RU2190915C2 - Device for saving and reproducing temperature conditions of electric motor being protected - Google Patents

Abstract

FIELD: electrical engineering; motor protective devices built around microprocessor units. SUBSTANCE: device has microprocessor unit with analog-to-digital converter; capacitor whose first lead is connected to analog-to-digital converter input; resistor connected in parallel with capacitor; first and second switches with control inputs connected to microprocessor inputs and with two leads, first one of first switch being connected to first capacitor lead and second one, to power supply terminal; first lead of second switch is connected to first capacitor lead and second one is designed for connecting to common conductor of power supply; newly introduced in device are second capacitor and second resistor connected in parallel with this capacitor; first leads of the both are connected to second leads of first capacitor and resistor and second leads are designed for connecting to common conductor of power supply. EFFECT: enhanced precision of motor cooling process simulation. 2 dwg

Description

 The invention relates to electrical devices and can be used in motor protection devices based on microprocessor technology.

 A device for maintaining the state of a digital overcurrent release with power from the protected circuit using an RC element [1].

 Its main disadvantage is the lack of reliability with respect to failures that occur during operation of the microprocessor protection device. In addition, this device is generally not able to reproduce the thermal state of the protected object.

 The closest in technical essence and taken as a prototype is a device for maintaining the state of microprocessor thermal protection of an electric motor according to the patent of the Russian Federation 1787302 [2], the circuit diagram of which is presented in figure 1. The device contains a microprocessor 1 with an analog-to-digital converter (ADC) 2, the first 3 and second 4 keys controlled by the microprocessor, a capacitor 5 and a resistor 6.

 In the device in question, the microprocessor, measuring the current consumed by the engine, calculates the temperature of its overheating, storing the calculated temperature in the random access memory (RAM). If the voltage on the capacitor 5 is less than the corresponding RAM content reflecting the thermal state of the engine, the capacitor 5 is charged through the first key 3 controlled by the microprocessor to the corresponding voltage value. If the voltage across the capacitor 5 is greater than the corresponding RAM content, discharge of the capacitor 5 through the second key 4 controlled by the microprocessor is performed to the required voltage value.

 In the event of a malfunction in the microprocessor protection and subsequent restoration of its operability after, for example, the device controlling the occurrence of malfunctions ("watchman"), the voltage across the capacitor 5, corresponding to the current thermal state of the engine, is converted by the ADC into a code that is entered in RAM and is used by the microprocessor as the current value of the overheating of the protected motor.

 In the event of a temporary disappearance of the voltage in the network from which the protected motor and microprocessor-based protection device are powered, the microprocessor does not work, and the cooling of the electric motor is simulated by the discharge of a capacitor 5 to a resistor 6 connected in parallel. entered in RAM and used by the microprocessor as the initial value of the overheating of the protected motor.

 However, in the latter case, engine overheating is reproduced with significant errors, since it is not possible to satisfactorily simulate the process of its cooling by the R-C circuit with one time constant. The motor in thermal terms is a rather complex object, and for a sufficiently accurate simulation of its thermal dynamics, a model with at least two time constants, which separately reflect the heat exchange processes of the stator winding and steel, is required.

 The disadvantage of the prototype device is unsatisfactory in accuracy modeling of the cooling process of the protected engine.

 The technical result of the proposed solution is to increase the accuracy of modeling the cooling process of the protected engine.

 The technical result is achieved in that in the device for storing and reproducing the thermal state of the protected motor, containing a microprocessor with an analog-to-digital converter, a capacitor, the first output of which is connected to the input of the analog-to-digital converter, a resistor connected in parallel to the capacitor, the first and second keys with control inputs connected to the outputs of the microprocessor, and two conclusions, while the first output of the first key is connected to the first output of the capacitor, and the second to the terminal for connecting the power source, the first terminal of the second key is connected to the first terminal of the capacitor, and the second is used to connect to the common wire of the power source, an additional second capacitor and a second resistor connected in parallel are introduced, and their first terminals are connected to the second terminals of the first capacitor and resistor, and the second conclusions are intended for connection to a common wire of a power source.

 The essence of the invention is to improve the device for reproducing the thermal state of the protected motor, achieved by using a dual-circuit physical model in it instead of the previously used single-circuit model, which makes it possible to simulate the cooling process of the motor winding much more accurately.

 Indeed, as indicated, for example, in [3], after shutdown, the temperature of the motor winding is very quickly equalized with the temperature of the stator steel, and then a regular cooling mode occurs, at which the temperature of the winding and steel decreases very slowly. It is not possible to model this complex R-C process by a circuit with one time constant, as proposed in [1 and 2]. Using the same model with two R-C circuits connected in series with different time constants, one can choose the values of these constants at which the real cooling process of the protected object will be modeled quite accurately.

 The claimed technical solution meets the criteria of "Novelty" and "Significant differences", since the distinguishing features of the invention are not common with the features of the prototype and are not found among known devices of a similar purpose.

Figure 2 shows a schematic diagram of the inventive device, where the following notation:
1 - microprocessor,
2 - analog-to-digital Converter (ADC),
3, 4 - keys,
5, 7 - capacitors,
6, 8 - resistors.

 The device contains a microprocessor with an analog-to-digital converter, the first 3 and second 4 keys controlled by the microprocessor, the first capacitor 5, shunted by the discharge resistor 6, and the second capacitor 7, shunted by the discharge resistor 8. The capacitors 5 and 7 are connected in series and form together with the resistors 6 and 8 a single chain, the processes of charge and discharge of which are described by equations with two time constants.

 The device operates as follows. Microprocessor 1, measuring the current consumed by the motor, calculates the temperature of its overheating, storing the calculated temperature in RAM. If the voltage on the capacitor chain 5-7 is less than the corresponding RAM content reflecting the thermal state of the engine, the capacitor chain 5-7 is charged through the first key 3 controlled by the microprocessor to the corresponding voltage value. If the voltage on the chain is greater than the corresponding RAM content, it is discharged through the second key 4 controlled by the microprocessor to the required voltage value.

 In the event of a malfunction in the microprocessor protection and subsequent restoration of its operability after, for example, a device controlling the occurrence of malfunctions ("watchman"), the voltage on the capacitor chain 5-7, corresponding to the current thermal state of the motor, is converted by the ADC into a code that entered in RAM and used by the microprocessor as the current value of the overheating of the protected motor.

 In the event of a temporary loss of voltage in the network from which the protected motor and microprocessor-based protection device are powered, the microprocessor does not work, and the cooling of the electric motor is modeled by the discharge of capacitors 5-7 to the shunting resistors 6 and 8. When power is restored, the voltage across the capacitor chain 5-7 using the ADC, it is converted into a code that is entered in RAM and used by the microprocessor as the initial value of the overheating of the protected motor.

 The chain of series-connected capacitors introduced into the device circuit with shunt resistors is a combination of two R-C circuits, which can have different discharge time constants, which allows you to correctly simulate the engine cooling process at all its stages. So, the parameters of the first R-C circuit, i.e. its time constant, it is possible to choose the appropriate process for the initial cooling of the winding to the temperature of the steel, and the parameters of the second circuit - slow cooling of the winding together with the steel of the stator motor. The ratio of the nominal values of the resistors 6 and 8 should be chosen so that the distribution of the voltage drops across them in a stationary mode corresponds to the distribution of the temperature excesses of the winding and steel. A model configured in this way will display the actual process of cooling the windings of a disconnected motor.

 Therefore, in the inventive device, in contrast to the prototype, not only the state of microprocessor thermal protection is preserved in the event of a malfunction in its operation or in the event of a short-term voltage failure, but also the cooling process of the winding of the protected motor in its disconnected state is reproduced quite accurately both in the short-term and and with prolonged stay in this state.

 The accuracy of simulation achieved in the claimed device expands its functionality, namely, it allows to successfully protect engines operating in modes with time intervals between adjacent on states.

Thus, the advantages of the proposed device in comparison with the prototype are as follows:
1. Improving the quality of microprocessor thermal protection by increasing the accuracy of modeling the cooling process of a disconnected motor.

 2. Providing a wider scope for the possible use of this protection by expanding it (functional capabilities to protect engines operating in modes with time-arbitrary intervals between adjacent on states.

Sources of information
1. The patent of Germany 3137496, H 02 H 7/08, 1987.

 2. RF patent 1787302, Н 02 Н 5/04, 7/08, Н 01 Н 69/01, publ. 01/07/93, BI 1.

 3. Kuznetsov R.S. Coordination of the protective characteristics of thermal apparatus and the overload capacity of induction motors // Electrotechnics, 1983, 5.

Claims (1)

 A device for storing and reproducing the thermal state of the protected electric motor, comprising a microprocessor with an analog-to-digital converter, a capacitor, the first output of which is connected to the input of the analog-to-digital converter, a resistor connected in parallel to the capacitor, the first and second switches with control inputs connected to the outputs of the microprocessor, and two conclusions, while the first output of the first key is connected to the first output of the capacitor, and the second to the terminal for connecting the power source, the first output the second key is connected to the first output of the capacitor, and the second is designed to connect to a common wire of the power source, characterized in that the second capacitor and a second resistor connected in parallel are additionally introduced, the first terminals being connected to the second terminals of the first capacitor and resistor, and the second conclusions connected to the common wire of the power source.

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