RU15620U1 - DEVICE FOR PROTECTING A THREE-PHASE ELECTRIC MOTOR FROM OPERATION IN TWO PHASES - Google Patents

Description

DEVICE FOR PROTECTING A THREE-PHASE ELECTRIC MOTOR FROM OPERATION IN TWO PHASES.

The utility model relates to relay protection devices for electric motors and can be used to automatically disconnect a three-phase electric motor from the supply network in the event of an out-of-phase operation mode or to signal the occurrence of this mode.

Known devices for protecting a three-phase electric motor from working in two phases. An example of such devices is a device that responds to the blowout of one of the fuses 1, p. 316, Fig. 14.3, a), a device that responds to the disappearance of current in one of the phases 1, p. 316, Fig. 14.3.6), a device, responsive to neutral displacement of the stator winding of the electric motor when phase 1 breaks, p. 316, Fig. 14.3, a).

These devices have the following disadvantages: the first of them does not provide protection if the two-phase operation is not caused by a burnout of the fuse-link, but by another reason, the second requires the installation of high-current relays and a large number of relay equipment, for the third it is necessary to have a neutral point of the stator winding of the electric motor and devices of the fourth group have insufficient sensitivity AND reliability of action, since when a line wire is broken, the interphase voltage does not disappear, but only decreases.

Closest to the claimed device is a circuit that responds to a violation of the symmetry of a three-phase voltage system at the terminals of an electric motor and uses the principle of creating an artificial zero point 2, p. 38, fig. 3, containing three identical capacitors, the first plates of which are connected to the phase terminals of the motor windings, and an electromechanical type relay, the breaking contact of which is included in the self-holding circuit of the magnetic starter winding, and the relay winding is connected between the connection point of the second capacitor plates and the zero point of the motor power supply in series with NC contact of push button switch.

Such a device is devoid of the above disadvantages. However, to carry it out, capacitors of significant capacitance (from 4 to 10 microfarads with an operating voltage of the relay 36 V.) for an operating voltage of at least twice the phase voltage of an electric motor, having large dimensions and cost, are required. The latter affects the size and cost of the entire protection device. In addition, having high sensitivity, such a device can sometimes turn off the electric motor as a result of breaking electrical symmetry caused by external asymmetric short circuits in the supply network.

bots on two phases due to the use of smaller capacitors in his circuit, as well as detuning from false positives for short-term violations of electrical symmetry caused by any reason, due to the introduction of a time delay in his work.

The essence of the utility model is as follows. The drawing shows a diagram of a device for protecting a three-phase motor from working in two phases. The device contains three identical capacitors 1, the first and second current-limiting resistors 1 and 22, the first and second rectifier bridges 3 and 32, an RC circuit 4 containing a resistor 4 and a capacitor 42, a thyristor optocoupler 5 and an electromechanical relay 6. The first plates of the capacitors 1 are connected to the phase terminals of the motor windings, and the second plates are interconnected. The connection point of the second capacitor plates 1 through the current-limiting resistor 2i and the inputs of the first rectifier bridge 3i are connected to the zero point of the motor power source. The diagonal of the first rectifier bridge 3 includes a series-connected resistor 4i and a capacitor 42 of RC circuit 4, parallel to the capacitor 42 of which a radiating part of the thyristor optocoupler 5 is connected through a current-limiting resistor 22. The power part of the thyristor optocoupler 5 is included in the diagonal of the second rectifier bridge 32, the inputs of which connected to the poles of the AC source through the winding of the electromechanical relay 6. The NC contact of the electromechanical relay 6 is included in the self-holding circuit of the magnetic coil winding skeleton electric motor.

The device operates as follows. With a full-phase power supply mode of the motor windings, there is no potential difference between the connection point of the second capacitor plates 1 and the zero point of the motor threading source. No current flows through the first current-limiting resistor 2i, the first rectifier bridge 3, the RC circuit 4, and the radiating part of the thyristor optocoupler 5. The thyristor optocoupler 5 is locked, as a result of which no current flows through the second rectifier bridge 32 and the winding of the electromechanical relay 6. The NC contact of the electromechanical relay 6 in the self-holding circuit of the magnetic motor starter is in the closed state.

When the motor is out of phase, a potential difference appears between the connection point of the second capacitor plates 1 and the zero point of the motor power source. A current starts flowing through the first current limiting resistor 2i, the first rectifier bridge 3b resistor 4i, and the capacitor 42RC of circuit 4. After the capacitor 42 is charged to the output voltage of the first rectifier bridge 3i, it is discharged along the circuit formed by the current-limiting resistor 22 and the radiating part of the thyristor optocoupler 5. The thyristor optocoupler 5 is unlocked, through its power part, the second rectifier bridge 32 and the winding of the electromechanical relay 6 flow current from the source alternating current. The electromechanical relay 6 is activated, its opening contact breaks the self-holding circuit of the winding of the magnetic starter. The latter, falling away, disconnects the electric motor from the mains.

With a short-term asymmetry of the voltage supplying the electric motor, the device does not have time to work due to the introduction of the RC circuit 4. The time required to charge the capacitor 42 introduces a delay in the operation of the protection device as a whole, providing the necessary detuning from the short-term asymmetry caused by any reason.

If the action of the device for switching on the Sitnal is necessary, then in parallel with the inputs of the second rectifier bridge Zg, instead of the electromechanical relay 6, it is necessary to turn on the signal lamp, or use the make contact of this relay to connect it, excluding the opening contact of the electromechanical relay 6 from the self-holding circuit of the winding of the magnetic motor starter of the electric motor.

A device for protecting a three-phase electric motor against operation on two phases of a similar type is manufactured and successfully operated at the Bogoslovsky Aluminum Plant OJSC. In the circuit, it used three capacitors with a capacity of 0.1 microfarads, which allowed to significantly reduce the size and reduce the cost of the device.

REFERENCES 1. Andreev V. A. Relay protection, automation and telemechanics in systems

Claims (1)

A device for protecting a three-phase electric motor from operating on two phases, containing three identical capacitors, the first plates of which are connected to the phase terminals of the motor windings, and the second plates are interconnected, an electromechanical relay, one terminal of the winding of which is connected to the first pole of the AC source, and disconnecting the contact is included in the self-holding circuit of the winding of the magnetic motor starter, characterized in that the first and second current-limiting resistors are additionally introduced into it , the first and second rectifier bridges, an RC circuit containing a resistor and a capacitor, and a thyristor optocoupler, while the connection point of the second capacitor plates through the first current-limiting resistor and the inputs of the first rectifier bridge are connected to the zero point of the motor power supply, and to the diagonal of the first rectifier bridge a series resistor and a capacitor of an RC circuit are connected, in parallel with the capacitor of which an emitting part of the thyristor optics is connected through a second current-limiting resistor ron, the power part of the thyristor optocoupler is included in the diagonal of the second rectifier bridge, and the second output of the electromechanical relay winding through the inputs of the second rectifier bridge is connected to the second pole of the AC source.