transformer (or even a new one) and take it apart for the wire. Here in the U.S., "doorbell transformers" for electric doorbells are readily available in hardware stores.

Now you need to wind the wire around the iron core. To get the highest number of turns in the smallest space the wire must be wound in even, smooth layers, leaving some extra wire length to make connections to later. Tape a few inches (5cm to 10cm) to one end of the core for one connection. Then start smoothly winding wire around the iron core, keep light tension on the wire so it is tight enough to not slide around. Each turn should be touching but not overlapping the prior turn.

This step is highly recommended but optional if the wire insulation is in good condition, there are no kinks in the wire, and you are being very careful. When you get to the end of a layer, wrap a thin sheet of paper around to cover the layer. Hold it in place with a bit of tape. This provides a stable surface for the next layer and helps avoid short circuits between the layers.

Now start the next winding layer. Make sure the wire winds around the core in the same direction as the previous layer did. Since you are starting at the other end of the core, it is easy to get this wrong if you turn the core around to get a better grip on it. Getting the winding direction the same is needed so that the magnetic field of the layers add to each other. If the winding direction changes between layers the magnetic fields will subtract from each other.

The layers do not have to be the same number of turns. When winding by hand, If you are putting on many layers it is often easiest to have fewer turns on the later layers so the windings do not fall off the ends of the magnet.

When you decide you have enough windings, leave some extra wire for connections and cover the finished coil with a couple layers of tape. This is to provide mechanical protection to the wire and to hold it in place during handling.

Just for your information, the strength of an electromagnet changes directly with the number of turns and the amount of current (Amps) flowing thru it; until it reaches an upper limit defined by the size and material of the core.

For a "First Prototype" this should be enough for you to get a feel of what is needed. Magnetic strength can be calculated based on the core size and material, the number of turns, and the amount of current thru it. If you get to a point where such calculations are needed, others here are much better at that than I am! Specifically, one @jim hardy comes to mind.

I know, spelled out in this amount of detail makes the whole thing sound boring and tedious. But if you are even slightly mechanically inclined it's not bad. Making an electromagnet is often a Grade school or Junior High school project!

Have Fun... and keep us updated.

Tom

I am extremely grateful for your detailed assistance.
Whilst I know how to make an electro magnet, it is my intention to purchase such due to my lack of skill. I recently had a stroke which makes any intricate work pretty difficult.
I am trying to research how signal generators work and if I would be able to link with an amplifier.
Certainly, your precise explanation has made it much easier for me to grasp.
Many thanks.

 

[sophiecentaur]

I am extremely grateful for your detailed assistance.
Whilst I know how to make an electro magnet, it is my intention to purchase such due to my lack of skill. I recently had a stroke which makes any intricate work pretty difficult.
I am trying to research how signal generators work and if I would be able to link with an amplifier.
Certainly, your precise explanation has made it much easier for me to grasp.
Many thanks.

Before you pile in and make this thing, it would be a good idea to say what the application is and to make sure that an electromagnet is actually what's needed.

 

[jim hardy]

Might be worth brushing up on the basics here...

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

You might find suitable secondhand solenoid or inductor coils someplace .
eg http://www.ebay.com/itm/Two-2-5-mH-Air-Core-Inductors-1-2R-with-free-shipping-/322808916088?hash=item4b28e92878:g:SlcAAOSwEQFZqzea

 

[Tom.G]

Or buy a spool of wire. As long as both ends are available you won't have to wind it. Then get or make a core to fit in the spool.

 

[sophiecentaur]

I still have reservations about this and about the possible context. A solenoid it for attracting metal. Changing the polarity will not change the direction of the force (attraction in both cases). Only if a permanent magnet is involved will the polarity of the electromagnet make a difference. We all have different pictures in our minds about this and so we have different preferences about the implementation. Where do the "two electromagnets" come into it?
@ Bo Cash please help us to help you by talking about the project in a useful amount of detail. You claim to know nothing about the subject so you probably do not know exactly what you need to know.

 

[Bo Cash]

Before you pile in and make this thing, it would be a good idea to say what the application is and to make sure that an electromagnet is actually what's needed.

Initially, I just want to see interaction when the polarity is rapidly switched.
If outcome is as I anticipate, I will then worry about how I would scale this up. IE High current, larger magnets, et cetera.
I just need to get ANY two electromagnets polarity simultaneously, rapidly controlled.

 

[Bo Cash]

I still have reservations about this and about the possible context. A solenoid it for attracting metal. Changing the polarity will not change the direction of the force (attraction in both cases). Only if a permanent magnet is involved will the polarity of the electromagnet make a difference. We all have different pictures in our minds about this and so we have different preferences about the implementation. Where do the "two electromagnets" come into it?
@ Bo Cash please help us to help you by talking about the project in a useful amount of detail. You claim to know nothing about the subject so you probably do not know exactly what you need to know.

I hate to sound mysterious but basically I just need to be able to control rapid switching of polarity of two electro magnets. (They will be interacting with permanent magnets.) In the beginning, virtually ANY size and power will be sufficient to see if my predictions warrant further investigation and larger and more powerful magnets.

 

[Bo Cash]

Might be worth brushing up on the basics here...

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html
View attachment 212963

You might find suitable secondhand solenoid or inductor coils someplace .
eg http://www.ebay.com/itm/Two-2-5-mH-Air-Core-Inductors-1-2R-with-free-shipping-/322808916088?hash=item4b28e92878:g:SlcAAOSwEQFZqzea

View attachment 212962

Thanks. I grasp the basics of magnetism and wish to be able to conduct experiments changing polarity rapidly.

 

[Bo Cash]

Are you handy with computers ? An Arduino or Raspberry guy perhaps?
if so
Take a look at this product
http://www.mouser.com/new/Infineon-Technologies/infineon-h-bridge-kit-2go/

it is a DC motor driver , built around
an H-Bridge that supplies voltage at either polarity to control direction of the motor, and at variable voltage by PWM
and a programmable microcomputer to control it

you'll have to add a power supply probably around fifteen volts seventy five watts

It should drive an electromagnet.
but you'll have to climb its learning curve . Fortunately it plugs into a PC's USB .
Here's the user manual
http://www.mouser.com/pdfdocs/Infineon-H-Bridge_Kit_2Go_UM-UM-v01_01-EN.pdf

look it over and see if it is something you feel comfortable tackling.

Thank you. A few people suggested using H bridge. I am ploughing through data trying to learn. Still unsure how it would be controlled to change polarity rapidly.

 

[Bo Cash]

Or buy a spool of wire. As long as both ends are available you won't have to wind it. Then get or make a core to fit in the spool.

Thanks. I don't have problem with electro magnets per se. Just how to rapidly change polarity. I am working on it.

 

[Bo Cash]

There's a variety of electromagnets on EBAY
http://www.ebay.com/bhp/electromagnet-12v

be aware though that it's the nature of inductance to resist changing polarity very often. As Berkeman pointed out , the bigger the slower.

You might be physically unable to make hundreds of reversals per second.

Thanks. I understand there may be a problem in achieving a high number of changes per second. This is the reason I require to experiment to discover what is actually attainable,

 

[Baluncore]

To change polarity rapidly you must meet two requirements.

Firstly; thick iron will not work. You must use a core that is laminated steel, iron powder or ferrite. That is because the magnetic field must quickly penetrate the magnetic core material. Laminations will give you up to 100 changes per second, iron powder will give 10 thousand CPS, ferrite will give you 10 million CPS.

Secondly; you must use fewer turns, with more current and less voltage. The limit to switching time is inductance which increases as the square of the number of turns.

Magnetic field is proportional to current times the number of turns of wire. To change magnetic field you must change current. The inductor voltage sets the rate of change of current; V = L ∙ di/dt ; which is why you need low inductance, few turns. Sudden changes of inductor current will need or produce high voltage spikes.

You might find the magnetic core and wire in second hand transformers, or from old electrical contactors in industrial scrap.

 

[Bo Cash]

Are you handy with computers ? An Arduino or Raspberry guy perhaps?
if so
Take a look at this product
http://www.mouser.com/new/Infineon-Technologies/infineon-h-bridge-kit-2go/

it is a DC motor driver , built around
an H-Bridge that supplies voltage at either polarity to control direction of the motor, and at variable voltage by PWM
and a programmable microcomputer to control it

you'll have to add a power supply probably around fifteen volts seventy five watts

It should drive an electromagnet.
but you'll have to climb its learning curve . Fortunately it plugs into a PC's USB .
Here's the user manual
http://www.mouser.com/pdfdocs/Infineon-H-Bridge_Kit_2Go_UM-UM-v01_01-EN.pdf

look it over and see if it is something you feel comfortable tackling.

Thanks. I would prefer to not have to use PC but use (apparently) signal generator and (apparently) audio amplifier. I am also checking other possibilities and may try similar to what you suggest.

 

[jim hardy]

Don't be locked into electronics.
Is what you want a rapidly alternating magnetic field ?

Why not just spin a permanent magnet ?
Find one about the right size
make a holder for it out of a dowel rod, chuck that in your electric drill or thrift store eggbeater..

Or rip the rotor&brush holders out of an old car alternator,

That could be supported in a 2X4 frame and spun by a motor .

Lastly, if this is one of those "Free Energy from Magnets" schemes you've been duped.

 

[sophiecentaur]

@Bo Cash I am wondering how you intend to measure the interaction between your two magnets. Are you planning to measure an alternating force? What sort of frequency are you contemplating? It would be possible to provide a field oscillating at tens of Megahertz. OTOH you could hear vibrations motion (vibration) up to around only 20kHz. To measure the vibrations you would need a transducer and some electronics. Etc. Etc.
If you are not careful and if you don’t plan the whole project you could end up with a number of articles that you have built or bought and that never produce any results.
Science at this level produces results that are very close to Theory. Learn a bit of theory and you can make up an experiment that could actually work.

 

[Bo Cash]

@Bo Cash I am wondering how you intend to measure the interaction between your two magnets. Are you planning to measure an alternating force? What sort of frequency are you contemplating? It would be possible to provide a field oscillating at tens of Megahertz. OTOH you could hear vibrations motion (vibration) up to around only 20kHz. To measure the vibrations you would need a transducer and some electronics. Etc. Etc.
If you are not careful and if you don’t plan the whole project you could end up with a number of articles that you have built or bought and that never produce any results.
Science at this level produces results that are very close to Theory. Learn a bit of theory and you can make up an experiment that could actually work.

Thank you for your response.
I know next to nothing about subject. so the frequency is, to me unimportant at this stage.
I just want to be able to rapidly change the polarity of ANY two electro magnets simultaneously. (Interaction with permanent magnets experiment.)
I will worry about technicalities if my anticipated results suggest I should advance further.

 

[Bo Cash]

Don't be locked into electronics.
I what you want a rapidly alternating magnetic field ?

Why not just spin a permanent magnet ?
Find one about the right size
make a holder for it out of a dowel rod, chuck that in your electric drill or thrift store eggbeater..

Or rip the rotor&brush holders out of an old car alternator,

View attachment 212976
That could be supported in a 2X4 frame and spun by a motor .

Lastly, if this is one of those "Free Energy from Magnets" schemes you've been duped.

Thanks. However, I require electro magnets.
I just want to be able to rapidly change the polarity of ANY two electro magnets simultaneously. (Interaction with permanent magnets experiment.)
I will worry about technicalities if my anticipated results suggest I should advance further.