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Small EM recommended specs info.

Small EM recommended specs info.

Small EM recommended specs info.

We need to create a remote controlled electromagnet for the pick-up arm of the submarine we're working on. EMs are pretty simple devices but I need some help to clarify a couple issues. The EMs in mind will be 1" or less in size and won't use more than 24Volts but 12Volts or less is more likely. I want to get the maximum magnetic flux I can out of this EM with the lowest possible voltage to get the job done.

1-I understand that to increase the magnetic flux of an EM, it can be done by
a)Increasing the current,
b)Adding a soft solid iron core rod with the highest percentage of iron possible,
c)Increasing the number of turns of the wire around the core.

a and b are easy. Regarding c, does the number of turns of a finer wire increase the magnetic flux as opposed to the same number of turns with thicker wire? If so, what is the limiting issue to how fine a wire can be used, amperage, voltage or something else? Can you provide a chart of recommended wire gauges for given amperage, voltages or the "something else" for EMs? This EM will be ON for very short periods of time - a second or less but often within 10 seconds.

2- Are there other core materials superior to soft iron for creating more magnetic flux? Can you provide a comparison chart of suitable materials?

3- Does anyone know where, besides Ebay, I can get a decent used magnetometer for a no more than a couple hundred $. I just need it to give me a measure of Teslas for making comparisons as I swap things around.

Thank you.


RE: Small EM recommended specs info.

"...with the lowest possible voltage..." is probably an incorrect goal (unless there are very strange circumstances). Electromagnets' strength is a function of current (and turns, core, geometry) - not voltage as such (except to drive the current through the resistance of the wire). So it would be relatively straightforward to design an amazingly strong electromagnet that requires only a few volts, but at an inconveniently high current. So right off the bat, you need to clarify your goal. Usually minimum power.

Next point, why wouldn't you just buy one? Granted, for underwater application you might need to process it (perhaps vacuum impregnated with something like epoxy, whatever is the usual approach...), but it would seem to be a better plan than trying to build something that outperforms what the experts offer.

We'll address wire gauge and current later.

RE: Small EM recommended specs info.

In general, you're aiming for amp-turns. If you think about it from first principles, there's no difference between a 10-turn coil carrying one amp, ten 1-turn coils each carrying one amp, and one thicker 1-turn coil carrying ten amps. It's all just electrons going in circles.

Wire gauge relates to maximum current, simply due to ohmic losses thus power going up as heat, heat causing excessive temperature, and damaging the materials.

If the duty cycle is only 1s on and 9s off, you could push the peak current much higher than would be usual for a given wire gauge.

In general: Beware duty cycle given as strict %. One month on and nine months off is still "10%", but it's a much different thing. Your 1s is pretty short, so it's within the thermal time constant for most macro sized hardware.

The water would provide a near infinite heat sink, depending on the physical design.

Wire insulation relates to maximum voltage. Most electromagnets are low voltage and thus can and do use enamelled wire.

Winding arrangements can reduce the voltage difference between adjacent wires. Usually you'd ignore this advantage and assume the worst.

RE: Small EM recommended specs info.

Thanks for the reply.

Since EMs are pretty basic devices, once we know the parameters, it would be easier and less expensive to make them because they will be fitting into another custom shaped device. So I'm trying to get the fundamentals clarified before moving ahead.

The power source will first from the sub. The EMs will also have a battery backup net to them in case the arm is broken off. We don't want to lose what it may be holding. So the batteries for the backup would need to also be small and last a while, say about 30 minutes. Ideally they would be button cell batteries and those are usually 1.5V or 3V, but I've seen small 12V batteries too. I don't know what amps these have. It sounds like your saying "current" is voltage.

If the thinner wire allows for more turns around the core, then I need to know the thinnest wire to use without it overheating under a given voltage - say 12V. If there are charts that provide recommendations for the possible combinations, I can zero in on a good starting point design from the start.


RE: Small EM recommended specs info.

Also, to give a better idea of size now, starting from the inside-out, the core might need to be between 1/16" to 1/18" diameter x 3/8" to 1/2" long. The maximum diameter would ideally be 1/4" max. So I need to get as many turns of wire around the core as possible.

Also, if I keep the winding zone to about 1/4" with non-wound ends on each end, does the magnetic flux change if the length of the non-wound ends are longer or shorter?


RE: Small EM recommended specs info.

The info you need is contained in a Table of American Wire Gauge (AWG) - one found on Wikipedia linked below. You'll need the physical size, a winding plan, math skills to work out the length, knowledge of Ohm's Law to calculate the resistance and thus the current as a function of voltage. Using very thin wire is attractive, but it can be very fragile and difficult to work with.

AWG Table on Wikipedia

I don't quite understand your plan for backup power. The switching scheme would probably be less reliable than the primary power. Why is the robot arm being torn off?

I don't understand how we went from a duty cycle of one second on, to holding something long enough to worry about backup power.

RE: Small EM recommended specs info.

Thanks for the wire chart, very helpful.

I'm back to current being about amps, not voltage, which is correct. It seems the goal is to maximize the amperage without overheating the coil wires.

The EMs will be used as a "grabber" on underwater iron based mineral structures to "walk" the sub in high currents. If conditions get too stressing on the arm it may break but we don't want to lose the hard earned position prematurely, so it will be tethered. We will be able to release as an option. But don't worry about this.

I just need the answers to the EM questions for now.


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