Protection of electric motors from emergency modes. Electrical protection of asynchronous electric motors. Reasons for hardware failures

Motor overload occurs in the following cases:

In case of prolonged start-up or self-start;

due to technological reasons and overload of mechanisms;

As a result of a break in one phase;

In case of damage to the mechanical part of the electric motor or mechanism, causing an increase in the torque M s and braking of the electric motor.

Overloads are stable and short-term. For the electric motor, only stable overloads are dangerous.

A significant increase in the motor current is also obtained in the event of a phase failure, which occurs, for example, in electric motors protected by fuses, when one of them burns out. At rated load, depending on the parameters of the electric motor, the increase in stator current in the event of a phase failure will be approximately (1.6 ÷ 2.5) I nom. This overload is sustainable. Also stable are overcurrents caused by mechanical damage to the electric motor or the mechanism rotated by it and overload of the mechanism.

The main danger of overcurrents for an electric motor is the accompanying increase in the temperature of individual parts and, first of all, the windings. An increase in temperature accelerates the wear of the winding insulation and reduces the life of the motor.

When deciding on the installation of overload protection on the electric motor and the nature of its action, they are guided by the conditions of its operation.

On the electric motors of mechanisms that are not subject to technological overloads (for example, electric motors of circulation, feed pumps, etc.) and do not have difficult starting or self-starting conditions, overload protection is not installed.

On electric motors subject to technological overloads (for example, electric motors of mills, crushers, haulage pumps, etc.), as well as on electric motors that cannot be self-started, overload protection must be installed.

Overload protection is performed with a shutdown action in case the self-starting of the electric motors is not ensured or the technological overload cannot be removed from the mechanism without stopping the electric motor.

Motor overload protection is performed with an action on the unloading of the mechanism or a signal, if the technological overload can be removed from the mechanism automatically or manually by personnel without stopping the mechanism and the electric motors are under the supervision of personnel.

On the electric motors of mechanisms that can have both an overload that can be eliminated during the operation of the mechanism, and an overload that cannot be eliminated without stopping the mechanism, it is advisable to provide for overcurrent protection with a shorter time delay for unloading the mechanism (if possible) and a longer time delay for turning off the electric motor . Responsible electric motors for auxiliary needs of power plants are under constant supervision of the personnel on duty, therefore their protection against overload is carried out mainly with the action on the signal.

Protection with thermal relay. Better than others can provide a characteristic approaching the overload characteristic of an electric motor, thermal relays that respond to the amount of heat generated in the resistance of its heating element.

Overload protection with current relays. To protect electric motors from overload, overcurrent protections are usually used using current relays with limited time delay characteristics of the RT-80 type or overcurrent protections made by a combination of instantaneous current relays and time relays.

The drive of executive mechanisms of various technological processes, as a rule, is carried out from electric motors.

The engine is one of the main components of the electric drive, which are most exposed to various adverse factors during operation.

The reasons for the probable deviations from the normal operation of the electric motor can be divided into three main groups:

problems in actuators causing braking and overloading of the drive motor;
violation of the quality of electricity supplying the electric motor;
defects that occur within the engine itself.

To ensure reliable operation, the electric motor must be equipped with automatic protections in the required amount, responding to dangerous deviations in operating parameters and overloads for any reason from the listed groups and acting to trip the circuit breaker.

The minimum volume of automatic motor protection devices is determined by the rules for the installation of electrical installations (PUE). Electric motors differ in rated power, supply voltage, type of current consumed, as well as design features.

In accordance with these differences, as well as based on the working conditions, for each model of the electric machine, the choice of automatic motor protection is made. Various types of automatic devices act both to open the circuit breaker and to turn on the warning signal.

According to the type of current consumed, electric motors are divided into:

alternating machines;

In everyday life and production, AC motors are common, which are asynchronous and synchronous.

According to the level of rated voltage, AC electrical machines are divided into two main groups - low-voltage, powered by voltages up to 1000 V and high-voltage, designed to operate in networks above 1000 V. The most widespread are asynchronous machines with a rated voltage of 0.4 kV.

They are protected by means of an automatic switch having electromagnetic and thermal releases from short circuits and overloads.

MAIN TYPES OF PROTECTION OF ASYNCHRONOUS ELECTRIC MOTORS UP TO 1000 V

Current cutoff.

Of all the emergency modes, the most dangerous is the phase-to-phase short circuit. This type of damage requires the immediate disconnection of the asynchronous motor by a switch from the mains.

In accordance with current regulations, asynchronous motors up to 1000 V must be protected against short circuits by fuses or electromagnetic and thermal releases of circuit breakers.

As usual, the rules lag behind the actual realities. At newly commissioned facilities, asynchronous electrical machines are equipped with remote multifunctional blocks of automatic relay protection of the electric motor based on microcontrollers that act to turn off the circuit breaker.

It does not change the main point. Automatic protection devices against phase-to-phase short circuits respond to overcurrents and do not have a time delay for opening the circuit breaker. Such devices are still called current cutoffs, protective relays are triggered by a short circuit in the stator winding or at the terminals of an induction motor.

The control of the flowing electric current is carried out by means of traditional current converters - current transformers (CT) or more modern electric current sensors.

The area of operation of the protective device is the section of the mains located after the CT or sensor. Usually, in addition to the asynchronous motor itself, there is also a supply cable in the protected area.

The current cutoff operation parameters must be reliably detuned from the starting currents. On the other hand, the automatic protective device must have sufficient sensitivity for interturn short circuits in any part of the stator winding of an asynchronous machine.

Overload.

This type of abnormal mode occurs when there is a malfunction or overload of the actuator. An overload of the engine can also occur due to its insufficient power. The overload mode is characterized by an increased level of current consumption with a relatively small multiplicity compared to the nominal value.

The current setting of the automatic motor overload protection is less than the value of the starting current parameters, therefore, detuning from the starting mode must be carried out by artificially delaying the operation time and turning off the circuit breaker.

The protection of the electric machine from overload can be implemented using the following devices:

thermal release of the automatic switch of protection of the electric motor;
remote protective kit with a current relay and a time relay that acts on the circuit breaker shutdown in case of overload;
block of complex protective automation of the engine on the microcontroller, when the switch acting on the release is triggered.

In the case of using a circuit breaker, you just need to select the machine that is suitable for the rated current and characteristic. The thermal release of the motor protection circuit breaker provides an integral dependence of the circuit breaker opening time on the magnitude of the current overload.

The protective automatic relay set with remote electromagnetic relays is configured for a fixed current and protection response time.

In this variant, in contrast to the thermal release, the current and time parameters are not related to each other. The output relays of the remote relay protection kits must act on the independent (not thermal) release of the circuit breaker.

OPEN PHASE PROTECTION

This type of automatic protective device is not prescribed by the PUE as mandatory, although it is highly desirable. When a three-phase electric motor operates in two phases, the windings gradually overheat, leading to the destruction of the insulation of the winding wire.

Such a mode can occur, for example, when contact is lost in one of the phases of the switch.

The worst thing in this situation is that the current consumed in this case can be comparable to the nominal value, that is, the current protection of the electric motor, including thermal-type releases that protect against overload, may not respond to this mode.

Some models of electrical machines contain built-in (temperature) winding sensors. Such modifications of electrical machines can be equipped with a special motor protection device that monitors the thermal state of the electrical machine.

Thermal protection devices can also help in case of overheating when operating on two phases.

PROTECTION DEVICES FOR ENGINES ABOVE 1000 VOLTS

The security of high-voltage electrical machines is provided only by remote relay devices. Thermal and electromagnetic releases are the prerogative of low voltage devices.

The principle of operation and calculation of current cut-off and overload protection settings is the same as for low-voltage machines. But besides this, there are specific protective devices that are not used at low voltages.

Protection against single-phase earth faults.

A feature of high voltage networks (6 - 10 kV) is the operation in the isolated neutral mode. In such networks, the value of I from the earth fault can be only a few amperes, which is outside the sensitivity zone of the maximum current overload protection.

Single-phase earth faults are characterized by the presence of zero-sequence currents flowing in the same direction in all three phases.

The earth motor protection relay (this is its name in the jargon of relayers) is connected to a special zero-sequence transformer, which is a torus (donut) through which the power cable passes.

In this case, the outlet of the shielding sheath of the high-voltage cable should not pass through the torus, otherwise, false trips of the device with the switch off take place.

The materials presented on the site are for informational purposes only and cannot be used as guidelines and normative documents.

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 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 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 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. 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.

The total land area is 2108 hectares, of which 1908 hectares are arable land, and 58 hectares are hayfields and pastures. 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 the breeders of the State Unitary Enterprise Plemptsezavod Blagovarsky have become widespread in the 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 performance of workSignature of supervisors 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" Planned 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 of 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.

Start 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

An electric motor, like any electrical device, is not immune from emergency situations. If measures are not taken in time, i.e. overload protection of the electric motor is not installed, then its breakdown can lead to failure of other elements.

( ArticleToC: enabled=yes )

The problem associated with the reliable protection of electric motors, as well as the devices in which they are installed, continues to be relevant in our time. This applies primarily to enterprises where the rules for the operation of mechanisms are often violated, which leads to overloads of worn-out mechanisms and accidents.

To avoid overloads, it is necessary to install protection, i.e. devices that can react in time and prevent an accident.

Since the asynchronous motor has received the greatest use, we will use its example to consider how to protect the motor from overload and overheating.

Five types of accidents are possible for them:

break in the phase stator winding (OF). There is a situation in 50% of accidents;
braking of the rotor, which occurs in 25% of cases (ZR);
decrease in resistance in the winding (PS);
poor engine cooling (BUT).

In the event of any of the listed types of accidents, there is a threat of engine breakdown, since it is overloaded. If protection is not installed, the current increases for a long time. But its sharp growth can occur during a short circuit. Based on the possible damage, protection of the electric motor against overloads is selected.

Types of overload protection

There are several of them:

thermal;
current;
temperature;
phase sensitive, etc.

To the first, i.e. The thermal protection of the electric motor includes the installation of a thermal relay that will open the contact in case of overheating.

Thermal overload protection reacting to rising temperatures. To install it, you need temperature sensors that will open the circuit in case of strong heating of the motor parts.

Current protection, which can be minimum and maximum. You can implement overload protection by using a current relay. In the first version, the relay is activated, opens the circuit if the permissible current value is exceeded in the stator winding.

In the second, the relays react to the disappearance of the current, caused, for example, by an open circuit.

Effective protection of the electric motor from increasing current in the stator winding, therefore, overheating is carried out using a circuit breaker.

The motor may fail due to overheating.

Why does it happen? Remembering school physics lessons, everyone understands that, flowing through a conductor, the current heats it up. The electric motor will not overheat at the rated current, the value of which is indicated on the housing.

If, for various reasons, the current in the winding begins to increase, the motor is in danger of overheating. If measures are not taken, it will fail due to a short circuit between the conductors, whose insulation has melted.

Therefore, it is necessary to prevent the growth of current, i.e. install a thermal relay - effective protection of the engine from overheating. Structurally, it is a thermal release, the bimetallic plates of which bend under the influence of heat, opening the circuit. To compensate for thermal dependence, the relay has a compensator, due to which a reverse deflection occurs.

The scale of the relay is calibrated in amperes and corresponds to the value of the rated current, and not to the value of the operating current. Depending on the design, the relays are mounted on shields, on magnetic starters or in a housing.

Properly selected, they will not only prevent overloading the electric motor, but also prevent phase imbalance and rotor jamming.

Car engine protection

Overheating of the electric motor also threatens car drivers with the onset of heat, and even with consequences of varying complexity - from a trip that will have to be canceled to a major overhaul of the motor, in which the piston in the cylinder can seize from overheating or the head is deformed.

While driving, the electric motor is cooled by air flow, and when the car gets into traffic jams, this does not happen, which causes overheating. To recognize it in time, you should periodically look at the temperature sensor (if any). As soon as the arrow is in the red zone, you must immediately stop to identify the cause.

You can not neglect the signal of the emergency light bulb, because behind it you will feel the smell of boiled coolant. Then, steam will appear from under the hood, indicating a critical situation.

How to be in a similar situation? Stop by turning off the electric motor and wait until the boil stops, open the hood. This usually takes up to 15 minutes. If there are no signs of leakage, add fluid to the radiator, and try to start the engine. If the temperature starts to rise sharply, they carefully move to find out the reason for a diagnostic service.

Reasons for overheating

In the first place are radiator malfunctions. These can be: simple pollution with poplar fluff, dust, foliage. Removing the pollution will solve the problem. It is more problematic to deal with internal contamination of the radiator - scale that appears when sealants are used.

The solution is to replace this element.

Then follow:

Depressurization of the system caused by a cracked hose, insufficiently tightened clamps, a malfunction of the heater faucet, an aged pump seal, etc.;
Faulty thermostat or faucet. It is easy to determine this if, with a hot engine, carefully feel the hose or radiator. If the hose is cold, the reason is the thermostat and it will need to be replaced;
A pump that is inefficient or not working at all. This leads to poor circulation through the cooling system;
Broken fan, i.e. not turning on due to a failed motor, clutch, sensor, loose wire. A non-rotating impeller also causes the motor to overheat;
Finally, insufficient sealing of the combustion chamber. These are the consequences of overheating, leading to the combustion of the head gasket, the formation of cracks and the deformation of the cylinder head and liner. If a leak is noticeable from the coolant reservoir, leading to a sharp increase in pressure when cooling is started, or an oily emulsion has appeared in the crankcase, then this is the reason.

In order not to get into a similar situation, it is necessary to carry out preventive measures that can save you from overheating and breakdown. The "weak link" is determined by the method of elimination, i.e. check suspicious details sequentially.

Overheating can be caused by an incorrectly selected operating mode, i.e. low gear and high rpm.

Protection against overheating of the motor-wheel

Motor - bicycle wheel also becomes unusable after "transferred" overheating. If on a hot day you drive at maximum power for some time at top speed, the windings of the motor-wheel will overheat and begin to melt, like any electric motor that experiences overload.

Next, it will be the turn of a short circuit and stop the engine, to restore the performance of which, rewind is needed. To prevent it, there are high power controllers that increase torque. Repairing a motor-wheel that has failed is an expensive operation, commensurate in financial costs with the purchase of a new one.

It would be theoretically possible to install a thermal sensor that will not allow overheating, but manufacturers do not do this for a number of reasons. One of them is the complication of the controller design and the increase in the cost of the motor-wheel as a whole. One thing remains - to carefully select the controller in accordance with the power of the motor-wheel.

Video: Engine overheating, causes of overheating.

MINISTRY OF AGRICULTURE OF THE RUSSIAN FEDERATION

BASHKIR STATE AGRARIAN UNIVERSITY

REPORT

on industrial operational practice

Faculty: Energy

Department: power supply and electrical applications

energy in agriculture

Specialty: 140106 Electrification and automation of agriculture

Full-time form of education

Course, group: EA 201/1

Arduvanov Ilgiz Radievich

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. Electric machines, depending on the type of current of the electrical installation in which they must work, are divided into direct current machines and alternating current 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 the operation of asynchronous electric motors, like any other electrical equipment, malfunctions can occur - malfunctions, often leading to emergency operation, damage to the engine. its premature failure.

Fig.1 Asynchronous motor

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, automatic switches with an electromagnetic release are used, selected in such a way that they withstand large starting overcurrents, but immediately work 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.

Fig.2 Thermal relay

In the event of thermal overloads, these contacts open, interrupting the power supply to the coil, which leads to the return of the group of power contacts to its original state - the electric motor is de-energized.

A simple and reliable way to protect the electric motor from phase failure is to add an additional magnetic starter to its connection circuit:

Fig.3 Wiring diagram for an additional magnetic starter

Turning on the circuit breaker 1 closes the power circuit of the coil of the magnetic starter 2 (the operating voltage of this coil should be ~ 380 V) and closes the power contacts 3 of this starter, through which (only one contact is used) power is supplied to the coil of the magnetic starter 4.

By turning on the “Start” button 6 through the “Stop” button 8, the power circuit of the coil 4 of the second magnetic starter is closed (its operating voltage can be either 380 or 220 V), its power contacts 5 are closed and voltage is applied to the motor. When the "Start" button 6 is released, the voltage from the power contacts 3 will go through the normally open block contact 7, ensuring the continuity of the power supply circuit of the magnetic starter coil.

As can be seen from this motor protection circuit, if for some reason one of the phases is missing, the voltage will not be supplied to the motor, which will prevent it from thermal overloads and premature failure.

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.

Fig.4 Disassembly and assembly of 3-phase asynchronous motors

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 modes include:

1) multi-phase (three- and two-phase) and single-phase short circuits in the motor windings; 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 engine 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 overloads of the electric motor due to the passage of increased currents through its windings: during overloads of the working mechanism 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 winding of the motor, mechanical damage in the motor or working mechanism, as well as thermal overload when the cooling conditions of the motor 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.

Fig.5

Types of protection for asynchronous motors

In order to protect the electric 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, performed in accordance with the "Electrical Installation Rules" (PUE). Depending on the nature of possible damage and abnormal operating modes, there are several main most common types of electrical protection for 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.

Protection of asynchronous electric motors against overload

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.

Fig.6 Winding shop

Protection of asynchronous electric motors against undervoltage or loss of voltage

Fig.7

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 recharge 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

SUE PPZ "Blagovarsky"

SUE "Plempticezavod Blagovarsky" is the assignee of the poultry farm "Blagovarskaya", 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. Breeding poultry farm "Blagovarsky" is located near the village of Yazykovo, Blagovarsky district of the Republic of Bashkortostan.

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

The structure of the plant includes:

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

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

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

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

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

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

Ducks created by the breeders of the State Unitary Enterprise “Plempticezavod Blagovarsky” have become widespread in the 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%.


	Workplace	Type of work	Work performance technology	Hands signature.	Note
		Assembly work.	Disassembly and assembly of 3-phase asynchronous motors.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Cabling.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Cabling.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Assembly of a grain crusher, installation of a water heater.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Installation of the lighting system.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.
08.07.12-09.07.12	Blagovarsky district, SUE "PPZ Blagovarsky"	Planned work.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Installation of a diesel power plant.


	Workplace	Type of work	Work performance technology	Hands signature.	Note
11.07.12-15.07.12	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Installation, maintenance of the ventilation system "Climate-47"
16.07.12-17.07.12	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement of automatic switches.
18.07.12-22.07.12	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, SUE "PPZ Blagovarsky"	Planned work.	Cleaning and cleaning of green spaces around the protected area of power lines.
24.07.12-29.07.12	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Installation and launch of AVM.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Disassembly and assembly of 3-phase asynchronous motors.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Installation of the lighting system.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Maintenance of transformers.
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, SUE "PPZ Blagovarsky"	Assembly work.	Replacement of automatic switches.

Start of practice 26.06.12 End of practice 04.08.12

CONCLUSION

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