High frequency electromagnets.

[Low-Q]

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]

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]

OK, I see. I will make some drawings. Maybe then someone will aswer my question

[Low-Q]

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]

....
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: X_L=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).

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