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Abstract

Content

Introduction

Efficient use of energy is one of the most important problems of the national economy. Its solution will reduce the consumption of energy and material resources in the production of industrial and agricultural products, reduce the large unproductive expenditures of the state and the population in the field of housing and communal services, and improve the environmental situation in the country. An important role in solving this problem is played by the electric drive, which is the main consumer of electrical energy.

In the general case, energy saving can be carried out both in the EC itself and in the technological processes it serves, where the mechanical energy produced by it is used. At the same time, the use of a regulated ES allows for the implementation of many technological processes, energy saving, sometimes many times exceeding the energy savings in the ES itself.

For example, the regulation of the speed of the conveyor belt due to the EP, which supplies parts to the quenching furnace, allows minimizing the amount of thermal energy for quenching, depending on their grade, quenching technology, and other factors. Highly speed &ndasp; controlled electronic coding can provide energy savings in working machines such as pumps, fans and compressors. Since these working machines are widely used in industry, in transport, in agriculture and in housing and utilities, consuming, according to various estimates, 30–40% of the generated electricity, energy saving in this area by means of ES is very effective.

1. Theme urgency

A very important point is to reduce energy costs. This is especially true of large enterprises, where costs are mainly carried out with the supply of electric motors. The use of frequency converters solves some of the problems associated with power consumption, but there are certain features. In most cases, using static frequency converters using pulse width modulation (PWM). The efficiency of modern IF is about 95%. The use of PWM introduces additional harmonic components, the presence of which adversely affects the performance and efficiency of the electric motor. Thus, the inverter affects the characteristics of blood pressure and interferes with the power supply. As a result, the efficiency (EFF) of the HELL connected to the inverter decreases. The presence of harmonics mainly increases electrical losses in copper. The increase in losses will lead to an increase in the temperature of the engine and, as a consequence, reduces its efficiency. In connection with these features today the study of energy performance and further cost reduction is a hot topic today.

2. The aim and the tasks of the research, the planned result

The purpose of the master's work is to study the energy modes of an adjustable electric drive working from a frequency converter, and to determine the conditions for ensuring the most energy efficient operation.

Tasks:

  1. The study of literary sources on the energy of electric drives.
  2. Experimental study of changes in the efficiency of the converter and the engine under different operating conditions.
  3. Investigation of the effect of the converter on the network.
  4. Determination of methods for increasing the efficiency of the frequency converter-asynchronous motor system and checking their efficiency.
  5. Development of guidelines for setting up a frequency converter &ndasp; asynchronous motor system to ensure high energy performance.

3. A Review of Literary Sources

3.1 Energy performance of the electric drive based on the frequency converter

The share of the electric drive accounts for about 70% of the total electricity generated. Therefore, the efficiency of the use of this electricity is of great technical and economic importance. Power to the electric drives (with the exception of the drives of transport or mobile machines) comes from an industrial AC network with a frequency of 50 Hz. Electric drives consume (and when operating in regenerative braking mode and give) active power from the network. Active power is expended on useful work and loss coverage throughout the entire electromechanical system of the working machine. Analyzing the efficiency of use of electrical energy, one should distinguish between the energy efficiency of the technological process itself, which is carried out by the working machine with electric drive, and the efficiency of the drive itself, characterized by its efficiency, which is the ratio of the output power Pout of this device to the input power Pinp, or the ratio of useful power Pfull (or energy) to the spent Pspent [1], [2]:

Formula – The ratio of output power to output power

where ΔP – is the loss in this device, Formula – Losses in the device in question

Since the power part of the electric drive consists of an electromotive, transmission and converter device, the efficiency of the electric drive.

Formula – Electric drive efficiency

In asynchronous motors with a power higher than 0.1 kW, the nominal efficiency is 0.85…0.9. With an increase in power, the nominal efficiency increases, and for large high-speed AC motors with a power of over 1000 kW, it can reach 0.97. Efficiency of electric motors significantly depends on the load on the motor shaft. For the analysis of this dependence, the method of separation of losses of the ΔP into constant K and variables V is used:

Formula – Analysis of efficiency depending on the load

For unregulated engine speeds, the permanent losses include [1]:
• loss in steel;
• mechanical losses, including self – ventilation;
• additional loss.

Variable loss V is calculated by the following formulas:
– for DC machines – V = Iя2 Rя;
– for asynchronous motors – V = 3I12r1 + 3I22r2;
where 3I12r1 – rub in the stator winding; 3I22r2 – losses in the rotor winding.

Losses in the rotor circuit

Formula – Losses in the rotor circuit

Roughly, we can assume that the losses in the stator windings are related to the losses in the rotor windings in proportion to r1/r2. Then the variable loss for asynchronous motors:

Formula – Variable Loss for Asynchronous Motors

Apparently, when working with part-load engine efficiency decreases. A typical curve for the efficiency of an induction motor as a function of load is shown in fig. 1. From fig. 1 that the overestimation of the installed engine power leads to a decrease in its operating efficiency, i. e. to unproductive power consumption. The efficiency of a converter device made on the basis of power semiconductor devices is quite high. The losses in the converter are mainly determined by the direct voltage drop in the semiconductor device. On average, we can assume that U = 2 V; for bridge circuits, U = 4,0 V.

Figure 1 – The dependence of the efficiency of the induction motor from the load

Figure 1 – The dependence of the efficiency of the induction motor from the load

Thus, the nominal loss for converters with a voltage of 440 V is 1%, and for converters with a voltage of 220 V – 2%. Taking into account the losses in the reactive elements of semiconductor converters, their efficiency can be taken as 0.95…0.98. Losses in mechanical transmission devices (gearbox, transmission, etc.) are determined mainly by friction forces. These losses, and, consequently, the efficiency of mechanical transmission depend on the type of bearings used, the processing class of gears, lubrication systems, etc. The efficiency of mechanical transmission essentially depends on the transmitted moment. Under the efficiency of the working machine (WM) understand the product of the efficiency of the electric drive nap by the efficiency of the working machine. So, for fan installation

Formula – Efficiency for fan installation

where ηfan.inst – aerodynamic efficiency of the fan; Q – fan performance, m3/s; H – head; Pspent – consumed electrical power, kW.

In the AC network, from which power is supplied to the electric drive, reactive power circulates, as a result of which the supply network is loaded with reactive current that does not create work. Reactive power is estimated to be cos φ, where the angle φ is the phase shift of the first harmonic of the current relative to the first harmonic of the voltage. In asynchronous squirrel cage motors, the nominal cos φ = 0.7…0.8. Underloading the induction motor leads to a further reduction in cos φ. In drives according to the system TC – M cos φ = cos α, which is determined by the delay, established by the system of pulse – phase control, opening of thyristors. Therefore, in drives TC – M at high speed cos φ in the AC power network will be high (0.8…0.9), as the speed decreases, as α increases, cos φ will decrease. When the TC – M drive is turned on, reactive power surges occur. In modern systems of adjustable electric drive, they seek to use unmanaged rectifiers, regulating the magnitude of the voltage supplied to the motor windings by pulse width methods. In this case, cos φ in the power supply will not be below 0.95. From the point of view of reactive power compensation of many electric power consumers, it is effective to use high power synchronous motors for unregulated electric drives, which, when overexcited, are capable of generating reactive power for its compensation in the enterprise power system.

3.2 Ensuring maximum achievable efficiency on the manufactured frequency converters.

For motors operating at full load, lowering the voltage results in a decrease in the speed. If the performance of the mechanisms depends on the engine speed, then it is recommended to maintain a voltage not lower than the rated voltage at the terminals of such engines. With a significant decrease in voltage, the moment of resistance of the mechanism may exceed the torque, which leads to the engine tilting, ie, to stop it. To avoid damage, the engine must be disconnected from the mains [4].

If the engine runs for a long time at a reduced voltage, then due to the accelerated wear of the insulation, the service life of the motor decreases [5]. Therefore, from the point of view of engine heating, negative voltage deviations are more dangerous in the considered limits.

For the analysis of various indicators of electrical equipment with him conducted instrumental studies, which give the following results.

In the process of research, the effect of frequency, voltage and temperature on the power consumption and operating characteristics of blood pressure is being studied [3].

The output values include: voltage, current, mains frequency, active and apparent power, efficiency of a three – phase inverter; voltage, frequency, torque on the shaft, linear currents, cos φ, rotational speed, the input active and useful power on the shaft, the efficiency of asynchronous motor (AM).

As the object of the study used AM with a rated power of 60, 1100, 1700 watts.

I. Comparison of the characteristics of an asynchronous motor when changing voltage and frequency using a three-phase inverter and synchronous generator (SG) [3].

Based on the results of studies given in [3], various hypotheses are put forward about the type of regression dependence between variables in order to select a regression equation. Then there are some results of single and multiple regression established during the conducted research:

– hhe efficiency of an asynchronous motor and the efficiency of a three-phase inverter do not significantly change with increasing voltage at fixed loads on the shaft;

– a change in frequency in the range from 45 to 52 Hz at fixed loads on the shaft significantly affects the efficiency of AM (η = – 0,002 ηf3 + 0,330 f2 – 16,11 f + 262,4; RI = 1)creating a local minimum at a frequency of 47 Hz, and a maximum at 51 Hz;

– the effect of frequency at the output of the inverter on its efficiency is also observed:

η = – 0,005 f 3 + 0,778 f 2 – 38,77 f + 643,6; RI = 1;

– elimination of the components (step-by-step regression analysis in the Statgraphics program) insignificantly worsens the predictive capabilities of the regression equation (the coefficient of determination has decreased). In this case, the value of the reduced coefficient of determination increases.

II. Analysis of results for voltage and frequency changes using a three – phase inverter [3]:

– performed multiple correlation and regression analysis in the software product Statistica for the dependence of the no load current on the line voltage and frequency.

The information part of the window indicates the following analysis parameters:

– coefficient of multiple correlation R = 0,99726;

– the coefficient of determination, showing the proportion of the total variation (relative to the sample mean dependent variable), which is explained by the constructed regression R2 = 0.9945;

III. When the load of the motor changes, both the current I1 and the power P1, and the rotor speed n2, slip s, efficiency η and cos φ1 change. The dependencies n2, s, М2, I1, cos φ1, η and 1 from P2 with U1 = const and f1 = const are called asynchronous motor performance data. Their approximate form (there were differences for different capacities) for significantly varying performance compared to natural [6] (solid lines) for AM is shown in Figure 2:

a (dotted line) – a tendency to their change with decreasing voltage with the help of an inverter;

b (dots) – with the help of SG of relatively low power;

c – are indicated when the inverter frequency decreases.

Рисунок 2 – Примерный вид значимо изменяющихся рабочих характеристик АД

Figure 2 – An approximate view of significantly changing the performance of blood pressure: a – when the voltage changes using a three-phase inverter; б – when voltage changes with the help of SG; в – when changing the frequency using an inverter
(animation: 6 frames; 7 cycles; 69 kilobytes)

Conclusion

  1. Asynchronous motor, powered by PWM voltage, has lower efficiency than when powered by sinusoidal voltage, due to increased losses caused by harmonics.
  2. During AM operation from frequency converters, the efficiency of the system as a whole, and not just the electric motor, should be evaluated.
  3. Each case must be properly analyzed taking into account the characteristics of both the motor and the converter, taking into account the following parameters: operating frequency, switching frequency, speed range, load and motor power, total harmonic distortion, etc .
  4. An increase in the switching frequency increases the efficiency of the motor and reduces the efficiency of the inverter (due to the increase in losses at the switching of the power switches).

This master's work is not completed yet. Final completion: June 2019. . The full text of the work and materials on the topic can be obtained from the author or his head after this date.

References

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