Magnetic field strength of an electromagnet (coil wound around a bobbin)

[PEMF_1]

Homework Statement

1) Calculating magnetic field intensity of an electro magnet
2) Calculating the maximum permissible wire length at a set power (Watts), and voltage (V)

Relevant Equations

Magnetic flux density (Tesla) = Force/(Amps per metre),
Gauss = Tesla x 10,000,
Pulling force of electro magnet = ((Coil turns x current)^2 x magnetic constant x Inner spool area) /(2x distance to target area^2),

I am trying to design an electromagnet which consists of a copper PVC sheathed wire wound around a cylindrical plastic spool of Circumference (C) = pi x diameter. The spool has a hollow body of diameter D1.

This wire has maximum length (L), cross sectional area A, resistivity P. The spool once the wire is fully round has the number of turns N (N = L / C).

The wire's voltage is 120V AC (V). The power going through the wire can range from 0 - 150W.

The spool has an cylindrical magnet contained within the hollow body of diameter D1. This magnet has a magnetic field intensity of G Gauss.

I am trying to calculate

(1) At maximum power of 150W, and at a wire cross sectional area of A, the maximum permitted length (L) of the wire.
(2) At maximum length (L) of the wire the resulting electro magnetic field intensity (Gauss) at a distance of 5cm (around 2 inches) G1
(3) The total magnetic field intensity of both the electro magnetic field (G1) and the magnet (G)

There are a few electrical equations and magnetic type equations I should use.

Please can someone help? Thanks

 

[Charles Link]

It is somewhat unclear what your apparatus is. You first say the spool is hollow, and then you say it contains a cylindrical magnet. Can you please provide a better description?Do you have two separate experimental apparatuses? It is unclear.

 

[PEMF_1]

The plastic spool's inner 'throat' is a hollow cylinder for which the outer walls are 75mm. Here is a weblink to the product (provided this forum will let me post a weblink): https://www.autoelectricsupplies.co.uk/product/985/category/188

 

[berkeman]

I am trying to design an electromagnet which consists of a copper PVC sheathed wire wound around a cylindrical plastic spool of Circumference (C) = pi x diameter. The spool has a hollow body of diameter D1.

Why are you using standard wire? If you want to wind a coil for an electromagnet, you should be using magnet wire (single build, probably). It has thinner insulation to allow more windings in the coil window area. Since the voltage from one layer of the coil to the next is a small fraction of the applied overall coil voltage, magnet wire is made with thin insulation.

The wire's voltage is 120V AC (V). The power going through the wire can range from 0 - 150W.

Are you really going to wind this coil, or is this just a paper exercise?

If you are really going to wind it, do you have access to a coil winder so that you can make the turns very uniform? Have you worked with AC Mains voltages much in the past? What kind of switch and fuse arrangement do you have? And how do you vary the power delivered from 0-150W?

 

[Charles Link]

Just a comment: If the OP is making a cylindrical magnet from a hollow core with just windings and no iron, they will find it considerably weaker (perhaps a factor of 100) than a permanent iron magnet of the same size.

 

[PEMF_1]

Why are you using standard wire? If you want to wind a coil for an electromagnet, you should be using magnet wire (single build, probably). It has thinner insulation to allow more windings in the coil window area. Since the voltage from one layer of the coil to the next is a small fraction of the applied overall coil voltage, magnet wire is made with thin insulation.

Are you really going to wind this coil, or is this just a paper exercise?

If you are really going to wind it, do you have access to a coil winder so that you can make the turns very uniform? Have you worked with AC Mains voltages much in the past? What kind of switch and fuse arrangement do you have? And how do you vary the power delivered from 0-150W?

I did not know there was such thing as magnet wire. I will use this instead for the coil windings. What type of magnet wire should I use?

Yes I am really going to wind this coil. I am actually going to do this.

I do not have a coil winder, where would I buy one?

I am going to connect the magnet wire (+ and - from the windings on the spool) to speaker wire. This speaker wire will then be connected up to + and - ports (Channel 1) on a stereo reciever. This stereo reciever can output around 75W per channel. The stereo reciever itself uses AC voltage and current on Channel 1. The stereo reciever itself is fuzed and connected to mains electric.

The stereo reciever is connected to a Windows 10 desktop computer using a 3.5mm jack to White Red aux cable. Using this cable I will use a piece of software called Scope. This Software can adjust the frequency (0-10Hz), waveform type (square) and % Duty Cycle (% of total power) of the stereo reciever.

 

[PEMF_1]

Just a comment: If the OP is making a cylindrical magnet from a hollow core with just windings and no iron, they will find it considerably weaker (perhaps a factor of 100) than a permanent iron magnet of the same size.

This is the magnet I am thinking of using. Code: NPI75M8-1, description '75mm dia N42 Neodymium Pot Magnet with M8 Internal Thread - 200kg Pull'. Website (if this forum allows it): https://www.first4magnets.com/pot-c...hread-200kg-pull-p13965#ps_0_14799|ps_1_15029.

This is the 'iron' you are referring to. The electro magnet should consist, after construction, of copper windings and a permanent magnet (the NPI75M8-1 magnet). I am trying to calculate the total magnetic field intensity of the above combination of coiled wire the NPI75M8-1 magnet.

 

[hutchphd]

Why are you doing this? What do you expect will happen? I think you may be disappointed with the result.

 

[Charles Link]

To give you some ballpark numbers, with a hollow core, $B \approx \mu_o n I$ where $n$ is the number of turns per unit length, and $n=10000$ per meter might be a healthy number. With a current of $I=1.0$ amps, and $\mu_o=4 \pi E-7$, that gives $B \approx .013$T. Meanwhile a typical iron magnet will have $B \approx 1.0$T (inside the magnet, and perhaps one half of that at the endface).

 

[PEMF_1]

Why are you doing this? What do you expect will happen? I think you may be disappointed with the result.

To make a Do It Yourself (DIY) Pulsed Electro Magnetic Frequency (PEMF) handheld device. This uses electro magnets with a field intensity of 3 Gauss upwards and is placed on the body. Has been known to improve health and general well being. Here is a weblink to the Youtube video I am following as a guide: .

 

[PEMF_1]

To give you some ballpark numbers, with a hollow core, B≈μonI where n is the number of turns per unit length, and n=10000 per meter might be a healthy number. With a current of I=1.0 amps, and μo=4πE−7, that gives B≈.013T. Meanwhile a typical iron magnet will have B≈1.0T.

Using this Magnetic calculator on website: https://www.kjmagnetics.com/fieldcalculator.asp, I used, for the NPI75M8-1 magnet (grade: n42, diameter: 2.95inches, thickness: 0.59 inches, and a measurement position of 5cm from object (so X: 2 inches, and Y: 2 inches). The resulting Gauss value was around 311.1

I am assuming, for a circumference of around 3.14cm x 7.5cm (23.55cm) around 800+ turns around the spool (n). What do you mean by n=1000 per meter?

 

[Charles Link]

Using this Magnetic calculator on website: https://www.kjmagnetics.com/fieldcalculator.asp, I used, for the NPI75M8-1 magnet (grade: n42, diameter: 2.95inches, thickness: 0.59 inches, and a measurement position of 5cm from object (so X: 2 inches, and Y: 2 inches). The resulting Gauss value was around 311.1

I am assuming, for a circumference of around 3.14cm x 7.5cm (23.55cm) around 800+ turns around the spool (n). What do you mean by n=1000 per meter?

Scratch that. Let me calculate for a minute or two... The numbers you inserted are for a disc magnet, with some distance from the disc. Generally the magnetic field is much weaker for those than that of a long cylinder.
$n=10000$ turns per meter would be ten layers with wire 1.0 mm thick. It would take much effort.

 

[PEMF_1]

What is the shape for which the magnetic field is strongest? The magnet could be either a 'pot' (disk/disc) shape or a cylinder, as long as it fits within through the spool it does not matter.

When the wire is fully wound around the disk or bobbin the resulting shape is 'disk' shaped.

What other shape other than disk/bobbin would create a stronger magnetic field with the same turns per meter (n)?

 

[Vanadium 50]

. Has been known to improve health and general well being.

Nonsense.

with a field intensity of 3 Gauss

The Earth's field is about 0.6 Gauss. Refrigerator magnets are around 100 Gauss. This is a very, very small field.

 

[Charles Link]

The windings do very little if you are inserting a permanent magnet in the core. To create an electromagnet you need to use iron (a non-permanent magnet) in the core, and the result is that even with moderate currents, you can create a magnet nearly as strong as a strong permanent magnet. Note that with a plain iron core, you can reverse the direction of the current, and the direction of the magnetic field will reverse. With a permanent magnet, you simply will get a magnetic field in one direction, and windings with currents will have very little effect.

 

[berkeman]

To make a Do It Yourself (DIY) Pulsed Electro Magnetic Frequency (PEMF) handheld device. This uses electro magnets with a field intensity of 3 Gauss upwards and is placed on the body. Has been known to improve health and general well being. Here is a weblink to the Youtube video I am following as a guide:

We don't allow YouTube videos as references in the technical forums here at PhysicsForums. References need to be from credible sources, like this one that I found at NCBI:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2670735/

Abstract
Exposure to a specific pulsed electromagnetic field (PEMF) has been shown to produce analgesic (antinociceptive) effects in many organisms. In a randomized, double-blind, sham-controlled clinical trial, patients with either chronic generalized pain from fibromyalgia (FM) or chronic localized musculoskeletal or inflammatory pain were exposed to a PEMF (400 μT) through a portable device fitted to their head during twice-daily 40 min treatments over seven days. The effect of this PEMF on pain reduction was recorded using a visual analogue scale. A differential effect of PEMF over sham treatment was noticed in patients with FM, which approached statistical significance (P=0.06) despite low numbers (n=17); this effect was not evident in those without FM (P=0.93; n=15). PEMF may be a novel, safe and effective therapeutic tool for use in at least certain subsets of patients with chronic, nonmalignant pain. Clearly, however, a larger randomized, double-blind clinical trial with just FM patients is warranted.

Keywords: Chronic pain, Fibromyalgia, Musculoskeletal pain, Neuromodulation, Pulsed magnetic field therapy, Randomized clinical trial

However, note that this early study was from 13 years ago, and I'm not finding much credible information after that (like the larger clinical trials that this article recommends be done).

The Wikipedia article cites the same infomation from 2007 and even says that the FDA cleared several devices for specific uses that were investigated in the clinical trials. But note that PEMF is not approved for broad ranges of pain, just the specific ones that were found by the small studies to have some efficacy.

https://en.wikipedia.org/wiki/Pulsed_electromagnetic_field_therapy

Pulsed electromagnetic field therapy


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(Redirected from PEMF)

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Pulsed electromagnetic field therapy
Drolet's 1990 Rhumart system, a PEMF device.
Other names	pulsed magnetic therapy, pulse magnetotherapy (PEMF)
[edit on Wikidata]

Pulsed electromagnetic field therapy (PEMFT, or PEMF therapy), also known as low field magnetic stimulation (LFMS) uses electromagnetic fields in an attempt to heal non-union fractures and depression.[1] By 2007 the FDA had cleared several such stimulation devices.[2]

In 2013 the U.S. Food and Drug Administration (FDA) warned a manufacturer for promoting the device for unapproved uses such as cerebral palsy and spinal cord injury.[3]

 

[PEMF_1]

The windings do very little if you are inserting a permanent magnet in the core. To create an electromagnet you need to use iron (a non-permanent magnet) in the core, and the result is that even with moderate currents, you can create a magnet nearly as strong as a strong permanent magnet. Note that with a plain iron core, you can reverse the direction of the current, and the direction of the magnetic field will reverse. With a permanent magnet, you simply will get a magnetic field in one direction, and windings with currents will have very little effect.

We don't allow YouTube videos as references in the technical forums here at PhysicsForums. References need to be from credible sources, like this one that I found at NCBI:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2670735/However, note that this early study was from 13 years ago, and I'm not finding much credible information after that (like the larger clinical trials that this article recommends be done).

The Wikipedia article cites the same infomation from 2007 and even says that the FDA cleared several devices for specific uses that were investigated in the clinical trials. But note that PEMF is not approved for broad ranges of pain, just the specific ones that were found by the small studies to have some efficacy.

https://en.wikipedia.org/wiki/Pulsed_electromagnetic_field_therapy

The windings do very little if you are inserting a permanent magnet in the core. To create an electromagnet you need to use iron (a non-permanent magnet) in the core, and the result is that even with moderate currents, you can create a magnet nearly as strong as a strong permanent magnet. Note that with a plain iron core, you can reverse the direction of the current, and the direction of the magnetic field will reverse. With a permanent magnet, you simply will get a magnetic field in one direction, and windings with currents will have very little effect.

To address your points:

1) As I am not interested in reversing the electro magnetic field direction, should I use a iron magnet or a permanent magnet?
2) The reason why I am wanting a 'magnet' (whether it be an iron or permanent) with windings so so that I can adjust the AC frequency of the windings. With just a magnet I cannot do this.
3) Please provide an equation that will let me calculate the combined magnetic field intensity from a magnet (whether it be permanent or iron) and the windings. Could the combined magnetic field intensity equation be: Magnetic Gauss/Tesla value + windings Gauss/Tesla value?

 

[berkeman]

To make a Do It Yourself (DIY) Pulsed Electro Magnetic Frequency (PEMF) handheld device. This uses electro magnets with a field intensity of 3 Gauss upwards and is placed on the body. Has been known to improve health and general well being. Here is a weblink to the Youtube video I am following as a guide:

Upon further review, this thread is now closed. As you can see from the information I posted above, the use of PEMF is confined to a few specific cases, and is not approved to "improve health and general well being". I would encourage you to find a local Mentor to help you with this project -- preferably one who has worked with medical device design in the past, and understands biomedical instrumentation.