SU1483544A2 - Device for protection of three-phase electric motor - Google Patents

Abstract

This invention relates to relay protection of electrical installations. The aim of the invention is to increase reliability by ensuring that the sensitivity of the overload in any of the phases is equalized. The goal is achieved in that the device contains two measuring current transformers 2 and 3 with three primary windings with a ratio of the number of turns 2: 2: 1. The first winding of each transformer 2 and 3 is connected in opposite to the second and third windings. The first winding of the first transformer 2 and the third winding of the second transformer 3 are included in the first phase, the second windings of both transformers 2 and 3 in the second phase, and the third winding of the first transformer 2 and the first winding of the second transformer 3 are included in the third power supply phase of the electric motor 1. 4 Il.

Description

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The invention relates to relay protection of electrical installations, namely to phase-sensitive protection of three-phase electric motors against emergency and abnormal conditions, and is an improvement of the invention according to the author. No. 1317540.

The whole invention is to increase reliability by ensuring the sensitivity of the device when overloaded in any of the phases of the supply network.

Fig. 1 is an electrical schematic diagram of a device for protecting a three-phase electric motor; in fig. 2–4 are vector diagrams of magnetic fluxes and the corresponding secondary voltages.

A device for protection of a three-phase electric motor 1 contains the measuring first 2 and second 3 current transformers with three primary windings each, with a third winding of the first 2 and first winding of the second 3 transformers included in the first phase of the protected motor. Electric motor 1, the second winding of the first 2 and the second 3 transformers - into the second phase, and the first winding of the first transformer 2 and the third winding of the second transformer 3 - into the third phase. The first windings of transformers 2 and 3 are turned on oppositely by the second and third windings of it, with the first and second windings of each transformer with the same number of turns, and a third with the number of turns less than two times. Between the secondary terminals 4 and 5 of the secondary windings of the first 2 and second 3 current transformers, an executive unit 6 is connected, to the terminals 7 and 8 of the first 2 and the terminals 9 and 10 of the second 3 current transformers, an annular phase detector is connected, which is connected in a series four chains, each of which contains serially connected diodes 11-14 and resistors 15-18, while pin 7 of the secondary winding of the first transformer 2 is connected to the common point of the anode of diode 11 and resistor 16, pin 8 to the common point of the anode of diode 13 and stories 18, pin 9 of the secondary winding of the second transformer 3 is connected to the common point of the anode - of diode 14 and resistor 15, pin 10 - with a common anode of the diode 12 and the point 17. rezis torus terminals 9 and 10 of the secondary

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winding of the second transformer 3 is connected to a threshold overload control unit, made, for example, on diode 19, resistors 20, 21, capacitor 22 and dynistor 23 and its output connected to two phototristor optocouplers 24 and 25 connected in series-LEDs, the photothyristors of which bypass resistors 18 and 16 two opposite chains of the ring phase detector, with the anodes of the photothyristors connected to the common points of the cathodes of the diodes 14 and 12 and the corresponding resistors 18 and 16.

The sensitivity of the device in case of single-phase asymmetrical overload is ensured by the fact that each of the current transformers is equipped with three primary windings that are included in different phases. With this design, an increase in any of the phase currents leads to an increase in the voltages on the secondary windings of both transformers. As a result of the increase in voltage, an overload control unit is activated, connected to the output of one of the transformers. This leads to the shunting of the resistors of the two opposite arms of the annular phase detector and to the actuation of the actuator as a result of the deep imbalance of the annular phase detector. In the known device, the current transformers contain only two primary windings, and the overload control unit is connected to the secondary winding of one of the transformers (for example, the second). Therefore, the overload control unit operates with single-phase overload only in those phases in which two primary windings of the second transformer are included, and does not work with single-phase asymmetrical overload in the third phase, which includes one of the windings of the first transformer.

By choosing the ratio of the number of turns of the primary windings of transformers 2: 2: 1, connecting the first windings oppositely to the second and third windings, and also by turning the windings into different phases of the motor supply, the formation of identical secondary voltages is obtained with a normal shift between them of approximately 90 ° in a normal symmetric mode (figure 2). The formation of such an angle between secondary voltages ensures stable operation of the actuator when the phase is broken, the rotor is wedged, and symmetric and asymmetrical overloads occur. Moreover, by choosing the ratio of the number of turns to 2: 2: 1, the difference in sensitivity by phases is reduced by about 1.5 times (as compared with the prototype.). This is explained as follows. For the prototype with the ratio of the number of turns 3: 1, the secondary voltage increases by 20% when the current increases, in proportion to which the magnetic flux f in the first phase changes by 27% or when the current in the second phase increases by 90 % (fig.Z). In the proposed device, a similar increase in the secondary voltage occurs when the current in the first phase increases by 100% or when the current in the second and third phases increases by 50 or 47%, respectively (Fig. 4). The difference in sensitivity in phases can be characterized by the ratio of the currents in question, which will be

90% for the prototype - 3.34, and for up to 100%, the lagged device is 2,13.

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Thus, in the proposed device is achieved reduced in

3 34

l 1.57 times the difference in phase sensitivity.

The device works as follows.

Since each of the measuring current transformers is equipped with three primary windings, which have a different number of turns and are included in different phases of the supply network, the first winding being connected oppositely to the second and third, the magnetic fluxes F, F2 / ff are summed in the cores of each current transformer created by the respective currents of the three phases (figure 2). When the ratio of the turns of the primary windings of the current transformers W -2: 2: 1 and the normal operation of the motor, the voltage U and U on the secondary windings of the transformers are equal in magnitude and shifted by an angle of about 90 ff.

When one of the phases of the supply network is broken, the voltages U1 and U are formed on the secondary windings of current transformers, which coincide in phase or are directed in opposite directions.

The executive body 6 (FIG. 1) works in this case, similarly to the prototype, due to the cosine characteristic of the ring phase detector. With symmetric overloads, the shear angle between the U and U voltages does not change, and the magnitude of the voltages increases, which triggers the overload control unit and shunts the resistors 16 and 18 of the ring phase detector. In this case, the actuator 6 operates in the same way as the proto5 type due to the unbalance of the annular phase of the detector.

In case of asymmetrical single-phase overloads, when a failure of the prototype is possible, the proposed device will ensure the shutdown of the electric motor, since an increase in any of the phase currents causes an increase in the secondary voltage and the triggering of the threshold control unit overload,

5 which leads to the shunting of the resistors 16 and 18 of the annular phase detector and to the response of the actuator 6 as a result of the unbalance of the annular phase detector.

0 The difference in sensitivity by phase, characterized by the ratio of the number of turns in the primary windings of the three phases will correspond to the ratio 2: 2: 1.

Thus, the proposed device, unlike the prototype, will provide protection for an electric motor in case of unacceptable asymmetric single-phase overload and will have a phase difference of about 1.5 times lower than the sensitivity.

Claims (1)

Invention Formula 5 Device for the protection of a three-phase electric motor according to auth.St. No. 1317540, characterized in that, in order to increase reliability by ensuring the sensitivity is equalized when overloaded in any of the phases, each of the current transformers contains an additional primary winding having terminals for inclusion in the corresponding free phase power 5 motor and containing two times fewer turns than the first and second primary windings of each of the current transformers, while the additional primary winding of each current transformer is pre-corresponding to the second primary is connected to turn on according to the coil. 2 g, gi J.2U Fi.Z F,