RU2302691C1 - Method for protecting induction motor against abnormal operating conditions - Google Patents

Abstract

FIELD: electrical engineering; induction motor protection against emergency operating conditions.

SUBSTANCE: proposed method for induction motor protection against emergency operation includes checkup of following two quantities: difference between measurement results of motor input power and measured power output corresponding to that found by calculations, as well as ratio of actual maximal electromagnetic torque that can be developed by motor to mechanical torque on motor shaft. Quantities under check are found including parameters of electrical energy for particular time moment.

EFFECT: enhanced reliability and improved checkup quality of motor protective gear.

1 cl, 2 dwg

Description

The invention relates to electrical engineering, namely to relay protection, and can be used to protect induction motors from emergency conditions.

A known method of protecting an induction motor from damage and dangerous operating conditions, in which currents and voltages at the supply end of the line to which the motor is connected, is measured, the active power at the input and output of the protected motor is determined, the torque on the motor shaft is measured, the active characteristic is determined by the operating characteristic the power consumed by the motor in normal mode with the measured mains voltage, the loss of active power in the motor is determined and a signal is generated protection if the specified losses exceed the specified value of the active power losses in normal operation / cm. A.S. 1594642, cl. Н02Н 7/08, 1990 /.

It is known that the active power at the input of an electric motor depends on the parameters of the quality of electric power / cm. Kurbatsky V.G. The quality of electricity and electromagnetic compatibility in electrical networks: a Training manual. - Bratsk: BrSTU, 1999. - p. 34, 36, 40 /. Since the difference in losses in the known method is defined as the difference between the active power at the input and the power consumed by the motor in normal mode with the measured mains voltage and torque on the motor shaft, such a difference will depend on the parameters of the quality of electricity

where P ₁ is the active power at the input of the electric motor in the current mode with the actual quality of electricity;

P ₁ ^' is the active power consumed by the electric motor from the network in normal mode, determined by the operating characteristic of the electric motor;

U ₂ is the voltage of the negative sequence;

I ₂ - current negative sequence;

u ₁ , u ₂ , ..., u _n - spectrum of harmonic voltage components from 1 to n;

i ₁ , i ₂ , ..., i _n - spectrum of harmonic components of the current from 1 to n;

δU - voltage deviation, determined according to GOST-13109-97.

For example, such a mode may occur when there is a deterioration in the quality of electricity, but within the limits described in GOST-13109-97 and there is no emergency mode, the operation of the electric motor is still permissible, however, the difference in losses can exceed the setting, which will lead to the motor being turned off. This is because the calculated operating characteristic is determined without taking into account the parameters of the quality of electricity, and with the appropriate torque on the shaft, the calculated power will be less than the measured active power at the input, so that the difference can exceed the setting. This example is illustrated in figure 1, which shows the operating characteristic M = f (P) of the engine 4A180M4, where 1 - taking into account only the voltage deviation at the supply end of the line; 2 - taking into account the parameters of the quality of electricity (with a voltage asymmetry coefficient in the reverse sequence equal to 1%, the coefficient of the 3rd harmonic component equal to 3%, voltage deviation equal to 3%).

Another case is possible when in emergency mode the active power generated in the form of heat in the place of damage is insignificant, and, accordingly, the difference in losses may be less than the tripping setpoint and the protection does not respond to emergency mode, for example, when the stator winding is closed in a loop. Based on this, the method has a low reliability.

The method also has a disadvantage, which is characterized by low speed, for example, when the rotor is jammed, the protection can react with a significant delay, which will lead to irreversible consequences, i.e. The method has a low quality control.

The technical result of the invention is to increase reliability and improve the quality of motor protection control.

The technical result is achieved by the fact that in the method of protecting an induction motor from emergency conditions, which consists in measuring the current and voltage at the supply end of the line to which the electric motor is connected, measuring the torque on the motor shaft, the angular rotation speed of the electric motor rotor, determining the active power at the input and the motor output, active power losses in it, the formation of a protection signal, if these losses exceed a predetermined value in normal operation, will complement the quality parameters of electric energy are separated out from the measured voltage at the supply end of the line, the maximum moment of the electric motor is determined, and a protection signal is generated that acts to turn off the electric motor if the measured torque on the electric motor shaft is close to the maximum electric motor moment, while the loss of active power and the maximum electric motor moment are determined taking into account the selected parameters of the quality of electricity at the current time.

The essence of the invention lies in the fact that two quantities are controlled.

The first controlled value is the difference between the active power at the input of the electric motor P ₁ and the active power consumed by the electric motor in normal mode P ₁ ^' at the corresponding parameters of the quality of electric power and the torque on the motor shaft, at the current time

The second controlled value is the ratio between the maximum moment M _max and the torque on the motor shaft M _mech , determined with the corresponding parameters of the quality of electricity at the current time

The dependence of the influence of electric power quality parameters on the maximum moment and active power at the motor input can be determined using elements of mathematical modeling of electric machines / cm. Kopylov I.P. Mathematical modeling of electrical machines. Textbook for universities. - 3rd ed., Revised. and add. - M .: Higher. Shk, 2001, p. 125-145 /.

_Max and P ₁ 'are determined at specified parameters of the quality of electricity, when simulating in real time, for a specific power of the electric motor, with small increments of the mechanical moment on the shaft of the electric motor. An example of the mechanical characteristics of the motor 4A132M4 at a sinusoidal, symmetrical and nominal supply voltage, in static mode, obtained by modeling, by which it is possible to determine the maximum moment, shown in figure 2. Alternately changing the parameters of the quality of electricity, you can get an array of data to determine M _max and P ₁ ^' . In such an array, one of the rows or columns is M _max and P ₁ ^' . The resulting array is programmed into the microcontroller of the device that implements this method.

The advantage of the proposed method over the known one is that it controls the mechanical moment on the motor shaft and compares it with the maximum possible moment that the electric motor can develop. The specified action allows you to protect the motor from tipping over and jamming of the rotor. Another advantage is that the maximum moment and losses of active power are determined taking into account the parameters of the quality of electricity, which increases the reliability of the proposed method when the deviation of the parameters of the quality of electricity from permissible.

Figure 3 shows a functional diagram of a device for implementing the proposed method.

The motor protection device consists of an electric motor 1, current sensors 2-4, voltage sensors 5-7, a torque sensor on the shaft of an electric motor 8, an angular velocity sensor 9; an analog filter that selects the parameters of the quality of electricity 10, analog-to-digital Converter 11, a digital microcontroller 12 and the Executive body 13. The outputs of the sensors 2, 3, 4, 8, 9 and the filter 10 are connected to the input of the analog-to-digital Converter 11, the output of which is connected to the input of the digital microcontroller 12, the output of which is connected to the input of the executive body 13, and the output of the latter is the output of the device as a whole.

The value of P _{1 is} determined by the microcontroller 11 according to the current and voltage values measured in the power supply circuit of the electric motor 1, and the values of P ₁ 'and M _{max are} determined by the microcontroller 11 from a given data set, taking into account the parameters of the quality of electric power and torque on the motor shaft.

The method is as follows. In normal mode, currents are measured using sensors 2-4, and voltage 5 at the supply end of the motor line is measured using sensors 5-7. Using the filter 10, power quality parameters are allocated. Using the converter 11, the analog signals from the sensors 2, 3, 4, 8, 9 and the filter 10 are converted to digital and fed to the input of the microcontroller 12. Using the current and voltage values measured in the power supply circuit of the electric motor, the value of P _{1 is} determined by the microcontroller 12. The selected parameters of the quality of electricity by the microcontroller 12 determines the values of P ₁ 'and M _max . From the values of P ₁ ′ and P ₁ , the difference (2) is determined by the microcontroller 12, the quotient (3) is determined from the values of M _max and M _fur . Since the electric motor operates in normal mode, the values (2) and (3) are less than the set value of the protection operation and the signal to turn off the electric motor by the microcontroller 12 is not generated.

In the event of an emergency mode, for example, a broken phase conductor supplying the electric motor, the value (2) will exceed the set value of the protection operation, and the microcontroller 12 will generate a protection signal to disconnect the electric motor from the network. When the mechanical power is poured onto the motor shaft, as a result of which the motor can be overloaded or stopped, the value (3) will exceed the set value of the protection operation, and the microcontroller 12 will generate a protection signal to disconnect the motor from the network.

The proposed method can be used to protect any asynchronous electric motor from emergency mode, especially an electric motor powered by voltage with low quality of electric power and subject to sharp surges of mechanical moment on its shaft.

Claims (1)

A method of protecting an induction motor from emergency conditions, which consists in measuring the current and voltage at the supply end of the line to which the motor is connected, measuring the torque on the motor shaft, the angular speed of rotation of the electric motor rotor, determining the active power at the input and output of the electric motor, active power losses in him, the formation of a protection signal, if these losses exceed a predetermined value in normal operation, characterized in that it is further isolated from the measurement At the supply end of the line, the parameters of electric power quality determine the maximum motor torque, if the measured torque on the motor shaft is close to the maximum motor moment, a protection signal is generated that acts to shut off the electric motor, while the loss of active power and the maximum electric motor moment are determined taking into account the allocated power quality parameters at the current time.

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