Optimum Length & Turns for a Search Coil of a Fluxmeter

In summary: I still don't think you've provided us with enough information to help you. For me, at least, the technical derivation of a constant B field in a simple coil doesn't help at all. So I focus on the quote above... What is moving? A stationary coil in a B-field of constant magnitude and direction will give you nothing. A DC current through a stationary coil likewise produces a stationary B field, and I don't see how that would help you detect a field. Metal detectors employ AC currents in coils to detect metals in their environment. There are many different types, but they all rely on some kind of coupling between the EM generated in the coil and
  • #1
Syed Khairi
2
0
Hello,

I'm investigating the relation of the length and number of turns of a search coil of a fluxmeter (measure magnetic flux). I got to an equation

B= qR/nA

where q is the quantity of charge passed through search coil
R is the combined resistance of search coil and fluxmeter
n is the number of turns of search coil
A is the cross sectional area of search coil

How can I get the optimum length & number of turns for a search coil?
 
  • #3
Syed Khairi said:
Hello,

I'm investigating the relation of the length and number of turns of a search coil of a fluxmeter (measure magnetic flux). I got to an equation

B= qR/nA

where q is the quantity of charge passed through search coil
R is the combined resistance of search coil and fluxmeter
n is the number of turns of search coil
A is the cross sectional area of search coil

How can I get the optimum length & number of turns for a search coil?
Welcome to the PF.

Can you say more about what you are trying to do? Are you wanting to make a coil sensor for B-field, or a Current Transformer (CT) that senses the current flowing in a wire passing through the coil (a 1:n transformer)? What bandwidth are you hoping to achieve? :smile:
 
  • #4
Hello berkeman,

I'm sorry for not giving much information. I am trying to make a coil sensor for B-field. I want to find the optimal design of a search coil which will be used to detect and measure magnetic field. The search coil will be connected to a galvanometer which will give readings in either current or flux.

I understand that the higher the number of turns/loops for a coil, the higher its magnetic flux. But I want to find more relation between number of turns/loops, wire length and radius of coil. I'm looking for an optimal design for maximum detection of magnetic flux.

The first part of this article (http://www.solitaryroad.com/c1049.html) shows the relation between magnetic field, resistance and number of turns/loops for search coil. I then assume the resistance of galvanometer is small and the total resistance only dependent on the search coil, hence using relation of

Resistance = Resistivity x Length of wire/Area of wire

Screen Shot 2015-11-26 at 12.24.26.png


Screen Shot 2015-11-26 at 12.31.09.png


Then, I'm stuck. Can anyone shed some light here? A different approach perhaps?

Thanks.

Syed
 
  • #5
Syed Khairi said:
Hello berkeman,

I'm sorry for not giving much information. I am trying to make a coil sensor for B-field. I want to find the optimal design of a search coil which will be used to detect and measure magnetic field. The search coil will be connected to a galvanometer which will give readings in either current or flux.

I still don't think you've provided us with enough information to help you. For me, at least, the technical derivation of a constant B field in a simple coil doesn't help at all. So I focus on the quote above... What is moving? A stationary coil in a B-field of constant magnitude and direction will give you nothing. A DC current through a stationary coil likewise produces a stationary B field, and I don't see how that would help you detect a field. Metal detectors employ AC currents in coils to detect metals in their environment. There are many different types, but they all rely on some kind of coupling between the EM generated in the coil and the current in the metal. But that's not magnetic detection or measurement.

I'll hazard a guess that you are describing what's known as a magnetometer. Again, there are different types. Flux gate, atomic magnetic resonance, magnetoresistance and Hall-effect devices are a few. They differ in sensitivity and other features. The least sensitive of these is the Hall-effect device. These are also the cheapest (naturally, the cheapest would be the least sensitive) and are used to detect fields in close proximity, like magnets on moving objects. They can be used on burglar alarms, in which a magnet on a window or door moves toward or away from the detector, or a tachometer that measures the relative velocity of a magnet on a rotating axis. The most sensitive are certain kinds of magnetic resonance detectors, in which the rate at which a proton or other subatomic particles precess around the B field direction can me measured. The measurement is a scalar one - only the strength of the field is measured, not the direction. Portable devices are used in geophysical prospecting for ores, in which case minute variations in the local strengths of the ambient field (i.e. the Earth's mag field and the field it induces in magnetic ore bodies beneath the planet's surface). Flux gate and magnetoresistance detectors are direction sensitive. The latter is used in electronic compasses. You can buy modules that work with microcontrollers from SparkFun and similar hobby electronic suppliers. Flux gate detectors are the only kind that I know use coils. In this case a coil wound on a ferromagnetic core carries alternate current. Because magnetic hysteresis in the ferromagnet is sensitive to asymmetries in the direction of the ambient magnetic field relative to the core, a second coil picks up a current at 2x the input frequency. That makes picking the relatively weak current out of the strong inducing current relatively easy. From the above, you can see how flux gate measurements are vectorial. They have been around since the 1930s and were developed to detect and locate submarines, they are also used in magnetic compasses. There are hobbyists who use them to find antiquities underground. I believe they can also be used in geophysical prospecting, though they are less sensitive than the magnetic resonance types. If you're interested, let me know and I can send you informational links.
 
Last edited by a moderator:

1. What is the optimum length for a search coil of a fluxmeter?

The optimum length for a search coil of a fluxmeter depends on the specific application and the target being measured. In general, longer search coils provide better sensitivity for detecting weaker magnetic fields, while shorter coils are better for measuring stronger magnetic fields. It is important to consider the desired sensitivity and the expected magnetic field strength when selecting the optimum length for a search coil.

2. How do turns affect the performance of a search coil?

The number of turns in a search coil affects the sensitivity and accuracy of a fluxmeter. More turns will increase the sensitivity to weaker magnetic fields, but also increase the noise level. Fewer turns will decrease the sensitivity but improve the accuracy for stronger magnetic fields. The optimum number of turns also depends on the application and target being measured.

3. Can the optimum length and turns for a search coil be determined mathematically?

Yes, there are mathematical equations that can be used to determine the optimum length and turns for a search coil of a fluxmeter. These equations take into account factors such as the magnetic field strength, desired sensitivity, and noise level. However, it is important to also consider practical limitations and potential sources of error when selecting the optimum length and turns.

4. Are there any other factors that should be considered when selecting the optimum length and turns for a search coil?

Yes, several other factors should be considered when selecting the optimum length and turns for a search coil. These include the size and shape of the target being measured, the distance between the search coil and the target, and the orientation of the target with respect to the search coil. It is important to carefully evaluate all of these factors in order to optimize the performance of the fluxmeter.

5. Can the optimum length and turns for a search coil change over time?

Yes, the optimum length and turns for a search coil can change over time. Factors such as environmental conditions, wear and tear on the equipment, and changes in the magnetic field being measured can all affect the performance of the search coil. Regular calibration and maintenance can help ensure that the optimum length and turns are maintained for accurate and reliable measurements.

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