a faster motor

I'm not much for motorology, but it sounds good to me! Sort like a synchronous setup only the rotor is 3 phase with a rotating field, no?

Benj

I really need to think about it more. Off the top of my head, I do think that such an arrangement works--sort of. In the the steady state with the rotor turn at double speed. the two rotating fields would be locked to each other. The problem I see is getting the motor up to speed.

At zero speed, the torque would average out to zero.

Bill

On Sat, 16 Jun 2007 13:58:56 -0700 Salmon Egg wrote: | On 6/16/07 10:08 AM, in article | snipped-for-privacy@n2g2000hse.googlegroups.com, "Benj" | wrote: | |> |> 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? |> |> I'm not much for motorology, but it sounds good to me! Sort like a |> synchronous setup only the rotor is 3 phase with a rotating field, |> no? |> |> Benj |> | I really need to think about it more. Off the top of my head, I do think | that such an arrangement works--sort of. In the the steady state with the | rotor turn at double speed. the two rotating fields would be locked to each | other. The problem I see is getting the motor up to speed. | | At zero speed, the torque would average out to zero.

Maybe additional phase angles could overcome the startup problem. Or maybe DC could be applied to one set of windings to make them appear as stationary magnets for a period of time.

What I'm wondering about next is ways to transfer the AC power into the rotor windings without brushes and such.

I think a wound rotor induction motor can have slip rings allowing you to externally add resistance to the rotor circuit. Slip rings could be used to drive the rotor I would think.

Oh, if that's the case you could presumably short the rotor circuit to allow the motor to start as an induction motor then apply whatever it takes to get the right currents into the rotor.

There is a long history of wound rotor induction motors. They use brushes and slip rings thereby avoiding the worst problems arising from commutators. Moreover, power from a source is not applied to the rotor. Rather, the induced emf in the rotor drives external resistors from the slip rings.

Bill

That will only get you near to synchronous speed, but I do not know how to exceed that. I am sure that there is room for invention.

Bill

| On 6/16/07 5:18 PM, in article snipped-for-privacy@news1.newsguy.com, | " snipped-for-privacy@ipal.net" wrote: | |> What I'm wondering about next is ways to transfer the AC power into the |> rotor windings without brushes and such. | | There is a long history of wound rotor induction motors. They use brushes | and slip rings thereby avoiding the worst problems arising from commutators. | Moreover, power from a source is not applied to the rotor. Rather, the | induced emf in the rotor drives external resistors from the slip rings.

But the induced EMF in the rotor is not counter-rotating. What I have been wondering about is if the two fields rotating in opposing directions can force the rotor to have the sum of the rotating rates. Apparently that might be so. The next issue that has been suggested is how to get it to start from zero rotation. But in my mind, these opposing fields are still going to end up pushing against each other to the effect of starting a rotation, as long as there are at least 3 phases with 6 winding poles (e.g. the opposite ends of each phase).

Instead of a slip ring, what about some kind of magnetic coupling that is continuous around the shaft to transfer just a single phase circuit over all possible rotation positions (then two more separate ones to transfer the other two phases) so that current is not flowing over moving contacts as a slip ring would have. Then these windings (that have no rotational sense of direction) would be wired within the rotator to the rotational windings to produced the counter (to the stator) rotating EMF.

The induced emf in the rotor must be considered, but it is not crux of the matter. External emf is applied to the rotor. This makes a stationary rotor behave like the rotor of a synchronous machine. The magnetic field of the stationary rotor will be rotating like the field from dc excited rotor turning at synchronous speed. If the rotor turns at double speed, the fields of the rotor and the stator will remain lined up.

I think such a motor will work. If someone does have the equipment, use a double speed motor to mechanically drive the rotor of such a machine at double synchronous speed. Then cut power to the drive motor. The machine should continue at double speed.

You may want to worry about overspeeding a real motor. If you have a variable frequency three-phase drive, you could run at half frequency and voltage to demonstrate the principle.

Bill

-- Support the troops. Impeach Bush. Oh, I forgot about Cheney.

This concept was perfected over 100 years ago; find an old motor engineering book and look up the "super-synchronous" connection for wound rotor motors. While you're at it, look up "concatenation" of wound rotor and induction motors, a related but different idea. Synchronous-induction frequency changers also used the concept of a wound-rotor motor with the rotor excited externally.

The fields would never lock up. At any speed they are going in the opposite directions. You would have double frequency torque pulsations but the average torque would always be 0. This is not particularly useful for any motor.

Note that the rotor field of a synchronous or induction motor rotates at the same speed as the stator field. While the two are not in phase at any given load, a steady state non-zero average torque is produced only when the phase difference is steady. Load changes simply change the relative phase of the stator and rotor fields. Note also the problems with a synchronous motor if the rotor speed is not synchronous. High pulsating torques and currents will occur at a frequency determined by the angular speed difference. This can be hard on the mounting bolts and can let the magic smoke out in short order.

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

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I am not convinced by this argument.

Consider a stator driven by a positive phase sequence. Say for argument's sake that the rotating field produced by the stator winding goes clockwise. (CW)

Forgetting the stator for the while being, consider a wound rotor being driven by a negative phase sequence. It produces a field that is counter-clockwise. (CCW) If the rotor is driven CW mechanically by an external motor at synchronous speed, the rotor field does not rotate at all. If the stator is being driven CW by the external motor at double synchronous speed, the rotor field will be rotating CW at synchronous speed.

Now consider the stator again. Its rotating field will rotate in the same direction and at the same speed as the field produced by the rotor. The question is: If the external drive motor stops driving the rotor, will the rotor continue to turn?

I think that the answer is yes.

Is there a problem? Again the answer is yes. If the rotor is driven from a low impedance source. The combination of the rotor and stator will act like an induction motor because of the shorted windings. At double synchronous speed, the slip will be -1.

While that complicates operation, it does not stop it. After all single phase induction motors work with counter rotating fields having slips close to 0 and -2.

Bill

-- Support the troops. Impeach Bush. Oh, I forgot about Cheney.

Some small wound-rotors can be started that way, but the currents are usually high when starting. To ease things a bit, an external resistor bank (typically wye connected) is put on the slip-rings to increase the resistance of the rotor circuit. This has the advantage of reducing the rotor current (duh...) but also increasing the stalled-rotor torque for faster acceleration up to speed. Some will have a couple of taps in the legs of the resistor bank to allow 'staging', by shorting out successive sections of the bank, you lower the current and shift the max-torque point higher and higher until you've reached 'shorted' condition.

But after that, you have a problem. If you had a variable-frequency 3-phase supply, you could feed very low-freq onto the rotor once it's accelerated up to near synchronous speed the 'conventional way'. Then gradually increase the frequency applied to the rotor until you're up to line frequency and make a 'cut-over' to applying line frequency to the rotor (opposite phase rotation as you suggest).

Don't really know what the torque-slip characteristics would look like, but in theory you would have a motor that runs near double the synchronous speed.

daestrom

Though if you have a speed drive I think you may as well just hook that up to the stator.

You are assuming an external drive on the rotor as well as an external excitation. I assume that you mean "if the rotor is driven CW by the external motor at double synchronous speed, the rotor field will be rotating CW at synchronous speed." If so, I agree. What happens when the external rotor drive is removed? There will be both forward and backward rotor fields. The stator field will induce rotor voltages and hence currents and torque corresponding to motor action at a slip of -1. The rotor field would produce torque corresponding to a slip of

1. Both torques would act to slow the machine (sketch torque -slip curves) This would result in fairly rapid deceleration to some speed below synchronous in one direction or the other depending on the relative strengths of the fields. There would be also pulsating torques.

Not quite a single phase machine but having the disadvantages and none of the advantages of a single phase machine along with a great deal of complication. Why bother?

I retract what I said before as the net average torque at any speed will depend on the balance between the sources and the fields that they produce- if equal at standstill, the motor will not start, if unequal, it will start and run one way or the other- rather poorly compared to conventional operation. You are right in saying it will run. It might even start without a mechanical boost. However it will not run at speeds above synchronous which is the whole point of the exercise. Thank you for pushing my thinking past a knee-jerk reaction.

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

Under the circumstance I describe, I think the motor would run, maybe not well, at double speed once you can get it to double speed.

This also brings up the effect of spatial harmonics. Especially with salient poles, the distribution of field is not sinusoidal. This means that there will be various spatial harmonics around the magnetic circuit with various phase sequences and rotational speeds.

Although I have not given such things great thought, it reminds me of two electrically related subjects. There are various forms of parametric devices that allow energy transfer from one frequency to another.

There are various waveguide structures in which spatial harmonics will propagate at different speeds. Electron beams with speeds close to the speed of the harmonics can have a traveling wave interaction with a spatial harmonic. In the case of a backward wave amplifier, an electron can interact with a spatial harmonic of the traveling wave that goes in a direction opposite to the direction of energy transfer.

Bill

-- Support the troops. Impeach Bush. Oh, I forgot about Cheney.

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.

1. 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.

---------------------------- "Salmon Egg" wrote in message news:C29E16B7.80F37% snipped-for-privacy@sbcglobal.net...

Where I disagree is in getting the motor to run at above synchronous speed once the external drive is removed. Assume that the rotor field is CCW and in the case of the stator windings shorted, the torque would try to drive the stator CCW, Since the stator is stubborn, the rotor is forced to rotate CW at some speed below synchronous. Now consider the conventional motor with the stator energised to cause CW rototation of the rotor. Again at something under synchronous speed. Superimpose the two and rotation is still below synchronous speed- in effect, the driving source is split between stator and rotor- both acting to turn the rotor CW. Drive the rotor above synchronous speed and the machine will generate feeding back to both stator and rotor. Generating torques will act to slow the machine so that on removal of the driving force and no removal of the electrical sources the speed will drop to something to below synchronous speed. At least that is how I see it today but may not see it tomorrow- I haven't yet dug into the basic equations with reversed rotor sources. I am reminded of the so-called "super-synchronous" machines in which the stator was free to rotate and with the rotor fixed or nearly so by a high torque load, would rotate backwards and would then be excited to run at synchronous speed backwards- then brakes were applied so that the stator was brought to a standstill, while still operating in synchronism, producing high torque to accelerate the rotor to synchronous speed in the forward direction.

As for spatial harmonics- True-particularly if one is considering rewiring of a salient pole machine rather than using a wound rotor induction machine. If one is doing so, then there are serious added problems of the sort that I first brought up due to saliency torques.

I won't comment on the backward wave amplifier as as that is outside of my area of interest or experience.

I have some difficulty following your argument.

If dc were supplied to one phase of rotor winding, the rotor would act like the field magnet of a synchronous motor.

With the slip rings shorted, the motor starts up just like an induction motor. When the slip gets close to zero, undo the short and send dc through part of the rotor winding. After an initial transient, you will have a synchronous motor running at synchronous speed. It may not be a good motor, but it is a motor nevertheless.

Now, leave the slip rings open and use an external mechanical drive to get the rotor up to double synchronous speed. Then apply negative sequence to the rotor and remove the mechanical drive, say with a clutch. The field of the rotor will be rotating at the same speed as that of the stator. The rotor has to go at double the speed to keep the fields from the rotor and stator aligned as the rotor rotates.

The combination acts just like a synchronous motor except that the rotor field is produced by th negative sequence ac rather that dc.

Bill

-- Support the troops. Impeach Bush. Oh, I forgot about Cheney.