why the rotor in a synchronous motor is excited by a dc source ?

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--------- Actually the field in the armature is stationary. The current switches in only the coils actually undergoing commutation while not switching in the other coils. At any instant of time, the rotor winding looks like a stationary solenoid. To clarify, the commutator brushes, ideally as other factors modify this somewhat under load, are positioned such that the current in an individual coil is switched when that coil is on the neutral axis- perpendicular to the field axis at the point where the voltage induced in that coil is 0. This also is the position that leads to maximum torque (the two fields at right angles) . The brushes can be moved but the result would be arcing at the brushes and a reduction of torque -neither are desirable.
Try a sketch of a Gramme ring machine which is essentially a toroid on an axial shaft and the toroid is wound with a continuous closed winding. This winding is connected to the world through contacts which are diametrically opposite. Note the current directions in the windings on the outside surfaces and note that as the ring moves, the overall winding appears stationary and the field due to this winding is stationary. You can now add an external field (at right angles to the contact axis and see what happens considering that the toroidal core shields the inside part of the winding so that only the outside part sees a flux. Early machines were built that way but better ways which didn't waste half the conductor length soon took over. --------------------

------------------------------- They certainly are in the industrial sector and part of the problem is that harmonics from one plant can interact with harmonics from another plant. In residential situations, there may well be, at some time, need for filters and pf correction. At present, metering is still done on the basis of kWh so there is no economic incentive for correction and loads are such that adverse effects are small.
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Don Kelly snipped-for-privacy@shawcross.ca
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wrote:

Home HVAC systems is another area that is going to variable speed systems as well. A/C compressors (that's air-conditioning, not alternating-current ;-) and air-blowers are more and more often variable speed for energy savings.
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The only true dc machine I know of that has been put to practical, is the homopolar machine. It is a variation of the Faraday disk. There is no reversal of current and no ac component to the current under steady load. It does require a slip ring. Faraday used a mercury trough.
A homopolar generator has been used to generate high current for testing rails.
Now that such machines are known to exist, it is likely that others can be devised. Apparently, there is not much motivation to do so.
Bill
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| The only true dc machine I know of that has been put to practical, is | the homopolar machine. It is a variation of the Faraday disk. There is | no reversal of current and no ac component to the current under steady | load. It does require a slip ring. Faraday used a mercury trough. | | A homopolar generator has been used to generate high current for testing | rails. | | Now that such machines are known to exist, it is likely that others can | be devised. Apparently, there is not much motivation to do so.
The fun part of this machine is that when you rotate the conductive disk, it generates electricity regardless of whether the radial magnets are stationary, or rotate with the disk (same angular velocity). If the conductive disk is stationary, you can rotate the magnets but this will not generate any current. So it matters that the disk is in motion but not the magnets. Thus moving the magnets does not constitute motion of the magnetic field. But moving a conductor within such a field, even if it moves from one point to another with the same flux, is what it takes to induce an electric potential.
http://en.wikipedia.org/wiki/Faraday_paradox
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Yes. Trying to understand those things make my head hurt though. Been working with the simple field-motion-current model for so many years, which won't explain these things, I guess I'll have to learn a new trick. :-)
daestrom
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wrote: |
| |> wrote: |> |> | The only true dc machine I know of that has been put to practical, is |> | the homopolar machine. It is a variation of the Faraday disk. There is |> | no reversal of current and no ac component to the current under steady |> | load. It does require a slip ring. Faraday used a mercury trough. |> | |> | A homopolar generator has been used to generate high current for testing |> | rails. |> | |> | Now that such machines are known to exist, it is likely that others can |> | be devised. Apparently, there is not much motivation to do so. |> |> The fun part of this machine is that when you rotate the conductive disk, |> it generates electricity regardless of whether the radial magnets are |> stationary, or rotate with the disk (same angular velocity). If the |> conductive disk is stationary, you can rotate the magnets but this will |> not generate any current. So it matters that the disk is in motion but |> not the magnets. Thus moving the magnets does not constitute motion of |> the magnetic field. But moving a conductor within such a field, even |> if it moves from one point to another with the same flux, is what it |> takes to induce an electric potential. |> |> http://en.wikipedia.org/wiki/Faraday_paradox |> | | Yes. Trying to understand those things make my head hurt though. Been | working with the simple field-motion-current model for so many years, which | won't explain these things, I guess I'll have to learn a new trick. :-)
So we could attach the permanent magnets to the spinning disk. As long as we have the field in the correct orientation to the disk, the whole thing can just spin on its axis and generate a radial potential. Now imagine the "disk" is fatter ... much fatter. Go all the way to a sphere. Make it as large as a planet that has a magnetic field. Because the distances are now very large, the radial potential being generated could be substantial. So where is the radial? Ground to sky is in that direction. Just how much this contribute to lightning I cannot say. Have an aspirin.
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This is a more interesting subject than BPH:-).
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|> The only true dc machine I know of that has been put to practical, is |> the homopolar machine. It is a variation of the Faraday disk. There is |> no reversal of current and no ac component to the current under steady |> load. It does require a slip ring. Faraday used a mercury trough. |> |> A homopolar generator has been used to generate high current for testing |> rails. |> |> Now that such machines are known to exist, it is likely that others can |> be devised. Apparently, there is not much motivation to do so. |> |> Bill | | This is a more interesting subject than BPH:-).
Yes, it is. Maybe we should start a new thread for it?
Here's another setup: Take a cylindrical magnet made of electrically conductive material with N and S at each of the circular ends (as opposed to one side and the other). At the axis of one or both circular ends attach it to a rotating mover (careful that this mover does not have its own magnetic fields, so avoid electric motors for now). Connect brushes to the outer edge of the magnet so they remain in contact when rotated, with one at the ends of one pole, and the other in the center (not at the opposite pole). Start the rotation and you get a voltage between those brush contacts. Simple enough?
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wrote:

I need a picture!
Bill
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| wrote: | |> Here's another setup: Take a cylindrical magnet made of electrically |> conductive material with N and S at each of the circular ends (as opposed |> to one side and the other). At the axis of one or both circular ends |> attach it to a rotating mover (careful that this mover does not have its |> own magnetic fields, so avoid electric motors for now). Connect brushes |> to the outer edge of the magnet so they remain in contact when rotated, |> with one at the ends of one pole, and the other in the center (not at the |> opposite pole). Start the rotation and you get a voltage between those |> brush contacts. Simple enough? | | I need a picture!
I need some picture making software that works the way my mind thinks. None out there I have seen do so. I've been working on my own software design, but it is nowhere complete, yet. I supposed maybe I need to get adapted to Pov-Ray.
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----------------------------

------------ There have been induction machines used which are doubly excited. Some of these used a form of slip frequency feedback. I believe the Schrage machine was one of these. All suffered from problems of size and relatively poor performance (I recall one machine which was about 3-4 times the size of a normal induction motor of the same rating). Certainly forcing a current at a desired "slip" frequency would do as you wish but blocking the normal slip currents would be a problem and may have been the cause of the poorer performance. Electronic drives supplying the rotor could be an improvement over these machines but if you can do this, you can go with a normal induction motor and use a VFD supply for the stator and avoid a lot of complications.
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----------------------------

------------ Consider a synchronous generator that is open circuited and the rotor excited with DC. Then there will be a voltage (or voltages for a polyphase machine) induced in the stator at a frequency linearly dependent on speed. If the machine is a 60 Hz machine, the frequency will be 60 Hz at synchronous speed. Now if the machine is run as a motor, it can only run at synchronous speed as at any other speed the generated internal voltage (or back emf) will be at , say 59 Hz, and the supply at 60 Hz, there will be a 1Hz beat frequency with peak torques and currents considerably higher than rated torque and current - considerable damage can result and if the machine is large enough, the effects can cause serious problems in the power system. There will also be 0 average torque so the motor will stall and try to pull up its mounting bolts and go walkabout-.
In theory one could change the speed of a motor by gradually changing the frequency of the rotor- say increasing from 0 to 1 Hz and if done slowly and carefully enough, the motor speed will drop so that the magnetic field of the rotor -as seen from the stator- will still be at synchronous speed as far as the stator is concerned, without actually pulling out of step -i.e. staying away from the maximum torque point.
This is complicated and not worth the effort.
If you want to change the speed- then use a variable frequency drive and, in that case it is easier to use the simpler and more forgiving induction motor. Note that in an induction motor the induced rotor voltages are at slip frequency and the currents will produce a magnetic field that is rotating at slip speed with respect to the rotor-and this field always appears at synchronous speed as seen by the stator.
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| ----------------------------
| |> why the rotor in a synchronous motor is excited by a dc source and |> stator with ac source?...why not the rotor is given ac excitation? | ------------ | Consider a synchronous generator that is open circuited and the rotor | excited with DC. Then there will be a voltage (or voltages for a polyphase | machine) induced in the stator at a frequency linearly dependent on speed. | If the machine is a 60 Hz machine, the frequency will be 60 Hz at | synchronous speed. Now if the machine is run as a motor, it can only run at | synchronous speed as at any other speed the generated internal voltage (or | back emf) will be at , say 59 Hz, and the supply at 60 Hz, there will be a | 1Hz beat frequency with peak torques and currents considerably higher than | rated torque and current - considerable damage can result and if the | machine is large enough, the effects can cause serious problems in the | power system. There will also be 0 average torque so the motor will stall | and try to pull up its mounting bolts and go walkabout-. | | In theory one could change the speed of a motor by gradually changing the | frequency of the rotor- say increasing from 0 to 1 Hz and if done slowly | and carefully enough, the motor speed will drop so that the magnetic field | of the rotor -as seen from the stator- will still be at synchronous speed as | far as the stator is concerned, without actually pulling out of step -i.e. | staying away from the maximum torque point. | | This is complicated and not worth the effort. | | If you want to change the speed- then use a variable frequency drive and, in | that case it is easier to use the simpler and more forgiving induction | motor. Note that in an induction motor the induced rotor voltages are at | slip frequency and the currents will produce a magnetic field that is | rotating at slip speed with respect to the rotor-and this field always | appears at synchronous speed as seen by the stator.
If one has a stator field rotating at 60 Hz and a rotor field not rotating, that should produce a 60 Hz mechanical rotation. So what if the stator field is rotating at 59 Hz and the rotor field is rotating at -1 Hz (the minus to indicating the opposite rotation direction)? Still 60 Hz mechanical rotation?
So how is having the stator field at 60 Hz and the rotor field at -60 Hz different from having the stator field at 119 Hz and the rotor field at -1 Hz?
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wrote:
| One point about this is that the synchronous rotor is typically salient pole | design and you can't get a magnetic field to rotate very easily on such a | design. If you went to a round-rotor type design, you may as well go to a | three-phase winding on the rotor with a third slip-ring. | | Otherwise the pulsations of the magnetic field on the rotor from a | single-phase winding would result in torque pulsations. With single-phase | and very low frequency on the rotor (say, 1 Hz for running near full speed), | the torsional vibrations would be severe.
One of the things I was thinking about for this was a motor with many poles but arranged so they don't all align at the same time. So while one set of poles is in exact positional alignment at one phase point of rotation, the others would not be. No energy would be applied at the poles which are at the point of alignment. But the other poles would have some slight spacing in their position, and the corresponding stator pole would be energized to pull the coming rotor pole, and push the leaving rotor pole. This way it could be designed so that there was always a constant power applied somewhere all the time. The DC or polyphase AC supply would then also see a constant power draw.
| But with a three-phase rotor and stator, I think your idea would work. But | as far as variable speed, you would need a VFVV drive to apply the right | frequency to the rotor. But if instead you just shorted the slip-rings and | applied the VFVV drive to the stator, you'd get the same result. And | wouldn't need a wound-rotor and you have the very common VFVV / induction | motor combination.
My interest in that design with a three phase energized stator and rotor was to see if an uncontrolled syncronous motor could operate at 7200 RPM on 60 Hz three phase power. And if it could, could it properly start up when power is instantly applied to an idle motor. And would it have an efficiency similar to a normal syncronuous motor.
| With your design, IIUC, you would apply 60 Hz to the rotor in the same | phase-sequence (direction) as the stator for starting and slowly decrease | the rotor frequency to accelerate the machine. Once at / near zero Hz, you | could reverse the phase-sequence (direction) and then ramp frequency back up | to 60 Hz as the rotor accelerates *above* synchronous speed.
Obviously this being a controlled motor. But if you have a controller, there wouldn't be any benefit I could see to the added complexity of having windings on the rotor and the coupling to energize them.
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----------------------------
wrote:

-------------- Don't worry about constant power- worry about unidirectional torque. What you describe can be done with some versions of stepper motors. Also, this is what you essentially have in a single phase shaded pole induction motor-without the complication of pole switching ------------.

-------------- No. - the rotor field would have to be at synchronous speed with respect to the stator in order to produce anything other than torque pulsations with a zero average and high currents. Note that a conventional synchronous motor doesn't have its field energized and starts as an induction motor- then the field is energized and as long as it is close enough to synchronous speed, it will lock in at synchronous speed. If the speed is not close enough- then it is floor shaking time. Similarly -if the machine was already at synchronous speed and you applied the same frequency AC to the rotor winding you would get the rotor field turning at a speed which was different from that of the stator field and the two fields would act much as they would with a startup from 0 with the field energised with DC. Not good. A very small motor might react quickly enough -for example, a 1/2 inch toy compass needle will sometimes start and accelerate to speed within less than a half cycle . Inertia of a larger mottor will not allow that. -------------------

-------- EXACTLY!
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|> My interest in that design with a three phase energized stator and rotor |> was to see if an uncontrolled syncronous motor could operate at 7200 RPM |> on 60 Hz three phase power. And if it could, could it properly start up |> when power is instantly applied to an idle motor. And would it have an |> efficiency similar to a normal syncronuous motor. | -------------- | No. - the rotor field would have to be at synchronous speed with respect to | the stator in order to produce anything other than torque pulsations with a | zero average and high currents. Note that a conventional synchronous motor | doesn't have its field energized and starts as an induction motor- then the | field is energized and as long as it is close enough to synchronous speed, | it will lock in at synchronous speed. If the speed is not close enough- then | it is floor shaking time. Similarly -if the machine was already at | synchronous speed and you applied the same frequency AC to the rotor winding | you would get the rotor field turning at a speed which was different from | that of the stator field and the two fields would act much as they would | with a startup from 0 with the field energised with DC. Not good. A very | small motor might react quickly enough -for example, a 1/2 inch toy compass | needle will sometimes start and accelerate to speed within less than a half | cycle . Inertia of a larger mottor will not allow that.
So a motor with permanent magnet rotor cannot simply be given three phase AC in the stator windings and expected to come up to speed, if I understand you right. While it might actually turn, it would be "bumping" against that (initially fast with respect to the rotor) rotating field. Is that where the shaking is from?
How does the syncronous motor of a clock work, then? Is it "small enough"?
But a motor under control, especially one with a positional feedback, can be brought up to speed by the controller managing the field by keeping it just ahead of the rotation enough to maintain the desired acceleration?
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wrote:

Motors like that have an Al 'squirrel cage' cast into the rotor as well as the PM. The squirrel cage acts like an induction motor and starts the thing spinning. Once it's up to just 1-2% slip, the slowly alternating torque of the PM is enough to accelerate it the rest of the way to synchronism. Once there, the squirrel cage does nothing and is just along for the ride.
Reluctance motors are often built the same way, with a small squirrel cage / induction rotor used for starting.

If you could turn the DC current of the rotor on/off at just the right times. But the rotor winding is highly inductive and it would be difficult to stop the current flow fast enough to avoid its magnetic field from creating a counter torque at the 'wrong' time.
In your 'twice synchronous speed' idea, you can't use a simple squirrel cage to accelerate the unit from a dead stop because it would develop counter-torque when you tried to go above sync-speed.
I still think your best bet would be a three-phase wound-rotor motor with VVVF applied to the rotor.
So in theory, you could do it. But in practice....
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wrote:

------------------ A hysteresis synchronous motor is actually better as it can develop maximum torque at all speeds up to synchronous. They have been used in clocks and the more expensive turntables (prior to speed feedbace controlled brushless DC machines) They are damned expensive per HP so are limited to small motors. -----------------

------------- In addition, even if the machine was brought up to syncronous speed and the rotor was supplied with line frequency- you would have +/- 100% slip -not good in a synchronous machine.
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GE had just this at the T-G test facility in Schenectady, N.Y. in the 70's or maybe 80's. It was used to test turbine generators. Basically it is (was?) a doubly fed wound rotor induction motor. The rating was something like 16,000HP at 3900 RPM (about). The rotor was fed from a cycloconverter and it probably could operate as slow as 0 speed for starting purposes. I don't know if the basic machine was 4 pole or 2 pole but my guess would be 4 pole but mechanically designed for 3900+ RPM. Put 60HZ on the stator and a DC voltage on the rotor and you get 1800 RPM, put 60HZ on the rotor and you get either 0 rpm or 3600 RPM. Put 63HZ on the rotor and you get either -300 or 3900 RPM. Make the rotor frequency adjustable between -60Hz and +63Hz (or is it 60Hz to -63Hz*) and you get 0 to 3900RPM.
* If the rotor field rotates the same direction and speed as the stator field, the rotor doesn't turn. Both windings have to have the same number of poles.
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