Building an ac electromagnet

I want to generate a sinusoidal electromagnetic field of a specificed frequency between 30-80 Hz. For that I was going to pass an ac current to the an electromagnet.

What do I generate an ac current of the specified frequency? How do I make sure the magnetic core of the electromagnet does not saturate between 30-80 Hz to produce a uniform sinusoidal electromagnetic field?

The magnetic field just needs to be measurable from 1-3 meters by a magnetometer. Do I need to worry about the electromagnet heating up because of this?

• I suppose, technically what you're asking for is a core that won't saturate at 30Hz (since if it doesn't saturate at 30Hz, it won't saturate at 80Hz). But also won't be too lossy at 80Hz (since the eddy losses are higher at higher frequencies). Jun 24, 2019 at 22:05
• @DKNguyen 80hz high enough for that? Jun 24, 2019 at 22:07
• I'm just saying that because my understanding is that that's technically what limits the high frequency operation of magnetic cores (not saturation). Practically though, I don't think eddy current losses are very bad at 80Hz but I am not a magnetics guy so I can't say for sure. Someone else will have to help you from here. All I can really tell you is pick a low frequency optimized core material (i.e. mainly one that does not saturate easily since I don't think you have to worry about hi freq losses) and wind it such that the currents you run it at don't saturate the core at the peaks. Jun 24, 2019 at 22:08
• If you are measuring a field from an open electromagnet it's almost impossible to saturate, so that is not a problem. How big a field do you want? For example if you were to take apart a larger 50Hz transformer (take the I core off) you'd have a perfectly viable electromagnet. Or there are any number of viable demagnetizers available for 50/60Hz that should work down to 30Hz without any saturation problems. If all you need is a small coverage field then even a watch demagnetizer might work for you with some modifications. Jun 24, 2019 at 22:23
• If you used a 110/25V AC transformer, you should be able to drive it (the 25V winding) from an audio amplifier and a signal generator. Lots of options. Jun 24, 2019 at 22:46

I want to generate a sinusoidal electromagnetic field of a specificed frequency between 30-80 Hz....What do I generate an ac current of the specified frequency?

You need to buy a signal generator or build an oscillator. If it must be capable of generating any frequency in the specified range, it must have an adjustment that covers the range. An amplifier will likely be required to provide sufficient power to drive the coil.

How do I make sure the magnetic core of the electromagnet does not saturate between 30-80 Hz...

The voltage must be adjustable to suit the frequency, winding and core design.

The magnetic field just needs to be measurable from 1-3 meters by a magnetometer.

That seems to imply that the primary objective is to generate a magnetic field in air rather than an iron core. Perhaps you need a magnetic core with a 1 to 3 meter air gap. If you can not make the 1 to 3 meter space an air gap in a longer iron path for the field, I don't believe there is any benefit to using an iron core.

You need to first determine the strength of the magnetic field. If that is not specified for you, you must determine the sensitivity of the magnetometer that you can afford to purchase and then determine how to design a coil that will produce the field that you can detect.

Do I need to worry about the electromagnet heating up because of this?

If you are going to use an iron core, you need to be concerned by heat generated in the iron by eddy currents and hysteresis. You also need to be concerned by heat generated in the wire by the flow of current. At a higher frequency, you would need to be concerned by skin-effect and eddy-current effects in the copper.

I believe that the question is too broad to allow a more specific answer.

All the energy in a gapped magneto or coil is in the air gap. Gapped cores being very small look like a point source from a distance and reduce in B field by 1/r^2.

Saturation depends on voltage and inductance producing a B field flux that exceeds the core Bsat rating. It does not depend on load current but rather the excitation current from V/L and relative permeability.

To solve your problem, you need a low current plane wave with the plane or diameter of the loop much bigger than the distance to the target. Thus a large air core loops of wire and a signal generator will easily be detected by a magnetometer the wire resistance and inductance such that R/L will be at least 10f frequency to create this field below 100Hz , which should be very easy.