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Abstract on the topic of graduation work

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Introduction

In modern control systems of both direct and alternating current motors, the most accurate determination of their energy and mechanical parameters is necessary for the implementation of the most high-quality and economic control and regulation[1]. It is not always possible to determine the parameters of the engine from the passport data, and if they are only basic information, such as: rated power , rated voltage , rated current , rated frequency and rated speed . For the correct regulation of the electric drive perevodchikam need to have more extensive knowledge about the machine, such as: active resistance stator and rotor inductance of stator and rotor, the flux linkage of the stator, the rotor, which cannot be found in reference materials. This knowledge plays a significant role in modern control systems of electric drives, so there is a need to accurately determine these parameters by calculation or in some other way. Currently, a huge popularity among electric drive control devices are (IF< / abbr>), which calculate the necessary parameters of the replacement circuit automatically according to pre-laid formulas and commands, thanks to which these devices are the best option in the world at the moment in terms of control and regulation of AC motors.

Depending on the different methods of calculating the parameters of the replacement circuit, absolutely different values can be obtained, so it is very important to choose the appropriate method for each engine individually[2].

1. Experimental determination of parameters and calculation of operating characteristics of asynchronous motors

1.1. Basic concepts

There are two methods of obtaining data for constructing the performance characteristics of asynchronous motors: the direct load method and the indirect method. The direct load method consists in an experimental study of the engine in the load range from idle to rated load mode with the measurement of the necessary parameters. This method is usually used for engines with a power of no more than 10-15 kW. With the growth of engine power, the task of its load becomes more complicated, unproductive power consumption and power grid load increase. The application of this method is also limited by the fact that it is not always possible to create a test facility due to the lack of the required equipment and the inadmissibility of overloading the power grid. A more universal indirect method is widely used, the use of which is not limited to the engine power. This method consists of performing two experiments: the no-load experience and the short-circuit experience.

These experiments are easily implemented, and they require a minimum power of the installed equipment. Experimental studies of asynchronous motors in operating modes, as a rule, are not carried out due to their high labor intensity and cost. Characteristics of asynchronous motors in operating modes are obtained by calculation according to the substitution scheme.

1.2. Idle experience

The asynchronous motor is powered in the experiment of H. H. through (IR < /abbr>) voltages (Fig. 1.1) or (AT< / abbr>), which allow you to change the voltage over a wide range, you can also use (TRN) or (NPF)[3]. In this case, the motor shaft must be free from mechanical load. In the scheme there are also measuring devices: ammeter, voltmeter and wattmeter, to remove the necessary physical quantities.

Scheme of switching on a three-phase asynchronous motor in the experiments of H. H. and K. Z..

Figure 1.1-Diagram of switching on a three-phase asynchronous motor in the experiments of H. H. and K. Z..

The experience begins with an increased supply voltage

Then gradually lower the voltage to 0.4 u1nom so as to take readings at 5-7 points. In this case, one of the measurements must correspond to the rated voltage U1nom. Measure linear values of voltages and currents and calculate their average values:

And then, depending on the connection scheme of the stator winding, the phase values of the voltage and current x x are determined.:

When connecting to a star

When connecting to a triangle

The wattmeter W measures the active power Ro consumed by the motor in the Xh mode, which includes electrical losses in the stator winding, magnetic losses in the stator core Pm and mechanical losses Pmex (W):

Here r1 is the active phase resistance of the stator winding (Ohms), measured immediately after disconnecting the motor from the mains so that the winding does not have time to cool down[4].

Sum of magnetic and mechanical losses of the motor (W)

The power factor for the regime of H. H.

Based on the results of measurements and calculations, the characteristics of xh are built.

Characteristics of an induction motor (3.0 kW, 220/380 V, 1430 rpm))

Рисунок 1.2 – Characteristics of an induction motor (3.0 kW, 220/380 V, 1430 rpm))

For asynchronous motors with a phase rotor, the voltage transformation coefficient between the stator and rotor windings is determined in the experiment of H. H. This coefficient can be determined with sufficient accuracy from the ratio of the arithmetic mean linear (interphase) stator voltages to similar rotor voltages.

1.3. Short circuit experience

The connection diagram of the asynchronous motor in the experiment of K. Z. remains, as in the experiment of H. H. (see Figure 1.1)[5]. But at the same time, the measuring devices must be selected in accordance with the limits of measuring current, voltage and power. The motor rotor should be rigidly fixed, having previously set it to the position corresponding to the average current of the kz. For this purpose, a small voltage is applied to the motor (Uk=0. 1u1nom) and slowly turning the rotor, monitor the reading of the ammeter, the arrow of which will fluctuate depending on the position of the motor rotor. This is explained by the mutual displacement of the tooth zones of the rotor and stator, which causes fluctuations in the inductive resistances of the motor windings.

The limit value of the stator current during the short-circuit test is set based on the permissible current load of the supply network and the possibility of conducting the experiment in the minimum time so as not to cause dangerous overheating of the engine. For engines with a power of up to 1 kW, it is possible to conduct an experiment starting from the rated voltage Uk=U1nom. In this case, the limit current Ik=(5 & ndash;7) Inom. For higher-power motors, the maximum current strength Ik=(2.5 & ndash;5) i1nom. When performing the experiment of KZ for educational purposes, you can limit yourself to the limit current Ik=(1.5 & ndash;2.5) I1nom. When performing the experiment of kz. it is desirable to connect the stator winding with a star.

Defining the range of variation of stator current with the experience of K. h., experience start with the limit values of this current, setting on an induction regulator voltage matching circuit. IR. Then gradually reduce the voltage to a value at which the current I reaches the lower limit of the specified range of its values. At the same time, the readings of the devices are taken for 5-7 points, one of which must correspond to the rated current of the stator (Ik=I1nom). The duration of the experience should be as short as possible. For this purpose, only one linear voltage is measured (for example, UKAV), since some asymmetry of linear stresses does not matter in the KZ experiment. Linear currents are measured in at least two linear wires (for example, IKA and IKV). The arithmetic mean of these two values is taken as the calculated value of the current. After taking the last readings of the instruments, the motor should be switched off and the active resistance of the stator winding phase should be measured immediately to determine the winding temperature. Linear voltages and currents are converted to phase Uk and Ik according to formulas similar to the experience of X. x.

The W wattmeter measures the active power of the Rc. Based on the obtained values of voltages Uk, currents Ik and powers Pk, the following parameters are calculated:

power factor at short-circuit.

total resistance of the short circuit.

active and inductive components of this resistance (ohms)

The measured and calculated values are entered in the table, and then the characteristics of the KZ are built (Fig. 1.3).

Characteristics of KZ on the example of an asynchronous motor (3.0 kW, 220/380 V, 1430 rpm)

Figure 1.3-Characteristics of an asynchronous motor (3.0 kW, 220/380 V, 1430 rpm)

During the short-circuit test, the motor windings quickly heat up to the operating temperature, since the motor is not ventilated when the rotor is stationary.

The winding temperature is usually determined by the resistance of the base, measured immediately after the experiment, according to the formula:

If the winding temperature is less than the calculated operating temperature for the corresponding class of heat resistance of the motor insulation, then the active resistance of the kz. rk is recalculated to the operating temperature:

Then the total resistance of the short circuit is recalculated to the operating temperature.

voltage short-circuit.

and the power of the kz.

The current and power of the circuit are converted to the rated voltage U1.

It should be borne in mind that such a recalculation is approximate, since at Uk = U1, the magnetic saturation of the cores (especially the tooth layers) occurs) stator and rotor; this leads to a decrease in the inductive resistance of the hc, which is not taken into account by the formulas.

The electromagnetic power in the KZ mode transmitted to the motor rotor is equal to the electrical losses in the rotor winding, so the electromagnetic moment during the kz experiment.

The magnetic losses in the KZ experiment are approximately determined by the characteristics of KH at a voltage U1=Uk. In the regime of H. H. the magnetic flux f more than in K. s., but if H. H. the magnetic losses occur only in the stator core, the mode indicator circuit. magnetic losses occur even in the rotor core, as f2=f1.

The initial starting torque is obtained by converting the moment Mk to the initial starting current Ip.

2. Determination of parameters of the asynchronous motor replacement circuit according to reference data

In the most complete reference books on asynchronous motors, the physical quantities necessary to determine the parameters of its replacement circuit are given[6].

The rated current of the motor stator is determined:

The basic resistance is calculated:

The parameters of the engine replacement circuit are found in physical quantities:

Active resistance of the stator winding:

The inductive reactance of the scattering of the stator winding:

Active resistance of the rotor winding, reduced to the stator winding:

Inductive scattering resistance of the rotor winding reduced to the stator winding:

Inductive resistance of the magnetization loop:

The parameters of the equivalent circuit allows to calculate the static characteristics of the asynchronous motor, for example by the formula Kloss, i.e. the saturation of teeth from stray fields and the displacement current in the squirrel cage rods.

3. Calculation of substitution scheme parameters by known methods

In Ulyanovsk State Technical University, to determine the parameters of the AD replacement scheme, a technique is used that allows you to determine the parameters of the replacement scheme under the following basic assumptions[7]:

–magnetic and mechanical losses in the engine are 0.02 hr n;

–the active resistances of the stator and rotor windings are assumed to be independent of the operating mode of the motor, i.e. the effects of current displacement are not taken into account.

Оthe idling current of the asynchronous motor is determined:

where I11 is the motor stator current at partial load, and

Active resistance of the rotor driven to the stator winding of an asynchronous motor:

где β – coefficient in the range 0,6–2,5;

Active resistance of the stator winding:

The inductive reactance of the short circuit:

Inductive phase dissipation resistance of the rotor winding:

Inductive phase dissipation resistance of the stator winding:

EMF of the magnetization branch induced by the air gap flow in the stator winding in the nominal mode:

Inductive resistance of the magnetization loop:

The parameters of the equivalent circuit allows to calculate the static characteristics of the asynchronous motor, for example by the formula Kloss, i.e. the saturation of teeth from stray fields and the displacement current in the squirrel cage rods.

Critical moment:

A convenient calculation method is presented in the work of Chepkunov R. A. " Determination of the parameters of the asynchronous motor replacement circuit according to the passport data when commissioning an asynchronous electric drive»

The calculation of the stator resistance:

The total relative energy losses in the stator and rotor in relation to the electricity consumed are equal:

The electrical power consumption is equal to:

The useful mechanical power on the motor shaft is equal to

The ratio of mechanical PM to electromagnetic PM is equal to the ratio of the rated motor speed to synchronous:

Hence the electromagnetic power is equal to:

The power loss in the stator is equal to the difference between the electrical and electromagnetic powers:

From here is the stator resistance:

However, the value of RS will be somewhat overestimated, since the electrical power of PE includes the loss power in the magnetic circuit, which is not taken into account in the substitution scheme.

Calculation of the scattering inductance:

The inrush current is equal to

where is ki – multiplicity of the starting current.

At the time of start-up, a current flows through the engine, many times higher than the idle current. Therefore, the influence of the magnetization loop is negligible and the replacement circuit is reduced to a series connection of the stator resistance RS, the rotor resistance RR and the scattering inductance LX. Such a scheme can be described by the equation:

From here:

Since the rotor resistance has not yet been determined, it is assumed to be equal to the stator resistance.

Calculating the rotor resistance:

The expression for EMF can be rewritten as:

Square EMF:

The voltage drop on the rotor is equal to:

The power loss in the rotor is equal to the difference between the electromagnetic and mechanical powers:

Сопротивление Ротора:

Calculate the magnetization inductance:

The reactive power balance equation is represented as:

Hence the inductance of the magnetizing:

Critical moment:

Conclusion

Based on the above methods, we can conclude that all methods differ from each other and when calculating the parameters of an asynchronous motor using these methods, we will get different values. This option is unacceptable, because for high-quality control, you need the most accurate engine parameters. Therefore, our task is to develop a method for calculating the parameters of asynchronous motors of various capacities and speeds, which would give the most accurate results.

Source list

  1. Мощинский Ю.А., Беспалов В.Я. Кирякин А.А. Определение параметров схемы замещения асинхронных двигателей по каталожным данным // Электричество.– 1998.– №4. – С. 38–42.
  2. Кувшинов А. И. Идентификация параметров схемы замещения асинхронного электродвигателя на основе экспертных оценок / А. И. Кувшинов, Н. И. Муха, О. А. Онищенко // Автоматизация судовых технических средств. – 2007. – Вып. 12. – С. 78–85.
  3. Усольцев А.А., Лукичев Д.В. Определение параметров модели асинхронного двигателя по справочным данням / Усольцев А.А., Лукичев Д.В. – Известия высших учебных заведений. Приборостроение. – 2008.– №10. Том 51– С.35–41.
  4. Ромашихин Ю.В., Родькин Д.И. Методы определения параметров машин переменного тока. // Вісник КДУ імені Михайла Остроградського. Випуск 4/2010 (63). Частина 3. – С. 140–143.
  5. Островский А.В. Безитерационная методика определения параметров схемы замещения асинхронного двигателя. // В сб. Праці ТДАТУ, 2012 вип.. 12 Т2. С. 66–72.
  6. Гридин В.М. Расчет параметров схемы замещения асинхронных двигателей по каталожным данным / Электричество.– 2012. № 5, С. 40–45.
  7. Макеев М.С., Кувшинов А.А. Алгоритм расчета параметров схемы замещения асинхронного двигателя по каталожным данным. // Вектор науки Тольяттинского государственного университета Выпуск № 1 (23). – 2013.– С. 108 –112.