Build electromagnets for electro-acoustic instrument project


#1

I’m in the process of sketching up and collecting knowledge on electromagnets for a project I’m working on. It’s basically going to be a organ-sounding electro-acoustic instrument. It’s an array of metal tines, taken from an old toy piano, where each tine has its own dedicated electromagnet, driving it and ressonating it. It’s more or less the same principle as this tine organ by Matt Steinke. I’ve tried to get a hold of him, but got no answer.

I’ve built various electro-acoustic instruments based on driving electromagnets with homemade pwm-modules, but I need these to be stronger and more precise, so I need to gain a proper understanding of how to build my own. Eventually the whole thing will be controlled through an Arduino and should accept MIDI input and so forth, but as of now, I need to get electromagnets down before I move on.

I’ve made an electromagnet recently, that works and will make the tine sing when hitting its fundamental frequency, but it’s not powerful enough and seems to heat up quite quickly.

I’ve made a less than ideal one recently using the following: An iron bolt as a core (M8 size), plastic washers, nut, a single layer of insulation tape around the core, enamelled cobber wire (unknown gauge, taken off a small transformer), wrapped in one or two layers of insulation tape. It’s wound using a hand drill, probably not ideally, since the wire had some twists and turns here and there coming off the transformer shell, but I tried to wind the coil in the same direction every time. I can’t be sure about amounts of turns using the method.


1: Bolt core and washers, no wire. 2: Transformer shell I took the wire from. 3: Wound and insulated electromagnet.

First off, I can’t say why I added insulation tape to the core before wrapping the wire around. Is this something I should stick to or avoid? I’ve thought about getting some small, industrial sewing machine bobbins, but I wouldn’t really know if that is preferable to just using an iron bolt for this project. Size is something of a factor, as the magnetic field of each electroamgnet has to, preferibly, only affect one tine each.

I want to order some cobber wire for the project, but I struggle to choose a gauge, as I’m unsure of what size fits my cores the best, how much current it will actually draw versus how much current the wire can actually handle before heating up too much. It will most likely be run through a mosfet being switched by an Arduino, so I can only assume I will have to supply the electromagnets with an external power supply, since 5v (or however much the Arduino can supply) isn’t nearly enough to get the power I need - I would think. In the project I linked to earlier, he says that he used a TI transistor array, ULN2803AP datasheet:http://www.kynix.com/uploadfiles/pdf9675/ULN2803AP.pdf, to power it and ramp it up to send 30v pulses. I would assume that is the way to go, correct?

Is it absolutely crucial that I know the amount of turns on the coil to find out how much current it can handle or is it possible to determine a safe margin through trial and error and rough calculations?

How much heat is expected? What constitutes overheating? When things start to smell and actually burn? Will slight overheating have any significant effect on the magnetic field?

What material is best to wrap around the coil after winding? As mentioned, I just added insulation tape, but I’ve seen other kind of tapes used. Is it for cooling? To ‘glue’ it down and hold it in place?