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permanent magnet as electromagnet

permanent magnet as electromagnet

(OP)
I have been searching all over for the answer to this and I cannot find it anywhere.

If I take a permanent magnet, and I wrap a whole bunch of turns around it with magnet wire (copper) and apply a DC voltage to it, will it strengthen the field and/or flux of the permanent magnet?



RE: permanent magnet as electromagnet

This is exactly how it was became a permanent magnet (magnetized).

RE: permanent magnet as electromagnet

Although not the way to use permanent magnets, a DC winding COULD strengthen the flux and flux denisty from two perspectives: 1. strengthen the permanent magnet itself by Bd-Br, Bd>=Br when the external field is positive; 2. contribution of the winding which serves as an air coil to add up the flux.

RE: permanent magnet as electromagnet

I have seen two applications sort of like this.
1. a few turns around the ends to help with shaping (or focusing) the field. You get more usable field this way.
2. a set of windings that are energized in the opposite polarity to the PM, but not enough to demag it. In this way you can 'turn off' the PM.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

RE: permanent magnet as electromagnet

(OP)
MagBen and EdStainless, thanks much for the replies!

This is very helpful

RE: permanent magnet as electromagnet

You can use superposition to approximate the combined effects. But the actual effect is greater than the sum of the parts--if the coil is boosting the field of the permanent magnet (PM) you'll get an effect greater than the sum due to shifting the PM operating point closer to Br.

In a quick FEA simulation I ran, where I put coils at each end of a PM (dia=2", length=4") and matched the coil currents to produce approximately the same field strength as the PM, the combined effect was 4% greater than the sum of the parts.

RE: permanent magnet as electromagnet

kreEng: What do you want the hybrid permanent magnet / electromagnet to do? Yes, wrapping wires around a cylidrincal magnet and running current through the coil can increase the magnetic output, but the reality is that you generally need to add a lot of current to make a significant effect. In addition, you can not turn the hybrid electromagnet off, nor can you reverse the polarity.

RE: permanent magnet as electromagnet

(OP)
MagMike, I am looking to make an electromagnet that I can apply a variable amount of voltage to, that will slowly or quickly, but smoothly attract another magnet, or something magnetic.

What I want to do is use some type of spring in-between the electromagnet and the other magnet, or magnetic item, and depending on how much voltage I apply, it will hopefully pull the object closer based on the magnetic field.

I will definitely have to play with the spring strength and size, but that is the idea.

I can if need be do this inside of a solenoid, which is where I am leaning because I believe by wrapping wire around an iron tube, I will get the best field, I think.

It does not have to be a spring, but the idea is instead of a solenoid that goes "clank", something that can close smoother and I can have some control over the speed via voltage/current.

Hopefully this makes sense.

RE: permanent magnet as electromagnet

You will have a hard time getting a "soft landing" unless you have a rather sophisticated control system. Due to the highly non-linear relationship of force to air gap the armature naturally wants to slam home.

Look into what has been done with electronic engine valve operation (cam-less engines) to keep them from beating themselves to death.

The simplest solution might be just to have an elastomeric button for your lift stop if you can get the performance and life you need. Without more info it's pretty much impossible to give reasonable recommendations.

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RE: permanent magnet as electromagnet

2
Hello KireEng, I've toyed with this problem of soft landing an armature a few times.

Using a spring is difficult because as dgallup says: "Due to the highly non-linear relationship of force to air gap the armature naturally wants to slam home."

A spring would work...but only if it was as wildly non-linear as a magnetic air gap. Another problem with that solution is that no sooner does the armature close on it's target than the spring tries to force it back the other way.....You need a latch.

In designing early HDAs, I had to solve a similar problem when parking the heads electromagnetically. After a lot of dead ends, I did this by creating a plastic slope that the flexible steel armature had to ride up in order to reach a flat landing place. The combination of elasticity of a flexible steel armature, and sliding friction as the armature traveled up the plastic slope were sufficient. At the top of the slope the armature rose over a final bump onto a flat area and that provided the latch. You might use something similar for your problem.

Another second way that showed promise - and I was actually going to use it in production for awhile - was to create an armature with a narrow "waist" and focus the magnetic field on that narrowing region so that less material was being attracted as the gap was reduced. It would still require a spring, but the world of usable springs was now much larger. The problem with that solution is twofold: Anything having a spring in it doesn't want to remain "parked", and any time you are dealing only with magnetic forces and narrow gaps then all of the tolerances become critical (and expensive).

Another third and common way - a very inexpensive way - to create a soft "bumper" without any friction and the associated wear is to allow the motion to try to force together two permanent magnets in the repelling orientation. For example, put one small magnet on the end of your armature and another similar permanent magnet positioned in the end of the cylinder so that the armature's motion will try to force the same poles together. In this way you can match the nonlinear attractive field you are using to actuate your solenoid with an equally nonlinear repulsive field to slow it down.

I've toyed with combining these second and third ways. In fact I still haven't entirely given up on it. That is, I still think that a combination of small permanent magnets and a shaped armature would allow the armature motion to be "tuned" for controlled closing. It's possible that a clever geometry could even deal with the problem of tight tolerances. I haven't pursued this method in production because it is limited to fairly slow systems. It produces induced eddy currents and rapidly repeating the armature motion causes heat to build up quickly. But it ought to work OK below some threshold of repeated motion.
Enjoy! Roger L.

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