EXPERIMENTAL STUDY OF THE EFFECT OF SOLID INCLUSIONS IN THE ROTOR OF A SYNCHRONOUS RELUCTANCE GEARED MOTOR ON ITS CHARACTERISTICS

A. A. Buimov, G. M. Goloshevsky, R. P. Laas

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

The intensive development of modern technology, and especially of such branches as radio electronics, aviation, atomic and rocket technology, sound and video recording, photography, and communications, has necessitated the creation, broad development, and introduction into production of highly reliable low-speed synchronous microelectric drives with a high degree of uniformity of rotational speed. In this connection, synchronous micromotors with electromagnetic speed reduction and a single winding on the stator—so-called synchronous reluctance geared motors (SRGMs)—are finding ever wider application.

Synchronous reluctance geared motors possess a number of very substantial advantages, the principal ones being low speed, high reliability, the possibility of smooth frequency control of speed, and simplicity and manufacturability of construction. However, the widespread use of such motors is being restrained by insufficiently good starting properties, the absence of recommendations on the selection and design of optimal constructions with improved starting characteristics, and the insufficient development of theory, in particular of questions of starting.

Starting synchronous reluctance geared motors that have no special starting devices is possible only at no load; in this case the rotor must be made with a small moment of inertia (of the smooth-rotor type), and the motor must have low rotational speeds (up to 100–200 revolutions per minute). For reliable starting of motors under load, it is necessary to equip them with special starting devices, for example additional starting windings—both disconnectable and non-disconnectable—located both in the stator slots and in the rotor; sometimes built-in starting asynchronous motors are used [1]. If part of the active length of the rotor is made of a magnetically hard material, then the value of the starting torque increases due to the hysteresis torque; this is the so-called hysteresis starting [2]. Asynchronous starting of synchronous reluctance geared motors is also possible. The asynchronous torque, which assists the synchronous torque in accelerating the rotor to the geared speed, is produced by currents induced either in a short-circuited winding placed in the slots of the laminated rotor, or in the solid body of a rotor made of ferromagnetic material [3].

The aim here is not to enumerate all the devices and methods currently existing in electrical engineering for starting geared motors—

... geared motors; the need for reliable, rapid starting of a SynRMGM led to the appearance of a comparatively large number of these methods. Each of them has its own shortcomings—among them an increase in weight, an increase in overall dimensions, a reduction in energy-performance indices, and a deterioration in the manufacturability of the motors; each also has certain advantages. The last of the starting methods mentioned, namely the asynchronous one, appears to be the most interesting; therefore, at the Department of General Electrical Engineering of the Tomsk Polytechnic Institute, work is being carried out on the study of asynchronous starting of synchronous reluctance geared motors with the aid of a solid rotor, as well as a short-circuited winding placed in the slots of a laminated rotor.

Studies of the starting process by means of a short-circuited rotor winding have shown that this type of starting means for a SynRMGM is not universal. A short-circuited cage contributes to a significant increase in the machine’s rated torque and its overload capacity, but at the same time, as the asynchronous component of the resulting electromagnetic torque increases, its synchronous component decreases, which is explained by the damping action of the rotor winding [4].

The foregoing, however, does not compromise the idea of asynchronous starting of a SynRMGM as a whole: asynchronous starting carried out with the aid of a solid rotor is free of the disadvantages listed. Of course, this method has not avoided them altogether; in particular, the use of a solid rotor is inevitably associated with additional losses and heating of the motor—this worsens its energy-performance indices. Nevertheless, reports are appearing in domestic and foreign literature indicating attempts to use solid and composite (combined) rotors for synchronous reluctance motors [5], so that the idea of using a solid rotor for starting a synchronous motor, including one of the geared type, is not new. Its implementation, however, requires detailed investigation and careful study.

To investigate the operation of synchronous reluctance geared motors with a solid rotor, several mock-up samples of motors of this type were tested. The motor stator is analogous to the stator of an ordinary asynchronous machine, with some modernization of the toothed zone; the motor rotor is assembled from plates insulated from one another; the configuration of the plate is shown in Fig. 1. For the experimental

Fig. 1. Configuration of rotor plates: 1 — shaft hole; 2 — toothed zone

studies, several rotors were manufactured having different plate thicknesses \(\Delta\), beginning with \(\Delta = 0.5\) mm and ending with a completely solid rotor with \(\Delta = 60\) mm. The motor has the following geometry:

\[ D_1 = 54 \text{ mm}; \qquad L_1 = 48 \text{ mm}, \qquad D_2 = 53.6 \text{ mm}, \qquad L_2 = 60 \text{ mm}, \]

\[ \delta = 0.2 \text{ mm}, \qquad Z_1 = 24, \qquad Z_2 = 26, \]

The solid rotor of a synchronous reluctance geared motor is accelerated under the action of the resultant electromagnetic torque, which consists, first, of the synchronous component of the torque, caused by the toothed construction of the stator and rotor (Fig. 1); the magnitude of this component of the torque depends on the geometry of the toothed zone and is practically independent of the value of \(\Delta\). The second component of the resultant electromagnetic torque is the asynchronous torque, arising as a consequence of the appearance of eddy currents in the rotor core. It is known that eddy currents arise in any electromagnetic core when the magnetic flux penetrating this core changes. The torque due to eddy currents, denoted above as the asynchronous component of the resultant torque, is by its nature analogous to the main electromagnetic torque arising in an ordinary asynchronous machine, for example with a phase-wound rotor, since it is likewise the result of the interaction of the eddy current with the rotating magnetic field. Only the role of the winding in this case is played by the solid rotor itself, in which the rotating magnetic field will induce currents. The asynchronous component of the torque, and consequently the resultant torque as a whole, depend strongly on the value of \(\Delta\). Determining the character of the dependence of the starting torque of the SRGM in asynchronous operation and of its energy indicators in synchronous operation on the value of \(\Delta\) is the subject of the present investigations.

The experiment on measuring the starting torque was carried out with the rotor braked, i.e., at \(s=1\), using lever scales. The results of the experiment are presented in Fig. 2 for several values of the supply voltage at a frequency of 50 Hz.

Fig. 2. Dependence of the starting torque of the SRGM on the thickness of the rotor plates at different voltages

The graph (Fig. 2) clearly demonstrates a significant increase in the starting torque of a synchronous reluctance geared motor with increasing thickness of the plates from which the rotor is assembled. With an increase in the supply voltage, this growth of torque becomes more intense.

At \(U=200\ \text{V}\), a voltage close to the rated value, increasing the plate thickness from \(\Delta=0.5\) to \(\Delta=5\) gives a threefold increase in the starting ...

torque, whereas the use of a sheet thickness \(\Delta = 20\) mm gives a sevenfold increase in the starting torque compared with an ordinary laminated rotor. Thus, one can state that there is a significant improvement in the starting properties of a synchronous reluctance geared motor as a consequence of using a solid rotor. It is obvious that the main role in starting a synchronous reluctance geared motor with a solid rotor is played by the asynchronous component of the torque; it is intended to bring the rotor up to the synchronous geared speed, and moreover must be of such a magnitude that, at the moment of transition to synchronous operation, it yields the dominant role to the synchronous torque. To achieve such an optimal relationship between the torque components, it is necessary to have quite definite rotor parameters.

It is known that the parameters of a solid rotor—the active resistance \(r_2\) and the inductive resistance \(x_2\)—because of the strongly pronounced surface effect, depend significantly on slip. Thus, in the case of \(f_1 = 50\) Hz, when starting an asynchronous machine with a solid rotor (\(s = 1\)), the equivalent depth of current penetration into the rotor is 3 mm, while at \(s = 0.02\) it is already 20 mm [6]. In contrast to asynchronous machines, the synchronous reluctance geared motor under consideration operates in the slip range from 1 to 0.9. Therefore, the depth of penetration of the current into the rotor body will not change substantially; nevertheless, when the motor is started and the slip changes from 1.0 to 0.9, the parameters of the solid rotor do change: the active resistance decreases and the inductive resistance increases [6].

Corresponding to the indicated change in parameters, the asynchronous component of the torque decreases. The principal and most difficult task is such an optimization of the parameters in which the asynchronous component at a slip of 0.9 decreases in magnitude so much that it is unable to increase the rotational speed above the synchronous geared speed and does not prevent synchronization.

Experiments carried out with rotors having different plate thicknesses \(\Delta\) showed that, beginning with \(\Delta = 20\) mm, a supersynchronous mode of operation is possible—the motor accelerates to a speed much higher than the geared speed. The probability of such a mode increases with increasing \(\Delta\), and with a completely solid rotor becomes one hundred percent. From Fig. 2 it is evident that the starting torque of a completely solid rotor (\(\Delta = 60\) mm) considerably exceeds the starting torque at smaller values of the rotor-plate thickness. An analysis of the operating characteristics (Figs. 3 and 4), however, shows that an increase in the torque \(M\) and in \(\cos \varphi\) with increasing thickness of the rotor plates is necessarily associated with an increase in the consumed power \(P_1\) and inevitably leads to a decrease in efficiency. The investigations showed that the maximum torque of the motor increases only up to a certain limit and then begins to decrease; the same occurs with the energy efficiency (Fig. 4).

All this makes it possible to conclude that the most rational plate thickness for such a rotor design is \(\Delta = 5 \div 15\) mm.

Considering the idea of asynchronous starting of an SRGM by means of solid inclusions in the rotor, it must be said that, structurally, the rotor may be designed in different ways; for example, it may be assembled from solid plates, as in the present case, where the thickness of the rotor plates must be determined when optimizing the parameters. The rotor may also be made in the form of a solid steel cup, that is, it is a type of hollow rotor. Finally, it may be an ordinary laminated rotor of a synchronous reluctance geared motor, but in this rotor it is necessary to include solid elements of definite shape, dimensions, and material.

Fig. 3. Operating characteristics of the SRRM at different rotor-plate thicknesses: 1 — \(\Delta=0.5\) mm; 2 — \(\Delta=5\) mm; 3 — \(\Delta=20\) mm; 4 — \(\Delta=30\) mm; 5 — \(\Delta=60\) mm

Fig. 4. Dependences of the energy efficiency [1] and maximum torque [2] on rotor-plate thickness at \(U=250\) V

Conclusions

1. The starting torque increases significantly when the thickness of the rotor plates is reduced. Thus, it proved entirely possible to improve the operation of the motor in the starting mode by using the idea of asynchronous starting of the SRRM with the aid of a solid rotor.

2. The torque developed by the motor does not increase without limit. Beginning with \(\Delta=30\) mm, the consumed power is spent to a greater extent on covering losses in the SRRM than on increasing the torque; the consequence of this is a decrease in the energy indicators and a reduction in the maximum torque.

1. By selecting the thickness of the solid plates, it is possible to obtain a motor with a specified starting torque and satisfactory energy performance. In the design of the “laminated rotor” considered here, the most advantageous thickness of the rotor plates is \(\Delta = 5 \div 15\) mm; in this case the motor operates reliably in synchronous mode and starts confidently even under load.

REFERENCES

1. Owczarek Jerzy. Miniaturowe silniki impulsowe. Wiadomości Elektrotechniczne, 1958, No. 4.

2. A. S. Kurakin. Starting of synchronous reluctance electric motors. Reports of the Scientific and Technical Conference on the Results of Research Work for 1966–1967. MEI, 1967.

3. A. S. Kurakin. A squirrel-cage winding as a starting means for synchronous reluctance motors. Elektromekhanika, 1969, No. 1.

4. A. A. Buimov, G. M. Goloshevskii, E. V. Kononenko. Starting of an SRRM with a squirrel-cage winding on the rotor. Izvestiya TPI, vol. 284, 1973.

5. M. S. Sarma, G. Soni. Experimental Study of Solid-Rotor and Composite-Rotor Induction Motors. Power Apparatus and Systems, 1972, Vol. PAS-91, No. 5, pp. 1812–1821.

6. A. I. Voldek. Electric Machines. Moscow–Leningrad, Energiya, 1966, p. 562.