Creating Rotating Magnetic Fields with Helmholtz Coils

[Jdo300]

Hi,

I want to know what the different methods out there to make a rotating field electrically. I am perticularly interested in the types that are produced by toroidally wound coils like the one here: http://upload.wikimedia.org/wikipedia/en/7/7a/RMFpatent.PNG but I want to know if it is better to use a ferrous or air core to produce the field on? I've also heard about Helmholtz coils being used to create the field. Which do you think would be best?

Thanks,
Jason O

 

[Jdo300]

Hello again,

I've narrowed my question now to asking if anyone here knows how to make a rotating field using sets of Helmholtz coils. I have heard of it being done but not in detail.

Thanks,
Jason O

 

[nbo10]

I might be missing something but have you tried rotating the helmholtz coils

 

[Jdo300]

Actually, the idea I am trying to test involves using static coils to electronically create a rotating field.

 

[Gokul43201]

What do you mean by a rotating magnetic field? Is $B(r,\phi, t)$ oscillatory? I can't make much out of the picture posted in the OP - do you have a rotating core in the toroid?

 

[Jdo300]

Hi Goku,

Actually, what I am trying to accomplish is similar to the rotating field that a standard AC motor creates. The only difference is the layout of my coils. I attached an illustration to show what I want to do. Pictured is a flat circular disk (Plexiglas or some other plastic) with two windings wrapped around it’s diameter at 90 degrees to each other. I would like to feed a signal into the coils so that they will produce a rotating field but I’m not sure how to wire them for this to work. I plan on using a sine wave function generator to power the fields. Could you or someone else here give me an idea of how to do this? I know that the input signals into the coils have to be 90 degrees out of phase with each other for it to work right but I’m not sure how to effectively create the second shifted signal.

Thanks,
Jason O

 

[NoTime]

I think if you just wind a second coil with the powered coil and connect that to the perpendicular coil.

 

[Jdo300]

I think if you just wind a second coil with the powered coil and connect that to the perpendicular coil.

Could you elaborate on that?

 

[NoTime]

Just a transformer and inductor.
Should have a 90 degree current lag (more or less due to coil resistance).

For more info try looking up shaded pole AC motor
Sometimes called a shorted turn motor.
These use this general idea to rotate the mag field so that the armature turns.

 

[marcusl]

I'd like to point out that your picture does not show Helmholtz coils. A Helmholtz coil is two circular windings separated by one radius, see the Wikipedia article for more information. Your arrangement will have only a tiny area of homogeneity right at the center. If homogeneity is unimportant to your application, carry on.

To produce a rotating field at angular frequency w=2*pi*f, drive your coils with sin(wt) and cos(wt). You can easily predict the direction of rotation by looking at the Argand diagram exp(iwt) in the complex plane.

 

[Jdo300]

Just a transformer and inductor.
Should have a 90 degree current lag (more or less due to coil resistance).

For more info try looking up shaded pole AC motor
Sometimes called a shorted turn motor.
These use this general idea to rotate the mag field so that the armature turns.

Ahhhh ok I see. Is there a way to figure out the number of turns I need for that coil to offset the current by 90 degrees, or will any coil work?

Thanks,
Jason O

 

[Jdo300]

I'd like to point out that your picture does not show Helmholtz coils. A Helmholtz coil is two circular windings separated by one radius, see the Wikipedia article for more information. Your arrangement will have only a tiny area of homogeneity right at the center. If homogeneity is unimportant to your application, carry on.

To produce a rotating field at angular frequency w=2*pi*f, drive your coils with sin(wt) and cos(wt). You can easily predict the direction of rotation by looking at the Argand diagram exp(iwt) in the complex plane.

Hi marcusl,

Yes I do realize that my final design is not of the Helmholtz type, but as you mentioned, I realized that I didn't need the homogenous field in the center. As for mathematical information, could you explain those equations a bit more. I am just now taking the EM part of physics and we haven't gotten to the chapter about magnetic fields yet.

Thanks,
Jason O

 

[Jdo300]

Just a transformer and inductor.
Should have a 90 degree current lag (more or less due to coil resistance).

For more info try looking up shaded pole AC motor
Sometimes called a shorted turn motor.
These use this general idea to rotate the mag field so that the armature turns.

Hi Again,

I just want to verify that I understand your explanation here. I drew a rough sketch of the circuit diagram for the coils. Could you check it for me and let me know if I am missing something?

Thank you,
Jason O

 

[marcusl]

Sorry, that's not going to work. NoTime led you a little astray with his comments. First, a shaded-pole motor is subtle and complex despite its extremely simple appearance. It induces eddy currents to produce phase shifts. Second, resistance can never give you a phase shift (you need reactances). In your drawing the coils are in series and each will carry exactly the same current.

One way I can think of generating a phase shift with passive components is to put a capacitor in series with each coil, put these two LC series combinations in parallel, and drive the parallel combination from your source. Choose the C's such that the resonant frequency of one L-C is below the drive frequency and the other above, by just enough to give a 90 degree phase shift in one current relative to the other. Two appropriate and convenient points are the half-power or -3 dB response points on each resonance curve (that is, +45 and -45 degrees relative to the resonances). There are certainly other ways. Please note that I'm not an electrical engineer or motor designer--someone else out there might have a better way.

As for number of turns, this turns out to be a little complicated also. The number of turns, the current and the winding geometry determine the strength of the magnetic field (which I imagine is important). The number of turns is constrained by the size of the former you are using and by the diameter of the wire. The diameter and length of the wire (which depend on the number of turns) determine the resistance, which, combined with the current, determine how hot the whole thing will get. The current is determined by the reactance, resistance, frequency and drive voltage. This determines the heat production and magnetic field strength, and so on around in circles. You can see that this is a multi-dimensional coupled optimization problem with many constraints, and actual values will depend critically on the specific case you are considering. I can't give you help with the details. E&M books will give you most of the formulas you need, if you can interpret and apply them. Perhaps you find books on coil and motor design for more direct help.

Good Luck!

 

[Averagesupernova]

... resistance can never give you a phase shift (you need reactances).

Not quite. Look up what is known as a split-phase motor. The starting winding uses a higher resistance wire. The inductance of the main winding vs. the resistance of the starting winding creates enough of a phase shift to make the motor start turning.

 

[marcusl]

I looked at motor webpages, and I see what you mean. By using a big resistance in the start winding the current lag is reduced compared to that in the main stator winding. I stand corrected.

I see two issues with using this to produce rotating magnetic fields in Helmholtz (or other) coils. First, the current in the secondary winding is smaller than that in the main winding because of the higher resistance and fewer turns. Second, the phase shift is about 30 degrees rather than 90, which is enough to start a small motor turning but in our case further reduces by about 70% the size of the perpendicular component. Both of these act to unbalance the main and perpendicular field strengths, creating a linear main field and reducing the strength of the rotating field. In other words, rather than a pure rotating field one will see a strong linear field along the axis of the main winding, with a small rotating component superimposed on it.

Does this sound right?

 

[NoTime]

Hi Again,

I just want to verify that I understand your explanation here. I drew a rough sketch of the circuit diagram for the coils. Could you check it for me and let me know if I am missing something?

Thank you,
Jason O

As others pointed out you can't just put the windings in series.
You need a third coil wound with the coil where you hook the two wire ends directly to the generator. The two coils together work as a transformer. The wire ends of the third coil is what you hook to other coil in your drawing.
You won't get exactly 90 degrees but it should be close.

 

[NoTime]

Sorry, that's not going to work. NoTime led you a little astray with his comments. First, a shaded-pole motor is subtle and complex despite its extremely simple appearance. It induces eddy currents to produce phase shifts.

Sure looks like I led him astay.

If you look at one of the shaded pole motors, you should notice a single loop of about #10 copper wire.
This is actually a shorted single turn secondary winding.
Yes eddy currents are induced in the armature.
But it is this single copper turn that provides the phase shift to start the rotation.

Note that once the armature is rotating, you no longer need the loop. There is enough angular momentum stored in the armature to maintain phase shifting by moving the location of the induced eddy current.

If you have a junk one, then cut the loop out.
It will no longer start by itself.
But, whatever direction you manually spin the drive shaft in, it will continue to rotate in that direction.

 

[NoTime]

Does this sound right?

Looks close to me, but its been a very long time since I've done any work in this area.

 

[Jdo300]

I looked at motor webpages, and I see what you mean. By using a big resistance in the start winding the current lag is reduced compared to that in the main stator winding. I stand corrected.

I see two issues with using this to produce rotating magnetic fields in Helmholtz (or other) coils. First, the current in the secondary winding is smaller than that in the main winding because of the higher resistance and fewer turns. Second, the phase shift is about 30 degrees rather than 90, which is enough to start a small motor turning but in our case further reduces by about 70% the size of the perpendicular component. Both of these act to unbalance the main and perpendicular field strengths, creating a linear main field and reducing the strength of the rotating field. In other words, rather than a pure rotating field one will see a strong linear field along the axis of the main winding, with a small rotating component superimposed on it.

Does this sound right?

Hello All,

I had a chance to fool around with this setup. I tried to use a capacitor to phse shift the coils (which did work), but then the amplitude of the phase shifted wave was much different than the non-shifed one.

Now I am wondering, if one were to use four coils wound in series to produce the rotating field, would the field become phse shifted if the coils were fed with the right frequency, even if it is something much higher than 60Hz? I know that the reactance of the coils starts to play a role at higher frequencies so could this work?

Thanks,
Jason O