ACCOUNTING FOR LOAD CONDITIONS IN THE DESIGN OF INDUCTION MOTORS

Yu. M. Bashagurov, E. K. Strelbitskii

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

Systematic underloading of induction motors in the electric drives of general-purpose metal-cutting machine tools significantly reduces the power factor of the network and creates a large unused power reserve.

The existence of this reserve is due not only to the natural desire of machine-tool designers to select the next larger electric motor from the existing range, but also to the difficulties that arise in determining the maximum power consumed by mechanisms with complex operating conditions. This reserve is to some extent inevitable, since it follows from the very nature of the variety of random loads under which general-purpose metal-cutting machine tools operate.

Ignorance of the actual operating conditions of drives leads to the fact that the power and maximum torque of the motor are usually selected with a considerable overestimate.

In addition, analysis of data from standard tests of motors of the AO2 series showed that the catalog values of the maximum torque for four-pole motors are underestimated on average by a factor of 1.3 compared with the actual values, and this creates an even greater power margin.

An economically correct choice of motors is possible only on the basis of the laws of distribution of loads on the shaft for particular groups of machine tools. The distribution laws can be obtained as a result of studying the operating conditions of motors in service.

Studies of the loads of the drives of basic machine tools should be a fundamental stage in the design of electric drives for a subsequent range of single-type machine tools of the same size, the magnitudes and nature of whose loads determine the choice of motors and predetermine in advance their technical-and-economic performance in operation. Serious attention must be given to the question of the correct choice of a motor as early as the machine-tool design stage, since replacing underloaded motors on the scale of the national economy encounters insurmountable difficulties, and it is possible to influence changes in the structure of installed motors only to a very insignificant degree.

Studies of the operating conditions of general-purpose metal-cutting machine tools, carried out in 1959–1968 at 17 large machine-building plants of the country, showed that the average load on motor shafts, even under the most severe loading conditions (machining of parts ...

large diameter, roughing, etc.) does not exceed 0.45 of the rated value [1—5].

In machine building, approximately 57 percent of the entire stock of machine tools is concentrated; of these, only 60—65 percent are in main production, where operation is carried out most intensively. One quarter of the entire stock is dispersed among small shops and workshops of small enterprises and educational institutions [6]. Overall, the utilization of the surveyed motors of the machine-tool stock, in terms of power, is 20—40 percent. In all, about 1500 lathes, milling, drilling, and other machine tools, and general-purpose semiautomatic machines of the most widespread models, were surveyed.

When comparing the results of studies of the load conditions of drives of similar machine tools, for the purpose of generalizing the statistical material, it is necessary to take the following into account.

In the published works, distributions are presented for various types of loads (on motor shafts, of consumed power, on one shaft and on the spindle of machine tools, etc.). This is explained by the fact that the studies were carried out by specialists in various branches of science and technology: power engineers, economists, machine-tool builders, electromechanics, and others. Naturally, the aims of the studies were different: accounting for loads only during machine time, selecting the most heavily loaded conditions, etc. The data themselves were obtained by different methods (statistical processing of technological materials, measurements with self-recording instruments, etc.), with differing accuracy, and from different samples.

Nevertheless, a certain regularity is observed in the statistical distributions of loads: all histograms and polygons of powers, in the general case, can be described by the Weibull law:

\[ f(p)=\frac{m}{x_0}p^{m-1}\exp\left(-\frac{p^m}{x_0}\right), \tag{1} \]

where \(m\) and \(x_0\) are distribution parameters.

The formation of a decreasing density of the load distribution is due to the approximately exponential law of distribution of the weights and dimensions of the parts making up the product, the predominance of finishing and semifinishing operations, and the presence of a significant proportion of passes and light-duty conditions (thread cutting, slotting grooves, chamfering, etc.).

Since the criterion quantity in selecting motors for universal machine tools is the magnitude of the maximum torque, in power distributions it is very important to know the overload region. The above-mentioned heavy-duty conditions, as a rule, do not increase the densities in the overload region, but only somewhat raise the mean power values.

Another important property of the distributions studied is the relative stability of their form and parameters, which makes it possible to classify the loading conditions of all metal-cutting equipment. Carrying out such a classification is of great theoretical and techno-economic significance.

The theoretical significance of the classification consists in the fact that we establish a certain rule, stable in time, based on the statistical regularities of the distribution of loading conditions.

In the techno-economic aspect, classification opens up broad possibilities for the rational selection, operation, and design of motors. As classification criteria, one should adopt, first, the type of distribution law and, second, the parameters of the process (the mean value and the correlation function).

Even now it can be asserted that the motor loads of mining equipment and of certain groups of metal-cutting automatic machines are distributed

according to the normal law, while the loads of universal metal-cutting equipment follow a law close to the exponential one. The motors of some automatic lines have a uniform load-distribution density.

In what follows, loads must be classified according to the degree of scatter and according to the degree of attenuation of the correlation function.

The effectiveness of studying load conditions has been shown by us using the example of a universal engine lathe, model 1K62. These machines, having the greatest weight in the stock of universal metal-cutting equipment, are characterized by the lowest average loads. The AO2-52-4 motor of 10 kW, installed on the machine, is loaded (not counting idle running) on average by 14 percent.

The distribution density of the power on the shaft obeys the exponential law

\[ f(P_2)=0.735 \exp(-0.735 P_2), \tag{2} \]

where \(P_2\) is in kW.

Taking into account that the annual output of 1K62 machines is about 20 thousand units, and that the law of distribution of loads on the shaft is sufficiently stable, it appears expedient to create a modification of asynchronous motors with increased overload capacity specifically for universal metal-cutting machines. Increasing the overload capacity by 1–2 steps of the power scale will make it possible correspondingly to reduce the rated power of the motors installed.

An important question in designing such a modification should be considered the determination of the required maximum torque, since increasing the latter within the same overall dimensions of the motor is accompanied by a deterioration of its energy indices. In this case one should not be guided by the greatest load in the given operating modes, taking into account that the region of the greatest overloads usually pertains to separate groups of machines in certain areas of production. It is therefore more rational to produce 1K62 machines with two motor power variants, i.e., to install a 10 kW motor only at the customer’s request.

In the present case it is necessary to have the maximum torque approximately equal to 11–12 kgm, which corresponds to the actual maximum torque of the AO2-51-4 motor.

The Tomsk Polytechnic Institute, jointly with the Special Design Bureau of Electrical Machine Building, designed such a motor for application to the 1K62 machine, on the basis of the series-produced AO2-42-4 motor, intended to replace the currently used AO2-52-4 motor. All the basic and mounting dimensions of the AO2-42-4 motor were retained.

The increase in overload capacity was achieved by increasing the magnetic flux by 14 percent, with some increase in the width of the stator and rotor teeth, so that the induction in the teeth and in the stator yoke was, respectively, 1.66 and 1.50 T.

When the steel is saturated, the effect appears of displacement of the flux from the stator yoke into the frame. Therefore additional investigations were carried out to determine the influence of additional losses in it on the efficiency of the motor.

The geometry of the tooth zone was optimized on an M-20 digital computer. As the objective function in optimizing the geometry, the sum of the manufacturing costs and the cost of losses at the average actual load was adopted.

An increase in the magnetic flux leads to an increase in the starting-current ratio \((I_k/I_n = 8 \div 10)\) and to a higher rate of temperature rise of the stator winding under short-circuit conditions \((T = 9 \div 12^\circ \mathrm{C}/\mathrm{s})\).

However, since the planned line of machine tools is expected to use thyristor starters and effective built-in protections, the indicated circumstance does not reduce the service life of the winding.

A prototype motor with the provisional designation AOM 2-42-4 was manufactured at SKBEM. Its principal data are given in Table 1.

The annual output of the proposed motor modification for the new line of lathes is 20 thousand units; according to ENIMS data, the expected benefit from introducing this output will amount to 300 thousand rubles per year.

Table 1

Note: experimental data are marked with an asterisk.

Conclusions

1. Regularities have been established for the loading conditions of electric motors in the drives of general-purpose metal-cutting machine tools, and a preliminary classification of them by equipment of the same type has been carried out.

2. Using the 1K62 machine-tool model as an example, the effectiveness has been demonstrated of creating a special modification of induction motors for driving general-purpose metal-cutting machine tools.

References

1. R. M. Pratusevich. Operating load conditions of general-purpose machine tools. Stanki i instrument, No. 6, 1960.

2. R. M. Pratusevich. Operating loads of machine tools. Stanki i instrument, No. 8, 1963.

3. ENIMS. Design loads of machine tools, guidance materials, 1964.

4. D. S. Lvov. Fundamentals of economic design of machines, Ekonomika, 1966.

5. Yu. M. Bashagurov, A. S. Gitman, E. K. Strelbitskii. Study of operating conditions of electric motors of general-purpose metal-cutting machine tools, Izvestiya TPI, No. 172, 1967.

6. L. M. Lych, M. V. Il’ienchenko, D. B. Rikberg, V. G. Shalit. Economics of the use of machine-building equipment, Tekhnika, 1967.