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building a safe ac electromagnet

E

ER

Jan 1, 1970
0
I would like to build an electromagnet that is powered directly from
household AC current (110V/60Hz), and I would like to know about any
precautions and design parameters I should keep in mind. The core will
be a metal bar approx. dimensions 6-inches x 1/2-inch x 1/4 inch.
I have no idea of the iron content.

Some questions I have:

1. Suggested gauge for magnet wire to use. I have plenty of 37 gauge
wire, but I can purchase other sizes if necessary.

2. How well should I insulate the core from the coil? Will one layer of
electrical tape suffice? or two layers? or should I use some other
insulating method?

3. Suggested minimum resistence of the coil. (I know the total impedence
will depend on the inductance as well, but I'm just going for a lower
bound on the minimal recommended impedence.) For instance, the coil
in a small AC powered aquarium air pump has a resistence of 6K ohms.
How much lower is it safe to go to?

4. How does a U-shaped core compare with a straight bar in terms a
inductance?

I would appreciate any pointers, help, suggestions you can give.
 
J

John Popelish

Jan 1, 1970
0
ER said:
I would like to build an electromagnet that is powered directly from
household AC current (110V/60Hz), and I would like to know about any
precautions and design parameters I should keep in mind. The core will
be a metal bar approx. dimensions 6-inches x 1/2-inch x 1/4 inch.
I have no idea of the iron content.

Some questions I have:

First, I have a question for you: What is the intended purpose of
this electromagnet?
1. Suggested gauge for magnet wire to use. I have plenty of 37 gauge
wire, but I can purchase other sizes if necessary.

That is probably way too fine. But, again, the best choice depends on
what you want this thing to do.
2. How well should I insulate the core from the coil? Will one layer of
electrical tape suffice? or two layers? or should I use some other
insulating method?

Since you are dealing with a possible death hazard, you don't want to
use any insulation that can fail by melting under the wire. I would
go with a layer of something that will smoke a while before it fails.
Kapton is wonderful. Make sure there are no sharp edges on the iron
that might pierce it. Paper and epoxy is pretty good. So is
fiberglass cloth tape and epoxy. Electrical tape is for covering wire
splices (on the outside, not underneath). You can wrap the outside of
the coil with electrical tape, though.
3. Suggested minimum resistence of the coil. (I know the total impedence
will depend on the inductance as well, but I'm just going for a lower
bound on the minimal recommended impedence.) For instance, the coil
in a small AC powered aquarium air pump has a resistence of 6K ohms.
How much lower is it safe to go to?

Are you expecting to saturate or remove the iron core at any time?
Resistance is all there is to limit the current under those
conditions, but it makes for a cooler running coil to put that
resistance somewhere else, such as a big light bulb in series. Its
glow also gives an indication of the current.
4. How does a U-shaped core compare with a straight bar in terms a
inductance?

The inductance is proportional the total flux, all other things being
constant. The air between the poles is the magnetic analog resistance
in an electric circuit. The more air between the poles, the more
magnetic resistance in the way of the flux. So getting the two poles
only half as far apart roughly halves the magnetic resistance, roughly
doubling the flux per amp, (very) roughly doubling the inductance.
I would appreciate any pointers, help, suggestions you can give.

Expect your iron core to get very hot, since it represents a massive
shorted single turn secondary with your coil being the primary of a
transformer. You have more questions to ask and answer before you
will understand what you are building.
 
A

Alan Boswell

Jan 1, 1970
0
ER
To make it safe, you should operate it from a 12 volt transformer, don't
mess with 110V. Lower voltage will mean using thicker wire, but it's
worth it for safety.
Alan
 
E

ER

Jan 1, 1970
0
John said:
First, I have a question for you: What is the intended purpose of
this electromagnet?

It's rather involved, so I created another posting.

Please have a look at the message I just posted in these same newsgroups
regarding "yet another mechanically scanned display".

Thanks,
 
E

ER

Jan 1, 1970
0
A said:
You mean like a hair clipper?

Well, I don't know exactly how a hair-clipper works, but I have taken
apart an aquarium air pump, and the mechanism seems to be very simple.
The AC power directly drives a small electromagnet which then causes
a permanent magnet on an arm to oscillate back and forth. The vibration
acts on a small diaphragm positioned on the center of the arm which
pushes air out.

regards,
 
S

Spehro Pefhany

Jan 1, 1970
0
Well, I don't know exactly how a hair-clipper works, but I have taken
apart an aquarium air pump, and the mechanism seems to be very simple.
The AC power directly drives a small electromagnet which then causes
a permanent magnet on an arm to oscillate back and forth. The vibration
acts on a small diaphragm positioned on the center of the arm which
pushes air out.

With a flapper valve. If you reverse it you can make a dandy little
vauum pump for vacuum tweezers etc.

Best regards,
Spehro Pefhany
 
A

A E

Jan 1, 1970
0
Spehro said:
With a flapper valve. If you reverse it you can make a dandy little
vauum pump for vacuum tweezers etc.

I picked up a small DC motor powered flapper valve air pump at a surplus store
in Toronto, and hooked it up to my desoldering gun. Makes a decent hot air
pencil. But the choppy quality of the air stream is not so great. And using a
pistol grip when you're used to a pencil grip is kind of awkward. But hey, it
was 2$.
 
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