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

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?

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A synchronous motor requires a continuous, constant magnetic field in the rotor. This field locks onto the rotating magnetic field produced in the stator by the AC input. In small synchronous motors the field is produced by permanent magnets in the rotor. In larger motors, the rotor field is produced by electromagnets, DC excited.
The power to the motor comes from the AC input which must be multi-phased to produce rotation. The DC excitation just produces the rotor field, but not power output. The DC power is usually a small percentage of the AC power.
If the rotor field were AC excited, the poles of the rotor would flip back and forth north to south to north at the AC frequency and could not lock onto the similarly flipping or rotating stator poles. North would lock onto south and south would lock onto north and since both were filliping at the same rate they would remain locked to each other and the motor would not rotate.
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In normal induction motor operation, the stator's rotating field induces a current in the rotor by transformer action. Because of the rotation, the rotor frequency is the slip frequency.
In a way, the motor can be considered to be a parametric convertor device. The pump is driving the stator. The signal is mechanical and at the shaft rotation speed. The idler is at the difference frequency (electrically speaking) current in the rotor. If you drive the rotor at dc, you force the slip frequency to be zero.
This gets me to think about a possible way to control motor speed. I would like to know if such a thing has been tried.
Consider an electronic drive that can vary in frequency. After starting a wound rotor machine, set the drive frequency to zero., You now have a synchronous motor. To change motor speed, change the rotor drive frequency. With a positive phase sequence (the same as the stator), the motor speed should be reduced. With a negative phase sequence, the speed should be increased.
It may be necessary to somehow block the slip current so as not to get normal induction torque. For synchronous operation, there is no steady state slip current.
Bill
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I don't know if anyone has tried to vary motor speed that way or not. It sound like it could work. But, in synchronous machines with DC rotors, the rotors are usually not laminated but sold iron. The winding inductance is usually high because nobody cares since they are usually DC excited. Because of this inductance and the lack of laminations, putting AC on these windings may be problematic, but it could work in principle.
Modern variable speed drives for induction motors do vary the frequency to control the speed. They rectify the line to DC then switch the DC with thyristors or transistors to three phase variable frequency AC to drive the windings. These are induction motors so there is no rotor excitation required. They also adjust the firing angle to control the motor voltage keeping the volt-time product constant.
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| In normal induction motor operation, the stator's rotating field induces | a current in the rotor by transformer action. Because of the rotation, | the rotor frequency is the slip frequency. | | In a way, the motor can be considered to be a parametric convertor | device. The pump is driving the stator. The signal is mechanical and at | the shaft rotation speed. The idler is at the difference frequency | (electrically speaking) current in the rotor. If you drive the rotor at | dc, you force the slip frequency to be zero. | | This gets me to think about a possible way to control motor speed. I | would like to know if such a thing has been tried. | | Consider an electronic drive that can vary in frequency. After starting | a wound rotor machine, set the drive frequency to zero., You now have a | synchronous motor. To change motor speed, change the rotor drive | frequency. With a positive phase sequence (the same as the stator), the | motor speed should be reduced. With a negative phase sequence, the speed | should be increased. | | It may be necessary to somehow block the slip current so as not to get | normal induction torque. For synchronous operation, there is no steady | state slip current.
I had suggested the idea a few months back to have a polyphase rotor field rotating in opposition to the field rotation of a polyphase stator as a way to get the rotor to run as double the syncronous speed, e.g. 6000 RPM for 50 Hz or 7200 RPM for 60 Hz. It got some suggestions that it would not work. At least one suggestion said that starting from 0 RPM would be a problem in syncronization (but I felt that if it had enough poles to make a nice uniform field, this might be avoided). Coupling the power into the rotor would be a question. I don't like the idea of slip rings. I was wondering if some way to couple power with a single phase 360 degrees around a magnetic "slip ring" might be doable (with 3 of them, 1 for each power phase). Maybe it might not work from a dead stop, but could work if initial rotation is started.
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|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net wrote:

Why not use a brushless DC motor driven at any speed you wish?
1. No slip rings. 2. Can be run at any reasonable speed. 3. Can use almost any available power input. 4. Can be put in servo loop to control speed to external standard.
http://www.st.com/stonline/books/pdf/docs/7209.pdf
<quote>
Brushless DC motors consist of two coaxial magnetic armatures separated by an air gap. In certain types of motor,
– The external armature, the stator, is fixed. – The internal armature, the rotor, is mobile (the rotor can also be external in certain cases). The stator is the induced part of the machine. The rotor is the inductor of the machine.
In brushless DC motors, the internal armature, the rotor, is a permanent magnet. This arma-ture is supplied by a constant current (DC). The external armature (stator) is polyphased (3 phases in our case) and is covered by poly-phased currents. The pulsation of these currents is ω. We say that the machine is a synchronous machine because, if Ω is the angular speed of the rotor, we have the relation: Ω=ω/p
In a Brushless DC motor, the rotor is a permanent magnet, this type of motor has almost the same properties and physical laws as a DC current machine.
An electric motor transforms electrical energy into mechanical energy. Two main characteris-tics of a brushless DC motor are: – It has an electromotive force proportional to its speed – The stator flux is synchronized with the permanent magnet rotor flux. The back electromotive force (as we will see in this document) is the basis of one the ways of driving brushless DC motors with the ST72141 microcontroller in sensorless mode.
</quote>
--
Virg Wall, P.E.


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| snipped-for-privacy@ipal.net wrote: | |> I had suggested the idea a few months back to have a polyphase rotor field |> rotating in opposition to the field rotation of a polyphase stator as a |> way to get the rotor to run as double the syncronous speed, e.g. 6000 RPM |> for 50 Hz or 7200 RPM for 60 Hz. It got some suggestions that it would |> not work. At least one suggestion said that starting from 0 RPM would be |> a problem in syncronization (but I felt that if it had enough poles to |> make a nice uniform field, this might be avoided). Coupling the power |> into the rotor would be a question. I don't like the idea of slip rings. |> I was wondering if some way to couple power with a single phase 360 degrees |> around a magnetic "slip ring" might be doable (with 3 of them, 1 for each |> power phase). Maybe it might not work from a dead stop, but could work if |> initial rotation is started. | | Why not use a brushless DC motor driven at any speed you wish? | | 1. No slip rings. | 2. Can be run at any reasonable speed. | 3. Can use almost any available power input. | 4. Can be put in servo loop to control speed to external standard.
That's certainly possible to do. But I was interested in exploring the effects of different configurations. Even if it wouldn't be practical, I was curious if it would work.
| In brushless DC motors, the internal armature, the rotor, is a permanent | magnet. This arma-ture is supplied by a constant current (DC). | The external armature (stator) is polyphased (3 phases in our case) and | is covered by poly-phased currents. The pulsation of these currents is ?. | We say that the machine is a synchronous machine because, if ? is the | angular speed of the rotor, we have the relation:
That's three phase DC, right?
| An electric motor transforms electrical energy into mechanical energy. | Two main characteris-tics of a brushless DC motor are: | ? It has an electromotive force proportional to its speed | ? The stator flux is synchronized with the permanent magnet rotor flux. | The back electromotive force (as we will see in this document) is the | basis of one the ways of driving brushless DC motors with the ST72141 | microcontroller in sensorless mode.
Well, at least it is syncronized to the DC.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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| http://www.st.com/stonline/books/pdf/docs/7209.pdf
I would not call that a DC motor. The motor is not getting DC; the controller is. The motor gets modulated pulsed AC. It may well be the standard practice to call this a DC motor, but if so, it is yet another case of terminology misapplied. I'd call it a "pulse control motor". A more clever controller could drive it from an incoming three phase AC supply directly, by switching in the supply phase that has the right voltage and polarity at that instant (in three phase AC power, there's always at least one phase in each polarity at any time). It sure would not be anything DC in that case.
--
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net wrote:

AC is alternating current because the current "allternates" the direction in which it flows. The current to the motor does not do so. The term "phase" does not only apply to one of the rotating vectors of a multiphase AC supply.
Everybody else calls them brushless DC motors, even the little ones in your computer case that drive cooling fans. These use Hall effect switches rather than back EMF for switching and do actually run on DC input. The current in their windings goes through "phases" as it is switched, but it's not AC. Three phase DC motors are used as the platter motors in hard drives, driven by the 12V DC supply. If this terminology is misapplied, it has been so for many years!
What do you do for a living that gives you so much time to think about this sort of thing? :-)
--
Virg Wall, K6EVE

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| snipped-for-privacy@ipal.net wrote:
|> |> | http://www.st.com/stonline/books/pdf/docs/7209.pdf |> |> I would not call that a DC motor. The motor is not getting DC; the |> controller is. The motor gets modulated pulsed AC. It may well be |> the standard practice to call this a DC motor, but if so, it is yet |> another case of terminology misapplied. I'd call it a "pulse control |> motor". A more clever controller could drive it from an incoming |> three phase AC supply directly, by switching in the supply phase that |> has the right voltage and polarity at that instant (in three phase AC |> power, there's always at least one phase in each polarity at any time). |> It sure would not be anything DC in that case. | | AC is alternating current because the current "allternates" the | direction in which it flows. The current to the motor does not do so. | The term "phase" does not only apply to one of the rotating vectors of a | multiphase AC supply.
It sure looked like it was alternating from some of the diagrams. Sure, not alternating in the way you get from the wall outlet. But the pulse go one way sometimes, and the other way other times. See figure 3 on page 7. See figure 4 on page 8.
The document showed an example where a simple north-sorth bar magnetic was figuratively used as the rotor. I suppose a pulse sequence only in one polarity could still pull it around in rotation and in sync. But it would operate better if there was a corresponding reverse pulse to pull the other end of the magnet. Additionally, reversing the field of each winding just as the magnet pole passes, so it pushes away afterwards, could help even more.
Also, even though the document describes "PWM" (pulse width modulation), some of the diagrams seem to suggest it is instead using "PDM" (pulse density modulation). PDM is where pulses at a very high rate are used, each at a fixed time interval, but varying in how many are on or off in a window of time, as opposed to a single pulse that varies in width. PDM can be used to more readily vary the average current in the winding over time, where desired. If the PDM rate is high enough, the winding inductance smooths it out. You could make a decent sine-like waveform that way. The advantage of PDM over PWM is that high switching rate allows less reactive component to smooth it out.
I would envision a much more sophisticated system for certain uses like independent automotive vehicle drives on all 4 wheels. The spacing of permanent magnet poles could be different than the spacing of windings so you don't have every pole aligning to every winding at the same time (to reduce vibration). For example, 18 magnet poles and 15 winding poles would allow all the magnets to alternate (even number) polarity, and the interval variation would be replicated three times to maintain a reasonable force balance around the rotor. In each 120 degree segment there would be 6 magnets and 5 windings. The windings would be energized according to which magnet poles are approaching/departing in rotation. For vehicular use, I would definitely want a feedback to confirm where the rotor is, since it is subject to variable load drag. There would be at least 4 windings per segment, 12 for the whole stator, that could be simultaneously energized.
| Everybody else calls them brushless DC motors, even the little ones in | your computer case that drive cooling fans. These use Hall effect | switches rather than back EMF for switching and do actually run on DC | input. The current in their windings goes through "phases" as it is | switched, but it's not AC. Three phase DC motors are used as the | platter motors in hard drives, driven by the 12V DC supply. If this | terminology is misapplied, it has been so for many years!
I guess the whole assembly can be called "DC" since it works on DC being fed to the whole assembly. There's still AC going into the motor windings according to the referenced PDF document.
FYI, this same design seems to be used in a VCR I once took apart to see where the lightning killed it. It was the video head motor, too. It had 12 separate windings.
| What do you do for a living that gives you so much time to think about | this sort of thing? :-)
It's a hobby thing. It's just like trying to figure out what chemicals are needed to go boom. But in this case it's what electricity is needed to go boom :-) Note that I don't actually build things unless I already know what it will (most likely) do. And even then, I find more of the fun in the thought process to design it. Electricity is not the only area I do thought experiments in.
--
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net writes:

I think the terminology is part history, and partly depends on the perspective of the person writing. For example a standard permanent-magnet DC motor with a commutator is universally called a "DC" motor, yet the armature windings are fed with approximately square-wave AC produced by the commutator (with a different phase shift for each coil in the armature). If you build the motor inside-out, with a permanent magnet rotor and the field windings fed polyphase square-wave AC switched according to shaft rotation, what is it? If you include the switching electronics as part of the "motor", then it's still powered by DC and has most of the same characteristics, so it gets called a DC motor. On the other hand, if you separate the electronics from the motor, then the motor is actually a polyphase synchronous motor being driven by a variable frequency generated by feedback from a shaft encoder.
I don't think there's any "correct" perspective, though some are more common than others.
    Dave
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| I think the terminology is part history, and partly depends on the | perspective of the person writing. For example a standard | permanent-magnet DC motor with a commutator is universally called a | "DC" motor, yet the armature windings are fed with approximately | square-wave AC produced by the commutator (with a different phase shift | for each coil in the armature). If you build the motor inside-out, | with a permanent magnet rotor and the field windings fed polyphase | square-wave AC switched according to shaft rotation, what is it? If you | include the switching electronics as part of the "motor", then it's | still powered by DC and has most of the same characteristics, so it gets | called a DC motor. On the other hand, if you separate the electronics | from the motor, then the motor is actually a polyphase synchronous motor | being driven by a variable frequency generated by feedback from a shaft | encoder. | | I don't think there's any "correct" perspective, though some are more | common than others.
This makes complete sense.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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All of the common electrical machines use alternating current. For induction and synchronous motors, the ac is used without conversion. For dc motors and generators the commutator is use to either rectify ac for dc distribution or invert dc to ac for generating torque.
Bill
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Salmon Egg wrote:

The little dry-cell motor you might have made as a Boy Scout did not convert anything to AC. The current in its single coil was reversed in the direction it flowed by its terminals contacting alternate brushes as it turned. It had an alternating magnetic field produced by passing DC through it in an alternating direction. It even produced torque!
With solid state power switches becoming cheaper, I think we'll see many "brushless DC motors" replacing induction or "universal" motors.
--
Virg Wall

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----------------------------
wrote:

-------------------- Passing DC through in an alternating direction IS AC. In this case it is not only AC but exhibits half wave symmetry with no DC component in the coil current (average over the "cycle" is 0. Sure, it is not a nice sine wave but that doesn't matter (and a square wave is actually better in this case). The commutator acts as an inverter in the motor as Salmon Egg correctly said.
Also, as a matter of fact, the rotor current does NOT produce an alternating magnetic field. The field is, ideally, stationary at right angles to the stator field -the maximum torque position. ---------------------

------ That is already happening with some fractional HP motors where speed control is required. Some of the newer washing machines with direct drive are an example. Also my electric lawnmower has a "brushless DC" motor. Large- industrial motors- probably not as the polyphase induction machine is too damned cheap, simple and reliable so why go with a more complicated device to do the same task?
--

Don Kelly snipped-for-privacy@shawcross.ca
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| Also, as a matter of fact, the rotor current does NOT produce an alternating | magnetic field. The field is, ideally, stationary at right angles to the | stator field -the maximum torque position.
I'm not sure it applies here, but in another post I think maybe what I was saying wasn't understood by someone. What I meant by a rotating field is one in which the structure of the field rotates around the axis due to changes in the power in the stator windings. A three phase syncronous or induction motor would have a rotating field. If you have a motor with windings at each of 12 positions, no matter what combination gets power, if that combination pattern is shifted to the next position in a subcycle of time, and ends up in the same configuration 30 degrees off, then this is also a rotating field.
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wrote:

Yes, but VWWALL said his dry-cell motor (such as a boy scout may build for a merit badge) has an "althernating magnetic field produced by passing DC through it in alternating direction."
I'm pretty sure *that* was what Don was replying to when he sayd, "the rotor current does NOT produce an alternating magnetic field."
Don's right of course, in the case of a classic DC motor with commutator, the rotor's magnetic field is stationary with respect to the frame and is 90 electrical degrees displaced from the magnetic field created by the field windings on the frame. As the armature current increases, this magnetic field distorts the main field created by the field windings and several issues come up. But that's a DC motor, not what you've been talking about.
daestrom
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| What I wrote as quoted applies to a commutator type DC machine. The | commutator switches coils in such a way that the rotor winding as a whole | (ideally) produces flux perpendicular to the field flux. | | The field doesn't rotate due to changes in the power. The field rotation is | provided by the exciting current which is reactive. The rotational speed or | magnitude of the field is independent of the power but its magnitude depends | on the applied voltage and frequency. The latter also determines the speed.
At least the simple versions of what I have seen of this type appear to be doing "flip flop" style field changes. It changes a full 180 degrees. The only reason a particular direction is sustained with be an external means to ensure it only goes one way. That could be a slight shift in angle between the commutator and field. Otherwise the existing velocity of the rotor is the direction.
Since I have readily envisioned much more complex controlled BLDC motors, even before I accepted that name for them, I could envision a commutator system that could achieve similar control. It could be very complex, and in mechanical form hard to design and maintain. That's why so many motors today are electronically controlled (along with other disadvantages of the BDC motor).
I'm still thinking through the idea of a motor with 18 permanent magnet poles and 15 stator winding poles, with electronic control.
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Don Kelly wrote:

I see what you mean--the use of the term inverter confused me. My specialty is communications. The AC I work with is usually several orders of magnitude higher in frequency than the one we've been discussing. With energy conservation becoming more important and with more energy being consumed by "electronics", we "low power" engineers have to be concerned.

It's only an alternating field in the armature. As you say, the purpose of the commutator, (inverter?), is to keep the rotator's field at its maximum torque position to the stator's field.

I worked as an apprentice machinist in a mill using overhead belts, at one time drive by water power. This had been replaced by electric motor power, but some individual machines were still belt driven. Modern machines seem to be using many smaller, precisely controlled motors.
A side effect to the use of this sort of thing in residences, is the effect on power distribution. Many switching power supplies have poor power factor, and compact fluorescent lamps compound the problem. I wonder how soon power factor correction will be a problem for domestic utility power supply. Harmonic currents on the neutral may also become a problem.
--
Virg Wall



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