INTRODUCTION

Electric machines are widely used at power stations, in industry, in transport, in aviation, in automatic control and regulation systems, and in everyday life. They convert mechanical energy into electrical energy (generators) and, conversely, electrical energy into mechanical energy.

Any electrical machine can be used as a generator or motor. This property is called reversibility. It can also be used to convert one kind of current into another (frequency, number of phases of alternating current, voltage) into the energy of another kind of current. Such machines are called converters. Electrical machines depending on the type of current electrical installation in which they must work are divided into DC machines and AC machines. AC machines can be either single-phase or multi-phase. The most widely used are asynchronous motors and synchronous motors and generators.

The principle of operation of electrical machines is based on the use of the laws of electromagnetic induction and electromagnetic forces.

Electric motors used in industry are produced in series, which are a series of electric machines of increasing power, having the same type of design and satisfying the general set of requirements. Special purpose series are widely used.

Protection of electric motors. Motor protection circuit

During operation asynchronous electric motors, like any other electrical equipment, malfunctions can occur - malfunctions, often leading to emergency operation, engine damage. its premature failure.

Fig.1

Before proceeding to the methods of protecting electric motors, it is worth considering the main and most common causes of emergency operation of asynchronous electric motors:

· Single-phase and phase-to-phase short circuits - in the cable, terminal box of the electric motor, in the stator winding (to the housing, interturn short circuits).

Short circuits are the most dangerous type of malfunction in the electric motor, because it is accompanied by the occurrence of very high currents, leading to overheating and burning of the stator windings.

· Thermal overloads of the electric motor - usually occur when the rotation of the shaft is very difficult (failure of the bearing, debris in the auger, starting the engine under too much load, or completely stopping it).

A common cause of thermal overload of an electric motor, leading to abnormal operation, is the loss of one of the supply phases. This leads to a significant increase in current (twice the rated current) in the stator windings of the other two phases.

The result of the thermal overload of the electric motor is overheating and destruction of the insulation of the stator windings, leading to the short circuit of the windings and the failure of the electric motor.

The protection of electric motors from current overloads consists in the timely de-energization of the electric motor when high currents appear in its power circuit or control circuit, i.e. in the event of short circuits. To protect electric motors from short circuits, fuse-links, electromagnetic relays, circuit breakers with an electromagnetic release, selected in such a way that they withstand high starting overcurrents, but immediately operate when short-circuit currents occur.

To protect electric motors from thermal overloads, a thermal relay is included in the electric motor connection circuit, which has control circuit contacts - voltage is applied to the magnetic starter coil through them.

Both AC and DC motors need protection from short circuits, thermal overheating and overloads caused by emergency situations or malfunctions in the technological process of which they are the power plants. To prevent such situations, the industry produces several types of devices, which, both separately and in combination with other means, form a motor protection unit.

Ways to protect electric motors from overloads

Besides, in modern schemes necessarily include elements that are designed for comprehensive protection of electrical equipment in the event of a power failure of one or more power phases. In such systems, in order to eliminate emergency situations and minimize damage when they occur, the measures provided for by the "Electrical Installation Rules" (PUE) are carried out.

Motor shutdown by current thermal relay

To prevent the failure of asynchronous electric motors that are used in mechanisms, machines and other equipment, where it is possible to increase the load on the mechanical part of the engine in the event of a violation technological process, use thermal overload protection devices. The thermal overload protection circuit, which is shown in the figure above, includes a thermal relay for the electric motor, which is the main device that implements an instantaneous or timed interruption of the power circuit.

The electric motor relay structurally consists of an adjustable or precisely set time setting mechanism, contactors and an electromagnetic coil and a thermal element, which is a sensor for the occurrence of critical parameters. Devices, in addition to the response time, can be regulated by the magnitude of the overload, which expands the possibilities of application, especially for those mechanisms in which, according to the technological process, a short-term increase in the load on the mechanical part of the electric motor is possible.
The disadvantages of the operation of thermal relays include the return to readiness function, which is implemented by automatic self-reset or manual control, and does not give the operator confidence in an unauthorized start-up of the electrical installation after operation.

The engine starting scheme is carried out using the start, stop buttons and an electromagnetic starter, the power supply of which they control, is shown in the figure. The start is realized by the starter contacts, which close when voltage is applied to the magnetic starter coil.

In this circuit, the current protection of the electric motor is implemented, this function is performed by a thermal relay that disconnects one of the winding terminals from the ground when the rated current flowing through all, two or one of the power phases is exceeded. The protective relay will disconnect the load even in the event of a short circuit in the power circuits to the electric motor. The thermal protective device operates on the principle of mechanical opening of the control terminals due to the heating of the corresponding elements.

There are other devices designed to turn off the electric motor in the event of an accident. lines of force and control circuits of short circuit currents. They come in several types, each of which produces an almost instantaneous tearing action without a temporary pause. Such equipment includes fuses, electrical, as well as electromagnetic relays.

Use of special electronic devices

There are sophisticated motor protection products that are used by experienced engineers in the design of electrical systems and are designed to simultaneously counteract emergencies, such as unauthorized, two-phase operation, undervoltage or overvoltage operation, single-phase short circuit electrical circuit to earth in systems with isolated neutral.

These include:

• frequency inverters,

• soft starters,

• contactless devices.

Use of frequency converters

The motor protection circuit implemented as part of the frequency converter shown in the figure below provides for the hardware capabilities of the device to counteract the failure of the motor by automatically reducing the current during start-up, stop, short circuits. In addition, protection of the electric motor by a frequency converter is possible by programming individual functions, such as the response time of thermal protection, which is activated from the motor temperature controller.

As part of its functions, the frequency converter also has radiator protection control and correction for high and low voltage, which can be caused in networks by third-party reasons.

The features of controlling the operation of electric motors in a system with frequency converters include the possibility remote control from a personal computer, which is connected using a standard protocol, and signal transmission to auxiliary controllers that process common process signals. You can learn more about the functions of frequency converters from the article about.

Soft starters and SIEP

With the reduction in the cost of devices in which the latest semiconductor elements are used, it becomes advisable to use soft starters and non-contact protection systems to protect asynchronous electric motors.

One of the most common ways to protect three-phase electric motors, both squirrel-cage and with a phase rotor, are electronic non-contact protection systems (CEP). The functional diagram, which shows an example of the implementation of the SIEP motor protection device, is shown below.

SIEP protects electric motors in the event of a break in any phase wire, an increase in current in excess of the rated current, mechanical jamming of the armature (rotor) and unacceptable voltage asymmetry between the phases. Implementation of functions is possible when shunts and current transformers L1, L2 and L3 are used in the circuit.

In addition, systems may include additional options, such as pre-start insulation resistance monitoring, remote temperature sensors and undercurrent protection.

The advantages of SIEP over frequency converters are direct data acquisition through inductive sensors, which eliminates the response delay, as well as a relatively low cost, provided that the devices have a protective purpose.

spruce thermal overloads. Overload protection should only be applied to electric motors of those operating mechanisms that may have abnormal load increases in case of disturbances in the working process.

Overload protection devices (thermal and temperature relays, electromagnetic relays, circuit breakers with a thermal release or clock mechanism) when an overload occurs, they turn off the engine with a certain time delay, the greater, the smaller the overload, and in some cases, with significant overloads, - and instantly.

Fig.6 Winding shop

Protection of asynchronous electric motors against undervoltage or loss of voltage

Protection against undervoltage or loss of voltage (zero protection) is carried out using one or more electromagnetic devices, acts to turn off the engine during a power failure or when the mains voltage drops below the set value and protects the engine from spontaneous switching on after the power failure is eliminated or restored normal voltage networks.

Special protection against operation on two phases protects the motor from overheating, as well as from "rollover", i.e. stopping under current due to a decrease in the torque developed by the motor, in the event of a break in one of the phases of the main circuit. The protection acts to turn off the engine. Both thermal and electromagnetic relays are used as protection devices. In the latter case, the protection may not have a time delay.

Fig. 7 Replacement, dismantling and maintenance of the ventilation system "Climate-47"

Other types of electrical protection of asynchronous motors

There are some other, less common types of protection (against overvoltage, single-phase earth faults in networks with isolated neutral, increased drive speed, etc.).

Electrical devices used to protect electric motors

Electrical protection devices can carry out one or several types of protection at once. So, some circuit breakers provide protection against short circuits and overload. Some of the protection devices, such as fuses, are single-acting devices and require replacement or recharging after each operation, others, such as electromagnetic and thermal relays, are multiple-acting devices. The latter differ in the method of returning to the ready state for devices with self-return and with manual return.

Selection of the type of electrical protection of electric motors

The choice of one or another type of protection or several at the same time is made in each specific case, taking into account the degree of responsibility of the drive, its power, operating conditions and maintenance procedures (presence or absence of permanent maintenance personnel). construction site, workshop, etc., identifying the most frequently occurring engine malfunctions and technological equipment. You should always strive to ensure that protection is as simple and reliable as possible in operation.

For each motor, regardless of its power and voltage, protection against short circuits must be provided. The following circumstances must be kept in mind here. On the one hand, protection must be adjusted against the starting and braking currents of the motor, which can be 5-10 times higher than its rated current. On the other hand, in a number of cases of short circuits, for example, with turn short circuits, short circuits between phases near the zero point of the stator winding, short circuits to the housing inside the motor, etc., the protection should operate at currents lower than the starting current. In such cases, it is recommended to use a soft starter (soft starter). Simultaneous fulfillment of these conflicting requirements with the help of simple and cheap means of protection is very difficult. Therefore, the protection system for low-voltage asynchronous motors is built on the conscious assumption that with some of the above-mentioned damages in the motor, the latter is not turned off by the protection immediately, but only in the process of developing these damages, after the current consumed by the motor from the network increases significantly.

One of the most important requirements for motor protection devices is its clear action during emergency and abnormal operation of engines and, at the same time, the inadmissibility of false alarms. Therefore, protective devices must be correctly selected and carefully adjusted.

SUE PPZ "Blagovarsky"

State Unitary Enterprise "Plempticezavod Blagovarsky" is the successor of the Blagovarskaya poultry farm, which was put into operation in 1977 as a commodity farm for the production of duck meat. In 1995, the poultry farm received the status of a state breeding poultry plant with the functions of a selection and genetic center for duck breeding. The Blagovarsky breeding plant is located near the village of Yazykovo, Blagovarsky district of the Republic of Bashkortostan.

General land area is 2108 hectares, of which arable land occupies 1908 hectares, and hayfields and pastures 58 hectares. The average number of ducks is 111.6 thousand heads, including 25.6 thousand heads of laying ducks.

The team employs 416 people, of which 76 are in the management apparatus.

The structure of the plant includes:

Workshop of the parent flock of ducks: has 30 buildings with the number of bird places for 110 thousand heads.

Shop for growing rearing young animals: has 6 buildings with the number of bird places for 54 thousand heads.

Hatchery: 3 workshops with a total capacity of 695520 pcs. eggs per bookmark.

Slaughter shop with a capacity of 6-7 thousand heads per shift.

Feed preparation workshop with a capacity of 50 tons per shift with a capacity of 450 tons.

Motor transport workshop: cars - 53, tractors - 30, agricultural machines 27.

In 1998, on the basis of the poultry breeding plant, a research and production system for duck breeding was created, uniting the work of poultry farms breeding ducks in 24 regions of the Russian Federation. More than 20 million breeding eggs and 15 million head of young ducks are sold through the scientific and production system. Breeding material is also supplied to such neighboring countries as Kazakhstan and Ukraine.

Ducks created by breeders of the State Unitary Enterprise Plemptsezavod Blagovarsky have become widespread in Russian Federation, they are successfully bred both in the Krasnodar and Primorsky Territories. The use of breeding ducks in the structure of the total number of ducks in Russia is about 80%.

DiaryDateWorkplaceType of workTechnology of work performanceSignature of supervisors Installation work. Disassembly and assembly of 3-phase asynchronous motors. 06/28/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement of automatic switches. 06/29/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Cabling. 06/30/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Cabling. 07/01/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Assembly of a grain crusher, installation of a water heater. 07/04/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement, dismantling and maintenance of the ventilation system "Climate-47" 05.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement, dismantling and maintenance of the ventilation system "Climate-47" 06.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Installation of the lighting system. 07/07/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Installation, maintenance of the ventilation system "Climate-47" 08.07.12-09.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Planned work. Cleaning and cleaning of green spaces around the protected area of ​​power lines. 07/10/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Installation of a diesel power plant.

DiaryDateWorkplaceType of workTechnology of performance of workSignature of supervisors.Note Installation, maintenance of the ventilation system "Climate-47" 16.07.12-17.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement of automatic switches. 18.07.12-22.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement, dismantling and maintenance of the ventilation system "Climate-47" 07/23/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Scheduled work. Cleaning and cleaning of green spaces around the protected area of ​​power lines. 24.07.12-29.07.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Installation and launch of AVM. 07/30/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Disassembly and assembly of 3-phase asynchronous motors. 07/31/12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Installation of the lighting system. 1.08.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Maintenance transformers. 2.08.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement, dismantling and maintenance of the ventilation system "Climate-47" 3.08.12-4.08.12 Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky" Installation work. Replacement of automatic switches.

Beginning of practice 26.06.12 End of practice 04.08.12

CONCLUSION

As a result of the production operational practice at the State Unitary Enterprise PPZ "Blagovarsky", I studied the structure of the enterprise, the scheme of the enterprise's power supply network, and also collected material on topics

During the operation of various electrical installations, emergency modes occur. The main ones are short circuits, technological overloads, open-phase modes, jamming of the rotor of an electric machine.

Emergency modes of operation of electric motors

Under short circuit the mode is understood when the overload current exceeds the rated current by several times. The overload mode is characterized by an excess of current by 1.5 - 1.8 times. Technological overloads lead to an increase in the temperature of the motor windings above the permissible level, its gradual destruction and failure.

Open phase mode (loss of phase) occurs in the event of a fuse blown in a phase, a wire break, or a contact failure. In this case, the currents are redistributed, increased currents begin to flow through the motor windings, the mechanism stops and the electric machine fails. The most sensitive to open-phase modes are electric motors of small and medium power, i.e., which are most often used in industry and agriculture.

Rotor jamming electrical machine can occur when the bearing is destroyed, the working machine is jammed. This is the hardest mode. The rate of rise in the temperature of the stator winding reaches 7 - 10 ° C per second, after 10 - 15 s the temperature of the motor goes beyond the permissible limits. This mode is most dangerous for engines of small and medium power.

The largest number of emergency outages of electric motors is due to technological overloads, jamming, destruction of the bearing unit. Up to 15% of failures occur due to phase failure and the occurrence of unacceptable voltage unbalance.

Types of electrical devices for protecting electric motors

To protect electrical equipment from emergency conditions, automatic switches, fuses, built-in temperature protection devices, phase-sensitive protection and other devices are commercially available.

When choosing the type of protection, specific operating conditions, speed, reliability, ease of use, and economic indicators are taken into account.

In electrical installations up to 1000 V, protection against short circuits is usually carried out fuses or electromagnetic overcurrent releases built into circuit breakers.

In addition, protection against short circuits of electric motors can be carried out by a current relay included in one of the stator phases directly or through a current transformer and a time relay.

overload protection are divided into two types: direct action protection, reacting to excess current, and indirect protection, reacting to temperature excess. The most common type of current protection used to protect electric motors from overloads (including jamming) are thermal relays. They are produced in the TRN, TRP, RTT, RTL series. Three-phase thermal relays RTT and RTL also protect against phase failure.

Phase sensitive protection (PS) protects against phase failure, jamming of the mechanism, short circuits, low insulation resistance of the electric motor.

Protection against overloads and jamming of the mechanism can also be carried out using special safety couplings. The specified type of protection is used on pressing equipment. To protect against phase failure, phase failure relays of the E-511, EL-8, EL-10 types, modern electronic and microprocessor relays are commercially available.

Indirect protection also includes built-in temperature protection UVTZ, which reacts not to the current value, but to the temperature of the motor winding, regardless of the reason that caused the heating. At present, for these purposes, modern electronic and microprocessor thermal relays are increasingly used, which react to changes in the resistance of thermistors built into the stator winding of the electric motor.

How to select the type of protection for electric motors

When choosing the type of protection, you must be guided by the following provisions:

the most critical electrical receivers, the failure of which can lead to great damage, subject to systematic pollution, or operating at high temperatures, as well as with a sharply changing load (crushers, sawmills, feed grinders), it is advisable to protect with built-in temperature protection and automatic switches or fuses.

Protection of low-power electric motors (up to 1.1 kW), which are serviced by highly qualified personnel, can be carried out by thermal relays and fuses.

Protection of electric motors of medium power (more than 1.1 kW) operating without attendants is recommended to be protected by phase-sensitive devices.

Thermal relays, phase-sensitive protection, built-in temperature protection work reliably at small overloads and long-term operating modes. The choice of the preferred apparatus in this case must be made taking into account economic indicators. Under variable loads with a load fluctuation period commensurate with the motor heating constant, thermal relays are unreliable and the built-in temperature protection or phase-sensitive protection should be used. Under random loads, they are more reliable protective devices acting as a function of temperature rather than current.

When the electric drive is connected to an open-phase network, a current close to the starting one passes through its windings, and the protective devices operate reliably. But if a phase break occurred after the motor was turned on, then the current strength depends on the load. Thermal relays in this case have a significant dead zone and it is better to use phase-sensitive protection and built-in temperature protection.

With prolonged starts, the use of thermal relays is undesirable. If starting at undervoltage, the thermal relay may falsely trip the motor.

When the rotor of an electric motor or a working machine is jammed, the current in its windings is 5-6 times higher than the nominal one. Thermal relays in this situation should turn off the electric motor within 1 - 2 s. However, the temperature protection for current overloads 1.6 times or more has a large dynamic error, so the electric motor may not be turned off, unacceptable overheating of the windings will occur and a sharp reduction in the life of the electric machine. Thermal relays and built-in thermal protection at large overloads operate with low efficiency. It is better to use phase-sensitive protection in such situations.

When using modern thermal relays RTT and RTL, the failure rate of electrical equipment is much lower than when using relays of the TRN, TRP type and in some cases is comparable to the failure rate when installing built-in thermal protection.

At present, for the protection of critical electric motors, modern ones are used, combining all types of protection and having the ability to flexibly adjust the operation parameters.

Application area various devices protection depends on the number of electrical equipment failures, the size of technological damage during shutdown, and the cost of purchasing protection equipment. To select the preferred option, a feasibility study is necessary.

Asynchronous motors of three-phase alternating current with voltage up to 500 V with powers from 0.05 to 350 - 400 kW are the most common type of electric motors.

Reliable and uninterrupted operation of electric motors is ensured primarily by their proper selection in terms of rated power, operating mode and form of execution. Equally important is the observance of the necessary requirements and rules when drawing up an electrical circuit, choosing ballasts, wires and cables, installing and operating an electric drive.

Emergency modes of operation of electric motors

Even for properly designed and operated electric drives, during their operation, there is always the possibility of emergency or abnormal modes for the engine and other electrical equipment.

Emergency conditions include:

1) multi-phase (three- and two-phase) and single-phase short circuits in the windings of the electric motor; multi-phase short circuits in the output box of the electric motor and in the external power circuit (in wires and cables, on the contacts of switching devices, in resistance boxes); phase short circuits to the case or neutral wire inside the motor or in an external circuit - in networks with a grounded neutral; short circuits in the control circuit; short circuits between the turns of the motor winding (turn circuits).

Short circuits are the most dangerous emergency conditions in electrical installations. In most cases, they occur due to breakdown or flashover of the insulation. Short-circuit currents sometimes reach values ​​that are tens and hundreds of times higher than the values ​​\u200b\u200bof normal mode currents, and their thermal effects and dynamic forces to which current-carrying parts are subjected can lead to damage to the entire electrical installation;

2) thermal overload of the electric motor due to the passage of increased currents through its windings: when the working mechanism is overloaded for technological reasons, especially difficult conditions for starting the engine under load or stalling, a prolonged decrease in mains voltage, a loss of one of the phases of the external power circuit or a wire break in the motor winding, mechanical damage in the engine or working mechanism, as well as thermal overloads when the engine cooling conditions deteriorate.

Thermal overloads cause, first of all, accelerated aging and destruction of the motor insulation, which leads to short circuits, i.e., to a serious accident and premature failure of the motor.

Types of protection for asynchronous motors

In order to protect the motor from damage in case of violation of normal operating conditions, as well as to disconnect the faulty motor from the network in a timely manner, thus preventing or limiting the development of an accident, protective equipment is provided.

The main and most effective means is the electrical protection of motors, carried out in accordance with

Depending on the nature of possible damage and abnormal modes of operation, there are several main most common types of electrical protection of asynchronous motors.

Protection of asynchronous electric motors against short circuits

Short circuit protection switches off the motor when short circuit currents appear in its power (main) circuit or in the control circuit.

Devices that provide protection against short circuits (fuses, electromagnetic relays, circuit breakers with an electromagnetic release) operate almost instantly, that is, without time delay.

The overload protection protects the motor from unacceptable overheating, especially in the event of relatively small but prolonged thermal overloads. Overload protection should only be applied to electric motors of those operating mechanisms that may have abnormal load increases in case of disturbances in the working process.

Overload protection devices (temperature and, electromagnetic relays, circuit breakers with a thermal release or clockwork) when an overload occurs, turn off the engine with a certain time delay, the greater the smaller the overload, and in some cases, with significant overloads, and instantly .

Protection of asynchronous electric motors against undervoltage or loss of voltage

Protection against undervoltage or loss of voltage (zero protection) is carried out using one or more electromagnetic devices, acts to turn off the engine during a power outage or a decrease in mains voltage below the set value and protects the engine from spontaneous switching on after the elimination of a power outage or restoration of normal mains voltage.

Special protection of asynchronous motors against two-phase operation protects the motor from overheating, as well as from “rollover”, i.e. stopping under current due to a decrease in the torque developed by the motor, in the event of a break in one of the phases of the main circuit. The protection acts to turn off the engine.

Both thermal and electromagnetic relays are used as protection devices. In the latter case, the protection may not have a time delay.

Other types of electrical protection of asynchronous motors

There are some other, less common types of protection (against overvoltage, single-phase earth faults in networks with isolated neutral, increased drive speed, etc.).

Electrical devices used to protect electric motors

Electrical protection devices can carry out one or several types of protection at once. So, some circuit breakers provide protection against short circuits and overload. Some of the protection devices, for example, are single-acting devices and require replacement or recharging after each operation, others, such as electromagnetic and thermal relays, are multiple-acting devices. The latter differ in the method of returning to the ready state for devices with self-return and with manual return.

The choice of the type of electrical protection of asynchronous motors

The choice of one or another type of protection or several at the same time is made in each specific case, taking into account the degree of responsibility of the drive, its power, operating conditions and maintenance procedure (presence or absence of permanent maintenance personnel).

Of great benefit can be the analysis of data on the accident rate of electrical equipment in a workshop, on a construction site, in a workshop, etc., and the identification of the most frequently repeated violations of the normal operation of engines and process equipment. You should always strive to ensure that protection is as simple and reliable as possible in operation.

For each motor, regardless of its power and voltage, protection against short circuits must be provided. The following circumstances must be kept in mind here. On the one hand, protection must be adjusted against the starting and braking currents of the motor, which can be 5-10 times higher than its rated current. On the other hand, in a number of cases of short circuits, for example, with turn short circuits, short circuits between phases near the zero point of the stator winding, short circuits to the housing inside the motor, etc., the protection should operate at currents lower than the starting current.

Simultaneous fulfillment of these conflicting requirements with the help of simple and cheap means of protection presents great difficulties. Therefore, the protection system for low-voltage asynchronous motors is built on the conscious assumption that with some of the above-mentioned damages in the motor, the latter is not turned off by the protection immediately, but only in the process of developing these damages, after the current consumed by the motor from the network increases significantly.

One of the most important requirements for motor protection devices - its clear action in emergency and abnormal modes of operation of engines and, at the same time, the inadmissibility of false positives. Therefore, protective devices must be correctly selected and carefully adjusted.