Abstract

Induction motor impedance vector measuring technique for rotor eccentricity definition in the operation condition

         The problems concerned with energy losses decreasing and raising reliability of induction motors (IMs) are of primary importance. Recently special attention gives to the equipment control on a condition. Advantage of the control on a condition consists that repair is made only that equipment to which it is need. Definition a condition method of motor indirect methods is developing. Measurements of signals such as: currents, voltages, speed, torque, noise, vibrations, magnetic fluxes and temperature can supply relevant fault information.
         It is known, as a result of eccentricity changes the efficiency of an IM can be reduced by 2.8 %, the breakdown and locked rotor torques are reduced within the limits 20 % and 8 % correspondingly, the slip can increase by 10%. The noticeable effect on the above mentioned operating variables becomes apparent as the eccentricity increases more than by 30-35 percent. The damages under such conditions require long-lasting IM reconstruction repair. Expenses in this case are similar to the new machine cost.

Production purposes and problems

         To the most well known methods of indirect control it is possible to refer a method of synchronous inductive resistance value control (which is measured by the motionless machine) [5], as well as short-circuit inductive resistance value and zero sequence control method [6], explored in DPI. The disadvantages of these methods are:
       - requirement of disconnecting a IM from the supply with working place preparation on the basis of the report;
       - there is a need of using the external sources of constant or alternating current for supplying a diagnostic influence on the IM;
      -significant time of a diagnostic test (up to 3-4 hours).
         At present the method of diagnosing air gap eccentricity based on a harmonic analysis of phase currents of the stator is applied. On the current spectrum is influenced by many factors including: electric supply, static and dynamic load conditions, noise, machine geometry and fault conditions and these conditions may lead to errors in fault detection [7].
         Also there are methods of vibrodiagnostic investigated in SPbGMTU [6].
         Specified methods did not find a broad application in industry. Thereby, the problem of creation of more efficient method of the air gap eccentricity checking is still actual. Use of transient of short-term switching-off IMs as test affecting has the important advantage before known methods.

Main notions and brief review of the work

         Increasing of asynchronous machine reliability is one of actual tasks of industrial energy. The questions of ensuring possible value of unevenness of the air gap eccentricity play major role. The air gap eccentricity is defined as a measure of breaking symmetry of air gap on the basis of direct measurements in face parts of stator on the known formula:
where δ_max, δ₀ - - average and maximum value of air gap, accordingly. [11]
         Because of minute value of the air gap this factor turns out to be highly sensitive to deformation of the bed locks, bearings panels and gradual wear of bearings of the machine under exploitation. [3] As it is known, when there is the air gap eccentricity in asynchronous machine the starting moment falls, unilateral magnetic attraction and vibration of rotor grow and efficiency falls because of increasing the steel losses.
         Determination of air gap on axis of each slot of rotor can be determined using the expression:

δ = δ₀(1 - ε cosφ),

where ε - rotor air gap eccentricity; φ - space (angular) coordinate, counted out from the spot of minimal air gap ?min (fig.1). [4]


Figure 1 - Space coordinate of IM air gap eccentricity
         Specified phenomena are stipulated by unevenness of sharing an attached voltage between spools of phases and breaking a symmetry of phase stator currents that causes growing of high harmonicas in the magnetic flux of air gap and increasing the inductances of machine.
         Normal transient process during short-time disconnection of the IM from the supply used as a test influence can give essential advantage versus known methods. Efficiency evaluation of express-method under investigation requires determination of a diagnostic parameter of machine, choice of possible diagnostic signal and evaluation of its sensitivity to air gap eccentricity change.

Results of studies

         During investigation the test program was created and the experiment was carried out on equipment of chair "electrical system" DonNTU.
         Investigation are carried out on IM MA36-41/8. At research possibility uniformly was used to change air-gap between stator and rotor windings. Schematically installation is presented on fig. 2.


Figure 2 - Experimental setup (A - the device changing an eccentricity of the IM, B - additional brushes).
         As the test influence the normal transient process during a hort-time disconnection of the IM from the supply was taken.
         The aims of this test were:
       - evaluation of harmonic components in the voltage and current of stator;
       - evaluation of harmonic components in the voltage on end of bar;
       - Анализа изменения значения полного сопротивления в комплексной плоскости.
       - the analysis of change a computing impedance in a complex plane.
         Primary estimation of change a computing impedance in a complex plane has shown, that there is a spread of meanings of resistance along a round fig. 3, but quantitatively to size up extent air gap eccentricity yet it was not possible.

Figure 3 - Change of a computing impedance in a complex plane (Red - ε =42,5%, Blue - ε = 65%, Green - ε = 85% Pink - ε =95% ).

Conclusion

         Thereby, this strategy gives theoretical base for the air gap eccentricity device development for industrial motors. This device will allow to give an evaluation of air gap eccentricity during a short period of measurements and data processing and take measures of its elimination.

References

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