SU1030907A1 - Device for protecting of a.c. motor - Google Patents

Abstract

DEVICE FOR PROTECTION OF AC MOTOR ELECTRIC MOTOR, containing a current transformer, plug, quad, thermal analog: :: 3, a log, an actuator, characterized in that, in order to increase reliability and accuracy, two current transformers are added to it, A quarator consists of a serially connected converter voltage - frequency, pulse generator, a current key and diode, and also from a voltage converter - current, the input of which is connected to the input of the voltage converter - frequency, vyhods output current key, and heat. The first analogue consists of three series-connected resistors and three capacitors, with the first capacitor connected in parallel to all three resistors, the second in parallel to the second and (L third resistors, and the third capacitor connected in parallel to the third resistor. CO O O O

Description

Inventive refers to electrical engineering, in particular to relay devices. zishty, and can be used to protect acHHXpOHJflbix electric motors against overload. It is known a device for an AC motor against an overload, containing current sensors ,; current relay, time relay and actuator 1 1. This device has insufficient performance. Jari significant overloads | and also does not allow to fully realize the overload capabilities of the electric motor. The device closest to the invention is to protect the AC electric motor against overload, containing a current sensor, rectifier, quad, performed on stabilizers, thermal analog, containing a circuit of linear and nonlinear resistance and capacitor, and the executive body. The voltage removed from the thermal analogue, which is proportional to the temperature of the motor windings, is fed to the actuator. The device allows you to more accurately realize the overload capacity of the C 1 motor. However, the principle used in the known device is the dependence of the response time of the overload protection device, as well as the use of high-capacity capacitors in a thermal analog (more than 1 Om m does not allow to have the protection characteristics corresponding to the overload characteristics of the motor, especially with a variable load curve, which does not provide the necessary reliability and accuracy of the device for protection. The use of a current transformer included in only one phase makes the protection device insensitive to asymmetric modes of the electric motor. The aim of the invention is to improve the reliability and accuracy of the device for protection of an AC electric motor. an AC current generator containing a current transformer, a rectifier, a quadrant, a thermal analogue, and an executive element, two current transformers are additionally introduced, and a square Soigrg 113 and later, | Pdio sudipsmifich voltage generator - frequency, pulse driver, current switch and diode, as well as from the voltage converter - current, the input of which is connected to the input of the voltage converter - frequency, and the output with the current switch output, thermal analogue consists of three series-connected resistors and three capacitors, with the first capacitor connected in parallel to all three resistors, the second - parallel to the second and third resistors, and the third capacitor. included parallel to the third resistor. FIG. 1 is a block diagram of a device for protecting an AC motor; in fig. 2 - scheme of thermal analogue. The protection device (Fig. 1) contains current transformers 1-3, rectifier 4, voltage-frequency converter 5, pulse shaper 6, current switch 7, voltage converter 8-current, diode 9, thermal analogue U and actuator 11. The thermal analog (Fig. 2) is made on resistors 12-14 and capacitors 15-17. The AC motor protection device operates as follows: From current transformers 1-3, a voltage proportional to the current of the electric motor is fed to rectifier 4. The rectified voltage controls the oscillation frequency of the inverter. The voltage 5 voltage is the frequency. From these oscillations, the pulse shaper 6 generates short pulses having a constant duration, which are then fed to the current switch 7, which closes for the duration of the pulse. The voltage from the output of the rectifier 4 is also supplied to the converter 8, a voltage, a current at the output of which the current is proportional to the current of the electric motor. The output current of the converter 8 is the voltage — the current goes to the current switch 7 when the latter is open. When closing the current switch 7, the current flows through the diode 9 to the thermal analog 10 and charges the capacitors of the thermal analog. The diode 9 serves to prevent the thermal analog capacitors from discharging to the current switch 7 when it is open.

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Thus, the thermal aiiajior 10 receives current pulses, the frequency and amplitude of which varies nponoiiionionno the current of the motor. The average value of the current supplied to the thermal analogue of Yu is proportional to the electric motor current squared. From the thermal analogue 1O, a voltage proportional to the temperature of the stator windings of the motor is supplied to the actuating element 11. When this voltage exceeds a certain value, the actuating element 11 switches off the electric motor.

The essence of the proposed thermal analogue (Fig. 2) is that in conditional division of the electric motor 11 into three parts - the stator winding, the stator and the capacitors 15-17 of the thermal analogue model, respectively, the heat capacity of these parts. Resistors 12 and 13 simulate the thermal coupling between the stator windings, the stator and the housing, and the resistor 14 simulates the thermal coupling between the motor case and the environment. If a thermal analogue is supplied with a current, the magnitude of which is proportional to the square of the current of the electric motor, then the voltage across the capacitors is at the temperature of those parts of the electric motor whose heat capacity these capacitors simulate. Proposed heat

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. “The tax models not only accumulate, but 1 distribute heat in the electrode, which allows more precisely to maintain proportionality between the 5 stator windings of the stator windings and the voltage on the thermal analog at any load curve of the electric motor at any time.

The quadrator with the proposed structure 10 allows generating a current, the value of which may be less than 10 A, which allows using stable and small-sized pressure detectors with a capacity of about 1 O microfarad in the thermal analog and, at the same time, obtaining stable time delays when protection trips for more than one hour.

An additional introduction to the device for protecting two current transformers 20 makes it more sensitive to asymmetrical modes of operation of the motor.

The proposed device for the protection of 2S as compared with the known one has a number of advantages: high reliability, speed, stability of characteristics. An additional advantage of the proposed device is 5shp its 3Q compactness, the possibility of installing a thermal relay in a magnetic starter in place without any additional installation work.

Yu

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Claims (1)

DEVICE FOR PROTECTING AN AC MOTOR, containing a current transformer, a rectifier, a quadrator, a thermal analog, an actuating element, characterized in that in order to increase reliability and accuracy, two current transformers are additionally introduced into it, and the quadrator consists of a voltage converter connected in series - the frequency of the pulse shaper, the current switch and the diode, as well as from the voltage - current converter, the input of which is connected to the voltage - frequency converter input, and the output is Odom current key, and heat. the new analog consists of three series-connected resistors and three capacitors, the first capacitor connected in parallel to all three resistors §, the second parallel to the second and | to the third resistors, and the third capacitor is connected in parallel with the third re-. I * to the zistor. I st 1 . 1030C