induction motor with superconducting rotor - produce any torque? 2

[electricpete]

Maybe not a practical question.

Let's say we built an induction motor with ideal superconducting rotor (zero resistance).
Is there any speed at which it would be capable of producing steady state torque?

I say the answer is no. And therefore any basic explanation of how an induction motor works which does not mention rotor resitance is incomplete.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jbartos]

Comment on electricpete (Electrical) Apr 10, 2004
jbartos talks about convergences as R->0 and s->0. I think that aolalde has well addressed that it depends upon the constraints imposed. If we constrain the motor to carry a given load, than as R->0 we can always pick a s proportionately smaller s to carry a given load. The problem is that as R->0 the values of s are constrained to a narrower and narrower band about s=0. The result is that when R=0 the torque speed curve has a discontinuity at s=0. It don't think we can talk about convergence and limits in the presence of a discontinuity.
///There is such mathematical phenomenon as convergence to a discontinuity from one side, e.g. left, and then from right. One sided limits are defined and exist. See for example:
Granino A. Korn, Ph.D., Theresa M. Korn, M.S. "Mathematical Handbook for Scientists and Engineers," McGraw-Hill Book Company, 1968, Section: 4.4-7 One-sided (Unilateral) Limits. Unilateral Continuity.
I studied this in my first undergraduate year in 1965/66 but from different books.\\

 

[jbartos]

Suggestion to electricpete (Electrical) Apr 10, 2004:
Visit

http://elsmar.com/ubb/Forum10/HTML/000182.html

for: "..for a one-sided lower limit is standard AIAG SPC methodology.""

http://forums.wolfram.com/mathgroup/archive/2003/Feb/msg00217.html

for: "However, if we use the affine extension (two-point compactification) of the reals, then the unilateral (one-sided) limits actually do exist. And Mathematica does these correctly."
etc. for more info regarding one sided limits that can be used from one side at a point of discontinuous functions with finite discontinuities.

 

[MarcSmith]

I apologise for the off topic interruption, but the above link for Elsmar.com somehow was parced by the software incorrectly and is an 'old forum' entry.

The thread is currently at:

http://Elsmar.com/Forums/showthread.php?t=2005

A search of the forums will also provide a number of other threads relating to one-sided specification limits.

Thanks!

Marc

Marc T. Smith

http://Elsmar.com

 

[nbucska]

The superconductor [SC] has R=0 -- there was a lead ring
kept in liquid He for decades with undiminished circulating
current. R<0 would give forever increasing current.

The resistance from hi to low: Fe--> Al-->Cu -->Ag --->Au. Consider SC a "super-Gold".

The current generates the magnetic field i.e. torque.
The slip is increasing until the voltage is high enough
to force high enough current to have high enough torque...

So the SC motor will work just fine only with less loss and less slip


<nbucska@pcperipherals.com>

 

[aolalde]

Rbucsa (electrical): Sorry but I think your concepts are baseless.
1. Gold (Au-- 14.55 Ohm*CM/FT) does not have higher conductivity than Silver (Ag—9.796 Ohm*CM/FT) or Copper (Cu—10.37 Ohm*CM/FT).
2. The magnetic field is generated by the stator winding. This thread is dealing with the rotor resistance ,
3. If the rotor resistance approaches zero the induced voltage required is minimum, then the slip does not increase but approaches zero.

 

[jbartos]

Comment: The superconducting induction motors can develop the high torque; however, different designs are used (see my posting on April 10, 2004 for """...a proper perception of the machine power conversion, including mathematical equations is needed. In fact, the superconductive induction motors are turning and producing torque-speed characteristics.""") Visit

http://ieeexplore.ieee.org/xpl/tocresult.jsp?isNumber=27268&page=16

for:
A superconducting induction motor using double-helix dipole coils
Goodzeit, C.L. Meinke, R.B. Ball, M.J.
Adv. Magnet Lab., Palm Bay, FL, USA;
This paper appears in: Applied Superconductivity, IEEE Transactions on

Publication Date: June 2003
On page(s): 2235- 2238
Volume: 13, Issue: 2
ISSN: 1051-8223

-----------------------------------------------------------
Abstract:
The double-helix dipole configuration, which uses concentric pairs of oppositely-tilted helical windings to generate a dipole field, is the basis of a design for a unique high-torque, low-rpm, compact superconducting motor. We describe an analysis method that uses coupled mutually-inductive equivalent circuits for each phase for computing the performance and operating characteristics of such a device. We discuss the performance and limitations of an example motor that uses NbTi superconducting cables for the stator and rotor. The example device is a 4-pole, 2-phase induction motor that produces typically 10 000 HP for a 1-m effective length rotor with a 0.4 m diameter rotor operating at /spl sim/300 rpm. In addition, the relatively large minimum bend radius of the double-helix coil configuration also facilitates the use of brittle superconducting materials such as HTS or A-15 for this application.

 

[nbucska]

aolalde:
Right about gold - my mistake, please skip it.

<nbucska@pcperipherals.com>

 

[nbucska]

aiolalde:
Both the stator and rotor generate magnetic field and their
interaction generates the force/torque.

My sentence "the slip is increasing" has nothing to do to
the resistance but to the self regulating of the rotor
current.

If there is no slip, no current is induced in the rotor i.e. there is no torque. If the motor is loaded, the slip
increases until the torque of the magnetic field of the current induced into the rotor is enough to overcome the
loading torque.

With superconductor, the required voltage is less by dV = i*R where R is the resistance of the normal conductor
compared with . In this case part of the slip generates the
torque and some part compensates for the resistive loss.


<nbucska@pcperipherals.com>

 

[Marke]

Hi ElectricPete

I suspect that it would be almost like a cross between a slip ring motor with the rings shorted and a synchronous motor. I would expect that you would need another means of starting it, or you would end up with a very high start current and almost no start torque. Once up to full speed, it would behave almost like a synchronous motor. Could you get a situation where the frequency of the current in the rotor was so close to zero that you could consider it to be a DC bias?? I suspect so.
Best regards

Mark Empson

http://www.lmphotonics.com

 

[sreid]

I can't resist taking a wild assed guess at this one. Superconducting wires not only exhibit zero resistance but also exhibit the Meissner Effect, the exclusion of external magnetic fields. I suspect that a current must be estableshed in the rotor prior to the rotor going superconducting. That current then flows continuously creating the rotor flux to interact with the stator flux. Since the rotor has no losses, there is no need for any slip to replace any lost energy in the rotor. The motor is then synchronous.

 

[DDT77]

Could we agree that the Pd= I2^2*r’2*(1-s)/s IM equivolent circuit falls apart here, and that we should analyze this in the d-q frame in the rotor reference frame?
If so, we can revert back to rotor flux cross rotor current.
this should allow us to detemine the flux -> back emf -> rotor current and therefore the torque. We might also note that the slope of the Torque, near zero slip, has near infinite negative slope, as shown in figures 3-4 of jbartos link

http://ieeexplore.ieee.org/xpl/tocresult.jsp?isNumber=27268&page=16

Torque characteristics of a motor using bulk superconductors in the rotor in the transient phase
Tsuboi, Y.; Ohsaki, H.
Page(s): 2210- 2213

 

[GOTWW]

Mr. bizzaro here can not pass this one up, with respect to DDT78 (not that I doubt these pearls, that you throw out to the swine). I have beem on the outskirts of some HTS and LTS programs. One is the High Temperature Super conducting antennae. These would simply not function with no external magnetic field. Anothed is LTS squid devices, extremely sensitive detectors of magnetic fields.

Just add another dimension to string theory, what the heck they are up to 14 or something already. Mr. Maxwell would be quite confused indeed.

 

[jbartos]

Suggestion: Visit

http://www.atp.nist.gov/eao/sp950-1/chap6.pdf

for:
“Superconducting Rotor”
(Patent No. 5,482,919: filed 9/15/1993,
granted 1/9/1996)
on Page 91

 

[jbartos]

Suggestion: Visit

http://patimg1.uspto.gov/.piw?Docid...ageNum=&Rtype=&SectionNum=&idkey=F6EDE6F0429C

for: Superconducting Rotor