Technical and Economic Feasibility Studies for the Development of Brushless Electromechanical Amplifiers (BEMA)

A. S. Baturin, A. I. Skorospeshkin, E. N. Podborskii

(Recommended by seminars of the Departments of Electrical Machines and Industrial Economics and Enterprise Organization)

The commutator electromechanical amplifiers (EMA) widely used in automation circuits have very substantial shortcomings caused by the presence of the commutator-brush assembly.

These shortcomings reduce the reliability of EMA, degrade their characteristics, and limit their areas of application.

Proceeding from this, at the Department of Electrical Machines of the Tomsk Polytechnic Institute, the development and investigation of various types of brushless electromechanical amplifiers (BEMA) is being carried out with the aim of increasing operational reliability and improving amplifier characteristics.

This article presents the results of a technical and economic analysis of the development of a brushless electromechanical amplifier (BEMA-12A). The comparison is made with the serially produced EMA-12A amplifier.

The most important requirements imposed on new designs are: technical excellence, economy in manufacture and operation. Final conclusions about economic efficiency are drawn as a result of a comprehensive assessment of the indicators of the above requirements. In this sequence the technical and economic analysis of the BEMA-12A will be carried out.

Since in the course of the analysis we shall compare the BEMA-12A with the EMA-12A, we present the technical and economic indicators for both amplifiers.

Table 1

Technical and economic indicators of commutator (EMA-12A) and brushless (BEMA-12A) electromechanical amplifiers

B. Economic indicators
1. In manufacture

On the basis of the data in the table of technical-and-economic indicators (Table 1) and of literary sources, let us evaluate the technical excellence of the BEMU-12A. A comparison of the principal technical indicators of the newly developed BEMU-12A and the base model, EMU-12A, shows that most of their data coincide. The most important qualitative indicators of electromechanical amplifiers are the amplification factor and response speed. For the amplifiers being compared these are identical. The figure of merit of the amplifiers (20000) is also the same. The exception is the higher power consumption of the drive motor (1945 W > 1900 W) and the related lower efficiency of the BEMU-12A (62% < 63%). At the same time, however, in the new amplifier design the no-load losses are 60 W lower than in the base model. Therefore the economy of one or the other amplifier depends on the ratio of operating time at rated load and at no load, as well as on the total operating time over the course of a year. This question will be resolved when operating costs are determined.

The new amplifier has a number of very important fundamental differences from the basic model EMU-12A.

Whereas the EMU-12A is an ordinary transverse-field electromechanical amplifier, the BEMU-12A consists of two synchronous generators combined in a single magnetic core. The excitation winding of the output stage of the BEMU is fed through rectifiers (D215A semiconductor diodes) from the first synchronous generator. Thus it has been possible to eliminate the sliding contact in the circuit. This is the main advantage of the new design, ensuring higher operating reliability, stability of output characteristics, and a broader field of application.

At the same time, by eliminating the commutator-brush assembly, the design of the new electromechanical amplifier has been simplified, has become more manufacturable, and, in addition, will have better operating indicators, since the introduction of semiconductor diodes into the circuit reduces the need for repairs.

An important advantage of the BEMU-12A is also that the new amplifier can operate without failure not only under normal conditions, but also under adverse operating conditions (operation in dusty rooms with increased humidity, reduced pressure, in an atmosphere contaminated by aggressive gases, vapors, etc.).

One of the technical advantages of the BEMU-12A is the possibility of increasing the amplification factor and improving response speed. This can be achieved by connecting, at the output, three capacitors of 20 μF each, type MBGO-3, in parallel with the stator windings. The latter measure will make it possible, with a lower control current, to obtain sufficient output powers with better response speed. The increase in the cost of the BEMU-12A due to this is about 2.5 rubles [8].

The elimination of the sliding contact from the BEMU-12A circuit opens up potential possibilities for creating a faster-acting and more economical design, while the low level of radio interference permits operation in the immediate vicinity of electronic circuits without causing large interference in the equipment. Such are the technical advantages of the BEMU-12A.

Let us consider the economy of the new design in manufacture (Table 1, section B, subsection I). An important indicator of economy in manufacture is production cost. Since in manufacturing the BEMU-12A a large number of parts and assemblies of the basic model EMU-12A are used, to determine the cost of the new amplifier the correction method most acceptable under these conditions was used. From the cost of the basic model (EMU-12A), the expenditures that will not occur in the new design were excluded, and the expenditures arising in connection with the addition and alteration of parts and assemblies were added. As a result, the manufacturing cost of the BEMU-12A was obtained. For comparison, we give Table 2, which presents figures for the cost of both designs.

As can be seen from Table 2, the cost of the BEMU-12A increased by 1% compared with the basic model EMU-12A. The increase in cost was caused mainly by the D215A semiconductor diodes [7]. As a result, the share of expenditures on basic materials and purchased items in the new design increased by 5.2%.

One of the important indicators of efficiency in manufacture is production labor intensity. In the new design, a reduction in manufacturing labor intensity compared with the EMU-12A of 15.8% has been achieved, which causes an increase in labor productivity of 18.7%. With an expected output in the second year after the assimilation of the BEMU-12A of 4,000 units per year, it is possible to release 25 workers.

Table 2

Manufacturing cost of EMU-12A and BEMU-12A

Along with this, the efficiency in the manufacture of electrical machines is characterized by the consumption of nonferrous metals, which are scarce materials. The total saving of nonferrous metals (with an output of 4000 units/year) in the manufacture of the BEMU-12A is 16456 kg/year. In addition, when switching to manufacture of the BEMU-12A it is possible to release fixed assets in the amount of about 5000 rubles.

All the changes listed above in the manufacture of the BEMU-12A are caused by the elimination from the design of the commutator assembly and brush mechanism, which require large labor expenditures and a significant quantity of scarce nonferrous metals [2].

The BEMU-12A will have the greatest efficiency in operation. The highest saving is obtained in repair costs, since the fundamental change in design reduces the repair-complexity category of the BEMU-12A to 3,4 conventional repair units as against 3,8 for the EMU-12A. Annual repair costs, calculated on the basis of the PPR standards [5], are reduced by almost 10%. In addition, the labor input of repair work is reduced by 11%, which raises labor productivity in repairs by 12,4%. All this is explained by the fact that in the new design there is no need to replace brushes and run them in, or to turn and undercut the commutator. Moreover, all calculations are given for two-shift operation under normal conditions. For difficult operating conditions, the absolute value of the saving in repair costs increases from 9,5 rubles/year under normal conditions to 24 rubles/year under difficult conditions.

Efficiency in the operation of electrical machines is characterized by the cost of electric-energy losses, which for the amplifiers under consideration can be determined from the formulas:

\[ \left\{ \begin{aligned} C_{\text{п}} &= t_1 \Pi_{\text{н}} K_{\text{м}} S + t_2 \Pi_{\text{х}} S,\\ T &= t_1 + t_2 \end{aligned} \right. \tag{1} \]

where \(C_{\text{п}}\) is the cost of electric-energy losses, rubles/year;
\(t_1\) is operating time at rated load, h/year;
\(\Pi_{\text{н}}\) is total losses at rated load, kW;
\(K_{\text{м}}\) is the power-utilization factor;
\(K_{\text{м}} = 0,7\) [4];
\(S\) is the cost of 1 kWh of electric energy, 0,01 rub./kWh [4];

$t_2$ — operating time in idle mode (the motor is running, while the generator does not deliver energy), h/year;
$\Pi_x$ — losses in no-load mode, kW;
$T$ — total operating time of the EMA per year, h/year.

For two-shift operation, which is most typical, the total operating time of the equipment, taking into account improvement in its utilization, is
$T = 4000$ h.

Substituting the corresponding numerical values into the system of equations (1), we obtain:

for the EMU-12A

\[ C_{\mathrm{p}1}=t_1\cdot 0.7\cdot 0.7\cdot 0.01+t_2\,0.168\cdot 0.01;\quad 4000=t_1+t_2; \tag{2} \]

for the BEMU-12A

\[ C_{\mathrm{p}2}=t_1\,0.745\cdot 0.7\cdot 0.01+t_2\,0.108\cdot 0.01;\quad 4000=t_1+t_2; \tag{3} \]

after transformations we shall have

\[ \begin{aligned} C_{\mathrm{p}1}&=0.0049\,t_1+0.00168\,t_2\\ 4000&=t_1+t_2 \end{aligned} \tag{2} \]

\[ \begin{aligned} C_{\mathrm{p}2}&=0.0052\,t_1+0.00108\,t_2\\ 4000&=t_1+t_2 \end{aligned} \tag{3} \]

Using equations (2) and (3), we construct a graph of the cost of electrical-energy losses in the coordinates $C_{\mathrm{p}}$ and $t_1$ (Fig. 1).

Fig. 1.

As is evident from the graph, in the region where $t_1<2667$ h, the cost of electrical-energy losses of the BEMU-12A is less than that of the EMU-12A; for $t_1>2667$ h, the reverse is true. In all cases equality of losses occurs at the point where $t_1=2t_2$, that is, the EMU-12A has lower losses only in the case when operation under load constitutes more than 66.67% of the total operating time per year. However, prolonged operation at nominal load is not typical for the overwhelming majority of EMAs. The absence of statistical data makes an exact solution of this question difficult. To determine the cost of electrical-energy losses, we shall take $t_1=t_2$. Under this assumption we obtain:

\[ C_{\mathrm{p}1}=0.0049\cdot 2000+0.00168\cdot 2000=13.16\ \text{rub./year} \tag{2} \]

\[ C_{\mathrm{p}2}=0.0052\cdot 2000+0.00108\cdot 2000=12.56\ \text{rub./year}. \tag{3} \]

We shall take these losses as the calculated ones.

The amount of depreciation charges is determined on the basis of the annual rate—10.2% [6]—and the initial cost (allowing for the enterprise’s profit, transportation and installation costs—1.15) of the EMA.

Substituting the data, we obtain:

\[ \text{for EMA-12A}\quad 10.2 \cdot \frac{218.51}{100} \cdot 1.15 = 25.65\ \text{rubles/year}; \]

\[ \text{for BEMA-12A}\quad 10.2 \cdot \frac{220.80}{100} \cdot 1.15 = 25.8\ \text{rubles/year}. \]

Since the $\cos \varphi$ and the efficiency of the EMAs being compared are either equal or differ only insignificantly, and also taking into account the small power consumption, we do not consider either the additional cost of static capacitors and the losses in them, or additional capital investments in power stations to compensate for these losses.

Having all the data listed above, it is possible to calculate the annual economic effect (savings) from introducing the BEMA-12A. In determining the savings, we proceed from the fact that in our case there is an insignificant increase in prime cost and an improvement in the quality of the BEMA-12A. But since for the manufacturer the BEMA-12A is working capital, and for the consumer it is fixed capital, the economic effect for the manufacturer and the consumer cannot be summed. On this basis, we determine the economic effect for the consumer, where the expenditures incurred by the manufacturer are also taken into account [3]:

\[ \begin{aligned} E_g &= \left[C_1' + E_n'\left(C_1 + E_n K_1\right)\right] - \left[C_2' + E_n'\left(C_2 + E_n K_2\right)\right] \\ &= C_1' - C_2' + E_n'\left[C_1 - C_2 + E_n\left(K_1 - K_2\right)\right] \\ &= [90.5 + 13.16 + 25.65] - (81 + 12.56 + 25.8)\cdot 4000 \\ &\quad + 0.15\left[(218.5 - 220.8)\cdot 4000 + 0.2\cdot 5000\right] = 39674\ \text{rubles/year}, \end{aligned} \]

where $E_g$ is the annual economic effect for the consumer, rubles/year;

$C_1'$ is the annual operating expenses for EMA-12A, rubles/year;

$E_n'$ is the coefficient of comparative economic efficiency for the consumer, $E_n' = 0.15$/year [3];

$C_2'$ is the annual operating expenses for BEMA-12A, rubles/year;

$C_1$ is the prime cost of the annual output of EMA-12A, rubles;

$C_2$ is the same for BEMA-12A, rubles;

$E_n$ is the coefficient of comparative economic efficiency for the EMA manufacturer, $E_n = 0.2\ \text{year}^{-1}$ [3];

$K_1$ is the capital expenditure in manufacturing EMA-12A, rubles;

$K_2$ is the same for BEMA-12A, rubles.

Thus, the total expected economic effect from introducing the BEMA-12A is 39,674 rubles/year.

At the same time, it is necessary to check the efficiency of expenditures on the development of the research topic on the BEMA-12A.

Development expenditures by year were: 1964—2,000 rubles, 1965—2,000 rubles, 1966—2,160, 1967—15,000 rubles (plan). The reduced expenditures for development are:

\[ K_{\text{pr}} = \sum_{i=1}^{t} k_i(1+E)^t = 2000(1+0.1)^4 + 2000(1+0.1)^3 + 2160(1+0.1)^2 + 15000(1+0.1) = 24460\ \text{rubles}, \tag{4} \]

where $K_{\text{pr}}$ is the total reduced expenditure on developing the BEMA-12A when adopted in 1968, rubles;

$K_i$ is the expenditure on developing the BEMA-12A in the corresponding years, rubles;

$t$ is the number of years during which $K_i$ produced no effect;

$E$ is the coefficient of economic efficiency for reducing expenditures, $E = 0.1\ \text{year}^{-1}$ [3].

Payback of the costs for developing the BEMU-12A:

\[ \frac{K_{\mathrm{pr}}}{E_{\mathrm{g}}}=\frac{24460}{39674}=0.62\ \text{years}. \]

The coefficient of economic efficiency of the costs for developing the BEMU-12A:

\[ \frac{E_{\mathrm{g}}}{K_{\mathrm{pr}}}=\frac{39674}{24460}=1.62\ \text{year}^{-1}, \]

that is, for each ruble spent in developing the BEMU-12A, 1.62 rubles of savings per year are expected. The efficiency obtained is close to the average actual value of the efficiency of scientific developments in the country.

Conclusions

1. The developed electric-machine amplifier BEMU-12A has the advantages inherent in brushless electric machines. Its basic characteristics are the same as those of serially produced amplifiers of the EMU-12A type.

2. The new amplifier is more manufacturable, since it has no commutator or brush mechanism. Manufacturing labor input is reduced by 15.8%. The saving of nonferrous metals amounts to 16,456 kg/year. In developing the BEMU-12A, a large number of parts and assemblies from the EMU-12A were used, which will ensure its rapid introduction into production. However, the manufacturing cost of the BEMU-12A has increased by 1%, mainly because of the D215A semiconductor diodes.

3. A large economic effect will be obtained from the use of the BEMU-12A in operation. Repair costs are reduced especially sharply. Moreover, the advantages of the BEMU-12A in comparison with the serial design are manifested most strongly under difficult operating conditions. Under normal operating conditions, repair costs are reduced by 10%, repair labor input is reduced by 11%. The BEMU-12A also has a lower cost of electrical-energy losses.

4. The total economic effect (saving) from introducing the BEMU-12A is expected to be 39,674 rubles/year.

5. The payback period for the costs of scientific development of the BEMU-12A is 0.62 years; the efficiency of expenditures on the scientific development is equal to 1.62 year\(^{-1}\), which is close to the average actual value for scientific developments in the country.

6. A methodology has been proposed for the technical and economic substantiation of the development of brushless electric-machine amplifiers.

REFERENCES

1. V. I. Radin. Electric-Machine Amplifiers. Gosenergoizdat, 1962.

2. P. A. Babadzhanyan, B. I. Lyusin. Design and Production of Commutators for Electric Machines. Gosenergoizdat, 1960.

3. Standard Methodology for Determining the Economic Efficiency of Capital Investments. Moscow, 1966.

4. G. I. Dudko. Basic provisions of a methodology for determining the economic efficiency of electrical machines and apparatus. In: Questions of Economics and Organization of Production at Enterprises of the Electrical-Engineering Industry. Moscow, 1963.

5. Unified System of Planned Preventive Maintenance and Rational Operation of Technological Equipment of Machine-Building Enterprises. Moscow, 1964.

6. P. R. Filippov. New Depreciation Norms. Moscow, 1963.

7. Price List No. 16-03. Wholesale Prices for Semiconductor Devices. Part 2. Moscow, 1966.

8. Price List No. 16-01. Wholesale Prices for Radio-Communication Equipment and General-Purpose Radio Components. Moscow, 1963.