Experimental Determination of Currents in the Rotor Bars of Synchronous Reluctance Motors

E. V. Kononenko, T. V. Chesheva

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

The rotor winding of synchronous reluctance motors is a cast aluminum squirrel cage. All slots of the rotor winding and the interpole spaces of the rotor core are filled with aluminum under pressure. In analytical studies of the synchronous operating mode of these machines, it becomes necessary to find the spatial distribution of currents in the bars of the rotor winding. In the pole pitch of the rotor of improved synchronous reluctance machines there are slots of various configurations. For this reason, doubts may arise as to the validity of the commonly adopted assumption of a sinusoidal distribution of the current components along the \(d\) and \(q\) axes. At present, experimental measurements of currents are performed mainly on the damper windings of large synchronous machines [1, 2, and others]. The currents in the bars of the damper windings of these machines are usually measured in two ways: either by the voltage drop across the bars, or by means of special current transformers placed on the end portions of the bars. However, the use of these methods for determining the currents in the bars of the rotor winding of synchronous reluctance machines is impossible because of the design and technological features of their rotors. In this connection, we carried out an experiment that made it possible to solve the stated problem with sufficient accuracy. The currents were measured by means of special current sensors, or so-called Rogowski magnetic belts [1, 3], embedded in the rotor in such a way that each sensor encircled one bar of the rotor winding. Structurally, these sensors were made as follows. PEV 0.57 wire was used as the base conductor, and PEV 0.06 wire as the winding wire. The winding was two-layer. The current constant of the sensor was determined by calibration.

The sensors were embedded as follows. From the end face of the rotor, the iron was milled out from the shaft to the short-circuiting ring by 5–10 mm. On the rotor surface, two adjacent grooves were turned, the height of which had to be not less than the diameter of the sensor. Through holes were drilled in the grooves on both sides of each bar, that is, in the rotor iron. These holes made it possible for the sensor to encircle any bar. The ends of the sensors were brought out to the end face of the shaft along a groove specially turned in it.

When determining the nature of the distribution of the current components along the \(d\) and \(q\) axes, the rotor of the machine was braked in such a way that

could also be simultaneously rotated through 180 degrees. A reduced voltage, corresponding to the rated current in the stator winding, was applied to two phases of the stator winding. Thus, a pulsating magnetic field is produced, which induces currents of network frequency in the rotor bars. The rotor was set so that one of the rotor axes coincided with the axis of the pulsating magnetic field. The currents flowing through the bars of the rotor winding induce e.m.f.s in the sensors, the magnitude of which was measured with a V6-2 selective microvoltmeter. The actual values of the current components in the bars of the rotor winding were calculated from the calibration curve of the sensors. The measurements were carried out with the stator winding supplied by voltages of different frequency (from 20 to 50 Hz). The experiment was performed on an improved synchronous reluctance motor having the following data: \(P_n = 4\) kW; \(U_n = 220\) V; \(I_n = 8.1\) A; \(\delta = 0.35\) mm; the rotor iron lamination is shown in Fig. 1. Fig. 2 gives the results of an experimental investigation of the distribution of currents over the bars of the rotor winding along the \(d\)- and \(q\)-axes, respectively. From Fig. 2 it is seen that the actual spatial distribution of the current components

Fig. 1. Cross section of the rotor of an SRM.

Fig. 2. Spatial distribution of the current components in the bars of the rotor winding along the \(d\)- and \(q\)-axes:
\(a\) — longitudinal current components;
\(b\) — transverse current components.

of the rotor winding along the \(d\) and \(q\) axes differs only very slightly from the usually assumed sinusoidal one: along the \(d\) axis the sinusoidal character is expressed most clearly, while along the \(q\) axis certain deviations from the sinusoid can be explained by experimental errors (bar 2, having a complex shape, was only partially covered by the sensor, which caused an understated value of the current in this bar).

The results obtained and the conclusions drawn are confirmed by the results of investigation and calculation of the parameters of the rotor winding of synchronous reluctance machines and may serve as an important basis for further studies of these machines.

REFERENCES

1. L. A. Medinskii. A method for measuring starting processes in the rotor cage of asynchronous motors, Elektrotekhnika, 1965, No. 4.

2. Ya. B. Danilevich, Yu. A. Kulik. Theory and Calculation of Damper Windings of Synchronous Machines, M.-L., 1962.

3. K. Heumann. Magnetische Spannungsmesser hoher Präzision, ETZ-A, Bd 83 (1962) H. II, S. 349—356; 158, 6QH.