ON IMPROVING THE FIELD STRUCTURE OF ARC-STATOR MACHINES

N. M. PIRUMYAN, M. G. REZIN

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

The fact of current asymmetry in the individual phases of the stator winding and the nonuniform distribution of induction along the length of the stator arc were noted already during the first tests of asynchronous motors with an open magnetic core. Subsequent investigations showed that the observed phenomenon is caused both by the very constructive arrangement of the stator windings and by the sum of physical processes associated with its openness and referred to as the longitudinal end effect.

In the present work only questions related to the longitudinal end effect in the primary circuit are considered. The influence of rotor currents on the distribution of the field in the air gap is not taken into account.

When the magnetic core is opened, two windings are located symmetrically, one at one end and the other at the other end of the magnetic core, while the third winding is symmetrical with respect to its center. In this case there is equality of two inductances and two mutual inductances.

As is known, the openness of the magnetic system leads to distortion of the distribution of the magnetic field in the air gap, where, along with the traveling field, a pulsating field appears. Owing to the presence of the pulsating field, the difference between the inductive resistances of the winding phases increases and the symmetry of the emf is distorted, so that, with a symmetrical system of applied voltages, the currents in the phases become asymmetrical. Currents of negative sequence appear. The presence of current asymmetry adversely affects the operation of motors with an open magnetic core. Thus, in [1] it is indicated that the specific power of special installations in asymmetrical systems may be several orders of magnitude smaller than in symmetrical ones. The presence of appreciable negative-sequence currents leads to the appearance of noise, additional heating of the stator, and vibrations, which have an adverse effect on the operating conditions of the motor. In installations for stirring and pumping liquid metals, vibrations may cause dangerous damage to the thermal insulation. Thus, compensation of the pulsating flux in the air gap and symmetrization of the system of phase currents are among the important problems in the theory of arc-stator machines.

Usually, questions of compensation of the pulsating component and symmetrization of phase currents are considered separately. Since in recent years the problem of combined action for eliminating the consequences of the longitudinal end effect has attracted the attention of a number of researchers [2, 3], consideration of this question is of interest.

This paper presents some results of an experimental study of a two-pole stator. The two-pole stator was chosen because the longitudinal end effect is most sharply expressed in it. The main winding is three-phase, double-layer, and with a diametral pitch; the number of slots per pole and phase is \(q=2\); the number of turns in the coil of the main winding and of the correcting winding is the same and equal to 25.

The distribution of the magnetic field along the length of the stator magnetic core was measured with the aid of loops located under the middle of the inductor teeth. The plane in which the loops were located was at a distance of \(\sim 1.5\) mm from the stator iron. All experiments were carried out under no-load conditions, with two methods of placing the correcting coils. In each case three variants were considered:

1. The correcting coils are disconnected.
2. The correcting coils are connected in series into the phase symmetrical with respect to the center of the magnetic core.
3. The correcting coils are connected, and the currents in the phases are equalized by changing one line voltage relative to another.

In the first method, the correcting coils were located closer to the air gap, on the basis of complete compensation of the fluxes shunting the ends of the magnetic core; in the second case, on the basis of the smallest possible difference between the phase currents.

In the uncompensated mode, the ratio of the positive-sequence currents \(I_1\) to the negative-sequence currents \(I_2\) reaches a considerable value: \(I_2/I_1=50\%\).

The use of compensation increases the induction of the forward-traveling field at constant phase voltage and simultaneously reduces the induction of the backward-traveling and pulsating fields (Fig. 1, curve 2). However,

Fig. 1

the use of correcting coils connected in series into phase “C” does not completely eliminate the pulsating and backward-traveling fields, but weakens them. The distribution of induction along the arc length of the magnetic core (curves 1, 2, Fig. 1) has a distinctive character, with a minimum at the center of the inductor \((x=0)\) and a maximum at \(x=\pm \frac{\tau}{2}\). At the same time, when only traveling and pulsating magnetic fields are present in the gap, the envelope curve of the induction has a maximum at \(x=0;\ \pm 2\tau\), etc., and minima at \(x=\pm \tau;\ 3\tau\), etc. [4].

Fig. 2

When symmetry of the currents in the phases is established, the induction of the reverse-rotating magnetic field is considerably smaller (Fig. 1, curve 3). In the second method, owing to the corresponding arrangement of the correcting coils, a minimum difference between the phase currents was achieved. In this case the induction curve has a more uniform character (Fig. 2, curve 2) than curve 2 in Fig. 1. The curve in Fig. 1 corresponds to the distribution of induction with the correcting coils disconnected. This indicates a significant reduction in the parasitic components of the induction.

The subsequent complete establishment of equality of the currents in the phases reduces the reverse-rotating field to a minimum. The curves no longer have any dips along the length of the stator arc (Fig. 2, curve 3).

Conclusions

1. With the second method of arranging the correcting coils, the distribution of induction in the air gap has a more uniform character than with the first.

2. Establishing equality of the currents by changing one line voltage, as compared with the others, does not substantially affect the pulsating component of the magnetic fluxes, as was noted earlier in [5].

3. Complete compensation of the pulsating field was achieved by shunting the fluxes from the side faces of the stator.

4. Establishing equality of the phase currents substantially reduces the reverse-rotating field.

5. The best result in compensating the edge effect is achieved when combined methods of action are used.

References

1. Proceedings of the VI Symposium on Engineering Problems of Magnetohydrodynamics. Pittsburgh, USA, April 1965, “Nauka,” Moscow, 1968, 102.

2. A. M. Andreev, V. A. Glukhikh, I. R. Kirillov. “Magnetohydrodynamics,” 1968, 4, 98.

3. CLOSED—CYCLE MHD WITH LIQUID—METAL WORKING FLUIDS. VIENNA, FOE, III P. 2047.

4. A. I. Voldek. NDBSh, “Electromechanics and Automation,” 1959, No. 2, 130.

5. Kh. I. Yanes, Kh. A. Tiismus et al. Experimental investigation of flat induction pumps. Izv. TPI, series A, No. 197, 215, 1962.