Physical Force of Electromagnets

Discussion in 'General Electronics Chat' started by thingmaker3, Jan 19, 2006.

1. thingmaker3 Thread Starter Retired Moderator

May 16, 2005
5,072
6
I would like to build something similar to a linear armature motor.

My google searches and book reading have come to naught on this issue - I'm obviously niether using the right search terms online nor checking the good chapters.

I wish to calculate physical force between two concentric coils given the ampere-turns of each and the distance.

Specifically:
if I wind N1 turns on a pipe (avg coil dia D1) and supply A1 amps dc through it,
and I have N2 turns on a rod inside the pipe (avg coil dia D2) with A2 amps dc through,
then how hard will the rod be pushed/pulled?
(Let's assume that there are L millimeters between the midpoint of each coil, and that each coil is much longer than D. I suspect that permeabilities of the pipe and rod will make a difference as well.)

2. beenthere Retired Moderator

Apr 20, 2004
15,815
282
Hi,

Life might be easier if you used a permanent magnet as the moving element. Life gets simpler if you don't have to move a significant length of somewhat stiff wire. As fas as newtons/tesla, anyone might say 'that depends'. The relationship is according to the inverse square of the distances, so excellent design and construction can make a very significant difference. Plus you have only one current to vary experimentally.

Data Products used to make disk storage. The DP-5045 had thirty-two 33' disks, and a five horse motor to spin it all. It used linear motors to run the head positioners - 8 heads per arm. The coil form was a rounded square. The positioner carriage carried a permanent magnet. There was significant clearance all around, something like 5 - 8 mm. The coils had a fixed voltage feed at the extreme ends, and had a reed relay make a path to ground at the desired track. Current was on the order of two amps. I had to use both hands and a hard pull (not well quantified - sorry) to move a positioner away from the track. Total motion was less than r, probably 40 - 50 cm.

Ever build a solenoid cannon? Just a winding on a dowel can put a washer through the ceiling.

3. thingmaker3 Thread Starter Retired Moderator

May 16, 2005
5,072
6
Thanks, Been! Your anecdotes are encouraging.

I've been wondering if I could build a hobbyist counterblow hammer using electromagnetics instead of pneumatics. I thought a little math ahead of time might save a few hours of wasted experiment.

I started with "1800 watt = 1327.6 foot pound-force/second = 8 pounds accelerated through 1 foot at a rate of... " and then started wondering about heat loss, mass of wire, and the like.

Your solenoid cannon is a "reluctance launcher," is it not? I was groping more toward something loosely inspired by the helical coil launcher.

I'll hit the books again with rare-earth magnets in mind.

4. beenthere Retired Moderator

Apr 20, 2004
15,815
282
Hi,

There's no doubt that the bookwork will start you off closer to where you want to be. However, as the old inverse square relationship is so sensitive to spacing, you might be better off seeing how little clearance you can manage. Having access to a machine shop is just beyond handy - especially for winding coils. Neatness really counts.

If you have a stereo buff friend or a DJ, they could be sources of rare earth magnets from blown speakers...