a faster motor

On 6/22/07 9:42 PM, in article Bw1fi.54986$xq1.33881@pd7urf1no, "Don Kelly"


If I interpret you correctly, connecting the slip rings to a low impedance negative phase sequence source still allows for induction motor action from emf in the rotor induced by the stator's rotating field. I agree.
The question then becomes: can the rotor rotate at double synchronous speed in the presence of a slip of -1 induced by the stator in the rotor. My answer is: I do not know for sure.
I do know that a single phase induction motor will simultaneously run with a low slip, close to zero, and a slip nearly equal to -2 corresponding the the two counter-rotating fields generated by single phase power applied to the stator. The high slip component arises the stator's field component rotating opposite to that of the rotor. The low slip component comes from the field component rotating in the same direction as the rotor. It is this low slip operation that provides high torque and completely overwhelms torque arising from the high slip interaction.
I can picture, in my mind, the same situation for the proposed motor. I picture the combination of positive phase sequence drive for the stator and negative sequence drive on the rotor as behaving like a synchronous motor running a double speed. While that is going on, there will be induction in the rotor from drive applied to the stator. When shorted out by the slip ring connection, it still is likely, as in the case of the single phase induction motor, there will be parasitic torques but that the machine will still work ok.
Bill
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I wish to point out that these various induction motors are similar to parametric amplifiers in many ways.
The pump is the three phase drive to the stator windings. The slip frequency in the rotor correspond to the idler frequency. The mechanical rotation of the rotor corresponds to the signal frequency. The sum of the idler and signal frequencies equals the pump frequency.
In essence, large torques are produced when various idler and signal frequencies add up to a pump frequency. This can happen for harmonics as well.
The kind of motor mode described here is that of an up-converter. The input consists of a pump and an idler derived from the same frequency. The signal (shaft rotation) frequency is the sum of the idler and pump frequencies.
This does not mean that there are no competitive parametric process taking place.
Bill
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| The question then becomes: can the rotor rotate at double synchronous speed | in the presence of a slip of -1 induced by the stator in the rotor. My | answer is: I do not know for sure.
Consider a motor where the design is the opposite of common practice. The stator has a pair of permanent magnets (or one pair of windings with smooth DC). The rotor has 6 or more poles/windings with each having a corresponding phase relative to its angular position. With those rotor winding currents creating a rotating field at the AC frequency, the rotor should turn in the opposing direction so the rotor fields align with the stator fields. This is just a syncronous motor turned inside out (with slip rings or such to pass the various phases into the rotor without effect by the rotor angle).
Now consider externally rotating the stator itself in the same direction as the rotor is turning. This would, of course, require an unusual mechanical assembly and another external mover source. Shouldn't these rotations add up and give the rotor net speed equal to the sum of the opposite of the rotor field rotation plus the stator rotation?
What the stator presented to the the rotor was a magnetic field rotating. Instead of creating this rotating field by mechanically rotating permanent magnets, let's create it by rotating the field itself through 6 or more poles of windings fed by AC power. I cannot see how this is any different.
What could be at issue is "getting started". Consider the mechanically rotating stator again, with the rotor windings open circuit. If current is suddenly applied to the rotor to create the magnetic field rotating in the opposite direction, how is this any different than a case where the stator is stationary and the applied current has twice the frequency aside from having to turn your head very fast to see it?
I'm convinced this design will rotate at twice the power frequency. I am also convinced that, with a reasonably smooth field structure (e.g not too much spatial harmonics), it can get started by directly applying the full frequency.
But perhaps 6 poles is insufficient for getting a good start. How about 12 poles, instead? This is not hard to achieve with 3 phase power. You simply have half the poles wired to L-N phases and the other have wired to L-L on the correct combination. This certainly complicates the windings on both the stator and rotor. But you only need 4 slip rings to do this on the rotor (3 were needed to do 6 poles). This is not complication in terms of control gear, however (unless you want to vary and control the speed, but in that case you should just produce a higher AC frequency with an ordinary motor).
| I can picture, in my mind, the same situation for the proposed motor. I | picture the combination of positive phase sequence drive for the stator and | negative sequence drive on the rotor as behaving like a synchronous motor | running a double speed. While that is going on, there will be induction in | the rotor from drive applied to the stator. When shorted out by the slip | ring connection, it still is likely, as in the case of the single phase | induction motor, there will be parasitic torques but that the machine will | still work ok.
Are you talking about inducing current into the stator windings that buck against the power being applied to them via the slip rings (which would not be shorted, but would be powered, in my proposed design)?
FYI, later on I want to extend this design idea to single phase through the use of simultaneous separate inductive and capacitive shaded poles on both the stator and rotor. My goal is an ultra high speed beltless fan.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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On 6/23/07 12:56 PM, in article snipped-for-privacy@news1.newsguy.com,

<snip>
I tend to agree with what you say.
In regard to some of the other things you say about getting even higher speeds, I refer back to the parametric interaction mentioned in another post. There will be parametric interactions between the various temporal harmonic and spatial harmonics. Some will be parasitic. The idler frequencies will be associated with the induced currents in the rotors. The engineering trick will be to sort out all these interactions. Many of these interactions have been noted by electrical machinery designers. Usually the interactions turn out to be detrimental.
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| On 6/23/07 12:56 PM, in article snipped-for-privacy@news1.newsguy.com,
|
|> |> | The question then becomes: can the rotor rotate at double synchronous speed |> | in the presence of a slip of -1 induced by the stator in the rotor. My |> | answer is: I do not know for sure. |> |> Consider a motor where the design is the opposite of common practice. The |> stator has a pair of permanent magnets (or one pair of windings with smooth |> DC). The rotor has 6 or more poles/windings with each having a corresponding |> phase relative to its angular position. With those rotor winding currents |> creating a rotating field at the AC frequency, the rotor should turn in the |> opposing direction so the rotor fields align with the stator fields. This |> is just a syncronous motor turned inside out (with slip rings or such to |> pass the various phases into the rotor without effect by the rotor angle). |> |> Now consider externally rotating the stator itself in the same direction as |> the rotor is turning. This would, of course, require an unusual mechanical |> assembly and another external mover source. Shouldn't these rotations add |> up and give the rotor net speed equal to the sum of the opposite of the rotor |> field rotation plus the stator rotation? | | <snip> | | I tend to agree with what you say. | | In regard to some of the other things you say about getting even higher | speeds, I refer back to the parametric interaction mentioned in another | post. There will be parametric interactions between the various temporal | harmonic and spatial harmonics. Some will be parasitic. The idler | frequencies will be associated with the induced currents in the rotors. The | engineering trick will be to sort out all these interactions. Many of these | interactions have been noted by electrical machinery designers. Usually the | interactions turn out to be detrimental.
But how are these different than a more traditional case where there is a field pulling or pushing against a field where one of them is stationary? Is the motion of what would otherwise be the stationary field the factor in this? Or is it the fact that there are N poles as opposed to 2 poles as might be the case with permanent magnets? Or is it simply the fact that both fields are produced by electric current as opposed to just one field against permanent magnets?
What if the stator was mechanically rotated in reverse of its field rotation such that the field became stationary? Wouldn't that let the rotor rotate at just 1x of the line frequency? Would these spatial harmonics be present here, too?
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On 6/23/07 6:28 PM, in article snipped-for-privacy@news1.newsguy.com,

What I am trying to point out is that motors are something like parametric amplifiers. You have a pump frequency that drives an idler and a signal. These lead to coupling between various phasor quantities. In a standard induction motor, the pump is the three-phase drive that excites the stator's rotating field. The desire signal is the rotation of the rotor. The difference in frequency (slip frequency) and the mechanical rotation add up to the pump frequency.
In addition, because the waveforms are not purely sinusoidal in time or around the stator, there will be spatial harmonics rotating at various speeds. There are possible strong interactions possible when the conditions for parametric amplification are met.
One particular case is that of the pulsating field of a single phase induction motor. It can be resolved into two counter-rotating fields (plus harmonics). Once started, there as a strong interaction or coupling between the rotation of one of these rotating fields and the low slip frequency in the rotor turning near synchronous speed. There is poor coupling between the other rotating field and the rotor.
The propose high speed motor we have been talking about is just another version in which there is strong coupling.
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| On 6/23/07 6:28 PM, in article snipped-for-privacy@news1.newsguy.com,
| |> But how are these different than a more traditional case where there is a |> field pulling or pushing against a field where one of them is stationary? |> Is the motion of what would otherwise be the stationary field the factor |> in this? Or is it the fact that there are N poles as opposed to 2 poles |> as might be the case with permanent magnets? Or is it simply the fact |> that both fields are produced by electric current as opposed to just one |> field against permanent magnets? |> |> What if the stator was mechanically rotated in reverse of its field rotation |> such that the field became stationary? Wouldn't that let the rotor rotate |> at just 1x of the line frequency? Would these spatial harmonics be present |> here, too? | | What I am trying to point out is that motors are something like parametric | amplifiers. You have a pump frequency that drives an idler and a signal. | These lead to coupling between various phasor quantities. In a standard | induction motor, the pump is the three-phase drive that excites the stator's | rotating field. The desire signal is the rotation of the rotor. The | difference in frequency (slip frequency) and the mechanical rotation add up | to the pump frequency. | | In addition, because the waveforms are not purely sinusoidal in time or | around the stator, there will be spatial harmonics rotating at various | speeds. There are possible strong interactions possible when the conditions | for parametric amplification are met. | | One particular case is that of the pulsating field of a single phase | induction motor. It can be resolved into two counter-rotating fields (plus | harmonics). Once started, there as a strong interaction or coupling between | the rotation of one of these rotating fields and the low slip frequency in | the rotor turning near synchronous speed. There is poor coupling between the | other rotating field and the rotor. | | The propose high speed motor we have been talking about is just another | version in which there is strong coupling.
But does this rule it out as a working motor? Or does it just mean issues to be dealt with in the design?
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On 6/24/07 9:57 AM, in article snipped-for-privacy@news3.newsguy.com,

I do not think it rules anything out. It will engineering to straighten these items out.
At first blush, single phase motors should not work. Although I have not worked on it, I am pretty sure that poor design could make a single phase induction motor impractical.
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| On 6/24/07 9:57 AM, in article snipped-for-privacy@news3.newsguy.com,
| |> But does this rule it out as a working motor? Or does it just mean issues |> to be dealt with in the design? | | I do not think it rules anything out. It will engineering to straighten | these items out. | | At first blush, single phase motors should not work. Although I have not | worked on it, I am pretty sure that poor design could make a single phase | induction motor impractical.
Why should they not work? For the same reason, a steam locomotive should not be able to get its drive wheels to rotate. But there is a mechanism that allows that to happen which seems analogous to the shaded pole of a single phase motor.
But doing the double winding double speed motor in single phase might be a lot harder. I'd think at the very minimum you'd have to have a shaded pole on both rotor windings and stator windings. And maybe a 2nd shaded pole at opposite phase angle.
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----------------------------
wrote:

The motor itself, is what is considered a doubly excited machine. Such machines have been built and the Schrage motor is an example. Typically such a motor was used for speed control applications. Typically they were about 3 times the size of a normal induction motor of the same rating and the advent of cheap and practical inverters made them obsolete pretty quickly. I do not recall any use of them at speeds over synchronous. Krause & Wasynczuk, "Electromechanical Motion Devices" 1989 Library ref TK153.K73 1989 develops induction motor theory without any restrictions on rotor excitation and makes reference to doubly excited machines (none in particular) only to then go into depth on the singly excited machine. However one statement made earlier is that a condition for steady state torque is that the frequency of the rotor currents has to be we-wm where we is the electrical frequency and wm is the mechanical speed. The induction motor model that they present is sufficient for analysis of a doubly excited machine such as this. On this basis, there is the possibility of synchronous motor torque at twice synchronous speed. Obviously this torque would be dependent on the phase difference between stator and rotor mmfs. However, there is also induction generator torque present and, a drive to get the rotor up to twice synchronous speed would have to develop torque to overcome this (and if available- then why not simply use it and throw away the doubly excited machine as an unnecessary complication?). When the drive is disconnected, then for any hope of synchronous operation the synchronous torque would have to exceed the induction generator torque. In the unlikely case that it does, there will be little capacity left to handle any mechanical load as it is still necessary to overcome the induction motor torque. Typically a synchronous motor can be brought up to near synchronous speed and when the field is energised it is pulled into synchronism and any induction motor action that occurs after that acts as damping of any oscillations. That wouldn't be the case here. In practice the most likely scenario would be that the motor would simply drop back below synchronous speed and behaves like a normal induction motor.
In the single phase motor, one can consider forward and backward "machines" with stators electrically in series and one having slip s and the other slip 2-s. One can then look at the torque speed characteristics of each "machine" and note that at standstill the torque is 0 but the slope of the torque speed curve is positive in this region. Hence, if the motor is given a kick in one direction- it will accelerate in that direction. Sometimes simply giving the shaft a twist by hand is sufficient. The forward torque will exceed the backward torque for speeds above 0 and up to near synchronous speed. Similarly the backward torque will dominate at speeds below 0.
Normally we look at the rotor from the stator. You can also look at the stator from the rotor and sure enough with the negative sequence excitation(as seen from the stator) the rotor tries to drive the stator in the backward direction- can't do it so there is an average forward torque on the rotor. So both stator and rotor excitation drive the rotor in the same direction. Will there be torque pulsations-maybe but I wouldn't be surprised if there weren't any. Likely the best way to analyse this is to consider a conventional machine with shorted rotor windings and determine the speed-torque characteristic (using the normal stator referenced model). Then consider the rotor energised and the stator shorted using a model referred to the rotor. We are looking at a linear model so we can superimpose the torque speed characteristics of the two machines taking into account the fact that the torques are in the same direction. Such a machine should start and run below synchronous speed without an external drive. There will be two different frequencies of currents in both stator and rotor so current waveforms will not be sinusoidal. That could be interesting.
Anyhow it is an interesting but impractical problem.
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Don Kelly snipped-for-privacy@shawcross.ca
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On 6/25/07 8:24 PM, in article tF%fi.63693$xq1.10277@pd7urf1no, "Don Kelly"
If all goes well, I will try to avoid further posting on the subject.
This all started with a suggested motor configuration by the original poster. Although his suggestion seemed ridiculous at first blush, I soon came around to thinking his way. Compared to other possibilities, I think this is a poor engineering solution. When I needed some higher speed for a project, I went to 400Hz.
Electrical machinery was only a tiny part, if even that much, of my electrical engineering career. Nevertheless, I was greatly impressed by the ingenuity of EEs who mostly had little advanced mathematical training. I find many conceptual similarities between electrical machinery and other aspects of EE. This include fields of antennas, synthetic array apertures, and spatial or temporal harmonics in microwave tubes.
Although I am not up-to-date, I am surprised that parametric interactions and mode coupling do not get more application to electrical machinery. By this I mean interaction between a pump frequency and other generated frequencies that allow nonlinear transfer of power from one frequency to another. In the simple situation of an induction motor, it is the transformation from the supply frequency to the slip and rotational frequencies. The hot shot field for this now is the nonlinear interaction be light for parametric amplification or frequency multiplication.
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OK - Let's toss the induction motor concept, since once I energize the rotor it bahaves as a synchronous machine.
Let's say I have a revolving armature synchronous machine. Stationary field is DC, rotor winding is 2 pole, synchronous speed is 3600 rpm @ 60Hz. Agreed??
Now, MECHANICALLY rotate the DC field at 60 rpm, in the same direction as the shaft rotation. The shaft speed will still be synchronous with respect to the field, but the field is rotating at 60 rpm. The shaft speed will be 3600 rpm PLUS the speed of the field, which is 60 rpm. The shaft speed will be 3660 rpm.
Now, re-wind the stationary field with a 3 phase 2 pole winding. Energize it from a 1 hz source. The three phase winding will create a rotating field, rotating at the rate of 60 rpm, exactly the same as resulted from mechanically rotating the DC field 60 rpm. Again, the shaft speed will be the SUM of the synchronous speed of the rotor and the revolving field speed of the stator.
I think we got off the track early on by assuming that we were energizing a standard DC synchronous field from an AC source. That is indeed highly problematic. But if both the field and armature are 3 phase AC windings the analysis is quite straightforward.
As a related proof of concept, consider the following:
Take for example a 25 to 60 cycle synchronous-induction frequency changer. (I grew up in Buffalo, NY the heart of 25 cycle territory, and I have seen and worked on dozens of these, up to 20,000 kw)
From the 25 cycle source drive a 10 pole synchronous motor. This will have a shaft speed of 300 rpm. On this shaft also place a 14 pole wound rotor machine. Energize the rotor from the 25 cycle source, in the direction of the shaft rotation. We now have a 14 pole rotor that is rotating at 300 rpm mechanically ( driven by the synchronous machine), but its field is rotating at 514.28 rpm with respect to the stator, because the 14 pole 25 cycle field at 214.28 rpm ADDS to the shaft speed. The 14 pole stator of the wound rotor machine sees the 514.28 and of course delivers 60 cycles. Even though the wound-rotor machine is called an induction machine, it is not, it has no slip and the frequency conversions are exact.
Point is, in a synchronous machine, the shaft speed is the sum of the field speed and the synchronous speed. In a standard synchronous machine the speed of the field is 0 (DC). There is absolutely no reason it can not be a positive or negative value, with the appropriate design.
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I am trying to follow the argument below but I am having difficulty
On 6/23/07 8:25 AM, in article snipped-for-privacy@4ax.com,

I presume that the machine you describe consists of the positive sequence three-phase stator in combination with the negative phase sequence driven rotor. (I am just trying to resolve possible ambiguity.)

I use the terms stator and rotor to get away from field and armature. For ac synchronous motors, the armature is stationary while the field magnet rotates.

At this points I have great difficulty resolving possible ambiguities.

Ambiguities have not gotten resolved.
Bill
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How do you expect the rotor field to follow or lock into the stator rotating field when the rotor field is rotating in the opposite direction???? It just would not start and can not run as a motor!!!
If the two fields have the same field rotation, the motor CAN run at 1200 rpm for 6 poles, and the 2 fields will be locked to each other. But it can NOT start. But once it is running, you will not be able to pull any mechanical power from the shaft because:
1. mechanical power output must be translated into a certain mechanical slip between the stator and rotor, or a phase angel.
2. In your arrangement, very likely the frequency is very rigid from the power source. Therefore, any mechanical load on the shaft of the motor will pull an enormous current that will trip all the protective devices, which I hope you will install.


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| How do you expect the rotor field to follow or lock into the stator rotating | field when the rotor field is rotating in the opposite direction???? | It just would not start and can not run as a motor!!!
What if the stator field is rotating very slow? It would be not much different than a set of stationary magnets attached to a rotating outer structure that to the interior rotor would appear to be a stator that is rotatting. As explained in another post in this thread, you get the SUM of the rotations created.
| If the two fields have the same field rotation, the motor CAN run at 1200 | rpm for 6 poles, and the 2 fields will be locked to each other. But it can | NOT start. But once it is running, you will not be able to pull any | mechanical power from the shaft because:
But they are opposite, and possibly 12 poles instead of 6 poles. One of the windings (the rotor) can turn, while the other is held stationary.
| 1. mechanical power output must be translated into a certain | mechanical slip between the stator and rotor, or a phase angel. | | 2. In your arrangement, very likely the frequency is very rigid from | the power source. Therefore, any mechanical load on the shaft of | the motor will pull an enormous current that will trip all the | protective devices, which I hope you will install.
Why would the current not be in proportion to the work done just as in a syncronous motor? If that load would require 1 HP to turn were a normal syncronous motor used, how much current at what power factor do you think this would have?
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| What if?? you are now introdusing a completely different issues . . .!! | I responded to the question as presented. For the field to rotate slow, | you must feed it with a slow frequency!!!
I was looking at a number of analogies. A mechanically rotating stator with permanent magnets vs. a physically stationary stator with a rotating field ... and perhaps at a different frequency than that applied to the rotor itself.
|> | If the two fields have the same field rotation, the motor CAN run at |> 1200 |> | rpm for 6 poles, and the 2 fields will be locked to each other. But it |> can |> | NOT start. But once it is running, you will not be able to pull any |> | mechanical power from the shaft because: |> |> But they are opposite, and possibly 12 poles instead of 6 poles. One of |> the windings (the rotor) can turn, while the other is held stationary. |> | In a ac induction motor, one rotating field must lock or follow another | one. | If one of the field is induced or fed through another source, it must be | in the same direction as the other . . . . same number of poles, etc.
I'm using the term "poles" as number of distinct windings. Is this not the correct usage? If I have 24 phase power, and fed that to 48 windings which were distributed around a stator at 7.5 degree intervals, how would that be different than 6 phase power to 12 windings at 30 degrees?
If I use DC power and energize the various poles of the stator at different levels such that I have 1 cycle of sinusoidal circular field intensity, would an AC energized rotor (twice as many poles as AC power phases) within that field start turning?
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On 16 Jun 2007 04:48:15 GMT, snipped-for-privacy@ipal.net wrote:

Is ther such a thing as a single pole motor?
    Poles                            Synchronous Speed          2                                            3600         4                                            1800         6                                            1200         8                                             900
        1 ???                                    7200 ???                    
Or, why not use a VFD and set to 400 HZ Yaskawa.com >>> V7 VFD Range: 0.00 to 400.0 Hz.
Motor may fly, but that's another story
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| On 16 Jun 2007 04:48:15 GMT, snipped-for-privacy@ipal.net wrote: | |>Suppose the following motor was built. The stator has 6 poles much like a |>3-phase syncronous motor would have. They are wired so the field rotates |>clockwise from the end being observed. Now suppose the rotor, instead of |>having permanent magnets, is coupled somehow to the 3-phase power source, |>and has 6 poles of its own, wired so its field rotate counter-clockwise. |>Would the motor run directly at 6000 (for 50 Hz) or 7200 (for 60 Hz) RPM? | | Is ther such a thing as a single pole motor? | | Poles Synchronous Speed | 2 3600 | 4 1800 | 6 1200 | 8 900 | | 1 ??? 7200 ??? | | | Or, why not use a VFD and set to 400 HZ | Yaskawa.com >>> V7 VFD Range: 0.00 to 400.0 Hz. | | Motor may fly, but that's another story
Assuming my proposed design actually works and can start from stopped even under load, it is almost certain that another design would be better.
I think we are talking different terms for "poles". For a basic syncronous three phase motor I would expect 6 poles. The poles that are 180 degrees apart from each other would be wired in the opposite sense so that one one has a maximum "N" magnetic sense, the other has a maximum "S" magnetic sense. That would be 3 pairs of windings, one pair for each phase. It would run at 3000 RPM at 50 Hz, 3600 RPM at 60 Hz, and fly apart at 24000 RPM at 400 Hz.
Now about 6 winding pairs. Do we call this 12 poles? I can get additional phase angles from the wye/star power source by having 3 windings pairs use L-N connections, and the other 3 winding pairs use L-L connections. They would need to have different numbers of turns to deal with the different voltages. Or ... I could just connect them in sequence so that the pairs at 90 degrees apart have the same phase angles. In this case it would run the motor at half speed.
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On 6/27/07 1:37 PM, in article snipped-for-privacy@news1.newsguy.com,

I would think that it is possible to make a three pole machine even if it may not work will. Think of a three phase transformer with only three legs.
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