Rotational field using 2 only signal input

Dear readers, please refer to the linked diagram and advise if I am doing this correctly. The object is to produce a rotating magnetic field using only two signals - the second being shifted 90 degrees from the first.

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This means only to amplifers are required.

How is different in effect from using a commutator (or four phase delayed waveforms ) to sweep from one coil to the next? IOW in the first case, all coils are active, but at differing phases. In the second, only one coil is active at a time.

IOW how does this affect the "type" of generated field?

In practice all the coils would be rotated so they are vertical.

Thank you,

Chris Warren

Reply to
pnielsen
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In the first case you will generate a field of constant intensity with a smoothly rotating direction.

In the second case you will generate a field that will have step changes in direction. Since you can't change the current through a coil infinitely fast, the field will not change direction instantaneously, but the direction will change in a series of jerky steps.

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Bill Sloman, Sydney
Reply to
Bill Sloman

What I don't quite understand though is any two opposing coils will still be fully activated. The only difference is the polarity.

Now this would constitute a rotational field if a magnetized rotor were placed inside, eg. it would spin. But from the perspective of pure intensity, it appears the field would simply be cylcing between an x and y axis.

Perhaps (undoubtedly)I am not visualizing this correctly. If so, a brief explanation would be much appreciated..

P Nielsen

Reply to
pnielsen

On a sunny day (Tue, 09 Apr 2013 12:41:08 +1000) it happened snipped-for-privacy@novelle.com wrote in :

Try driving with a sine wave. For any inductance I = t.U/L (current rizes linear with time if coils have zero R) so you get a ramp / sawtooth.

For the rest think of it as an oscilloscope with magnetic deflection. To make a nice circle you need 2 90 degrees shifted sine waves.

This would give a square standing on one of its points:

/\ \/

BTW with magnetic deflection, the field moves the electrons sideways...

Old CRT stuff.

Reply to
Jan Panteltje

And the magnitude of the field which will be sqrt(2) times larger when both X and Y coils are active simultaneously.

To make it smoothly vary in direction you have to drive the coils with sine and cosine waveforms so that the magnetic field rotates.

The waveform you have drawn is;

X Y

1 0 1 1 0 1 0 0

The four state drive should be:

X Y 1 0 0 1

-1 0 0 -1

A full cycle of 8 states should look like

X Y 1 0 a a 0 1

-a a

-1 0

-a -a 0 -1 a -a

Where a is 1/sqrt(2)

If you use I.sin(wt) and I.cos(wt) as the drive currents then the magnetic field rotates.

--
Regards, 
Martin Brown
Reply to
Martin Brown

Your coils are series *opposing*! The field in the centre would be zero!

Jeroen Belleman

Reply to
Jeroen Belleman

The waveforms in my drawing are already 90 degrees offset, which I believe corresponds to sine/cosine. Are you saying here these need to be actual sinewaves and that squarewaves will not work?

Sorry, I am not familliar with this notation. Would it be posible to elaborate? What does the "-" denote?

For those of us who are not proficient in expressions, can you please explain this another way?

P Nielsen

Reply to
pnielsen

Yes. The reason you can make the magnetic field inside the two sets of coils rotate is because of the identity sin^2(x) + cos^2(x) = 1

Negative numbers! Current flowing the opposite sense to "+"

You actually need to drive the coils with a sinusoidal current or at least a piecewise approximation to one for this to work.

--
Regards, 
Martin Brown
Reply to
Martin Brown

sine waves will give smoother operation. with square waves it will rotate but may buzz more that it would with a sine wave drive.

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Reply to
Jasen Betts

Thank you for clarifying those points.

So, as I understand, the two sinewaves need to be bipolar as well (?)

Not to doubt you, but here is the article I got the idea from. Seems they are using squarewaves. What is the difference in derived field between this and what you have described?

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P Nielsen

Reply to
pnielsen

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As drawn they show both sets of coils with the same drive waveform so it wouldn't work at all. Unfortunately not uncommon in EDN. No wonder you are confused if you are using that as a reference!

Try the Wiki for stepper motors which is closer to what you want:

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And the waveform diagrams for different drive styles.

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Regards, 
Martin Brown
Reply to
Martin Brown

It's a good idea.If you put a bipolar sine wave through a coil, the current flowing is limited by the inductance and the resistance of the coil - they are in quadrature. If you drive an off-set sine wave through the coil, the DC component (which is what gives you your off- set) only sees the resistance of the coil.

The intensity of the field fluctuates as it rotates.

The usual half-baked EDN design idea - it looks as if the editors have thrown away a lot of what the authors originally wrote. With the toys available, the authors could have produced a pulse-width modulated approximation to a sine wave - one of Don Lancaster's "magic sine waves" - relying on the inductance of the coils to suppress the higher frequency harmonics that this approach generates.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

Actually, zero amplifiers are required; just get an old shaded-pole motor, and remove the rotor and gearbox. The stator has a four-arm magnet structure with heavy shorted windings (the 'shade' of the shaded pole) that acts as a 90 degree inductor/resistor phase shifter. Plug it into AC, and it produces the rotating field.

Reply to
whit3rd

Four, if you're really going to drive 0, 180, 90 and 270. A three-phase motor only needs three amplifiers, and draws power more smoothly from the supply.

Not too different from a commutated motor, except that there is no feedback shown. Stepping from one coil to the next makes each amplifier simpler, but gets you less torque and power for the copper expended in the motor.

Depending on how the coils are arranged, not much.

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Reply to
Tim Wescott

I am still a bit confused. Are you saying the four coils, wired up as shown in the EDN article, can be driven using one bipolar sinewave and one bipolar and cosine wave?

This seems to indicate that only two push/pull amps are required. Of cousre, in this case, there would be no common ground to the coils as shown in my little sketch.

IOW if I replace the two squarewave signals shown in the EDN article with two 90 degree offset, bibolar sinewaves will a smooth rotational field be produced?

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P Nielsen

Reply to
pnielsen

This does seem to be the group concensus.

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Bill Sloman, Sydney
Reply to
Bill Sloman

OK, then it actually is possible to drive four coils with two amps, assuming they are push/pull, as is most consumer gear. That allows for the use of standard stereo equipment and sound card generated signals.

If anyone else who responded disagrees, or sees any drawbacks with this method, please let me know.

Thanks,

P Nielsen

Reply to
pnielsen

Two-phase quadrature supplies, and motors, were common at one time. They had been around as long as three-phase. Mature technology.

They died when standardized 3-phase grid supplies became the norm, along with the plethora of local voltages.

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Reply to
Fred Abse

What are you trying to do?

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Regards, 
Martin Brown
Reply to
Martin Brown

Only two bipolar amps are needed the identity cos(180) = -1 is very useful. You just connect opposing coils in series so they generate a reinforced solenoidal field in the gap between them.

(get it wrong and they cancel out)

One coil acting at a time is kind of clunky.

--
Regards, 
Martin Brown
Reply to
Martin Brown

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