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CCWKen
02-24-2015, 04:32 PM
There seems to be a lot of electrical questions lately so I hope one more won't hurt. :)

I need to wind two coils on a U-shaped core, one on each leg, to form an electro magnet for a low voltage AC powered horn. I want the EM to be as strong as possible but I'm confused about the direction of the coil windings. The coils will be wired in series. How should they be wound? In opposite directions or the same direction? I'm thinking they should be the same direction like one continuous coil to get a North and South pole on each half of the cycle.

The horn is of a simple construction consisting of an AC (magneto powered) electro magnet that pulls on a metal leaf. The leaf rests against a diaphragm so when the coil is energized through a half cycle, the leaf is pulled away from the diaphragm. The zero crossover point allows the leaf to snap back against the diaphragm resulting in the sound. Basically, it just a buzz.

Lew Hartswick
02-24-2015, 05:51 PM
Wind them separate and then you can reverse one of them if the fields cancel out just by reversing which end of the second coil is connected to the first.
...lew...

ironmonger
02-24-2015, 07:07 PM
The coils must have opposite magnetic polarity facing each other. One coils start winding at the top of one leg of the 'U' and the other coil with the end winding at the top of the other leg of the 'U'. The coils would be the same if they were driven by DC.

But you might look at these (http://www.mpja.com/30-120V-AC_DC-Electronic-Alarm/productinfo/31917%20SU).
or this (http://www.mpja.com/6-18VDC-Electronic-Alarm-Panel-Mount/productinfo/20188%20SU) which could be driven by AC with the addition of a small diode.

paul

ironnut
02-24-2015, 07:11 PM
I think your thinking is correct. If the core was straight and you wound the wire onto it and then bent it into the U shape, I don't think it would be wise to wind part of the coil in one direction and then wind in the other direction for the remainder before bending into the U shape. The 2 coils solution suggested by Lew would certainly be a safer way to do it.

Gordon

PStechPaul
02-24-2015, 08:12 PM
If you are using AC from a magneto, it may have a variable frequency, duty cycle, and voltage. Thus the magnetic field strength will depend on these factors as well as the number of turns and the material of the core, which determine its inductance. The current will also be limited by the resistance of the coil. This is similar to old telephone ringers:

http://en.wikipedia.org/wiki/Magneto

http://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Magnet_%28PSF%29.png/220px-Magnet_%28PSF%29.png (http://bbs.homeshopmachinist.net/wiki/File:Magnet_(PSF).png)

J Tiers
02-24-2015, 08:52 PM
The U-shape has nothing to do with polarity.

Just wind one coil and then go on and wind the next in same direction *around the core*.

If you were to take the core, and hold it in front of you, with one leg toward you and the open end of the U to your right....Imagine starting at one side, drawing little arrows on the outside surface, pointing "up"....... then turn the core so the open end of the U is away from you, and keep drawing the arrows pointing up.... then turn so the open part is to the left, and continue drawing the arrows....still pointing up.

You would have a core with arrows on 3 sides, all pointing up. That's how you would wind the wire, also.

At any given instant, the end of one leg will be north, and the end of the opposite leg will be south. That's what you want in order to make an electromagnet to develop a field at the open end of the U.

CCWKen
02-24-2015, 11:39 PM
Thanks guys.

The coils are wound over rectangular "spools" then slide down over each core stack leg so it's not just one layer in a single direction. Each spool will have 3 layers of about 30 turns on each layer. The spools are held in place on the core stack by bending a portion of the outer plates back a little to flare a tab. Each core leg covered by a spool measures about 3/16" W x 15/16" L x 1 1/8" D. So they're pretty small coils.

This is a horn for an antique car. The electrical system is driven by a magneto in the engine so the voltage and frequency does change with engine speed. It can range from about 7v/3kHz at idle to about 32v/16kHz at high end. High end being about 35-40mph. :)

PStechPaul
02-25-2015, 12:05 AM
That should make an interesting sound if you mash the horn button while accelerating and decelerating. Not quite an A-OOH-GAH, but more like a 1950s cop car siren, maybe. Make sure you get a video with sound! :D

dp
02-25-2015, 01:16 AM
Wind them as if you were winding them on a straight core. That is to say, don't change the direction of the winding. If you bend such a core in a U it will still work fine.

Paul Alciatore
02-25-2015, 03:22 AM
What everyone else is saying, put another way. If you are winding them on spools and then placing them on the two arms of the core, wind them the same way. Assuming you have a single layer of wire, connect the two wires nearest to the U bend together and apply power to the two wires near the poles.

This works because they need to continue in the same direction around the U bend. So they are wound in the same direction. But you do need to pay attention to the ends that you connect.