ENERGY EFFICIENCY DETERMINATION OF LOADING-BACK SYSTEM OF ELECTRIC TRACTION MACHINES

Dep. «Electric Rolling Stock of Railways», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 31, e-mail afanasof@ukr.net Dep. «Electric Rolling Stock of Railways», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipropetrovsk, Ukraine, 49010, tel. +38 (056) 373 15 31


Introduction
Requirements of the relevant standards and regulations for repair of locomotives provide for acceptance testing of each newly manufactured electric traction machine or machine after repair [3].These tests are an important and integral part of technical process of production or repair of electric traction machine, the material costs for which are included in the cost of final product [10].
Technical quality control, which is conducted during the acceptance tests of electric traction machines, ultimately determines the reliability and dependability of the hauling plant as a whole and, consequently, the cost-effectiveness of railway transportations of mainline and industrial vehicles.
The loading-back systems (where the energy exchange between the tested machines takes place) provide high energy efficiency of the tests with relatively low total power of power supply sources.External power sources in such loading-back systems are needed only to cover the power losses in the tested electric machines [4].
Reduction of power consumption for acceptance post-repair tests of electric traction machines is one of the urgent problems on repair enterprises of traction rolling stock of mainline and industrial vehicles.Thermal tests of electric traction machines on the test bench of loading-back are the most energy-intensive part of the entire test program.Energy consumption for this type of tests can be reduced both by increasing the energy efficiency of the loading-back system and by optimizing the mode of loading of electric traction machines [11,13,12].

Purpose
The purpose of this research is to develop the methods for determining the energy efficiency of loading-back systems of electric traction machines.

Methodology
The term energy efficiency is generally understood as the rational use of energy resources during some technological process.
The most known index of energy efficiency of a device performing useful work is its efficiency coefficient.That is the ratio of this useful work to the energy consumption from the network.
Despite the fact that the loading-back system does not perform useful work, the fact of the load current flow of electric machines and rotation of their anchors is the purpose of loading-back process, namely, providing the tests of electric machines tests under loading [9].
Thus, the energy efficiency of the loading-back system can be considered as the ratio of the total energy of mechanical and electromotive forces providing anchors rotation and the flow of currents in tested electric machines to the total energy consumed during the test from the external network.
In general, the energy efficiency coefficient of loading-back system can be represented as where пол А и затр А are the useful and consumed energy accordingly.Consumed energy затр А does not require detailed explanation, but, nevertheless, one should note that it will be considered as the total amount of electricity consumed during the test of electric traction machines by all the sources of system from the three-phase AC network.
The issue of what should be considered as a useful energy consumed for the test requires de-tailed observation.If the purpose of loading-back is to ensure the load current flow and anchors (rotors) rotation, the useful power is the total power of all kinds of losses in the tested electric machines.Covering all the power losses in electric machines is the main purpose of energy sources in the system of loading-back [8].From this point of view, the useful power of sources of loading-back system is пол дг where дг P ∆ ∑ is the total power losses in the tested electric machines (engine and generator).
Then the energy efficiency coefficient of the loading-back system is ∑ -the total power consumed by the loading-back system from the network; 1 t -is the testing time.
Total power consumption P ∑ is consumed to cover the total power losses in the tested electric machines The energy efficiency of the loading-back system, as it was shown above, is the ratio of the total power losses in the tested electric traction machines meeting the parameters of adopted loading mode to the total power consumed by the power supply sources of loading-back system from the network.The nature of dependence of this indicator on the structure of the loading-back system can be determined by the universal scheme of energy transformations shown in Fig. 1.
This scheme is universal and takes into account all the possible options to cover the power losses in the tested electric machines [9]: -direct covering of electric losses; -indirect covering of electric losses; -direct covering of idling losses; -indirect covering of idling losses.The scheme provides the variants of electrical and mechanical power transmission in the main power conversion loop using converters of electric and mechanical power, respectively.
Electric power source ES compensates electric losses in loading-back system by direct process, when the electric power source ES' compensates idling losses by indirect way.The source of mechanical power MS covers idling losses in the system directly, when the source of mechanical power MS' covers electrical losses indirectly.All four possible power sources are connected to the network «N».
The tested engine is conventionally divided into two parts: Д and Д'.A conditional part Д is included into the main loop of power transformations, and the part D' is a conditional converter of ES' source electric power into mechanical power transmitted to generator G [5].The tested generator is also conditionally divided into two parts: G and G'.Conditional part G is included into the main loop of power transformations, when the part G' is a conditional converter of MS' source mechanical power into electric power transmitted to the engine E.
Transmission of power иэ P of ES source to the engine E is carried out directly, when the transmission of power иэ P′ of the ES' source to generator G is carried out through the engine E' and mechanical power converter and the MPC.Transmission of mechanical power д P from the tested engine E to the tested generator G is carried out through the MPC converter; when transmission of the electric power г P from the tested generator G to the engine E is carried out though the EPC converter.
Energy transformation loop, including the conditional part of engine E, conditional part of generator G, and the converters MPC and EPC is the main one.Total losses in the tested engine and generator caused by transformations in this loop, represent a useful power of the loading-back system.In accordance with the energy transformation scheme shown in Fig. 1, the power balance can be represented as Power losses in the sources and converters are determined by their efficiency and net power at the output required for the operation of loading-back system in the set mode [7].
Note that the diagram in Fig. 1 is universal and takes into account all possible energy transformations in the loading-back system.For a particular electromechanical loading-back system most of elements of the given energy scheme will be absent. The The energy efficiency coefficient of mechanical power им P′ transformation into electrical one г P′ in the generator G' can be represented as [1] г пр2 им It should be noted that the additional losses д P′ ∆ and г P′ ∆ are connected with energy process parameters in the main loop, i.e. they are dependent on д P and г P correspondingly.Also note that in almost all variants of loading-back systems at least one of the indirect compensation methods of at least one of the loss types in the tested electric machines is used [9].
Coefficients пр1 η and пр2 η together with efficiency of all power sources and converters determine the total energy efficiency of loading-back system эфс k .

Originality and practical value
It was introduced the concept and proposed the method of analytical determination of the energy efficiency of the loading-back system of traction electric machines.It differs by considering the efficiency of power sources and converters, as well as energy efficiency coefficient of indirect methods to cover losses.
The proposed evaluation methodology of energy efficiency of loading-back system can be used to solve the choice problem of rational circuit design of stands for acceptance testing of electric traction machines of mainline and industrial vehicles.

Conclusions
Energy efficiency of loading-back system is determined by efficiency of power sources and converters, as well as the energy efficiency of indirect methods of loss cover.The losses depend on the share of particular type of loss in the total power loss in the tested electric traction machine.
Reduction of electric energy consumption for electric traction machines testing can be achieved by means of both the choice of rational loading modes [6], and the optimization of the structure of loading-back system.
Minimization of total energy consumption for testing of electric traction machines can be achieved by reducing the number of successive power transformations in auxiliary devices or refusal from such transformations.The most rational seems to be a solution to cover all the losses by a single source of energy.This can be a source of both electrical and mechanical power [9] LIST OF REFERENCE LINKS

Fig. 1 .
Fig. 1.Universal scheme of energy transformations in the loading-back system of electric traction machines

4 P
are the powers, consumed from the network by the sources ES, ES', MS, MS' accordingly.