SU970543A1 - Device for thermal protection of electric motor - Google Patents

Description

The invention relates to electrical engineering and can be used to protect an electric motor from overheating during abnormal modes of operation.

A device for thermal protection of an electric motor is known, comprising a capacitive voltage divider, a rectifying bridge, a smoothing filter, a voltage stabilizer, a transistor converter connected to thermal sensors, and an executive relay and device for protecting the motor against temperature rise, containing a power source, a thermal sensor placed in the windings of a protected electric motor, the output of which is connected through a threshold element and an OR element to an actuating element, an integrator, a generator, pulses and delay element fl.

The disadvantages of these devices are the need for a source of stabilized power, greater inertia and low reliability, since they contain a large number of semiconductor devices.

The closest to the proposed technical essence is

a device for thermal protection of an electric motor containing a power source, three thermal sensors mounted on the windings of an electric motor and connected to the input of a control unit with a current switch on transistors, to the output of which an actuator is connected - thyristor 2 ..

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The disadvantage of this device is the need for additional windings for the power source, and therefore the complexity of manufacturing.

The purpose of the invention is to simplify the device while at the same time increasing the reliability of its operation.

This goal is achieved by the fact that a device for temperature zagchity of an electric motor, containing three thermal sensors installed on the stator windings of an electric motor, two capacitors and a rectifier, to the output of which the actuator is connected, is additionally equipped with three resistors, one of the phases being connected through the first capacitor The first resistor, the second capacitor, the thermal sensors, and the third resistor are connected to another phase, which is ahead of that, and the connection point of the first resis ora a second condenser through the second resistor is connected to the third phase lagging by first .If this connection point of the first capacitor with lane jBHM resistor and the third resistor with a temperature sensor connected to the rectifier. In addition, a relay was used as an executive body. Fig. 1 is a schematic diagram of the device; in fig. 2 and 3 are vector diagrams of currents and voltages. The device contains the first capacitor 1, the first, the second, and the third resistors 2-4, the second capacitor 5 the stator of the electric motor 6, the thermal sensors 7-9 with a positive temperature coefficient (for example, posistor), the rectifying bridge 10 and the actuator 11 - the relay In the device, phase A is connected via series-connected capacitor 1, resistor 2, capacitor 5, temperature sensors 7-9 and resistor 4 to phase C, the connection point of resistor 2 to capacitor 5 is connected via resistor 3 to phase B, and capacitor 1 with resistor 2 and voltage 4 torus with temperature sensor 9 are connected to the input of the bridge rectifying unit 10, whose output is connected to the actuator body - the relay apparatus 11 operates as follows ... During normal operation of the motor, the temperature of the stator windings is below the threshold and the resistance of the temperature sensors 7-9 is many times lower than the resistance of the capacitor 5, In this case, the three-phase network voltage is distributed on the active-reactive elements of the fabric, as shown in FIG. 2. The voltage U on the resistor 2 (due to the sequential connection of the capacitor 1 and the resistor 2 to phase A) is shifted Ь4 in phase to the front side with respect to the voltage, id, and the help of a series of series-connected resistors 4, temperature sensors 7-9 and the capacitor 5, the voltage U on the capacitor 5 and the temperature sensors 7-9, is out of phase with respect to the voltage Ug. Thus, the voltage Ug and turn out to be equal in magnitude and practically coincide in phase, and their voltage after rectification by bridge 1 is applied to the executive element of relay 11. The voltage difference Ug and Uy is close to zero and therefore current flows through relay 11 less tripping current. In order for all phases I to have the correct alternation and allowable temperature of the stator windings Ufty, and Uj-6tinH to be equal in magnitude and coincide in phase, it is necessary that the following ratios between the Element Elements of the device be fulfilled; TfJ R1; R, 4 (Vi OUSE) I; Rj Xc-; where R1 is the resistance of resistor 2; 1 / ujCi - capacitance of capacitor 1 (Y - network frequency, С1 - capacitor capacity 1); - capacitive impedance 1 / w1 capacitor 5; Rn is the resistance of the resistor 3; Rj is the resistance of resistor 4; R is the resistance of thermal sensors at temperature below the threshold. In this case, the difference between the tension Uft and Uj is close to zero. In case of motor overloads, the temperature of the stator windings reaches the threshold value and, accordingly, the resistance of the sensors (thermal sensors) 7–9 (one or all) increases dramatically, and the voltage Uy– increases. Between the voltages UR and Uj, a phase shift appears (Fig. 3), respectively, their difference increases and the current through the relay 11 exceeds the response threshold. When this relay 11 is triggered and its contact turns off the motor. - The proposed device provides an accelerated shutdown of the electric motor in the case of an incomplete phase ra; bots. When the phase B is broken, the point O (Fig. 2) will shift along the Ug vector, which will result in a phase shift of about 60 al-degrees between the vectors UR and j. and, accordingly, to an increase in current through the relay 11, which is triggered. When a phase A or C is disconnected, one of the UR or U lines is uncompensated and sufficient to turn on the relay 11. The device also protects the electric motor from wake-up switching when the phase sequence changes, which can lead to failure of the mechanism of the technological line serviced by it and subsequent jamming the motor. When changing the phase rotation (Fig. 1)

between the vectors Ug and Uj-, a phase shift of approximately 120 electrical grad appears. The difference in voltage IJu / i and Uj (after rectification by bridge 10) will be sufficient to turn on relay 11.

The device performs self-monitoring of the health of the thermal sensor circuit and in the event of a disconnection between the thermal sensors, the winding of the relay 11 is turned on through a resistor 4 and a capacitor 1 between phase A and B. As a result, the relay 11 is activated by disconnecting the electrical equipment.

Thus, the proposed device allows the motor to be protected from overheating in hazardous modes by reducing the inertia of the device operation, and the simplicity of performance and the independence of the response parameters from voltage fluctuations of the network give the device high reliability.

: The economic feasibility of applying the invention is determined by the increase in the service life of an electric motor between overhauls and a decrease in the total number of electrical equipment failures.

In addition, the proposed device provides for the use of dp protection of electric motors and other electrical equipment in agriculture.

Claims (2)

1. A device for thermal protection of an electric motor, comprising three thermal sensors mounted on the stator windings of an electric motor, two capacitors and a coupler, to the output of which an actuator is connected, in order to simplify 0 device while improving the reliability of its work, it is additionally equipped with three resistors, with one of the phases through a series-connected first capacitor, first resistor, second 5 capacitor, thermal sensors and a third. the resistor is connected to a different phase, leading the aforementioned, and the connection point of the first resistor with the second capacitor through the second resistor. 0 is connected to the third phase, which is lagging behind the first one, while the connection points of the first capacitor with the first resistor and the third resistor with a thermal sensor are connected to the rectifier. five 2. The device according to claim 1, which is based on the use of a relay as an executive body. Information sources, 0 taken into account in the examination 1. USSR author's certificate No. 691986, cl. .N 02. N 5/04, 1978. 2. Authors certificate USSR 675518, cl. H 02 5/04, 1977. Un isG Us fig.Z