INCREASING THE OVERLOAD CAPACITY OF ASYNCHRONOUS SHORT-CIRCUITED MOTORS

Yu. M. Bashagurov, A. S. Gitman, E. K. Strelbitskii

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

Studies of the loading regimes of electric motors of universal metal-cutting equipment, carried out in recent years at NIIMASH and ENIMS, have shown that the average load of the motors is 10–25 percent of rated power [1, 2]. Similar, but more acceptable for electrical-machine builders, studies were carried out by the Department of Electrical Machines of the Tomsk Polytechnic Institute on IK62 model machine tools [3]. It has been established that the density of the load distribution on the shaft \(P_2\) is well approximated by the Weibull law

\[ f(p_2)=\frac{m}{x_0}p_2^{m-1}\exp\left(-\frac{p_2^m}{x_0}\right). \tag{1} \]

It was experimentally established that, for random loading regimes of motors on universal machine tools, \(m=0.91\), and

\[ x_0=\left(\frac{P_2}{b_m}\right)^m, \tag{2} \]

where

\[ b_m=\Gamma\left(\frac{1}{m}+1\right) \]

\(P_2\) is the average load on the motor shaft.

An AO 2-52-4 motor of 10 kW power, installed on an IK62 machine tool, is loaded on average by 10 percent.

When motors are selected for continuous load, the limiting factors, as a rule, are the temperature of the stator winding and the magnitude of the starting torque. In the motors under consideration, the indicated limitations are not limiting, and the choice of motor is made practically according to the value of the pull-out torque, whose catalog value for motors of the AO2 series is 170–220 percent of the rated torque.

If one takes into account that the number of motors annually installed on metal-cutting machine tools as main drives is 120–180 thousand, and that the law of load distribution on the shaft is sufficiently stable, then the expediency becomes obvious of creating a modification of asynchronous electric motors with increased overload capacity specifically for universal metal-cutting...

equipment. 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 being installed.

The Tomsk Polytechnic Institute, jointly with the Special Design Bureau of Electrical Machine Building, designed such a motor for application to the 1K62 machine tool on the basis of the standard AO 2-42-4 motor, intended to replace the AO 2-52-4 motor now in use. All the main and overall mounting dimensions of the AO 2-42-4 motor were retained.

To increase the overload capacity, the magnetic flux was increased by 27 percent. This was accomplished by reducing the number of turns of the stator winding while somewhat increasing the tooth width of the stator and rotor, so that the induction in the teeth was 1.8 T. The geometry of the tooth zone was optimized on the M-20 electronic digital computer. The increase in magnetic flux leads to a larger starting current \((I_k/I_n = 9 \div 10)\) and to a high rate of temperature rise of the stator winding with the rotor locked \((10 \div 12^\circ\mathrm{C}/\mathrm{sec})\).

In order to keep these indices at the level of standard motors, a special device was used, proposed as early as 1926 by Academician K. I. Shenfer [4]. A steel disk is fitted onto the motor shaft between the end face of the rotor, which has no ventilation blades, and the bearing shield; in the short-circuit mode this disk is attracted to the rotor end face and, by shunting the short-circuiting rings, increases the leakage flux of the end parts by a factor of 8–10. In normal operating modes the disk is pressed away from the rotor by a spring. Depending on how much it is necessary to reduce the short-circuit current, disks are installed on one or on both sides. In the first case the disk is installed on the side of the external fan.

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

Table 1

Note: experimental data are marked with an asterisk.

Conclusions

1. The starting and maximum torques of the AOM 2-42-4 motor correspond to the analogous indices of the AO 2-51-4 motor, which is intended to be installed in the future on 1K62 machine tools.

2. The rate of temperature rise of the stator winding in the locked-rotor state of the AOM 2-42-4 motor conforms to the standards for this index.

3. The temperature rise of the stator winding in the rated mode corresponds to insulation class E.

4. Although the rated efficiency and power factor of the AOM 2-42-4 motor are lower than those of the AO 2-51-4 motor, the corresponding average

the performance characteristics of the AOM 2-42-4 motor in the operating modes of the 1K62 machine tools will be higher than those of the AO 2-51-4 and AO 2-52-4 motors.

1. The weight and cost of the AOM 2-42-4 motor are significantly lower than those of the AO 2-51-4 motor.

2. The device described may prove effective in mass-produced general-purpose motors for reducing the rate of temperature rise and increasing reliability.

REFERENCES

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

2. D. S. L’vov. Fundamentals of the Economic Design of Machines. “Ekonomika,” 1966.

3. Yu. M. Bashagurov, A. S. Gitman, E. K. Strel’bitskii. Investigation of the operating modes of electric motors of universal metal-cutting machine tools. Izvestiya TPI, No. 172, 1967.

4. K. I. Shenfer. Patent invention No. 7507, class 21 d², 20, issued 31.01.29.