RU2291538C2 - Electric motor protective gear - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: proposed protective gear designed for overcurrent and overtemperature protection of electric motors and also for preventing ground insulation worsening due to inadmissible reduction of its resistance has overcurrent protection channel, overtemperature protection channel, and ground fault protection channel. These channels are interconnected in parallel and connected through OR gate to final relay whose output is connected to control circuit contacts of magnetic stator whose power contacts are designed to insert motor phases in line. OR gate, final relay, and magnetic stator constitute actuating device of motor protective gear. Differential current transformer is connected to power phase leads in line section between current transformers and motor.

EFFECT: enhanced reliability and reduced installation and maintenance charges of protective gear.

1 cl, 7 dwg

Description

The invention relates to electrical engineering and can be used to protect the motor from unacceptable current and thermal overload, as well as deterioration of the insulation of the winding relative to the motor housing due to an unacceptable decrease in its resistance.

A device is known for protecting an electric motor from overheating and overload, comprising a current sensor of a protected electric motor, the output of which is connected to an input of an actuating element and a shunt thyristor, a key and a delay element connected in series, a first potentiometer and a zener diode connected in series. Parallel to the zener diode, a series-connected engine temperature sensor, a second potentiometer, and an ambient temperature sensor are connected. The output of the second potentiometer is connected to one of the key inputs, to the other input of which, through the logical operator OR, the output of the delay element and the current cutoff element are connected [1].

The disadvantage of this device is the low reliability of the protection due to the mismatch of the time-current characteristics of the device with the thermal characteristics of the engine.

A device for protecting an electric motor with a pump drive, comprising a temperature sensor, a voltage source, a low-pass filter, a first battery, a control unit and a pulse generator [2]. The low-pass filter input is designed to connect to the zero point of the secondary winding of the power transformer. A temperature sensor installed near the stator windings of the submersible motor is connected in series with the diode between the zero point of the motor winding and its grounded housing. The control unit is made in the form of two comparison units, the outputs of which are connected to the actuators.

This device eliminates the influence of insulation resistance on the measurement of the heating temperature of the stator windings of the electric motor.

The disadvantage of this device is the lack of protective elements of the submersible motor from the pump supply failure, controlled by the pressure in the well, which can lead to "dry friction" in the pump, an increase in the gas factor of the pumped liquid and, consequently, to a deterioration in the cooling of the engine and power cable .

A device is known for protecting an electric motor from abnormal operating modes, comprising a current sensor in the form of a current transformer, which is configured to be mounted on an electric motor to provide thermal contact with the electric motor at a temperature control point, wherein the core of the current transformer is made of ferromagnetic material with a given Curie point, which allows to expand the functionality of the device by providing thermal protection of the electric motor [3]. In this case, the Curie point is equal to the maximum permissible motor temperature at the location of the current transformer.

In this device, when the maximum permissible temperature is reached by the electric motor, the core of the current transformer loses its magnetic properties when the Curie point is reached. This leads to disconnection of the magnetic starter and electric motor from the network. In the overload mode of the electric motor by current, when the permissible value is exceeded, or when the phase is broken, the motor also disconnects from the network.

However, when the motor is running, the value of the permissible excess current above the nominal value largely depends on the duration of this current. So, the shorter the duration of the action, the greater the magnitude of the current can be passed through the motor windings without accidental consequences, while with large exposure times, the current value must be reduced. In addition, when using this device, it is possible to start the electric motor in operation with a reduced insulation resistance of the winding relative to the housing, which occurs when moisture or other aggressive environments in which the equipment and the electric motor are operating are hit. This device will not be able to protect the motor from the resulting emergency.

Closest to the technical nature of the present invention is a device for protecting an electric motor and driven equipment, containing protection channels against current and thermal overload, including a current sensor, made in the form of a current transformer and mounted on a phase wire, a magnetic starter, the power contacts of which are designed to be connected to an electric motor phase supply line, magnetic starter control elements, and an executive relay with an output contact, and the input of the executive relay is connected it is connected to the output of the current sensor, and its output contact is included in the control circuit of the magnetic starter, and a contact element mounted on the electric motor responding to thermal overload of the electric motor, a channel for protection against current overload, containing an electronic unit for the formation of time-current characteristics, a channel for protection against thermal overload , the channel of protection against emergency load shedding of driven equipment and the channel of protection against starting the electric motor with insufficient insulation resistance of the winding relative to the housing motors are interconnected in parallel and connected via a logical OR operator to an executive relay, the output of which is connected to the contacts of the control circuit of the magnetic starter [4].

In this device, the current overload protection channel contains an integrator, to the input of which current sensors installed on two phase wires are connected in parallel with an amplifier and a rectifier, and the electronic unit for generating the time-current characteristics includes a unit that converts the magnitude of the current overload factor to the pulse repetition rate, for example, in a quadratic dependence, which is connected to a pulse counter. The overload protection channel contains a warning indicator "CURRENT OVERLOAD", illuminated when the pulse counter is turned on, and an alarm indicator "CURRENT OVERLOAD", illuminated when the pulse counter is full.

The motor protection channel against thermal overload contains a contact element made in the form of a temperature sensor, and the “HEAT OVERLOAD” alarm indicator, which lights up when the temperature measured by the sensor exceeds the setpoint temperature. The channel for protection against emergency load shedding of driven equipment contains the DRY STROKE alarm indicator that lights up when the motor current decreases below the minimum current setting value. The channel for protecting the motor from starting with insufficient insulation resistance of the winding relative to the motor housing contains a constant voltage source and an alarm indicator "ISOLATION", illuminated when the insulation resistance of the winding decreases below the value of the resistance setting.

In the known prototype device provides comprehensive protection of the electric motor from:

- unacceptable current overload caused by the breakdown of one of the phases of the stator winding or power line, phase voltage asymmetry, technological overload, jamming of the rotor or inter-turn short circuits;

- abnormal disappearance of the load, for example, due to the operation of the pump in the "dry run" mode;

- lowering the insulation resistance of the winding relative to the motor housing below an acceptable level;

- unacceptable thermal overload of the engine caused by the deterioration of its cooling conditions.

In the prototype device, the temperature sensor of the motor protection channel from thermal overload is connected by conductors to the protective device. But in many systems, the protective device and the electric motor with a temperature sensor installed on it are at a considerable distance from each other. For example, when a submersible motor is operating in a well, a protective device is located on the ground. The need to use long conductors for the temperature sensor reduces the reliability of the protective device due to the possibility of their breakage or damage, puts increased maintenance requirements and complicates the operation of the remote motor.

In addition, for the operation of the motor protection channel from the reduced insulation resistance of the winding relative to the motor housing, a constant voltage source is required. This protection channel only prevents the motor from starting up when the insulation resistance value is lower, and during protection of the motor this protection channel does not work. But it is in the process of prolonged operation of the electric motor that deterioration of the insulation state can occur, for example, due to aging, operation in a moist, contaminated or aggressive environment, etc., which will lead to an emergency and damage to the electric motor.

The objective of the invention is to increase the reliability of protection, reducing the cost of installation and maintenance of the motor protection device.

The problem is solved in that in the known motor protection device comprising a current overload protection channel, including a current sensor mounted on a phase conductor of the power supply line, made in the form of a current transformer, and an "CURRENT OVERLOAD" alarm indicator, a thermal overload protection channel including “HEATER OVERLOAD” alarm indicator, illuminated when the threshold value measured by the temperature sensor is exceeded, and the protection channel from the reduced insulation resistance of the winding relative to motor housing, including the "ISOLATION" alarm indicator, illuminated when the insulation resistance of the winding decreases below a threshold value, connected together in parallel and connected by a logical operator OR to an executive relay, the output of which is connected to the contacts of the control circuit of the magnetic starter, the power contacts of which are designed to turn on in the power line of the phases of the motor, according to the invention, the phase wires of the power line in the area between the transformers the coil and the electric motor are covered by a differential current transformer, the IZOLYATSIA emergency indicator lights up when a half-wave leakage current occurs in the ground wire connected to the motor housing, and the thermal overload protection channel with the “HEATER OVERLOAD” alarm indicator illuminates when a single-half-earth leakage current occurs the wire contains an electrical circuit connected to the ends of the various phases of the stator winding with two facets connected in series limiting resistors and a temperature sensor made in the form of a posistor, the resistance of which increases with increasing temperature, and an electric branch with a reverse diode and an electric branch with a dinistor and an optocoupler LED, the photo-thyristor of which is connected to the electrical circuit node located between the limiting resistors, are connected in parallel with the posistor, and motor housing.

The presence of a differential current transformer, covering the phase wires of the power line exiting the motor, allows you to automatically register a decrease in the insulation resistance of the winding relative to the motor housing below the permissible value and the excess of the motor temperature above the permissible value. In the presence of these emergency factors, a deviation from zero of the sum of the currents in the phase supply lines (instantaneous values or vectors of effective values) arises due to the appearance of a leakage current in the grounding wire connected to the motor housing. A nonzero sum of currents in the phase power lines causes the appearance of a differential current in this transformer, and hence an electromotive force in its secondary winding, which, by means of an executive relay, disconnects the main contacts of the magnetic starter, disconnecting the electric motor from the network.

With reliable insulation between the winding and the motor housing and a low motor temperature not exceeding the permissible value, leakage currents are practically absent. Moreover, in the specified current transformer, the sum of the currents in the phase supply lines will have a zero value and there will be no differential current.

A thermal overload protection channel, comprising an electric circuit connected to the ends of the various phases of the stator winding with two limiting resistors in series and a temperature sensor made in the form of a posistor, in parallel with which an electric branch with a reverse diode and an electric branch with a dinistor and an optocoupler LED are connected, whose photo-thyristor is connected to an electric circuit node located between the limiting resistors and the motor housing, which allows increasing Resistors above a certain level cause a leakage current to appear in the ground wire.

Since the IZOLYATSIA alarm indicator lights up when a half-wave leakage current occurs in the grounding wire, and the “HEAT OVERLOAD” indicator lights up when a half-wave leakage current occurs, the cause of the accident and the motor disconnection from the mains can be easily established.

The absence of test leads from the electric motor to the protection device reduces the cost of installing and maintaining the protection device. This is especially noticeable when there is a significant distance between the engine and the protective device or when the motor is in an inaccessible place, in an aggressive environment, etc.

Figure 1 presents the General structural diagram of a motor protection device;

figure 2 is an electrical diagram of a motor protection device;

figure 3 - temperature characteristic of the resistance of the posistor;

figure 4 - the nature of the current i in the grounding wire with the glow of the alarm indicator "INSULATION";

figure 5 - the nature of the current i in the ground wire with the glow of the alarm indicator "HEAT OVERLOAD".

figure 6 - the nature of the electromotive force e in the secondary winding of the differential transformer with the glow of the alarm indicator "ISOLATION";

Fig.7 - the nature of the electromotive force e in the secondary winding of the differential transformer with the glow of the alarm indicator "HEAT OVERLOAD".

The motor protection device consists of a channel 1 for protection against current overload with the emergency indicator "CURRENT OVERLOAD" 2, channel 3 for protection against thermal overload with the alarm indicator "HEAT OVERLOAD" 4 and channel 5 for protection against reduced insulation resistance of the winding relative to the motor housing with emergency indicator " INSULATION "6. These channels are connected in parallel with each other and connected by means of the logical operator OR 7 to the executive relay 8, the output of which is connected to the contacts of the control circuit magnetically the first starter 9, the main contacts 10 of which are intended to be included in the power supply of the phases 11 of the electric motor 12. The device contains a lock of the contacts of the starter 13, the START button 14 and the STOP button 15 of the magnetic starter 9. The logical operator OR 7, the actuating relay 8 and the magnetic starter 9 form an actuator 16 of a motor protective device. To protect the motor 12 from short circuit currents and disconnect from the power source 17, as well as to supply power to the actuating relay 8, an automatic switch 18. The channel 1 of the protection against current overload contains current sensors 19 installed on the phase wires 11, made in the form of current transformers .

The phase wires of the power line 11 in the area between the current transformers 19 and the electric motor 12 are covered by a differential current transformer 20. The electric motor contains a three-phase winding of the stator 21 and the housing 22 connected to the grounding wire 23.

The thermal overload protection channel 3 contains an electrical circuit connected to the ends of the various phases of the stator winding 21 with limiting resistors 24 and 25 connected in series and a temperature sensor 26 made in the form of a posistor, the resistance R _{T of} which increases with increasing temperature (Fig. 3) . In parallel with the posistor 26, an electric branch with a reverse diode 27 is connected, protecting the dynistor 28 and the optocoupler LED 30 against reverse voltage 30. The phototyristor 31 of the optocoupler 30 is connected to an electric circuit assembly 32 located between the limiting resistors 24 and 25 and the motor housing 22. Channel 3 thermal overload protection is located in block 33.

Figure 2 shows the equivalent insulation resistance 34 of the winding 21 relative to the housing 22 of the motor 12.

The motor protection device operates as follows.

If one of the emergency factors occurs:

1. excess current in the motor winding of a threshold value;

2. excess temperature threshold value;

3. lowering the insulation resistance of the winding relative to the motor housing below a threshold value,

the logical operator OR 7 gives a signal to the executive relay 8, which, in turn, gives a signal to the control circuit of the magnetic starter 9, the main contacts 10 of which disconnect the power supply phase of the motor 12.

Channel 1 protection against current overload works as follows. If the current measured by at least one of the current sensors 19 does not exceed the threshold value, then the “CURRENT OVERLOAD” alarm indicator 2 does not light up and the signal is not supplied to the logical operator OR 7. In this case, the normal operation of the electric motor 12 is ensured. If the current measured at least one of the current sensors 19 on the phase 11 power supply line exceeds the threshold value, the “CURRENT OVERLOAD” alarm indicator 2 lights up and a signal is sent to the logical OR 7 operator, followed by disconnection of line n Tania phases 11 motor 12.

With reliable insulation between the three-phase winding of the stator 21 and the housing 22 of the motor 12, the equivalent resistance 34 of this insulation is large and the leakage current through the ground wire 23 to the ground is practically absent.

At a low temperature of the motor 12, measured by a temperature sensor 26 and not exceeding the permissible value, there is no leakage current either. This is due to the fact that at this temperature, the resistance of the sensor - posistor R _T 26 low (figure 3). In the electric circuit connected to the ends of the various phases of the stator winding 21, current flows through the limiting resistors 24 and 25 and the temperature sensor 26, without affecting the sum of the currents in the phase conductors of the power line 11 covered by the differential transformer 20. Since the resistance of the resistor R _T 26 is small then there is little voltage drop on it. In this case, the dinistor 28 is in the closed state, i.e. its resistance is large and the current through it and through the optocoupler 30 does not flow.

In the absence of leakage current through the ground wire 23, the sum of the currents in the power lines of the phases 11 (instantaneous values or vectors of effective values) is zero and the differential current in the current transformer 20 is zero. This ensures the normal operation of the electric motor 12.

If the insulation condition between the three-phase winding 21 and the housing 22 occurs in the electric motor 12, i.e. Since its equivalent resistance 34 becomes less than the permissible value, a leakage current occurs through the grounding wire 23 to the ground (Fig. 4). Due to the leakage current, the sum of the currents in the power lines of the phases 11 is not equal to zero and a differential current will appear in the current transformer 20. The presence of the differential current of the transformer 20 causes the appearance of an electromotive force in its secondary winding (Fig.6). In this case, the IZOLYATSIA 6 alarm indicator lights up and a signal is sent to the logical operator OR 7 with the subsequent disconnection of the power line of phases 11 of the electric motor 12. Since the two-half-wave (harmonic) leakage current flows in the grounding wire 23 connected to the motor housing 22, the nature the electromotive force in the secondary winding of the transformer 20 is of a similar nature with a phase shift.

With increasing temperature of the electric motor 12 above the permissible value, the resistance of the current sensor - the resistor R _T 26 increases sharply (figure 3). In this case, the voltage drop on this element 26 also increases. As a result, the voltage on the dynistor 26 increases and it opens, i.e. its resistance becomes small and through it and the LED 29 of the optocoupler 30 current begins to flow. The flow of current through the LED 29 leads to the opening of the photo thyristor 31 of the optocoupler 30. Since the resistance of the photo thyristor 31 becomes small, a leakage current flows through it and the ground wire 23 to the ground (Fig. 5). Due to the leakage current, the sum of the currents in the power lines of the phases 11 is not equal to zero and a differential current will appear in the current transformer 20. The presence of the differential current of the transformer 20 causes the appearance of an electromotive force in its secondary winding (Fig.7). In this case, the “HEATER OVERLOAD” alarm indicator 4 lights up and a signal is sent to the logical operator OR 7 with the subsequent disconnection of the power line of phases 11 of the electric motor 12. Since the ground wire 23 connected to the motor housing 22 flows a half-wave leakage current, the nature of the electromotive force in the secondary winding of the transformer 20 is not harmonious.

The different nature of the leakage currents and the corresponding electromotive forces in the secondary winding of the differential transformer makes it easy to identify the cause of the accident and disconnect the motor from the network.

The proposed device provides comprehensive protection of the motor 12 during its operation from various heterogeneous factors (unacceptable current and thermal overload, lowering the insulation resistance of the winding relative to the motor housing below the acceptable level), which guarantees high reliability of protection. The protective device operates without the use of special wires from the measuring sensors mounted on the electric motor to the actuator 16 remote from it.

The protection device was manufactured, passed an experimental test in various conditions, showing high reliability.

Information sources

1. A.S. USSR No. 600654, MKI N 02 N 7/08, 1978

2. A.S. USSR No. 1302369, MKI N 02 N 7/08, 1985

3. A.S. USSR No. 1764118, MKI N 02 N 7/08, 1992

4. Pat. Ukraine №61537 A, MKI N 02 Н 7/08, Н 02 Н 5/04, з. No.2002211652 of February 25, 2003 (prototype).

Claims (1)

An electric motor protection device comprising a current overload protection channel, including a current sensor mounted on a phase conductor of the power supply line, made in the form of a current transformer, and an "CURRENT OVERLOAD" alarm indicator, a thermal overload protection channel, including a "HEAT OVERLOAD" alarm indicator, illuminated when the temperature measured by the sensor exceeds the threshold value, and the protection channel from the reduced insulation resistance of the winding relative to the motor housing, including the emergency ind IKOLATOR ikator, illuminated when the insulation resistance of the winding decreases below a threshold value, connected together in parallel and connected by a logical operator OR to an executive relay, the output of which is connected to the contacts of the control circuit of the magnetic starter, the power contacts of which are designed to be connected to the motor phase supply line characterized in that the phase wires of the power line in the area between the current transformers and the electric motor are covered by a differential transformer rum current, the “ISOLATION” alarm indicator lights up when a half-wave leakage current occurs in the ground wire connected to the motor housing, and the thermal overload protection channel with the “WARM OVERLOAD” alarm indicator lights up when a half-wave leakage current occurs in the ground wire, it contains the ends of the various phases of the stator winding an electrical circuit with two limiting resistors and a temperature sensor connected in series to it ide thermistor whose resistance increases with increasing temperature, the parallel thermistor incorporated electric branch with a reflux diode and electrical branch dinistorov LED and photocoupler, fototiristory which is connected to the circuit node disposed between the limiting resistor and the motor housing.

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