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

On Sun, 06 Apr 2008 02:25:59 GMT Don Kelly wrote:

|> 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?

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.

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

| 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.

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....

daestrom

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.

daestrom

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

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

|> | 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. |>

|>

| | 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.

This is a more interesting subject than BPH:-).

On Sun, 6 Apr 2008 08:16:19 -0700 (PDT) Rich256 wrote: | On Apr 6, 12:24?am, Salmon Egg 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. |>

|> 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?

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.

---------------------------------------------- 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.

You can't just push power into any motor as the losses and mechanical load on the motor determines the power required. The load says "I want" and the electrical supply tries to satisfy it. In a generator the external load says "I want" and the generator tries to supply it, in turn saying "I want" to the mechanical driver.

At no load, ignoring losses, there is a current in each phase that produces the magnetisation required by the voltage (by Faraday's law - leading to Erms (sine wave) =4.44fN(phi_max)) This current is in phase with the flux produced by the current and this flux leads the voltage by 90 degrees. Using a combination of 3 phase windings, distributed sinusoidally around the stator gives a constant magnitude rotating field. So does the use of a true N phase setup. In a single phase motor there will be a forward and a backward field which cancel at standstill so the motor needs some means of biasing this in one direction or the other. This backward field does result in more vibration than in the case of a polyphase machine. If you have your structure and sequentially switch windings, you will also have a rotating field but it may not be constant in magnitude because the flux is not actually distributed sinusoidally. It isn't actually sinusoidal in conventional 3 phase machines because the conductors have to be in discrete slots. The windings are designed to eliminate the lowest harmonics- the 5th and 7th. Touch the terminal of a scope with your finger -using your body as a capacitive pickup and you can see the harmonics (amplified- they are not as big as they look). This may not be as easy with your device which still appears to be essentially a stepper. >

--

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

--------- 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.

------------------ 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.

I need a picture!

Bill

| 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.

On Sun, 06 Apr 2008 23:41:38 -0700 Salmon Egg wrote: | In article , snipped-for-privacy@ipal.net | 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.