Experience in Assessing the Economic Efficiency of Individual AC Drives for Roller Conveyor Rollers
Abstract Generated abstract
This paper evaluates the economic efficiency of alternative individual AC drives for roller conveyor rollers in rolling mill equipment. Using plant reporting data, price lists, design materials, and calculated operating expenses, it compares geared 50 Hz drives, drives with frequency converters and reduced-frequency motors, and direct drives with multipole low-speed motors across 37 facilities at major metallurgical plants. The analysis indicates that frequency-converter drives are economically advantageous in most examined cases, especially with motors rated around 20 Hz and for smaller frame sizes, while geared drives may remain competitive where lightweight reducers are used. It also suggests that low-speed industrial-frequency motors are promising when designed within weight limits determined by minimum calculated costs.
Full Text
Experience in Evaluating the Economic Efficiency of Individual AC Drives for Roller Conveyor Rollers
P. P. Chinenov
(Presented by the scientific seminar of the Department of Electric Power Stations)
In the equipment of rolling mills, an individual drive for roller conveyor rollers can have a significant share and a sharply differing variety of designs. Therefore, the question of choosing the most economically efficient variant becomes highly relevant and of practical value.
Taking into account established practice, as well as the near-term prospects, it appears advisable first of all to study the economic efficiency and areas of application of the following types of drive:
a) a geared drive with 50 Hz electric motors;
b) a drive using frequency converters and industrial- and reduced-frequency electric motors;
c) a drive with multipole slow-speed electric motors at 50 Hz and direct transmission.
The assigned task provided for obtaining national-economic indicators of the economic efficiency of the variants, and also, as far as possible, determining the areas of their most rational application.
Because of the imperfection of the accounting system operating at enterprises, the composition of costs was determined only by those components that directly and decisively affect the degree of their economic efficiency. In accordance with this, capital costs included costs for power and working machines, buildings, and structures. For operating expenses, on the same basis, deductions for depreciation, costs of current repairs, maintenance of fixed assets, and losses of electrical energy were taken into account.
Capital costs were calculated according to the reporting data of plants or current price lists, and the components of operating expenses were found as follows.
Depreciation deductions were taken from the sums of capital investments using the actual percentage of deductions for this cost direction according to factory calculations or according to established deduction rates. Costs for current repairs and maintenance of fixed assets were determined by the method of applying a percentage to capital investments in the amounts of the actual ratio of these components for the reporting year. Costs for losses of electrical energy were calculated according to the long-term operating mode, proceeding from the equipment adopted.
Special difficulties, owing to the absence of the necessary initial data, arose—
plants, taking account of costs and their influence on the lubrication system. Therefore, the necessary techno-economic indicators were determined indirectly, from operating data and design materials. This was done as follows. Capital costs were taken from the materials of design organizations, in which the required facilities were sought out, while operating expenses were taken from the reporting documentation of plants where these facilities were in operation. Then the specific capital investments and operating costs per lubrication point were calculated, and from this the total costs for the variant were obtained, depending on the number of lubrication points of the equipment being considered.
Let us note some of the most essential features and assumptions in the adopted calculation method.
To obtain a nonindustrial frequency in the corresponding drive variants, a single (calculated) type of converter unit was adopted. In this case, the power and electric-energy losses were recalculated as a function of the actually required frequency. The reserve capacity of the converters was provided in accordance with existing design practice. The demand factor of the consumers was adopted from operating data. The selection of electric motors was based on the condition of complete interchangeability between the existing electric motors and the newly proposed ones. The necessary construction component was taken into account. As the base variant, as a rule, the variant of the existing drive was adopted. The annual working-time fund was averaged. The analysis was carried out only for those facilities that have electric motors of the AR series. In all, 37 facilities at leading metallurgical plants were examined.
The question of the possibility of using low-speed electric motors at a frequency of 50 Hz with direct transmission, owing to the limited range of parameters of the experimental series produced and the absence of any sufficiently extensive operating data, was resolved from the standpoint of the most probable prospect. For this purpose, from among the variants already considered, the variant with gear transmission was adopted as the initial one, and for it the costs relating to the mechanical part of the drive directly associated with the use of gear reducers were determined. This part of the costs is absent in the variant with low-speed motors and therefore appears as a certain saving of capital investments. By the amount of this saving, the possible increase in the weight of each electric motor in the variant with low-speed machines was calculated, since they require greater material consumption. The admissibility of such a calculation is determined by the relatively high stability of the specific cost of one kilogram of weight (0.9–1 rubles/kg) for the majority of roller-conveyor electric motors. The subsequent calculation was carried out according to the scheme adopted above. In this case, the costs of electric-energy losses were found from the specific value of the averaged nominal losses per kilogram of machine weight.
The indicators obtained made it possible, by the method of successive approximation, to establish such a unit weight of the motor at which the “calculated costs” of the variant with low-speed electric motors would be no higher than the smallest of those obtained for the competing variants.
The most characteristic results of comparing the different drive variants are given below. Analysis of these results showed the absolute economic efficiency of the drive with frequency converters in almost all the cases considered. At the same time, electric motors with a rated frequency of 20 Hz have a clear advantage over electric motors of industrial frequency. The greatest economic
Table 6
Economic efficiency of drive-roller variants
| No. | Object name | Electric-motor type | Replaced variant: reducer; number of electric motors, pcs.; total power, kW | Replaced variant: total capital expenditures (numerator) and operating costs (denominator), thousand rubles | Replaced variant: released capital expenditures upon conversion to thyristor electric drives, thousand rubles | Variant with frequency converters: total capital expenditures (numerator) and operating costs (denominator), thousand rubles | Variant with frequency converters: increase in all electric-drive equipment due to account of release of capital expenditures, kW | Variant with frequency converters: greatest permissible capital investments under the minimum condition of costs, thousand rubles | Variant with frequency converters: total capital expenditures (numerator) and operating costs (denominator), thousand rubles | Estimated costs, thousand rubles: variant with thyristor electric drives | Estimated costs, thousand rubles: reducer variant | Estimated costs, thousand rubles: variant with frequency converters |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Continuous mill 2800 of the Chelyabinsk Metallurgical Plant | ARP73-10 | 472 / 355 | 505.2 / 80.8 | 318 | 332.7 / 49.9 | 1029 | 610 / 255 | 294.1 / 58.82 | 113.62 | 181.96 | 114.04 |
| 2 | The same, Kommunarsk Foundry Plant | Same | 1602 / 355 | 1375.7 / 293 | 889.4 | 816.6 / 197.9 | 910 | 435 / 80 | 712 / 218.2 | 360.6 | 568.14 | 361.2 |
| 3 | The same, Nizhny Tagil Metallurgical Combine | ARP73-10 | 710 / 355 | 676.2 / 103.3 | 423 | 419.1 / 64.7 | 951 | 525 / 170 | 380.9 / 71.7 | 147.88 | 238.54 | 148.5 |
| 4 | Rail-and-beam mill 800 and billet department of Nizhny Tagil Metallurgical Combine | ARF42-6 | 777 / 65 | 399.7 / 61.8 | 333.2 | 351.7 / 61.8 | 391 | 395 / 330 | 314.3 / 56.8 | 119.04 | 141.7 | 131.3 |
* The denominator of column 9 (the weight of the electric drive) is taken as the difference between the numerators of columns 9 and 4.
Continuation of table
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9* | 10 | 11 | 12 | 13 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5 | 650 heavy-section mill of the Nizhny Tagil Metallurgical Combine | ARP53-10 | $\frac{308}{150}$ | $\frac{154.3}{29.4}$ | 101.8 | $\frac{178.3}{37}$ | 481 | $\frac{460}{310}$ | $\frac{145.1}{30.2}$ | 59.22 | 60.26 | 72.6 |
| 6 | 300-2 section-rolling mill of the Chelyabinsk Metallurgical Plant | ARP53-12 | $\frac{179}{150}$ | $\frac{167.2}{26.5}$ | 136.7 | $\frac{106.9}{21.5}$ | 914 | $\frac{540}{390}$ | $\frac{99}{23.1}$ | 42.9 | 59.94 | 42.9 |
| 7 | 1450 sheet mill of the Magnitogorsk Combine | ARF43-4 | $\frac{100}{70}$ | $\frac{75.6}{12.7}$ | 66.2 | $\frac{59.7}{15.7}$ | 579 | $\frac{530}{460}$ | $\frac{54.2}{15.8}$ | 26.64 | 27.82 | 27.64 |
| 8 | 1220 electric tube-welding mill of the Chelyabinsk Tube-Rolling Plant | ARP43-10 | $\frac{755}{70}$ | $\frac{450.6}{59.6}$ | 373.8 | $\frac{375.4}{70.5}$ | 424 | $\frac{424}{354}$ | $\frac{328}{57.7}$ | 123.3 | 149.32 | 145.58 |
the effect was obtained at an operating frequency within 10 cycles, with a tendency toward further growth as the number of rollers increases.
The calculations established that a drive with frequency converters is economically expedient for electric motors only up to and including the 7th frame size. Judging from the data obtained, the advantage of this variant is maintained until the excess cost of the electric motor goes beyond approximately 180–200 rubles.
The area of application of a geared drive with low-weight reducers (80–100 kg) may expand, since the competing variant usually has an anticipatory increase in costs due to the more powerful electric motors required by the condition of obtaining identical energy and technological effects.
The main component that reduces the economic effect of the geared drive is the capital cost of the liquid-lubrication system. It is important to note the small influence on the economic efficiency of all drive modifications with frequency converters of the electric-motor design, the operating mode, the value of PV% and the working-time fund.
The calculations carried out for variants with low-speed electric motors make it possible, with a high degree of probability, to conclude that the transition to the indicated scheme is always expedient if the variant being replaced has electric motors no larger than the 6th frame size. This follows from the fact that, for electric motors up to the 7th frame size, the weight reserve needed to reduce the speed proved to be very substantial (330–440 kg), with a very small influence on this indicator from the total number of rollers taken into account. It is not excluded that in individual cases it will be economically advantageous to replace drives with electric motors of the 7th frame size as well, since the weight reserve in this case also reaches almost 260 kg if the number of rollers is no more than 600–700 units. With a further increase in the number of rollers, the indicated weight reserve drops sharply. But in all cases the economic effect from reducing the size of such electric motors will be the greater, the smaller the unit weight of the machine being replaced.
As a result, the following conclusions may be drawn:
-
For a wide range of production conditions, an individual drive for roller-conveyor rollers with frequency converters has substantial economic advantages over a drive with a reduction gear.
-
The use of low-speed electric motors of industrial frequency is highly promising if they are designed with allowance for limiting weight standards based on the minimum of “calculated costs.”