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Eccentric Rotor unbalanced magnetic pull increases with load?

Eccentric Rotor unbalanced magnetic pull increases with load?

Eccentric Rotor unbalanced magnetic pull increases with load?

(OP)
Ref 1 - "Calculation and measurement of unbalanced magnetic pull in cage induction motors with eccentric rotors. Part 1 : Analytical model"  by A.C.Smith and D.G. Dorrell, IEE Proc.-Electr.P ower Appl., Vol. 143, No. 3, May 1996

Ref 2 - "Calculation and measurement of unbalanced magnetic pull in cage induction motors with eccentric rotors. Part 2: Experimental investigation"  by A.C.Smith and D.G. Dorrell, IEE Proc -Electr Power Appl , Vol 143, No 3, May 1996

Ref 1 states:

Quote (Ref 1):

The steady component of UMP acts predominantly along the axis of the narrowest airgap. However, as the load torque is increased, it has been found experimentally that the magnitude of the steady UMP increases with load. This is difficult to understand if one considers the UMP to be derived from the fundamental air-gap field. The fundamental resultant airgap field reduces in magnitude as the rotor speed reduces and the UMP would as a consequence also be expected to reduce. The force mechanism is clearly more involved and the permeance harmonics have to be considered....[followed by lots of math]

Ref 2 provides experimental results including Figure 10 = "Variation of UMP with slip for a ten-pole series-connected machine with cage rotor" which is attached.  From my eyeball it shows unbalanced magnetic pull vs slip for various values of eccentricity.  It shows u.m.p. going to going to zero at zero slip which does not seem like a credible result.   I am not sure how the motor was tested with such high slip, although reduced voltage is mentioned.

All of the discussion applies to "static eccentricity" i.e. position of rotor does not orbit.

My questions:
1 – Has anyone ever heard of unbalanced magnetic pull increasing with load?
2 – What would be the explanation for such phenomenon?
3 – Do any references address this subject?

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RE: Eccentric Rotor unbalanced magnetic pull increases with load?

It is a very interesting issue, indeed, and I am sorry I can't dedicate more time to study it. I found in the Web a few articles connected with, as for instance:
http://www.springerlink.com/content/q407g5431718mw08/
THE CALCULATION OF UNILATERAL MAGNETIC ATTRACTION FORCES IN ELECTRIC DRIVE MOTORS FOR SUBMERSIBLE DEEP-WELL PUMPS.
http://lib.tkk.fi/Diss/2003/isbn9512266830/article2.pdf
FINITE-ELEMENT CALCULATION OF UNBALANCED MAGNETIC PULL AND CIRCULATING CURRENT BETWEEN PARALLEL WINDINGS IN INDUCTION MOTOR WITH NON-UNIFORM ECCENTRIC ROTOR by A. Tenhunen [2001]
http://frauman.com/pierre/files/Article%20-%20Frauman%20Pierre.pdf
Effects of the Slot-Harmonics on the Unbalanced Magnetic Pull  by P. Frauman, A. Burakov, A. Arkkio [2006]
The force, or UMP, is the result of an interaction between two magnetic field waves, the wave numbers of which differ by one.
In "Elektrische Mashinen" by R. Richter [1954] vol. IV ch. H4a is written: "The unilateral magnetic attraction force [UMP] is produced by interaction of two unsynchronized wave [two harmonics of 1 difference in order] .The sources are the stator and rotor slots and may be an interaction of two stator magnetic wave or two rotor magnetic wave or one from stator and second from rotor. In any case the force will be proportional with the field generated by currents so it will increase with the current"
There is a formula F=C*K*I1^2 (332) where C = FUNCTION (Dia, Length, number of turns per phase) and K is a function of winding spreading way. So for a certain motor only I1 [stator current] is variable.
http://www.pdfqueen.com/pdf/ri/richter,-rudolf-electrische-maschinen/6/
If we'll compare the Fig.10[ from A. C. Smith, D. G. Dorrell  article] with fig 2 of Tenhunen it seems that fig.10 is for starting phenomenon [from right to left from s=0.8 to s=0] and fig.2 is for variable static load. As the current depends on slip the UMP depends also on slip.

 

RE: Eccentric Rotor unbalanced magnetic pull increases with load?

(OP)
Thanks.  It seems some other references tend to corroborate this surprising result.  I will take a time to review that.

Which item was supposed to be relevant at this link?
http://www.pdfqueen.com/pdf/ri/richter,-rudolf-electrische-maschinen/6/
 

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RE: Eccentric Rotor unbalanced magnetic pull increases with load?

Actually, I have the translated book only. I never downloaded this from the web as from my location I can't get any server of "Download Cave". But I think the "full" has to be good. You have to find the volume:
Elektrische Maschinen von Rudolf Richter Vierter Band Die Induktionsmaschinen Zweite, verbesserte Ausslage Verlag Birkhauser Basel/Stuttgart 1954 .Then H chapter "Die Kuplungs und Harmonische Krafts" and then you have to find the following:

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