DEVELOPMENT OF A SERIES OF THREE-PHASE ASYNCHRONOUS ELECTRIC MOTORS FOR POWER TOOLS

V. A. Arkhipenko, N. I. Pashinskaya, A. I. Gulyaeva

(Presented by the scientific seminar of the departments of electrical machines and general electrical engineering)

With the development of industry and construction, the need of the national economy for electrified tools grows with each year. Thus, according to data from the All-Union Scientific Research Institute of Construction and Mechanized Tools, the demand for power tools driven by asynchronous motors with frequencies of 50 and 200 Hz in 1970–1975 amounts to 17 million units.

Modern portable power tools of domestic manufacture are produced both with commutator AC motors of the KN type and with three-phase asynchronous motors of the AN type (mains frequency 50 Hz) and AP type (frequency 200 Hz). Electric motors of the KN, AN, and AP types are manufactured in accordance with the requirements of GOST 10085-62. The principal drawback of domestic machines is insufficiently reliable protection against electric shock. Electric motors for tools are produced for voltages of 220 and 36 V. When working with tools at a voltage of 220 V, protective means against electric shock are used (mats, gloves, etc.), which creates certain inconveniences and reduces labor productivity.

At present, in accordance with the recommendations of the International Commission for Testing Electrical Equipment, in order to improve electrical safety and convenience of operation, tools are being developed and introduced which, in addition to functional insulation, have protective insulation against electric shock. Further improvement of the series of electric motors for tools, as well as the need to improve work safety, makes it expedient to design a new unified series of machines with double insulation (functional and protective).

In designing a unified series of three-phase asynchronous electric motors with a frequency of 50 Hz, with powers of 120, 180, 250, 370, 550, 750, and 1100 W, with a synchronous rotational speed of 3000 rpm, with double insulation at a voltage of 220/380 V and without double insulation at a voltage of 36 V, and also with a frequency of 200 Hz, with powers of 25, 40, 60, 90, 120, 180, 250, 370, 550, 750, 1100, 1500, and 2200 W, with a synchronous speed of 12000 rpm, at a voltage of 220 V, with double insulation from 36 V, without double insulation, a large volume of computational and theoretical work was carried out to determine the optimal relationships of the design and techno-economic parameters of each motor. This problem was solved with the aid of the “Minsk-22” electronic computer, which made it possible—

then, in an automatic search, to determine all the necessary optimal parameters of the motor series. It is known that the direction of the search and the development are determined by the selected optimization criteria. In developing an optimal series of motors for power tools and in compiling the calculation procedure, the following questions were investigated:

a) obtaining motors with minimum total costs for their manufacture and operation;

b) ensuring guaranteed operating and service characteristics;

c) operational reliability;

d) electrical safety of the motor.

The problem of ensuring the electrical safety of motors for power tools with a voltage of 220/380 V was solved by introducing protective insulation between the stator stack and the frame and between the shaft and the rotor core. Electric motors with a voltage of 36 V were calculated without protective insulation. The electric motors are of a protected design, ventilated by axial ducts located between the stator core and the protective insulation in electric motors with a voltage of 220/380 V and between the stator core and the frame in electric motors with a voltage of 36 V. Cooling is provided by a fan built into the electric motor. The general appearance of the motor with double insulation is shown in Fig. 1.

Fig. 1. General appearance of an electric motor with double insulation

Taking into account the great demand for, and the use of, motors mainly in power tools, the principal optimization criteria adopted were obtaining the minimum total cost in manufacture and operation and the minimum weight of the motors. The attainment of the necessary operating and service indices in calculating a series of motors according to the optimization criteria was ensured by limiting values of the starting-current ratio \(k_i\), the starting- and maximum-torque ratios \(k_p, k_m\), the established temperature rise of the stator winding \(\Theta_{\text{w. avg.}}^\circ\mathrm{C}\), and also the rate of temperature rise in short-circuit mode, \(\mathrm{T}^\circ\mathrm{C}/\mathrm{s}\). In view of the difficulty of describing the reliability criterion by analytical relationships and the absence of sufficiently substantiated data, the calculation does not provide for a search according to the reliability criterion. Motors satisfying the specified values of the limiting indices were obtained as a result of simultaneous variation of the outer diameter of the stator, the active length of the stator stack, the bore diameter, the width and height of the tooth, and also the induction in the air gap. The range of variation of the varied quantities

wide. The methodology included electromagnetic, ventilation, and thermal calculations, as well as calculation of the weight and cost of the motor.

To determine the optimal parameters of the motors on the electronic digital computer, a series of search calculations was carried out. The search calculations were performed according to the criterion of the minimum total cost and minimum total weight of each electric motor in the designed series; i.e., ideal motors with minimum costs and weights were determined. In this case, each motor of the series had its own dimensions, differing from the others, which made it difficult to make them unified.

To unify the series, the change in total cost, taking into account demand for 1970–1975, as a function of the outside diameter of the stator of motors calculated for minimum cost and minimum weight, was presented in a diagram (Fig. 2). On the basis of this diagram, basic motors having the greatest total cost were selected.

Fig. 2. Dependence of the total cost of the ideal series on the outside diameter of the stator.
Motor 200 Hz, 36 V — by minimum cost, — — — by minimum weight. Motors 50 Hz, 220 V — by minimum cost, — — — by minimum weight.

By total cost is meant the product of the cost of the optimal motor by the demand for 1970–1975. The outside diameters of the basic motors were taken close to dimensions corresponding to economical cutting of sheet material. Motors having a low total cost but close to the basic motors in their outside diameter were calculated at the diameter of the basic motors. Some motors were calculated for several diameters; for example, a 50 Hz, 220/380 V motor with a power of 1100 W was calculated at diameters of 102 and 120 mm; a 200 Hz, 36 V motor with a power of 370 W was calculated at diameters of 56, 63, and 70 mm.

After an economic comparison of the variants, 6 overall diameters were adopted: 42, 56, 70, 76, 92, and 102 mm, which, with their smallest number, made it possible to obtain an optimal series of motors that was only 1% more expensive than the ideal one.

For the purpose of assessing the possibility of modernizing the existing series, a search calculation of motors on active iron was carried out (motors of the AN and AP series). The optimal variant in this case was determined by changing the length of the active part of the magnetic flux. It was found that the cost of the series in this case is greater by 12–12.5 million rubles, and the weight of active materials is greater by 12%.

A comparison of the energy characteristics of the motors being developed, the existing motors, and motors calculated on the basis of the existing AN and AP series is given in Fig. 3. It is found that the newly developed motors are significantly superior in their parameters to the existing ones. Thus, for motors of frequency 50 Hz the efficiency value is higher by 10%; for powers of 120 and 180 W, and for other powers the increase in efficiency is somewhat smaller. In addition, the volume of the active part of these motors is smaller by 6–10%, and the weight of winding copper is smaller by 6–8% than in the existing 50-Hz motors of the AN series. For motors of frequency 200 Hz, the energy characteristics are also higher overall, while the volume of the active part and the weight of copper are lower.

It should be noted that motors of frequency 200 Hz have a lower value of the rate of temperature rise during short circuit (\(T\)). For the motors being designed, the value of \(T\) does not exceed \(\frac{12^\circ C}{\mathrm{s}}\),

Fig. 3. Values of \(\eta\) and \(\cos\psi\) for the series of electric motors \(U = 220\) V.
Motors 220 Hz: 1—1 designed, 3—3 based on AP-type motors, 5—5 existing AP-type motors. Motors 50 Hz: 2—2 designed, 4—4 based on AN-type motors, 6—6 existing AN-type motors.

whereas for existing motors \(T = 18 \div 22^\circ\mathrm{C}/\mathrm{s}\), which leads to a reduction in the reliability of the existing motors.

The energy characteristics of motors calculated on the basis of existing ones differ only slightly from those of the designed series (on the order of \(1 \div 3\%\)); however, the weight of active materials is greater than for the optimal motors. Thus, in motors with a frequency of \(50\ \mathrm{Hz}\), the increase in copper weight reaches 50%, while the weight of active iron is reduced by only 10%. Motors with a frequency of \(200\ \mathrm{Hz}\) have an increase in the weight of active iron of \(18 \div 30\%\) with practically unchanged weight of active copper. Overall, for the entire series based on existing motors, the increase in winding-copper weight in comparison with the optimal series is 20%, and that of active iron is 9%.

A comparison of the energy and starting characteristics of motors of the designed series with foreign models is given below.

Table 1

It follows from this that, in terms of their characteristics, the designed motors are on a level with the best foreign models.

Thus, the developed unified series of three-phase asynchronous electric motors for hand-held power tools surpasses the existing motors in its characteristics, is on a level with the best foreign models, and meets the modern requirements imposed on power tools for ensuring electrical safety.