Wyniki wyszukiwania - Biblioteka Nauki

1

Wyznaczanie prądu rozruchowego i momentu rozruchowego dla silników asynchronicznych

100%

Zieliński W. G.

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tom Nr 80

147-150

EN

Calculation of the starting parameters for three-phase asynchronous motors is a very important issue as these parameters determine whether a motor can be satisfactorily used in a driving system and characterize it from the point of view of supply mains. The article discusses methods of determining initial starting current and initial starting torque with the omission of issues connected with electromagnetic transient processes, the influence of thermal states, and the effect of parasitic torques. Direct determination of these parameters is relatively complex, which is why they are often determined in an indirect way. Two methods are described, which allow to establish the aforementioned parameters: the tangent method and the short-circuit reactance distribution method. The methods consist in an extrapolation of short circuit characteristics obtained at reduced voltage. The results of research and the comparison of parameter calculations refer to two induction motors: squirrel cage and slip ring.

2

Racjonalna praca silników asynchronicznych klatkowych przy zmianie skojarzenia uzwojeń z trójkąta w gwiazdę

100%

Zieliński W. G.

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tom Nr 77

125-128

EN

Three-phase asynchronous motor with a squirrel-cage rotor that is the most often applied in the industry has its disadvantages, which get revealed at a low-load motor operation or at its idling. High current input and low power factor at idling are among the disadvantages. Electrical energy saving is possible owing to the increased energy indices such as efficiency or power factor. The power factor value to a considerable degree depends on the load. Hence, in cases when the load varies within a wide range an adequate voltage reduction causes its increase. When voltage is reduced such quantities as: torque overload capacity, slip, rotor current and total power losses increase. The motor supply voltage can be varied in a continuous or a step mode. In some cases in order to considerably enhance energy indices and obtain significant energy savings it is enough to apply a two-step voltage regulation. It is mostly when the time-load run of the motor approaches intermittent running with idling breaks. The paper presents a solution that makes possible to enhance energy indices that is to make the motor operation more efficient with the application of a delta-to-star change of the winding connection. The method consists in that at the startup and full-load phase the motor operates at the delta connection of the windings, while at lower load values a switch to the star connection is performed. Additionally, runs of quantities that are characteristic for an efficient motor operation (power factor, efficiency, slip, current input) and have been obtained based on laboratory measurements performed at the delta-to-star switch of the stator winding connection have been determined.

3

Metoda wyznaczania rezystancji wirnika z uwzględnieniem wypierania prądu dla silników szybkoobrotowych

63%

Zielińska M. J. , Zieliński W. G.

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tom Nr 80

155-157

EN

High speed motors are supplied with 400-1500Hz frequency voltage. They operate in special conditions and have a structure and properties that differentiate them from classic machines. In particular, they are distinguished by the high frequency of supply voltage, intensive cooling introduced to reduce the size of the motor, and closed rotor slots used to minimize vibrations and noise. In the discussed motors, the closed rotor slot was replaced with an open slot due to saturation of the magnetic circuit by the slot leakage flux. The resistance increase coefficient for rotor cage bar as a function of current flowing through the bar was estimated and then total rotor resistance was determined. Subsequently, the resistance calculated on the basis of geometric dimensions with skin effect taken under consideration was verified against results obtained from measurements. It was proved that short-circuit resistance is not a constant and depends on the value and frequency of current. It also includes a component which derives from additional losses occurring in solid construction elements and in rotor cage bars. The paper also presents descriptions showing changes in short-circuit resistance as a function of the motor's current determined on the basis of measurements with parametrically variable frequency.

4

Ocena oszczędności energii elektrycznej przy skokowej regulacji napięcia w silniku asynchronicznym klatkowym

63%

Zielińska M. J. , Zieliński W. G.

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tom Nr 77

121-124

EN

In order to enhance energy indices and obtain electrical energy savings for an asynchronous squirrel-cage motor operating on a long-time basis at a load lower than the rated load it needs to reduce its supply voltage. Efficient operation obtained that way can apply to a single motor or a set of motors driving the same machines. Motor load can vary within a wide range depending on the operation kind. Particularly disadvantageous is operation at little load or at idling, when energy indices of the motor are very low. In such a case it is necessary to apply voltage that can be varied depending on the load factor. When voltage is changed simultaneously with a load change the motor runs at higher values of the indices that decide over electrical energy consumption. At the rated load a motor should be supplied with rated voltage and when the load gets reduced the supply voltage should also be adequately decreased. The lowest voltage is required at the idle run. Relative value of the voltage is then of ca 0.3. Voltage reduction brings about a decrease of voltage-dependent energy losses and an increase of efficiency and the power factor, which leads to the power input reduction and consequently to electrical energy savings. The paper presents motor operation conditions that should be met at a sudden voltage change and rules for enhancing energy indices on the basis of algorithms that maintain such parameters as: minimal current, maximal power factor and maximum efficiency at a preset level. Additionally, runs of the basic motor quantities such as: current input, power factor, efficiency, and slip at step-regulated supply voltage have been determined

5

Wykorzystanie metody pośredniej do określania momentu krytycznego silnika indukcyjnego

51%

Zieliński W. G. , Zielińska M. J. , Zieliński W.

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tom Nr 80

151-154

EN

Pull-out torque and the corresponding pull-out slip of induction motors is determined, with a fair degree of accuracy, by measurements in the short-circuit and idle-running states. This refers to motors without the skin effect in the cage bar or those in which the phenomenon has been omitted. The paper compares analytical methods for determining pull-out torque based on the method of equivalent graphs and a simplified Kloss formula. The analysis was carried out for two induction motors: squirrel cage and slip ring. Pull-out slip and torque diagrams were used. An advantage of this method is the fact that determination of pull-out torque does not require the knowledge of pull-out slip.