RU2537744C1 - Method of diagnostics of induction motor stator windings insulation - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method means measurement of impedance of motor stator windings at rated speed of motor rotor rotation, and measurement of enclosure leakage currant of the induction motor together with voltage measurements on the motor body relatively to artificial zero point created by zero-sequence voltage filter connection to the power networks phases. Resistance measurements are performed by indirectly using the values of currents and voltages on the motor stator windings. The motor rotor speed is measured using the speed transmitter.

EFFECT: diagnostic of turn-to-turn damages and decreasing of insulation resistance relatively to motor body at earlier stage of development, monitoring of PE conductor integrity in TN-S, TN-C-S systems, or combined protective and null work conductors PEN in TN-C systems or PE conductor in TT earth system.

1 dwg

Description

The invention relates to electrical engineering, and in particular to means for improving the reliability of power supply to industrial enterprises and diagnosing the state of insulation of windings of stators of asynchronous electric motors.

A known method for diagnosing the technical condition of an electric motor by its electrical parameters, which consists in the fact that in three phases of the electric motor, a continuous measurement of the supply voltage and phase current is carried out. The signals of the instantaneous values of current and voltage produce a continuous calculation of the instantaneous values of the consumed capacities for a period; spectrograms of the consumed capacities are constructed. Calculate the magnitude of the energy loss in the engine at each of the frequencies of the spectrum. Assessment of the technical condition of the engine is carried out by comparing the resulting spectrograms of losses with similar spectrograms obtained on a known-good engine (see patent RU No. 2425391, IPC G01R 31/34, published on July 27, 2011).

The disadvantage of this method is the high error when reducing the insulation resistance of the motor windings relative to the housing, which does not lead to a significant current imbalance, according to which the state of the electric motor is estimated, as well as with the possible influence of network interference on the shape of the voltage supplying the electric motor.

The closest in technical essence is the method of functional diagnostics, taken as a prototype, which consists in the fact that they measure the total resistance of the stator windings of the electric motor, as well as measure the insulation resistance relative to the casing of the induction motor. The resistance values of the motor windings are determined indirectly by the current values of the phase currents and voltages. The value of insulation resistance relative to the motor housing is evaluated by the current value of the leakage current to the housing (see patent RU No. 2351048, IPC Н02Н 7/08, G01R 31/34, publ. 03/27/2009).

The disadvantage of this method is the high error in assessing the state of insulation of the windings relative to the motor case in case of a failure to connect the grounding conductor body, as well as the deterioration of electrical safety for maintenance personnel due to the possibility of a dangerous potential appearing on the motor body.

The technical result is the creation of an effective and reliable method for diagnosing the technical condition of induction motors and the provision of measures to improve the safety of their operation.

The technical result is achieved in that the method for diagnosing asynchronous electric motors, which consists in measuring the instantaneous values of currents in the windings and voltages at the terminals of the electric motor, converting them into voltages proportional to the current and voltage, recording the received signals, as well as measuring the leakage current to the motor casing, differs the fact that they additionally measure the voltage on the motor housing relative to the artificial zero point obtained using the voltage filter of a zero sequence connected to the terminals of the electric motor, and a comparison of the total resistances of the stator windings of the electric motor, determined from the obtained data of instantaneous values of currents and voltages in the windings, with each other and with those obtained with a known-good electric motor, is performed at the rated frequency of rotation of the rotor of the electric motor, and, if the resistance stator windings of an asynchronous electric motor are lower than permissible, or the voltage on the housing is higher than permissible, or the leakage current has exceeded the permissible Inu, it sends a signal to switch off the motor and an informational message.

The essence of the invention is that in a device for diagnosing an electric motor, the current values of the stator currents and voltages of an asynchronous electric motor are measured at the rated frequency of rotation of the electric motor, leakage current to the housing and voltage across the housing relative to the artificial zero point formed by the zero-sequence voltage filter connected to the supply phases network. The values of the currents and voltages of the stator of the electric motor allow you to calculate the total resistance of the stator windings and compare these values with the values recorded on a known good motor, and a comparison between each other.

$$\left(\begin{array}{c}Z\mathrm{from}t\mathrm{one}=\frac{U\mathrm{from}t\mathrm{one}}{I\mathrm{from}t\mathrm{one}}\\ Z\mathrm{from}t2=\frac{U\mathrm{from}t2}{I\mathrm{from}t2}\\ Z\mathrm{from}t3=\frac{U\mathrm{from}t3}{I\mathrm{from}t3}\end{array}\right)\text{(one)}$$

where Zst1, Zst2, Zst3 are the total resistances of the stator windings of the electric motor;

Ust1, Ust2, Ust3 - phase voltage of the supply network;

Ist1, Ist2, Ist3 - phase currents of the stator of an asynchronous electric motor.

A comparison of the resistances between each other will reveal a change in the technical condition of one or two windings, which may be a consequence of inter-turn faults. Comparison of the resistance values with the values recorded on a serviceable electric motor will allow to detect distributed damage, such as insulation wetting or wear, which are characterized by a general decrease in resistance. Comparison of the leakage current from the stator windings to the motor casing with the setpoint will allow early detection and monitoring of the rate of development of single-phase faults on the casing, while the simultaneous monitoring of the voltage between the motor casing and the artificial zero point of the zero-sequence voltage filter will allow you to control the integrity of the earth conductor connections of the motor casing, violation of which the leakage current may be absent or have a greatly reduced value, which leads to incorrect water about the state of isolation and exposes service personnel to danger of electric shock.

Figure 1 presents the block diagram of the device.

The block diagram contains an electromagnetic contactor 1, a first microcontroller 2, an analog-to-digital converter of the first microcontroller 3, a three-phase current-to-voltage converter 4, a three-phase voltage divider 5, a zero-sequence voltage filter 6, a single-phase voltage divider 7, an amplifier 8, an analog-to-digital converter of the second microcontroller 9, second microcontroller 10, speed sensor 11, three-phase asynchronous electric motor 12, current transformers 13, 14, 15, 16, liquid crystal display 17, button Keyboard 18.

A three-phase voltage divider 5, a zero-sequence voltage filter 6, and a three-phase current to voltage converter 4 receiving currents from three current transformers 13, 14, 15 are connected to the phases of a three-phase asynchronous electric motor; Converter 4 and divider 5 supply the converted signal to analog-to-digital converter 3, having a common zero point with the converter 4 and the divider 5, the analog-to-digital converter 3 is connected to the first microcontroller 2, which has the ability to disconnect the contactor by opening the circuit Contactor coil power. The zero-sequence voltage filter 6 is connected to a single-phase voltage divider 7, which is connected to the casing of an induction motor. From a single-phase voltage divider 7, the converted voltage is supplied to the first input of the analog-to-digital converter 9, while the measured leakage current Iout is received at the second input, which is removed from the current transformer 16 and amplified in the amplifier 8. The analog-to-digital converter 9 is connected to the second microcontroller 10 , which also receives a signal from the speed sensor 11. The second microcontroller 10 is connected to the first microcontroller 2. The first microcontroller 2 is connected to the keypad 18, liquid crystal kim display 17 and the control coil of the electromagnetic contactor 1.

The device operates as follows.

Before starting the electric motor, it is necessary to enter in the first microcontroller 2 the values of the nominal rotational speed of the electric motor rotor and the setpoint for switching off the electromagnetic contactor 1 using the keypad 18 and the liquid crystal display 17.

After starting the induction motor 12 with an electromagnetic contactor 1, the analog-to-digital converter 3 receives the measured and converted signals from the converter 4 (receiving signals from current transformers 13, 14, 15) and the divider 5, the obtained data is converted by the analog-to-digital converter 3 into digital form and transmits to the first microcontroller 2, which, when the rotational speed of the rotor of the electric motor is equal to the nominal (data about the frequency of rotation are received in the first microcontroller 2 from the second microcontroller 10), S THE calculation according to formula 1, the impedances of the stator windings of the induction motor 12.

Current transformer 16 supplies the measured signal proportional to the leakage current to the motor housing 2 to the amplifier 8, then the amplified signal is fed to the measuring input of the analog-to-digital converter 9, which, in turn, receives the signal from the divider 7, to the input of which data the voltage between the casing of the induction motor and the voltage filter of the zero sequence 6. The received signals analog-to-digital Converter 9 converts from analog to digital and transmits a second microcontrol ller 10, which also receives data on the frequency of rotation of the rotor of the electric motor from the sensor 11. The second microcontroller 10 in connection with the first microcontroller 2 transmits data on the frequency of rotation of the rotor of the electric motor, the magnitude of the leakage current to the housing and the voltage on the motor housing relative to the zero-voltage filter 6 .

Next, the first microcontroller 2 compares the values of the total resistances of the windings of the induction motor with the values recorded on a known-good motor at the rated speed, and compares the resistances with each other. In the case of a uniform decrease in the total resistance of the stator windings of the induction motor 12 below the settings, a signal is issued to disable the electromagnetic contactor 1, and a message on the general decrease in the total resistance is displayed. If, when comparing the total resistances, an unacceptable imbalance (exceeding the set value set before starting work) was revealed, then the first microcontroller 2 gives a signal to turn off the electromagnetic contactor 1, and a message appears on the display 17 about the occurrence of inter-turn faults in one of the stator windings of the asynchronous motor 12 . In the next step, the first microcontroller 2 compares the value of the leakage current to the motor housing with the setting entered before starting work. If the leakage current exceeds the permissible value, the first microcontroller 2 sends a signal to turn off the electromagnetic contactor 1, and a message appears on the display 17 about a decrease in the insulation resistance of the stator winding relative to the housing. After that, and in the case of a satisfactory value of the leakage current, the first microcontroller 2 compares with the setpoint the voltage value on the motor housing 12 relative to the artificial zero point 6 (which was transmitted to it from the second microcontroller 10 along with the leakage current and speed data). The presence of voltage, the value of which exceeded the setting, may indicate a deterioration of the connections in the protective conductor circuit, up to the breakage of the connection of the casing of the induction motor 12 with the protective conductor. In this case, the results of measuring the leakage current resistance by the current transformer 16 should not be considered plausible, in addition, the potential on the case can be dangerous for maintenance personnel, so the first microcontroller 2 generates a signal to turn off the electromagnetic contactor 1, and displays a message on the display of the presence of a dangerous potential on the motor housing.

The whole chain of measurements and comparisons is carried out with a certain frequency during the operation of the electric motor at the nominal speed. It is permissible to carry out a comparison with the set point of the leakage current and potential on the motor casing and not at the rated frequency of rotation of the rotor of the electric motor. Microcontroller 2 gives signals about the state of isolation in the form of messages on the display after each measurement and comparison.

The proposed method allows to diagnose the insulation state of the stator windings of an induction motor with high accuracy and reliability, as well as to monitor the integrity of the protective earth conductor of the earth of the motor housing, which increases the level of safety of staff.

Claims (1)

A method for diagnosing asynchronous electric motors, which consists in measuring the instantaneous values of currents in the windings and voltages at the terminals of the electric motor, converting them into voltages proportional to the current and voltage, recording the received signals, as well as measuring the leakage current to the motor housing, characterized in that they additionally measure voltage on the motor housing relative to the artificial zero point obtained using the zero sequence voltage filter, connected about to the terminals of the electric motor, and a comparison of the total resistances of the stator windings of the electric motor, determined from the obtained data of instantaneous values of currents and voltages in the windings, with each other and with those obtained with a known-good electric motor, is performed at the rated frequency of rotation of the rotor of the electric motor, and if the resistance of the stator windings is asynchronous the electric motor is lower than the permissible one, or the voltage on the housing is higher than the permissible one, or the leakage current has exceeded the permissible value, then they form a signal to turn off the electric electric motor and informational message.