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### Author Topic: High frequency electromagnets.  (Read 13332 times)

#### Low-Q

• Hero Member
• Posts: 2840
##### High frequency electromagnets.
« on: January 24, 2011, 06:30:13 PM »
Hi,

I have a question about electromagnets. If we apply an electromagnet DC, Let's say 1A @ 10V (This is just random numbers).
OK, the electromagnet consumes 10W power.

Now, if we apply the same electromagnet an AC signal at very high frequency, and let it still be 10V. Now, the power consumption is quite low because there is a phase shift between current and voltage - the impedance has increased but not the resistance in the coil.

I have learned that the current flow are the same regardless of frequency at the same voltage in the same coil / electromagnet.

So, if the current is determing the magnetic strength in the electromagnet, would it be possible to make a strong electromagnet even at very low power consumption?

Vidar

#### Low-Q

• Hero Member
• Posts: 2840
##### Re: High frequency electromagnets.
« Reply #1 on: January 24, 2011, 10:36:52 PM »
My idea is to spend virtually no energy in supplying a flow of high current through an electromagnet. Further, this electromagnet can be used in a motor. The magnetic force are capable of doing work beyond its power consumption. Well, that is the idea.

Vidar

#### Low-Q

• Hero Member
• Posts: 2840
##### Re: High frequency electromagnets.
« Reply #2 on: January 25, 2011, 06:25:21 PM »
OK, I see. I will make some drawings. Maybe then someone will aswer my question

#### Low-Q

• Hero Member
• Posts: 2840
##### Re: High frequency electromagnets.
« Reply #3 on: January 25, 2011, 07:58:21 PM »
No pictures, but I made an electromagnet with some nails and magnetwire. I made also a CD with different frequencies; 250, 500, 1000, 2000, 4000, 8000, and 16000Hz.
I used a 1200W audio amplifier, and powered the electromagnet.

At 250Hz, the electromagnet did attract a ballbearing I have. Also at 500Hz, but with less force. At 1000Hz there was not enough magnetism to keep the ballbearing from falling.

One lesson learned. It takes time to build up a magnetic field. The faster the polarity changes, the less magnetic force it will be.

Vidar

#### gyulasun

• Hero Member
• Posts: 4084
##### Re: High frequency electromagnets.
« Reply #4 on: January 25, 2011, 09:42:06 PM »
....
I have learned that the current flow are the same regardless of frequency at the same voltage in the same coil / electromagnet.

Hi Vidar,

What you wrote above is not ok you seem to disregard a coil increasing inductive reactance in the function of the frequency applied.  Just look up coils inductive reactance formula: XL=2*Pi*f*L   Pi=3.14 f=frequency  L=coil self inductance
So the current entering your coil at a constant 10V AC input voltage will continuosly decrease with increasing the frequency.  When this increasing frequency goes as high as the coils parallel resonant frequency, the current will be at its minimum possible (just like in any tank circuit).

Quote
So, if the current is determing the magnetic strength in the electromagnet, would it be possible to make a strong electromagnet even at very low power consumption?

Vidar

Well, a possibility is to increase the number of turns for the coil, and use a high permeability core for the magnet (consider to avoid core saturation).  Unfortunately the copper loss gradually increases as you make more and more turns, so you wish to choose a wire diameter that gives a practically still acceptable size and volume for the whole electromagnet.
You found in your AF power amplifier test that the strength decreased with increasing frequency: it must have been due to the increasing inductive reactance, making less and less current draw.

rgds,  Gyula

#### fritz

• Sr. Member
• Posts: 424
##### Re: High frequency electromagnets.
« Reply #5 on: January 26, 2011, 08:43:19 PM »
If you apply 1A with a 10V voltage source - you need a (DC) coil resistance of about 10 Ohm.
This magnet consumes 10 Watts permanent and heats up.
On turning on the current will ramp up in a linear function up to 1 Amp.

The strength of the magnet is proportional to your current.
If I start to play with a permanent magnet disturbing my electromagnetic field - you will see that this 1 Amp will start to fluctuate - because we induce an extra current - which might add or subtract from the undisturbed current.

If you want an electromagnet with pretty no power - you have to bring the resistance / voltage down.
If you have a superconducting coil with 0.000001 Ohm and a current of 1Amps - your power consumption will drop from 10 Watt to 100uW - by the factor of 100.000.

If you start to think about AC - you have to keep the following in mind:
*)Losses in the core
*)Extra losses due to reactive power
*)higher resistance due to skin effect...
*)Less current due to inductivity

Additional - you will experience losses in the material you want to attract.
If you exceed the maximum flux density and rated frequency of a core material - the losses may increase dramatically - and the stuff will start to heat up. If we talk about metal this would be kind of induction heating.

So nothing gets better with AC.

#### mscoffman

• Hero Member
• Posts: 1377
##### Re: High frequency electromagnets.
« Reply #6 on: January 26, 2011, 09:04:39 PM »
I just remember that "Magnetic Field Strength of a coil is proportional
to ampere(*)turns".  The way to use less current is to emplace more
wire coil turns. The phase shift caused by the coil means that an AC
signal will be more reluctant to flow into coil above and beyond it's DC
resistance. That means to get more field strength can only occur with
more AC power excitation.  The magnetic poles switch (N/S) with AC.
You can change the reluctance (inductance) of the coil upward by
adding a core material. But there is lowest reluctance - is called the
reluctance-of-free-space which is an universal physical constant.

:S:MarkSCoffman

#### gyulasun

• Hero Member
• Posts: 4084
##### Re: High frequency electromagnets.
« Reply #7 on: January 26, 2011, 10:22:42 PM »

Hi Vidar,

One more thing that just occured to me: Nikola Tesla showed just the kind of electromagnet you may have been looking for:
http://www.tfcbooks.com/patents/512340.htm

He 'embedded' capacitance between two parallel guided wires and made a coil from the double wires. And obviously the resonant frequency dependens on the mechanical closeness of the parallel wires and the dielectric properties of the insulator materials between the wires and of course the wires length (number of turns).  Tesla wrote this setup is good for coils in general so not exclusively for flat "pancake" shaped coils.
By choosing the AC frequency of the input current to be the same as the resultant resonant frequency of the double wires connected as he showed, the inductive reactance would not "resist" the current flow like in conventional coils.

rgds,  Gyula

ps: I fully agree with fritz and mscoffman posts.

#### fritz

• Sr. Member
• Posts: 424
##### Re: High frequency electromagnets.
« Reply #8 on: January 26, 2011, 11:17:11 PM »
I just remember that "Magnetic Field Strength of a coil is proportional
to ampere(*)turns".  The way to use less current is to emplace more
wire coil turns. The phase shift caused by the coil means that an AC
signal will be more reluctant to flow into coil above and beyond it's DC
resistance. That means to get more field strength can only occur with
more AC power excitation.  The magnetic poles switch (N/S) with AC.
You can change the reluctance (inductance) of the coil upward by
adding a core material. But there is lowest reluctance - is called the
reluctance-of-free-space which is an universal physical constant.

:S:MarkSCoffman

On switched dc I get the "investment" in the reluctance (while switching on and field establishes) back on turning off (if IÂ´m so clever to recycle the energy from the back EMF on turning off)
But charging and decharging the field causes extra current - which leads to  extra losses as long we have a resistive coil.

If I have a perfect supraconducting coil in vacuum  - I have still reluctance - but no losses -I invest the energy once and can take it back.

Take for example the SMES - where an inductive system is used to store energy. Normally you can do this only with capacitors - using superconductors - you can do the same with coils.

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

rgds.

#### fritz

• Sr. Member
• Posts: 424
##### Re: High frequency electromagnets.
« Reply #9 on: January 27, 2011, 12:32:34 AM »
To round it up - an alternating magnetic field will induce an em field.
So even if we have a perfect coil we will experience losses due to em radiation.
This is pretty nothing at AC - which is for me somewhere between 10 - 400Hz.
If we come into the Khz range this part will increase.
Depending on the matching from our "magnetic" antenna (our coil) to the free field impedance
(120pi Ohms) this will start to suck up energy.
Typicalwise, electromagnets have a very high inductivity - so aproaching 1Mhz - the only radiating thing
will be the feed wires - and we will end up with a dipole (because the coil is high-Z).
rgds.