RU2400004C1 - Device for protection of three-phase electric motor against open-phase mode - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: device comprises filter of negative sequence voltage, containing resistors, the first and second capacitors, two current reed relays, two current transformers and components included into responder - rectifying bridge, the third and fourth capacitors, additional resistor and threshold element, besides primary windings of current transformers are connected into two phase wires, to secondary windings there are windings of current reed relays connected, closing contacts of which are connected in parallel to resistors of negative sequence voltage filter, to output of which rectifying bridge is connected, to bridge output the following components are connected in parallel - the third capacitor, additional resistor, and via threshold element shunted with the fourth capacitor, actuating element, contact of which is shunted in circuit of magnetic starter coil, besides in phases of supply line there are three transformers installed, which convert current into voltage, and are connected to inlets of negative sequence voltage filter, accordingly.

EFFECT: improved reliability of device operation as a result of actuation in case of supply grid phase rupture downstream area of negative sequence voltage filter connection.

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Description

The invention relates to electrical engineering, and more specifically to the protection of electric motors from asymmetries of supply voltage, as well as from an increase in current in phases during overloads, can be used to protect vehicle generators and power lines.

A device for protecting an electric motor is known, which contains a negative sequence voltage filter, three resistors are connected to its inputs, one of which is temperature-dependent and has a direct connection with the motor winding, and the second resistor leads are connected to the junction of two coils of which each phase motor winding consists [ one].

The disadvantage of this device is the complexity of installation due to the need to connect the motor windings with a voltage filter of the negative sequence.

Closest to the image in technical essence is a device for protecting consumers of electric energy, which contains a negative sequence voltage filter consisting of resistors, first and second capacitors, two current reed relays, two current transformers and a rectifier bridge, a third and fourth capacitor, which are part of the reacting element , an additional resistor and a threshold element, and the primary windings of the current transformers are included in two phase wires, to the secondary windings The windings of the current reed relays are connected, the closing contacts of which are connected in parallel to the resistors of the negative sequence voltage filter, to the output of which a rectifier bridge is connected, the third capacitor, an additional resistor and an actuating element through a threshold element shunted by the fourth capacitor are connected in parallel [2].

A disadvantage of the known circuit is the inability to operate the device when the phase of the supply network is disconnected behind the connection point of the reverse sequence voltage filter.

The objective of the invention is to increase the reliability of the device due to the operation when the phase failure of the supply network after the connection of the voltage filter of the negative sequence.

This object is achieved in that this device for protecting a three-phase electric motor from an out-of-phase mode, comprising a negative sequence voltage filter, consisting of resistors, first and second capacitors, two current reed relays, two current transformers and a rectifier bridge, third and the fourth capacitor, an additional resistor and a threshold element, and the primary windings of the current transformers are included in two phase wires, to the secondary windings are connected to the windings of current reed relays, the closing contacts of which are connected in parallel to the resistors of the reverse sequence voltage filter, to the output of which a rectifier bridge is connected, a third capacitor, an additional resistor are connected in parallel to the bridge output, and an actuator whose contact is switched on through a threshold element shunted by the fourth capacitor in the phases of the coil of the magnetic starter, in the phases of the supply line there are three transformers that convert the current to voltage, o coils are connected in star and are connected to the input of loop filter voltage sequence.

Figure 1 presents a diagram of the proposed device for protecting a three-phase electric motor from an in-phase mode; figure 2 is a vector diagram in the normal mode of operation of the electric motor; figure 3 is a vector diagram in emergency operation of the electric motor.

A device for protecting a three-phase motor 7 from an out-of-phase mode (Fig. 1) contains a negative sequence voltage filter 1 having two arms, in one of which a capacitor 2 and a resistor 3 are connected in series, and in the other, a capacitor 4 and resistors 5 and 6 are connected in series. In one phase of the electric motor 7, a current transformer 8 is installed, to the secondary winding of which a relay coil with a reed switch 9 is connected, the contacts of which shunt the resistor 5. In another phase of the electric motor 7, a current transformer 10 is installed, to the second the main winding of which is connected to the winding of the second relay with a reed switch 11, the contacts of which shunt the resistor 6. To the outputs of the voltage filter of the negative sequence 1, a reactive element 12 is connected containing a rectifier diode bridge 13, to which a capacitor 14, a resistor 15 and through a threshold element 16 are connected, shunted capacitor 17, the actuator is an output relay 18, the contacts of which 24 act on the coil of the starter 23, the power contacts 19 of which connect the electric motor 7. To the inputs of the voltage filter hydrochloric sequence 1 connected transformers 20, 21 and 22 that convert current into voltage, which windings are connected in star.

A device for protecting a three-phase motor from an in-phase mode operates as follows.

After the power contacts of the starter 19 are turned on, a symmetrical nominal three-phase current flows through the power circuits of the electric motor 7. In this case, the secondary current of the current transformer 8 and 10 is not sufficient to close the contacts of the current relays with the reed switches 9 and 11. Since the symmetric voltage from the transformers 20, 21, 22, which convert the current to voltage, is supplied to the voltage filter of the negative sequence 1, then its output appears a very small unbalance voltage due to the spread in the parameters of the filter parts (the potentials of points m and n on the topographic diagram of FIG. 2 coincide). In the reacting element 12, the capacitor 14 is charged through the diode bridge 13 to an unbalance voltage. This voltage is not enough to open the threshold element 16, and the actuator - the output relay 18 does not work.

Short-term asymmetry of the supply voltage leads to an asymmetry of phase currents and to an increase in voltage at the output of the diode bridge 13, but the capacitor 14 is charged with a time delay, so the threshold element 16 does not have time to open, the output relay 18 does not work. After restoration of the supply voltage, the capacitor 14 is discharged to the resistor 15.

In the event that high-frequency oscillations occur in the power network, the capacitor 14 cannot filter them out, which can cause a false opening of the threshold element 16 and the output relay 18. So that the threshold element 16 does not false, it is shunted by the capacitor 17.

Long-term asymmetry of the supply voltage leads to an asymmetry of the phase currents of the electric motor and to an asymmetry of the voltages at the output of the transformers that convert the current to voltage. This asymmetrical voltage is applied to the input of the negative sequence voltage filter and increases the voltage at the output of the negative sequence voltage filter 1. At the same time, the capacitor 14 is charged through the diode bridge 13. When the voltage across the capacitor 14 reaches the opening value of the threshold element 16, the resistance of the threshold element decreases stepwise 16. Then the capacitor 14 is discharged through the threshold element 16 to the actuator - the output relay 18, the contacts of which 24 open the circuit the coil 23 of the magnetic starter, the power contacts of which 19 turn off the motor 7.

In those cases when there is a symmetrical overload of the electric motor 7, or the phase breaks in the winding of the electric motor 7 itself, the current in the phases increases. Then the secondary current of current transformers 8 or 10 also increases, current relays with a reed switch 9 and 11 operate every half-period of a current of industrial frequency. One or both resistors 5 and 6 are bridged each half cycle, the negative sequence voltage filter 1 is unbalanced. In this case, the voltage at the output of the negative sequence voltage filter increases, the capacitor 14 is charged through the diode bridge 13 to the opening voltage of the threshold element 16. Then, the actuating element - relay 18 is activated and disconnects the electric motor 7 through the power contacts of the starter 19.

In the normal mode of operation of the electric motor 7, the current in all phases is the same, therefore, the voltage at the output of the transformers that convert the current to voltage is symmetrical. As a result, at the input of the voltage filter of the negative sequence 1, the voltage in all phases is the same, and the points m and n coincide in the vector diagram (Fig. 2).

If, for example, phase B of the supply network is interrupted, the current in phase B disappears behind the connection point of the voltage filter for the negative sequence 1, as a result of which the voltage at the transformer 21 disappears, converting the current into the voltage of this phase. An asymmetrical voltage is applied to the input of the voltage filter of the negative sequence 1, it is unbalanced, a voltage appears between points m and n (Fig. 3), the threshold element 16 trips, the output relay 18 trips and opens its contacts 24 in the coil circuit of the magnetic starter 23, its power contacts 19 will open by disconnecting the motor 7.

Information sources

1. USSR author's certificate on application No. 2944682 / 24-07, cl. H02H 7/08, dated 29.12.80.

2. USSR author's certificate No. 1053209, cl. Н02Н 7/08, dated 12.03.82.

Claims (1)

A device for protecting a three-phase electric motor from an out-of-phase mode, comprising a negative sequence voltage filter, consisting of resistors, first and second capacitors, two current reed relays, two current transformers and a rectifier bridge, a third and fourth capacitor, an additional resistor and a threshold resistor element, and the primary windings of current transformers are included in two phase wires, the windings of current reed current relays are connected to the secondary windings whose closing contacts are connected in parallel with the resistors of the negative sequence voltage filter, to the output of which a rectifier bridge is connected, a third capacitor, an additional resistor and, through a threshold element shunted by the fourth capacitor, an actuator whose contact is shunted in the circuit of the magnetic starter coil, are connected in parallel to the output of the bridge , characterized in that in the phases of the supply line there are three transformers that convert the current to voltage, and are connected to the inputs of the fil тра of the negative sequence voltage, respectively.

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