3-phase wound rotor motors - why rotor always 3-phase? 4

[electricpete]

This is a spinoff from another thread on wound rotor motors.

Typically a three-phase (stator) wound rotor motor also has three phases on the rotor.

I don’t see any reason why it has to be that way. To my thinking for a three-phase stator, the wound rotor could be be 1-phase, 2-phase, 3-phase, 4-phase, 5-phase, 6-phase etc. Wound with the same number of poles as the stator. One group per phase per pole. If odd number phases, subsequent pole-phase groups in the same phase are opposite polarity (same as three-phase winding). If even number phases, all groups wired the same polarity.

If single phase I don’t think there is any problem with starting torque because the 3-phase stator winding creates a forward rotating field. There may be forward and reverse rotating fields from the rotor, but no stator field for the reverse rotating field to interact with. The only problem I suspect single-phase might be the harmonics. Also if it were an oddball number of poles, it might be a little unfamiliar to the winders.

What do you guys think? Is there any compelling reason that the rotor of a 3-phase-stator wound rotor motor needs to be wound as three-phase?

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[electricpete]

I meant oddball number of phases, not oddball number of poles.

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[Marke]

Hi Pete

on the surface, I see no reason for the three phase requirement, but I wonder if there is a mechanism in the effective rotating field in the rotor. If we consider the stator, we know that in order to have a rotating magnetic field in one direction, we need to have more than one phase. If we take a single phase motor and disable the start winding and apply voltage we have two counter rotating fields. The start winding is required togive us one field in one direction.

Now look at the rotor.During start, we induce a current to flow in the rotor. If we have a single winding, surley that would produce two counter rotating fields in the rotor? Is this likely to affect torque and slip losses? Just a thought.
The torque is as result of the interaction of the stator field and the rotor field and I wonder if the effective counter rotating rotor field from a single winding may cause problems. Just a thought to ponder on!!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[motorspert]

The number of rotor phases can be any number. if you think of a cage induction motor with say 48 slots on the rotor,it effectively has 48 phases all shorted together. All the slipring do is allow extra resistane to be added.

Single phase wouldnt work as it doesnt rotate and I don't think 2 gives a good use of the rotor or might be noisy. A low number of phases is required to reduce length of sliprings and more importantly costs of rings, cables etc.

Richard

 

[electricpete]

Thanks guys. It sounds like we are looking at it the same way except for the case of single phase.

For single phase here is the general way I view things:

Any single phase winding [with proper spacing around the stator] produces forward and reverse rotating fields.

Non-zero average torque is only produced by interaction of rotor poles which have the same speed, direction, and number of poles as each other.

Now examine single phase motor. The stator has forward and reverse rotating field. At standstill this induces equal forward and reverse rotating currents and fields in the rotor. The torque from the forward rotating field of the stator interfacting with forward rotating field from rotor cancels (equal/opposite) the torque from the reverse rotating field of the stator interacting with reverse rotating field of the rotor.

Now examine three-phase motor. We combine three single-phase windings with proper phase relationship. The forward set of rotating fields are in-phase and are additive among the three phases. The backward set of rotating field of each individual phase is non-zero but the phase relationship not in phase and is such that they sum to zero.

Now, my view of three-phase stator combined with single-phase rotor. Look at starting conditions. The three-phase stator produces only a forward rotating field. This will induce single-phase currents in the single-phase rotor will produce forward and reverse rotating fields. The forward rotating field of the rotor interacts with the forward rotating field of the stator to produce torque. The reverse rotating field of the rotor has no reverse rotating stator field to interact with and cannot produce torque. The result is we have a non-zero forward torque and the rotor accelerates.

In summary I conclude the three-phase stator single-phase rotor is not subject to the zero-torque-at-start problem that we associate with simple single-phase motors. The reason is that the three phase stator does not produce any reverse rotating field so there can be no reverse torque to cancel the forward torque.

What do you guys think?

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[Marke]

Hi Pete

If we ignore the stator for a moment, and just look at the rotor, we will have an AC current flowing through the rotor winding which wil create two counter rotating magnetic fields. On of these fields will be at the same speed as the stator field and wil create a positive torque. The second field will be at the stator speed minus double the slip. There would be a positive torque filed generated, but I expect that there would be a torque ripple at double slip frequency. Just my thoughts. What do you think?

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

I love this,

I had never thought about the question put by Pete. Why do we always take things for granted? Or very often...

I try to see it like this. Get rid of the stator windings and replace them with a huge rotating magnet. Then put a wire loop in the rotating field. There will be a torque, surely. But that torque varies as the field varies. That is obvious during start when the loop moves in and out of the flux just as fast as the magnet turns.

As the loop rotor accelerates, it moves in and out of the field with slip frequency. The torque will still be a sine function, i.e. heavily pulsating and not very useful. I think that a three-phase winding adds the varying torques from the three phases to produce a constant, non pulsating torque.

Look: No Math!



Gunnar Englund

http://www.gke.org

 

[electricpete]

I agree with your comment about pulsating torque.

The forward component of the stator field interacts with forward component of the rotor field to give a constant non-zero torque.

The forward component of the stator field interacts with reverse component of the rotor field to give a time-varying (twice slip frequency) zero-average torque.

Add together the two and you have a time-varying ripple on top of a non-zero average torque.

The single-phase motor has this same time-varying zero average torque (after start) but it doesn't generally cause any problems that I know of. So maybe it would be a disadvantage but not a show-stopper?

So there are tradeoffs.

I can imagine in some cases if there was an interest to cut cost that going to single-phase rotor would be attractive in terms of reducing the complexity of the external controls by a factor of 3.

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[Skogsgurra]

Pete,

I think that it would make the "show" a very bad one.

Assuming a two percent slip would mean that the loop moves periodically out of and into the flux at a rate of once per second. The torque ripple would then be a sine torque with the frequency 1 Hz. And that ripple really goes to zero. Not down to half peak torque or something like that. No, it goes to true zero when the loop is perpendicular to the field lines.

As I said before, I have never thought about this before and it is remarkable how one just accepts things without having a good feel for the underlying physics. We should ask questions like these more often. There was a discussion a few weeks ago about a mysterious motor with two slip-rings. I didn't follow that discussion closely and I cannot find it any more. I think that something useful might be in there - if I could find it.



Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Or twice per second. If you really think about it.

Gunnar Englund

http://www.gke.org

 

[Marke]

So the bottom line would appear to be that the single phase wound rotor should work, but with a noticable torque ripple.
Advantage - small improvement in cost.
Disadvantage - torque ripple.

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

That makes sense to me.

The torque ripple would be comparable to what we see in single-phase motors, right?

Maybe not good for crane and conveyor type applications (unless there is a large rotating inertia to help dampen the oscillations).

The application here thread237-130091 was clarified to be a pump application at a mixer OEM facility.

Also possibility of exciting torsional resonance especially if there is a wide range of speed.

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[Skogsgurra]

No, not similar to what you see in a single phase motor.

There is a big difference. The single phase motor (let's say that it is single phase without any auxiliary winding) has two opposing fluxes - both of same strength but opposite directions. Once started in one direction, the rotor continues to spin in that direction and can also be loaded.

The reason is that the rotor winding has a much lower impedance for the currents induced by the slip frequency (which is in the order of sub-Hz to a few Hz) and a very high impedance for the currents induced by the flux moving in the opposite direction (which induces a current with close to 100 Hz).

The reason for the difference in impedance is, of course, that the rotor bars are mostly inductive (heavy gauge and iron core). That also makes the rotor current for the opposite direction almost exactly 90 degrees out ouf phase with flux so that the braking torque is close to nil.

Wait a minute: When I think about it, why couldn't a single phase rotor also be arranged to always have active turns in the flux? Where did I go wrong?? Hmm... Anyone?

Gunnar Englund

http://www.gke.org

 

[electricpete]

I believe your last question is geared towards explaining torque pulsation in a single-phase motor.

To my thinking, torque pulsation in single phase motor with relatively small slip is due to the reverse component of the rotor field interacting with forward component of the stator field which gives rise to 2*slip torque pulsations. This would be the exact same interaction that would apply to three-phase stator single phase rotor motor.

In theory the single phase motor has one other opportunity for pulsations that the 3-phase stator single phase rotor motor does not - namely interaction of reverse stator field with forward rotor field. With a small slip the frequency would be very high (close to 2*line frequency) such that it would not be noticeable (easily dampened by the inertia).

I could be wrong. Most of what I learned about forward and reverse rotating interpretation of single-phase motor I learned from you gunnar. Some day I am going to crack open the very detailed appendix on this subject in Liwschitz-Garak. But today is not the day. (last time I tried I gave up).

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[Skogsgurra]

Yeah, let's have a beer or two..

Gunnar Englund

http://www.gke.org