TECHNICAL-ECONOMIC ASSESSMENT OF A BRUSHLESS ELECTRIC-MACHINE AMPLIFIER WITH ADJUSTABLE FREQUENCY (BEMA-AF)

A. S. BATURIN, A. I. SKOROSPESHKIN, B. E. TROFIMENKO, Sh. S. ROIZ

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

One of the economical motors for the actuating mechanism of an automated electric drive is the squirrel-cage asynchronous alternating-current motor, which has comparatively high operating reliability and low manufacturing cost. In many cases, when it is necessary to obtain smooth variation of the rotational speed of an asynchronous motor over a wide range, frequency control of speed is used. As sources of alternating current of adjustable frequency, synchronous and asynchronous generators, commutator and single-armature converters, and various circuits of static converters are used. At present, much attention is being devoted to the development of circuits of static frequency converters as the most promising.

A promising source of adjustable frequency may also be a brushless electric-machine alternating-current amplifier of adjustable frequency (BEMA-AF), being developed at the Department of Electrical Machines of the Tomsk Polytechnic Institute.

The BEMA-AF consists of three main units:

1) a drive motor (squirrel-cage asynchronous);

2) an electric-machine amplifier (a three-stage electric machine);

3) a controlled semiconductor commutator (CSC).

The principle of operation of the BEMA-AF is as follows. A direct-current voltage is supplied to the control winding (the first stage of the amplifier) through the CSC. As a result, a stationary magnetic flux is created in space. With the aid of the CSC, the magnetic flux becomes rotating with a certain adjustable speed, which can be varied depending on the switching frequency of the CSC. The first stage excites the second, which is an asynchronous generator. The latter is a purely inductive load of the first stage and generates active power into the third stage, which is the output stage and performs the functions of contactlessness. The frequency at the output of the amplifier varies depending on the frequency at the input.

At the same time, the BEMA-AF provides power amplification at the output.

The technical and economic indices of the BEMA-AF must be compared with analogous indices of designs that make it possible to perform the same functions with identical output parameters. The most acceptable for comparison with the BEMA-AF may be a static converter of three-phase current using planar transisto-

... in [1]. Table 1 gives the techno-economic indicators of the BEMA-AF and of a static converter.

Table 1

Techno-economic indicators of a brushless EMU with adjustable frequency (BEMA-AF) and of a static three-phase AC converter using planar transistors.

A comparison of the technical indicators (Table 1, Section A) makes it possible to establish that the BEMA-AF and the static converter have approximately the same output powers (1.2 and 1.1 kW). A comparison of the other technical parameters makes it possible to draw a number of conclusions about the advantages and disadvantages of the designs under consideration.

The principal advantages of the BEMA-AF should include:

a) low power of the controlled semiconductor commutator;

b) a satisfactory form of the current and voltage curves, obtained owing to the machine part;

c) the possibility of operation under significant overloads (it permits a short-circuit mode);

d) a sufficiently high output voltage;

e) comparatively good response speed.

The disadvantages of the BEMA-AF are as follows:

a) a low amplification factor (12);

b) a comparatively large weight of the unit as a whole (100 kg);

c) comparatively large dimensions of the unit;

d) the presence of rotating parts.

The main advantages of the static frequency converter using planar transistors in comparison with the BEMA-AF are:

a) better response speed;

b) small overall dimensions and weight;

c) a higher efficiency of the unit;

d) absence of rotating parts.

At the same time, a static frequency converter has a number of substantial shortcomings that reduce the effectiveness of its application.

The shortcomings of a static converter include:

a) the semiconductor elements are rated for the full load power;

b) unsatisfactory waveforms of the current and voltage curves at the output (additional filters are installed for smoothing, making the circuit more expensive);

c) sensitivity to overloads;

d) low output voltage (without a transformer at the output).

The inclusion of an additional transformer narrows the frequency-control range;

e) inadmissibility of operation without load (no-load operation is impossible).

Power semiconductor elements in static frequency converters are under more severe conditions (than in the BEMA-AF), do not permit short circuits, and are sensitive to commutation switching.

Analyzing the economic indicators (Table 1, Section B), we find that the approximate values of manufacturing cost (determined by the aggregated method [2]) for the BEMA-AF and the static converter amount to large sums. The high manufacturing cost is explained by the high prices of semiconductor devices. Thus, the manufacturing cost of the control-and-conversion unit for the BEMA-AF is 1500 rubles, which is about 86 percent of the cost of the entire unit. In this case, the most expensive elements (Table 2)—thyristors, diodes VK2-10, and semiconductor triodes—make up 42 percent of the manufacturing cost of the control-and-conversion-unit circuit, or more than 90 percent of the cost of purchased products and basic materials. With an increase in the scale of production of semiconductor devices, as well as with further improvement of the technology of their manufacture, their cost and prices will decrease; this, in turn, will make it possible to reduce significantly the cost of products whose circuits include semiconductors. From this standpoint, it may be expected that such products are promising, all the more so because the use of semiconductor devices makes it possible to create contactless circuits with high efficiency, good response speed, and small dimensions.

A comparison of the cost of the BEMA-AF and the static converter (Table 1) shows that the latter is the more expensive item to manufacture. At the same time, however, it should also be borne in mind that the manufacture of the static frequency converter consumes considerably less scarce copper.

The annual operating expenses for the static converter also somewhat exceed the expenses for the BEMA-AF. This is explained mainly by the large depreciation charges for the static converter.

All economic calculations were made for two-shift operation under normal operating conditions.

Since the efficiency, power factor, and also the copper consumption for the variants being compared are not the same, the differences should be taken into account in monetary terms. Table 3 gives the values of the principal economic indicators and their correction.

As can be seen from Table 3, even after correction of the economic indicators, the BEMA-AF has lower one-time costs and operating expenses. It is important to note that, if the one-time costs of the BEMA-AF-12A are 1.27 times lower, then the operating expen-

Table 2

Calculation of the cost of purchased products and basic materials
for the BEMA-AF-12A control circuit

—only by a factor of 1.06. This ratio is explained by the lower cost of electrical-energy losses in the static converter, owing to its higher efficiency.

Table 3

Main economic indicators of the BEMA-AF-12A and of a frequency static converter using planar transistors, with adjustment taken into account

Conclusions

1. The brushless electric machine amplifier with adjustable frequency (BEMA-AF-12A) provides power amplification and a wide range of output-frequency regulation.

2. In comparison with a static three-phase-current converter using planar transistors, the BEMA-AF-12A has a number of substantial advantages:

a) low input power of the controlled semiconductor switch (CSS);

b) satisfactory shapes of the voltage and current curves at the output, owing to the machine part;

c) the possibility of operation under considerable overloads (it permits a short-circuit mode);

d) a sufficiently high output voltage;

e) comparatively good speed of response;

f) lower manufacturing cost and operating expenses (under two-shift operation and normal operating conditions);

1. The BEMA-AF-12A may find application in non-reversing circuits of automated drives for controlling asynchronous squirrel-cage motors. In this case there should be no stringent requirements on the dimensions and weight of the BEMA-AF unit.

References

1. M. B. Kyunovalov. Static converters of three-phase alternating current using planar transistors. Dissertation, Tomsk, 1963.

2. A. S. Konson. Economic substantiation of designs for electrical machines, apparatus, and instruments. Gosenergoizdat, Moscow–Leningrad, 1963.

3. P. R. Filippov. New depreciation standards. Moscow, 1963.

4. Unified system of scheduled preventive maintenance and rational operation of technological equipment at machine-building enterprises. Moscow, 1964.

5. I. A. Syromyatnikov. Operating modes of asynchronous and synchronous electric motors. Gosenergoizdat, 1963.

6. Journal Vestnik statistiki, No. 9, 1966.