Increasing the Overload Capacity of Squirrel-Cage Induction Motors
Abstract Generated abstract
The paper addresses the mismatch between the low average loading of induction motors on universal metal-cutting machine tools and the high rated power selected to satisfy pull-out torque requirements. Using previously established Weibull load-distribution data for 1K62 machine tools, the authors argue for a squirrel-cage induction motor modification with increased overload capacity, and describe a prototype AOM 2-42-4 motor based on the standard AO 2-42-4 frame. The design increases magnetic flux and uses a steel-disk device to limit short-circuit current and locked-rotor heating. Experimental comparisons indicate that the prototype achieves starting and maximum torque comparable to larger motors, meets heating limits, and may offer lower weight and cost with better average operating performance in the intended duty cycle.
Full Text
Increasing the Overload Capacity of Squirrel-Cage Induction 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 conditions of electric motors of universal metal-cutting equipment, carried out in recent years at NIIMASH and ENIMS, have shown that the average load of motors amounts to 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 model 1K62 machine tools [3]. It was established that the density of the distribution of loads 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 conditions 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 with a power of 10 kW, installed on a 1K62 machine tool, is loaded on average to 10 percent.
When selecting motors 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 practically made according to the value of the pull-out torque, whose catalog value for motors of the AO2 series is 170–220 percent of rated torque.
If one takes into account that the number of motors installed annually on metal-cutting machine tools as main drives is 120–180 thousand, and that the law of distribution of the load on the shaft is sufficiently stable, then the advisability of creating a modification of induction motors with increased overload capacity specifically for universal metal-cutting...
equipment. Increasing the overload capacity by 1–2 steps of the power scale makes it possible correspondingly to reduce the rated power of the motors being installed.
The Tomsk Polytechnic Institute, jointly with the Special Design Bureau for Electrical Machine Building, designed such a motor for use with 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 being used. All the principal 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, with some increase in the width of the stator and rotor teeth, 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. Increasing the magnetic flux leads to a higher 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 \((10 \div 12^\circ \mathrm{C}/\mathrm{s})\).
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 mounted on the motor shaft between the end face of the rotor, which has no ventilation blades, and the bearing shield; in the short-circuit mode it is attracted to the end face of the rotor and, by shunting the short-circuiting rings, increases the leakage flux of the end portions 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 the short-circuit current must be reduced, 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 motor with the provisional designation AOM 2-42-4 was manufactured by SKBEM. Its principal data are given in Table 1.
Table 1
| AO 2-42-4 | AOM 2-42-4 | AO 2-51-4 | AO 2-52-4 | |
|---|---|---|---|---|
| \(P_2\), kW | 5.5 | 5.5 | 7.5 | 10 |
| \(M_m\), kgm | 7.4 | 13.2* | 10.0 | 13.3 |
| \(M_m/M_n\) | 2.00 | 3.57* | 2.00 | 2.00 |
| \(M_p\), kgm | 5.5 | 7.0* | 7.0 | 9.3 |
| \(M_p/M_n\) | 1.5 | 1.9* | 1.4 | 1.4 |
| \(I_n\), A | 11.0 | 12.5 | 14.5 | 19.2 |
| \(\cos \varphi_n\) | 0.86 | 0.80 | 0.88 | 0.88 |
| \(\eta_n\), % | 88 | 86.5 | 89 | 90 |
| Weight, kg | 68 | 69 | 90.5 | 104 |
| Cost, rubles | 52 | — | 84 | 100 |
Note: experimental data are marked with an asterisk.
Conclusions
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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 expected to be installed in the future on 1K62 machine tools.
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The rate of temperature rise of the stator winding in the locked condition of the AOM 2-42-4 motor corresponds to the standards for this index.
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The temperature rise of the stator winding in the rated mode corresponds to insulation class E.
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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 indicators of the AOM 2-42-4 motor in the operating modes of the 1K62 machines will be higher than those of the AO 2-51-4 and AO 2-52-4 motors.
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The weight and cost of the AOM 2-42-4 motor are considerably lower than those of the AO 2-51-4 motor.
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The device described may prove effective in standard general-purpose motors for reducing the rate of temperature rise and increasing reliability.
REFERENCES
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R. M. Pratusevich. Operating loading conditions of universal machine tools. Stanki i instrument, No. 6, 1960.
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D. S. Lvov. Fundamentals of the Economic Design of Machines. “Ekonomika,” 1966.
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Yu. M. Bashagurov, A. S. Gitman, E. K. Strelbitskii. Study of the operating modes of electric motors of universal metal-cutting machine tools. Izvestiya TPI, No. 172, 1967.
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K. I. Shenfer. Patent invention No. 7507, class 21 d², 20, issued 31.01.29.