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# Electromagnet with 30mA

## Electromagnet with 30mA

(OP)
Hi,

I'm a kind of "crazy inventor", and have now certain kind of "break through" invention on my mind .

I have surfed hours on the web, finding answer what one can get for electromagnet with 30mA and 50-100 turns coil if you can freely select the core material and the shape of core. I have not found the answer. Because the answer is easy to be given for a prfessional, I hope some of you could give it!

"what one can get" equals, more or less to, how big force can be gotten!

Thanks!

Tapio

### RE: Electromagnet with 30mA

First of all it depends on the required characteristics of the force.  A simple solenoid type electromagnet (just copper and iron) will give a force non-linear with airgap between electromagnet and target - very low force at large airgap increasing sharply as the airgap closes, and high holding force when closed.  Alternatively, a linear actuator (copper, iron and a permanent magnet) can give a higher average force and one that is linear with displacement.

### RE: Electromagnet with 30mA

(OP)
I would need about 1-2mm movement and linear force!

### RE: Electromagnet with 30mA

There have been some similar threads regarding flux density and force calculations on this site recently, e.g.
If you are looking for the equation for force across an airgap:

Force (in Newtons) F = B2*A/(2*?0)
where B is the flux density in the airgap (Tesla)
A = area of pole face (or area of airgap in direction perpendicular to flux) (m2)
?0 = permeability of free space (=4*pi*10-7)
i.e. all in SI units

If you assume a typical flux density in the airgap of 1.0T, you can rewrite this equation as:
force/area = 0.4N/mm²
BUT - to get a flux density of 1.0T for an airgap of 1.0mm, rearranging the equation for reluctance of an airgap (given in one of the above threads) to give ampere-turns:
NI = B*g/?0  where g is the airgap (in meters)
= 800  Ampere-turns
Multiply that by 2 (say) to account for reluctance in the rest of the magnetic circuit (>2 if you have another airgap elsewhere) gives 1600Ampere-turns.

I'm afraid you only have 1.5 to 3 A-t from you initial post, so the force you can expect is going to be tiny,
something like less than 1microNewton per mm².

### RE: Electromagnet with 30mA

(OP)

Let's continue this threat.

How about if I'll use a permanent magnet as an anchor? So the actual force cames from permanent magnet, and the electromagnet is only used as a controller?

Is there any, even hypothetical, way to increase the 1microNewton per mm2 with that kind of construction?

### RE: Electromagnet with 30mA

Yes, this is where it gets a bit difficult, for me anyway.  Certainly a suitable permanent magnet can provide very high forces, but how do you control that with such a low electrical input (I assume you have a limitation on the available electrical power).  A limited motion PM actuator such as a voice-coil actuator - you could cut one out of an old loudspeaker - might be worth considering but designing it would be complicated, as would calculating the force characteristic.  Possibly you might find some info on-line by "yahooing" on "voice coil actuator", such as:
http://www.beikimco.com/appguides/actuators.php

Perhaps as you require only a very small displacement, you could employ a step-down geared system.

### RE: Electromagnet with 30mA

As a fellow inventor I understand your frunstration but
try to describe the problem well enough to help finding
a solution.

<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS

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