Analysis of the Results of Inspection and Examination of DC Machines and Electromechanical Converters under Operating Conditions
Abstract Generated abstract
This paper reports an operating-condition inspection of 920 DC machines and 102 electromechanical converters to assess their technical condition, service life, operating practices, documentation, and repair organization. The examination used equipment inspections, review of data sheets and failure logs, discussions with maintenance personnel, partial or complete disassembly, and measurements. The results indicate that armature windings were the least reliable component in the DC machines, with failures linked to interturn short circuits, winding breaks, commutator connection unsoldering, insulation degradation in aggressive or humid environments, bearing problems, contamination, and maintenance violations, while no converter failures were recorded. The study also finds that repair practices were often technically inadequate and economically unjustified for “P” series machines, since repair costs could exceed the list price by two to three times.
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ANALYSIS OF THE RESULTS OF INSPECTION AND EXAMINATION OF DC MACHINES AND ELECTROMECHANICAL CONVERTERS UNDER OPERATING CONDITIONS
A. N. NIMIROVSKII
(Presented by the scientific seminar of the Department of Electrical Machines)
The purpose of the examination and inspection was to determine the actual technical condition of electrical machines, the conditions and modes of operation, compliance with operating instructions, the degree of suitability of the accompanying documentation, etc. At the same time, the question of the technical level of the organization of repair-and-restoration work was studied, both in terms of quality and economy.
The examination process was carried out by inspecting electrical equipment, becoming acquainted with the data sheets, holding discussions with specialists servicing the electrical equipment, and studying entries in failure logs. At almost every enterprise where the products were inspected, it was possible not only to carry out an external inspection, but also to disassemble them completely or partially, with subsequent performance of the necessary measurements. In all, 920 DC machines and 102 electromechanical converters were examined.
The following results were obtained from the inspection of DC machines.
Only 16 machines had worked out their service life specified in the technical specifications (10–12 thousand hours). Three machines had operated for 16–17 thousand hours. Thirty machines failed without having worked out the assigned service life. 890 machines continue to operate. The inspection of 102 electromechanical converters gave the following results:
service life of less than 10,000 hours — 75 units;
service life of more than 10,000 hours — 13 units;
service life of more than 13,000 hours — 10 units;
service life of more than 20,000 hours — 2 units;
service life of more than 35,000 hours — 2 units.
There were no converters that had failed.
The main causes of failure of DC machines of the first and second frame sizes are interturn short circuits, breaks in the armature winding, and unsoldering of the points where the winding is connected to the commutator (7 cases were recorded).
In electric motors operated under conditions of prolonged exposure to an aggressive environment or under conditions of humidity exceeding the requirements stipulated in the technical specifications for these products, a reduction in insulation resistance is observed.
Motors operated under conditions equivalent to a marine climate work satisfactorily, and no decrease in insulation resistance was observed from the measurements carried out (the comparison was made with the values in the data sheets, entered—
…at the manufacturer’s plant). The insulation resistance of the windings is 100–500 megohms.
Cases were also noted of destruction of bearings on the drive side after approximately 1000–2000 hours, i.e., during the running-in period. It should also be noted that RGM-grade wires, serving as intercoil connections and also running from the coils to the traverse, lose their elasticity and become brittle. Sometimes, sagging and shifting under the influence of vibrational forces and the ventilation air flow, the lead ends touch the rotating parts of the armature, thereby causing failure of the machine.
During operation, considerable contamination by fibrous, oily coal dust is observed in the internal volume of the machine’s magnetic system, and especially at the inlet of the ventilation air flow. This fact should apparently be taken into account and considered in the design and calculation of machines, since machines calculated at the upper limits for overheating, because of deterioration in ventilation and heat-transfer conditions, may significantly reduce their service life and sometimes lead to failure.
No destruction of the enamel coating (PKÉ-19 enamel) of armatures and pole coils was observed. During sample inspections, no violations were found in the dimensions of the diameters of shaft journals or of the holes in bearing shields.
The glass-micanite frames installed between the frame and the pole coils, after more than 10 thousand hours of operation, showed fraying and contamination with coal dust. The fraying was caused by the action of the ventilation air flow.
During inspection of bearing units, isolated cases of corrosion were observed on the outer race of the bearing and, correspondingly, on the surface in the shield bore. This was probably caused by electrocontact erosion in the presence of bearing currents.
In addition to the above, certain characteristic violations by maintenance personnel of the instructions for operation of electrical equipment should be noted, which clearly reduces its reliability and durability. These violations amount to the following:
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Mixing of different grades of lubricants in bearing units is permitted (1-13, CIATIM-221, VNII NP-242, etc.).
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Replacement or addition of lubricant is carried out irregularly. For example, the P52M electric motor operated 13,270 hours on the lubricant that had been placed in it at the factory (replacement was supposed to be carried out after 4000 hours).
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During preventive inspection of machines, the installation of different grades of brushes and of different dimensions in height in one and the same brushgear is permitted, which, as is known, leads to increased wear of the commutator.
In the course of examining the products, special attention was also paid to the level of organization of production of repair-and-restoration work on electrical equipment. It was established that even specialized shops for repairing electric motors do not have the full complement of technological equipment necessary for high-quality repair work. The standard of repair-and-production work is also at a low level. After repair, tests are not carried out in full and, as a rule, are performed at no load. After repairs have been performed, electrical shops give no guarantees of repair quality. The total costs incurred by repair shops at various enterprises in different regions of the country for machines of the “P” series amount to from 104% to 380% of the list price of the product.
Conclusions
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In DC machines, the armature winding has the lowest reliability.
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Repair of electric machines of the “P” series, frames I–VI, even when carried out at specialized enterprises, should be considered economically unjustified, since the cost of repair exceeds the list price of these products by a factor of 2–3.