Electromagnet Question

[Xaverius]

@Honk,

If either electromagnet is energized momentarily it can fire a small permanent magnet from close proximity (zero inches away) to about 12 inches away vertically.  If the same electromagnet is energized for the same amount of time without the permanent magnet in place, does it consume the same electrical energy?  So far my gross test set up appears to show that the electromagnet consumes the same electrical energy in both cases.  Is my equipment not sensitive/sophisticated enough to measure the difference, or do both cases consume the same amount of electrical energy?

Thanks,

M.

Hi, sorry for the late reply, it's been a long weekend.  Actually the amount of energy consumed by the electromagnets should be the same regardless of whether a permanent magnet is attached

Think of it this way:  compare the electromagnet to a cannon, the black powder inside the cannon barrel is the electricity, the cannon ball is the permanent magnet.  If the barrel is pointing straight up then compare the gravitational attraction of the cannon ball to the magnetic attraction of the electro and permanent magnets.  Now fire the cannon using 10 lbs of black powder.  The black powder produces X energy.  If a cannon ball is in the barrel, then it will obtain the energy of the black powder.  If NO cannon ball is in the barrel, the black powder will still produce X energy, only this time the energy will be dipersed into the atmosphere as heat.

Also, to calculate the amount of energy in the electromagnet use: E=.5xLx(I)^2

E=Energy
L=Inductance in Henries
I= Amperage

To calculate L use: u x n^2 xA/l

u=permeabilty

[Xaverius]

continued.......

n=number of turns of wire
A=cross-sectional area of the electromagnet
l=length of the electromagnet

u=4 x pi x10^-7 for free space/air/vacuum, multiply this number by 50 for ordinary unpurified iron, most electrical steels like in your electromagnet you would multiply by around 2000, sometimes more.

By understanding more about the energy of the electromagnet you can understand better how it affects the permanent magnet.  Hope this helps.

[mondrasek]

But doesn't that mean we are gaining the potential energy of the raised perminant magnet for "free"?

Case one:  Energize an ideal solenoid using an ideal power source for .1 sec.  Then remove the power source and substitute an ideal load.  Electrical power in = Electrical power out.

Case two:  Energize an ideal solenoid using an ideal power source for .1 sec., but this time have a permanent magnet initially inside the solenoid so that when the solenoid is energized the permanent magnet is accelerated upwards against gravity (doing work).  Then remove the power source and substitute an ideal load.  Also, capture the permanent at the the apex of it's trajectory.  Power in = Power out + Potential Energy of the raised permenant magnet mass.

I believe this uses the permanent magnetic field, and not the electrical power, to do work.

My idea for a simple demonstration unit involves reclaiming the electrical power used to excite the perminant magnet rather than power a load.  Does anyone know of a simple circuit design that would allow me to pulse the electromagnet for a short time (simple mechanical or duration adjustable electrical switch) and then capture the induced EMF from the solenoid once the pulse is removed and the field collapses?

Thanks again!

M.

[Honk]

You can use a simple H-Bridge to pulse your electromagnet in either direction.
See page 8 at this pdf on the H-Bridge design.
http://www.powernucleus.com/application_note/topologies_overview.pdf

But if you dont need to reverse the field you can just use a plain two transistor forward stage.
http://www.coremaster.com/appnotes/an107.pdf

The inductive kickback is recovered and clamped to the input voltage by the parallel diodes within
the transistors or extra diodes externally placed outside if using IGBT transistors instead of Mosfets.

If you place a permanent magnet within a magnetic circuit (core) it will shift the hyseresis (B/H curve) of the
material (magnetizing your core) and it takes the same or more power to reverse the field by the windings....
...thus no free power in this type of design.

[mondrasek]

Thanks Honk.

The permenant magnet is place within the windings of the solenoid and held so it's center is just above the center of the solenoid.  When the solenoid is energized it accelerates the magnet away using the repulsion of the magnetic fields.  The result is Kinetic Energy in the mass of the moving permanent magnet and Potential Energy in the mass of the permanent magnet if it is raised vertically in our gravitational field.

My simple test set up uses a solenoid from a Mac air valve rated at 24 V.  Resistance is ~75 Ohms depending on temperature so it draws around ~320 mA.  Using a MacDonalds drinking straw as a guide, I can place a 1/4" dia x 3/8" long neo permanent magnet inside the solenoid.  When I energize the solenoid momentarily the neo fires up the straw and out the other end, a total travel of about 18 inches.  Unless that reaction consumed the power used to energize the solenoid, it came only from the permanent magnetic field.  It did not reduce the permanent magnetic field.

M.