Protection of asynchronous motors from emergency modes. Electrical protection of asynchronous motors. level at shutdown

In order to avoid unexpected failures, costly repairs and subsequent losses due to motor downtime, it is very important to equip the motor with a protective device.

Motor protection has three levels:

External short-circuit protection of the installation ... External protection devices are usually fuses of various types or short-circuit protection relays. Protective devices of this type are mandatory and officially approved, they are installed in accordance with the safety regulations.

External overload protection , i.e. protection against overloads of the pump motor, and, consequently, prevention of damage and malfunction of the electric motor. This is over current protection.

Built-in motor protection with overheating protection to avoid damage and malfunction of the electric motor. The built-in protection device always requires an external switch, and some types of built-in motor protection even require an overload relay.

Possible Engine Failure Conditions

During operation, various malfunctions may occur. Therefore, it is very important to anticipate the possibility of failure and its causes in advance and protect the motor as best as possible. The following is a list of failure conditions under which damage to the motor can be avoided:

Poor quality of power supply:

High voltage

Undervoltage

Unbalanced voltage / current (surges)

Frequency change

Incorrect installation, violation of storage conditions or malfunction of the electric motor itself

A gradual increase in temperature and its going beyond the permissible limit:

Insufficient cooling

High ambient temperature

Reduced atmospheric pressure (operation at high altitudes above sea level)

High fluid temperature

Too high viscosity of the working fluid

Frequent switching on / off of the electric motor

Too large moment of inertia of the load (different for each pump)

A sharp rise in temperature:

Locked rotor

Phase loss

To protect the network against overloads and short circuits when any of the above failure conditions occur, it is necessary to determine which network protection device will be used. It should automatically disconnect power from the mains. A fuse is the simplest device that has two functions. As a rule, the fuses are interconnected by means of an emergency switch that can disconnect the motor from the mains supply. On the following pages, we will look at three types of fuses in terms of their principle of operation and applications: fuse switch, fast acting fuses and time delay fuses.

The fuse switch is an emergency switch and a fuse combined in a single housing. The circuit breaker can be used to open and close the circuit manually, while the fuse protects the motor against overcurrent. Switches are generally used in connection with service activities when it is necessary to interrupt the power supply.

The emergency switch has a separate cover. This cover protects personnel from accidental contact with electrical terminals and also protects the switch from oxidation. Some EPOs have built-in fuses, others are supplied without built-in fuses and are equipped with a switch only.

The overcurrent protection device (fuse) must distinguish between overcurrent and short circuit. For example, minor short-term overcurrents are perfectly acceptable. However, with a further increase in current, the protection device must be triggered immediately. It is very important to prevent short circuits immediately. A fused circuit breaker is an example of a device used for overcurrent protection. Correctly sized fuses in the breaker will open the circuit in the event of overcurrents.

Fast-blow fuses

Fast acting fuses provide excellent short circuit protection. However, short-term overloads, such as the starting current of an electric motor, can cause this type of fuse to break. Therefore, fast acting fuses are best used in networks that are not subject to significant transient currents. Typically, these fuses will withstand about 500% of their rated current for one quarter of a second. After this time, the fuse insert melts and the circuit is opened. Therefore, in circuits where the inrush current often exceeds 500% of the fuse rating, fast blowing fuses are not recommended.

Time delay fuses

This type of fuse provides both overload and short-circuit protection. As a rule, they allow 5 times the rated current for 10 seconds, and even higher currents for a shorter time. This is usually enough to keep the motor running without opening the fuse. On the other hand, if overloads occur that last longer than the melting time of the fusible element, the circuit will also open.

The fuse time is the time it takes for the fuse (wire) to melt for the circuit to open. For fuses, the response time is inversely proportional to the current value - this means that the greater the overcurrent, the shorter the time period for the circuit to trip.

In general, it can be said that pump motors have a very short acceleration time: less than 1 second. In this regard, time delay fuses with a rated current corresponding to the full load current of the motor are suitable for motors.

The illustration on the right shows the principle of the fuse operating time characteristic. The abscissa shows the relationship between actual current and full load current: if the motor draws full load current or less, the fuse will not open. But when the current is 10 times the full load current, the fuse will open almost instantly (0.01 s). The response time is plotted on the ordinate axis.

During start-up, a fairly large current flows through the induction motor. In very rare cases, this will result in a shutdown by means of relays or fuses. Various methods of starting the motor are used to reduce the starting current.

What is a circuit breaker and how does it work?

The circuit breaker is an overcurrent protection device. It automatically opens and closes the circuit at the set overcurrent value. If the circuit breaker is used within its operating range, opening and closing does not harm it. Once an overload has occurred, the circuit breaker can be easily reactivated by simply resetting it.

There are two types of circuit breakers: thermal and magnetic.

Thermal circuit breakers

Thermal circuit breakers are the most reliable and economical type of protection devices that are suitable for electric motors. They can handle the high current amplitudes that occur when the motor is started and protect the motor from malfunctions such as locked rotor.

Magnetic circuit breakers

Magnetic circuit breakers are accurate, reliable and economical. The magnetic circuit breaker is resistant to temperature changes, i.e. changes in ambient temperature do not affect its response limit. Compared to thermal circuit breakers, magnetic circuit breakers have a more precise response time. The table shows the characteristics of two types of circuit breakers.

Circuit breaker operating range

The circuit breakers differ in the level of the trip current. This means that you should always select a circuit breaker that can withstand the highest short-circuit current that can occur in a given system.

Overload relay functions

Overload relay:

When starting the electric motor, they are allowed to withstand temporary overloads without breaking the circuit.

Open the motor circuit if the current exceeds the maximum permissible value and there is a threat of damage to the motor.

Set to the initial position automatically or manually after overload elimination.

IEC and NEMA standardize overload relay trip classes.

In general, overload relays react to overload conditions according to the pickup characteristic. For any standard (NEMA or IEC), product classifications determine how long the relay takes to trip on overload. The most common classes are 10, 20 and 30. The numeric designation reflects the time required for the relay to operate. A class 10 overload relay operates in 10 seconds or less at 600% full load current, a class 20 relay operates in 20 seconds or less, and a class 30 relay operates in 30 seconds or less.

The slope of the tripping characteristic depends on the protection class of the motor. IEC motors are usually tailored to a specific application. This means that the overload relay can handle excess current that is very close to the maximum performance of the relay. Class 10 is the most common class for IEC motors. NEMA motors have a larger internal capacitor, so Class 20 is more commonly used.

Class 10 relays are commonly used for pump motors since the acceleration time of the motors is about 0.1-1 seconds. Many high inertia industrial loads require a Class 20 relay to operate.

Fuses serve to protect the installation against damage that could be caused by a short circuit. Therefore, the fuses must be of sufficient capacity. Lower currents are isolated with an overload relay. Here, the rated current of the fuse does not correspond to the operating range of the electric motor, but to the current that can damage the weakest components of the installation. As mentioned earlier, the fuse provides short circuit protection but not low current overload protection.

The figure shows the most important parameters that form the basis for the coordinated operation of fuses in combination with an overload relay.

It is very important that the fuse blows before other parts of the installation are thermally damaged by short circuits.

Modern outdoor motor protection relays

Advanced external motor protection systems also provide protection against overvoltage, phase imbalance, limit the number of on / off switches, and eliminate vibration. In addition, they allow the stator and bearing temperatures to be monitored via a temperature sensor (PT100), to measure the insulation resistance and to record the ambient temperature. In addition to this, advanced external motor protection systems can receive and process the signal from the built-in thermal protection. Later in this chapter, we will look at a thermal protector.

External motor protection relays are designed to protect three-phase electric motors in the event of a threat of motor damage for a short or longer period of operation. In addition to protecting the motor, the external protection relay has a number of features that provide protection for the electric motor in various situations:

Provides a signal before a malfunction occurs as a result of the entire process

Diagnoses malfunctions that have occurred

Allows you to check the operation of the relay during maintenance

Monitors bearing temperature and vibration

An overload relay can be connected to a central building management system for continuous monitoring and online troubleshooting. If an external protection relay is installed in the overload relay, there is less downtime due to process interruption due to breakdown. This is achieved by quickly detecting malfunctions and avoiding damage to the motor.

For example, an electric motor can be protected against:

Overload

Rotor blocking

Jamming

Frequent restarts

Open phase

Short to ground

Overheating (via a signal from the motor through a PT100 sensor or thermistors)

Low current

Overload warning

External overload relay setting

The full load current at the specific voltage indicated on the nameplate is the guideline for setting the overload relay. Since there are different voltages in the networks of different countries, electric motors for pumps can be used both at 50 Hz and at 60 Hz in a wide range of voltages. For this reason, the current range is indicated on the motor nameplate. If we know the voltage, we can calculate the exact current carrying capacity.

Calculation example

Knowing the exact voltage for the installation, you can calculate the full load current at 254/440 Y B, 60 Hz.

The data is displayed on the nameplate as shown in the illustration.

Calculations for 60 Hz

The voltage gain is determined by the following equations:

Calculation of the actual full load current (I):

(Current values for delta and star connections at minimum voltages)

(Current values for delta and star connections at maximum voltages)

The first formula can now be used to calculate the full load current:

I for "triangle":

I for "star":

The values for the full load current correspond to the permissible full load current of the motor at 254 Δ / 440 Y V, 60 Hz.

Attention : the external motor overload relay is always set to the rated current indicated on the nameplate.

However, if the motors are designed with a load factor that is then indicated on the nameplate, e.g. 1.15, the setpoint current for the overload relay can be increased by 15% compared to the full load current or service factor amps (SFA). ), which is usually indicated on the name plate.

Why is integrated motor protection needed if the motor is already equipped with overload relays and fuses? In some cases, the overload relay will not register the motor overload. For example, in situations:

When the motor is closed (insufficiently cooled) and slowly heats up to a dangerous temperature.

At high ambient temperatures.

When the external motor protection is set too high a trip current or is set incorrectly.

When the motor is restarted several times over a short period of time and the inrush current heats up the motor, ultimately damaging it.

The level of protection that internal protection can provide is specified in IEC 60034-11.

TP designation

TP stands for thermal protection. There are different types of thermal protection, which are indicated by the TP code (TPxxx). The code includes:

The type of thermal overload for which the thermal protection has been designed (1st digit)

Number of levels and type of action (2nd digit)

In pump motors, the most common TP designations are:

TP 111: Gradual overload protection

TP 211: Protection against both fast and gradual overload.

Designation	Technical load and its variants (1st digit)	Number of levels and functional area (2nd digit)
TP 111	Only slowly (constant overload)	1 level when disabled
TR 112
TP 121
TP 122
TR 211	Slow and fast (constant overload, blockage)	1 level when disabled
TR 212
TR 221 TR 222		2 levels on alarm and shutdown

TR 311 TR 321	Only fast (blocking)	1 level when disabled

Display of the permissible temperature level when the motor is exposed to high temperatures. Category 2 allows higher temperatures than category 1.

All Grundfos single-phase motors are equipped with motor overcurrent and temperature protection in accordance with IEC 60034-11. The TP 211 motor protection type means that it reacts to both gradual and rapid temperature increases.

Resetting data in the device and returning to the initial position is carried out automatically. Grundfos MG three-phase motors from 3.0 kW are equipped with a PTC temperature sensor as standard.

These motors have been tested and approved as TP 211 motors, which respond to both slow and fast temperature rises. Other electric motors used for Grundfos pumps (MMG models D and E, Siemens, etc.) can be classified as TP 211, but they generally have protection type TP 111.

The information on the name plate must always be observed. Information on the type of protection for a particular motor can be found on the name plate - TP (thermal protection) according to IEC 60034-11. Typically, internal protection can be provided with two types of protection devices: Thermal protectors or thermistors.

Thermal protection devices built into the terminal box

Thermal protectors, or thermostats, use a snap action disc-type bimetallic circuit breaker to open and close the circuit when a certain temperature is reached. Thermal protectors are also referred to as "Klixons" (from Texas Instruments). As soon as the bimetallic disc reaches the set temperature, it opens or closes the group of contacts in the connected control circuit. The thermostats are equipped with contacts for normally open or normally closed operation, but the same device cannot be used for both modes. The thermostats are pre-calibrated by the manufacturer and cannot be changed. The discs are hermetically sealed and located on the terminal block.

The thermostat can supply voltage to the alarm circuit - if it is normally open, or the thermostat can de-energize the motor - if it is normally closed and connected in series with the contactor. Since thermostats are located on the outer surface of the coil ends, they react to the temperature at the location. With regard to three-phase electric motors, thermostats are considered unstable protection under braking conditions or other conditions of rapid temperature changes. In single-phase motors, thermostats are used to protect against locked rotor.

Thermal circuit breaker built into the windings

Thermal protectors can also be built into the windings, see illustration.

They act as a mains switch for both single-phase and three-phase motors. In single-phase motors up to 1.1 kW, the thermal protector is installed directly in the main circuit to act as a winding protector. Klixon and Thermic are examples of thermal circuit breakers. These devices are also called PTO (Protection Thermique a Ouverture).

Indoor installation

Single-phase motors use one single thermal circuit breaker. In three-phase electric motors - two series-connected switches located between the phases of the electric motor. Thus, all three phases are in contact with the thermal switch. Thermal circuit breakers can be installed at the end of the windings, but this tends to increase the response time. The switches must be connected to an external control system. This protects the motor against gradual overload. For thermal circuit breakers, an amplifier relay is not required.

Thermal switches DO NOT PROTECT the motor if the rotor is locked.

The principle of operation of the thermal circuit breaker

The graph on the right shows resistance versus temperature for a standard thermal circuit breaker. Each manufacturer has its own characteristic. TN usually lies in the range 150-160 ° C.

Connection

Connection of a three-phase electric motor with built-in thermal switch and overload relay.

TP notation on the graph

Protection according to IEC 60034-11:

TP 111 (gradual overload). In order to provide protection in the event of a locked rotor, the motor must be equipped with an overload relay.

The second type of internal protection is thermistors, or positive temperature coefficient (PTC) sensors. Thermistors are built into the windings of the electric motor and protect it from locked rotor, prolonged overload and high ambient temperatures. Thermal protection is provided by monitoring the temperature of the motor windings using PTC sensors. If the temperature of the windings exceeds the shutdown temperature, the resistance of the sensor changes according to the temperature change.

As a result of this change, the internal relays de-energize the control loop of the external contactor. The electric motor is cooled down and the acceptable temperature of the electric motor winding is restored, the resistance of the sensor is reduced to the initial level. At this point, the control module is automatically reset to its original position, unless it has previously been configured to reset and manually re-enable.

If the thermistors are self-installed at the ends of the coil, the protection can only be classified as TP 111. The reason is that the thermistors do not have full contact with the ends of the coil and therefore cannot react as quickly as if they were originally built into the winding.

The thermistor temperature sensing system consists of positive temperature coefficient (PTC) sensors in series and a solid state electronic switch in an enclosed control box. The set of sensors consists of three - one per phase. The resistance in the sensor remains relatively low and constant over a wide temperature range, with a sharp increase at the response temperature. In such cases, the sensor acts as a solid state thermal circuit breaker and de-energizes the monitor relay. The relay opens the control circuit of the entire mechanism to disconnect the protected equipment. When the winding temperature is restored to an acceptable value, the control unit can be manually reset.

All Grundfos motors from 3 kW and above are equipped with thermistors. A positive temperature coefficient (PTC) thermistor system is considered to be fault tolerant because a failure of the sensor or disconnection of the sensor wire creates infinite resistance and the system responds in the same way as when the temperature rises - the monitoring relay is de-energized.

The principle of operation of the thermistor

The critical resistance / temperature relationships for motor protection sensors are defined in DIN 44081 / DIN 44082.

The DIN curve shows the resistance in thermistor sensors versus temperature.

Compared to PTO, thermistors have the following advantages:

Faster response due to lower volume and weight

Better contact with the motor winding

Sensors are installed on each phase

Provides protection when the rotor is blocked

TP designation for motor with PTC

TP 211 motor protection is only realized when the PTC thermistors are fully fitted at the ends of the windings at the factory. The TP 111 protection can only be realized by self-installation on site. The motor must be tested and certified to comply with the TP 211 marking. If a motor with PTC thermistors has TP 111 protection, it must be equipped with an overload relay to prevent the consequences of seizure.

Compound

The figures on the right show the connection diagrams for a three-phase electric motor equipped with PTC thermistors with Siemens trip units. To implement protection against both gradual and rapid overload, we recommend the following connection options for motors equipped with PTC sensors with TP 211 and TP 111 protection.

If the motor with thermistor is marked TP 111, this means that the motor is only protected against gradual overload. In order to protect the motor against rapid overload, the motor must be equipped with an overload relay. The overload relay must be connected in series with the PTC relay.

Motor TP 211 is only protected if the PTC thermistor is fully integrated into the windings. TP 111 protection is only realized when connected independently.

The thermistors are designed in accordance with DIN 44082 and can withstand a load of Umax 2.5 V DC. All disconnecting elements are designed to receive signals from DIN 44082 thermistors, i.e. thermistors from Siemens.

note: It is very important that the built-in PTC device is connected in series with the overload relay. Repeated re-energizing of the overload relay can lead to burnout in the event of a motor stall or high inertia start. Therefore, it is very important that the temperature and current consumption data of the PTC device and relay

In an electric motor, as in many other electrical devices, accidents can occur. If measures are not taken in time, then in the worst case, due to a breakdown of the electric motor, other elements of the power system may also fail.

The most widespread are asynchronous electric motors. There are 5 main types of accidents in asynchronous motors:

phase loss OF stator winding of the motor (probability of occurrence 40-50%);
braking of the rotor ZR (20-25%);
technological overloads TP (8-10%);
lowering the insulation resistance of the winding PS (10-15%);
violation of engine cooling BUT (8-10%).

Any of these types of accidents can lead to failure of the electric motor, and a short circuit in the motor is dangerous for the supply network.

Emergency modes such as OF, ZR, TP and BUT are capable of causing an overcurrent in the stator winding. As a result, the current increases to 7 I and more over a fairly long period of time.

A short circuit in the motor can lead to an increase in current of more than 12 I for a very short period of time (about 10 ms).

Taking into account possible damage, and select the required protection.

Motor overload protection. Basic types.

Thermal protection- carried out by heating the winding of the heating element with a current and affecting it on the bimetallic plate, which in turn opens the contact in the control circuit of the contactor or starter. Thermal protection is carried out using thermal relays.

Temperature protection- reacts to an increase in the temperature of the hottest parts of the engine using built-in temperature sensors (for example, PTC thermistors). Through temperature protection devices (UVTZ), it acts on the control circuit of the contactor or starter and turns off the motor.

Overcurrent protection- reacts to an increase in the current in the stator winding and when it reaches the current, the settings will disconnect the control circuit of the contactor or starter. It is carried out using maximum current relays.

Undercurrent protection- reacts to the disappearance of current in the stator winding of the motor, for example, in the event of an open circuit. After that, a signal is given to disconnect the control circuit of the contactor or starter. It is carried out using minimum current relays.

Phase-sensitive protection- reacts to a change in the phase angle between the currents in the three-phase circuit of the stator winding of the motor. When the phase angle changes within the setting (for example, in case of phase failure, the angle increases to 180º), a signal is given to disconnect the contactor or starter control circuit. It is carried out using phase-sensitive relays of the FUZ type.

Overload protection efficiency table:

№	Overload protection type	Reliability of protection
№	Overload protection type	reliably	less reliable	not reliable
1	Thermal protection	TP	OF; ZR	BUT; PS
2	Temperature protection	TP; BUT	OF; ZR	PS
3	Overcurrent protection	ZR	TP	OF; BUT; PS
4	Undercurrent protection	OF	—	BUT; PS; TP; ZR
5	Phase-sensitive protection	TP; OF; ZR	—	BUT; PS

One of the most effective means of protecting the motor is a circuit breaker.

A circuit breaker with overcurrent protection, which will protect the motor from an excessive increase in current in the stator winding circuit, for example, in case of phase failure or insulation damage. In doing so, it protects the supply circuit against a short circuit in the motor.

The circuit breaker, which includes a thermal release, an undervoltage release, is able to protect the motor from other abnormal conditions.

Currently, it is one of the most effective protective devices for induction motors and the circuits in which they operate.

General rules for choosing protection for induction motors.

All motors must be short-circuit protected and motors operating in S1 mode must have overcurrent protection.

Electric motors, the windings of which switch from "delta" to "star" at start-up, should preferably be protected with three-pole thermal relays with accelerated response in open-phase modes. For electric motors operating in intermittent modes, it is recommended to provide built-in temperature protection. Motors operating in short-time S2 mode with possible rotor braking without technological damage should be equipped with thermal protection. In the event that the rotor braking entails technological damage, temperature protection should be applied.

Thermal relays are mainly designed to protect motors in S1 mode. It is permissible to use them for the S2 mode, if an increase in the duration of the working period is excluded. For S3 mode, the use of thermal relays is allowed in exceptional cases with a motor load factor of no more than 0.7.

Single-pole relays (two relays), two-pole and three-pole relays can be used to protect the star-connected motor windings. Protection of delta-connected windings should be carried out by three-pole relays with accelerated operation in open-phase modes.

For multi-speed motors, it is necessary to provide separate relays at each speed stage if it is necessary to fully use the power at each stage or one relay with a setpoint selected by the current of the highest speed stage for motors with fan load.

The rated current of the thermal elements of the relay must be selected according to the rated motor current so that the rated motor current is between the minimum and maximum relay current settings.

An overload of the electric motor occurs in the following cases:

with prolonged start-up or self-start;
for technological reasons, for mechanisms with fluctuating loads (hoists, rolling mills, etc.);
when overloading the mechanism that occurs in coal mills and crushers when raw coal enters them and on other mechanisms of a similar type;
as a result of the breakage of one phase;
in case of damage to the mechanical part of the electric motor or mechanism, causing an increase in torque M with and braking the electric motor.

Overloads are stable and short-term.

Only persistent overloads are dangerous for the electric motor.

Overcurrents caused by starting or self-starting of the electric motor are short-term and self-destruct when the normal rotation speed is reached. These currents can be dangerous only if the process of deployment of the electric motor is delayed or if, during self-starting, it turns out that M d.< М с. нач. . В последнем случае электродвигатель развернуться не сможет и длительно будет обтекаться пусковым током.

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

The main danger of overcurrents for an electric motor is the accompanying increase in the temperature of individual parts and, first of all, of the windings.

An increase in temperature accelerates the deterioration of the winding insulation and thus reduces the service life of the motor.

The overload capacity of an electric motor is determined by the characteristic of the relationship between the magnitude of the overcurrent and the permissible time of its flow:

t = T a-1 / k 2 -1

where t -permissible overload duration, sec;

T- heating time constant, sec;

a- coefficient depending on the type of motor insulation, as well as the frequency and nature of overcurrents; for asynchronous electric motors on average a = 1.3;

k- frequency of overcurrent - the ratio of this current to the rated current of the motor, i.e. k = I / I nom

Previously, overload protection was installed with a shutdown action on all electric motors, which in some cases led to incorrect shutdowns of electric motors.

Currently, when deciding on the installation of overload protection on an electric motor, they are guided by the conditions of its operation:

on electric motors of mechanisms that are not damaged by technological overloads (for example, electric motors of circulation, feed pumps, etc.) and do not have severe starting or self-starting conditions, overload protection is not installed.
on electric motors subject to technological overloads (for example, electric motors of mills, crushers, dredging pumps, etc.), as well as on electric motors, which self-starting is not ensured, overload protection must be installed.
overload protection is performed with a shutdown action in the event that self-starting of the electric motor is not ensured or the technological overload cannot be removed from the mechanism without stopping the electric motor.
overload protection of the electric motor is performed with an effect 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 electric motors of mechanisms that can have both an overload eliminated during the operation of the mechanism and an overload, the elimination of which is impossible without stopping the mechanism, it is advisable to provide for the action of 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 from overload is carried out mainly with an effect on the signal.

Protection of electric motors subject to technological overload, it is desirable to have such that, on the one hand, it protects against unacceptable overloads, and on the other hand, makes it possible to use the overload characteristic of the electric motor to the fullest, taking into account the previous load and the ambient temperature.

MINISTRY OF AGRICULTURE OF THE RUSSIAN FEDERATION

BASHKIR STATE AGRARIAN UNIVERSITY

REPORT

on industrial operational practice

Faculty: Energy

Department: Power Supply and Electrical Application

energy in agriculture

Specialty: 140106 Agricultural electrification and automation

Full-time form of education

Course, group: EA 201/1

Arduvanov Ilgiz Radievich

INTRODUCTION

Electric machines are widely used in power plants, in industry, in transport, in aviation, in automatic control and regulation systems, 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 both a generator and a motor. This property is called reversibility. It can also be used to convert one type of current to another (frequency, number of phases of alternating current, voltage) into energy of another type of current. Such machines are called converters. Electric machines, depending on the kind of current of the 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 electric machines is based on the use of the laws of electromagnetic induction and electromagnetic forces.

Electric motors used in industry and in everyday life are produced in series, which are a series of electric machines of increasing power, having the same design and meeting a general set of requirements. Series of special purpose are widely used.

Protection of electric motors. Motor protection circuit

When operating 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 moving on to 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, in the terminal box of the electric motor, in the stator winding (on the case, interturn short circuits).

Short circuits are the most dangerous type of malfunction in an electric motor, since they are accompanied by the occurrence of very high currents, leading to overheating and combustion 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 getting into the auger, starting the engine under too heavy 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 nominal) in the stator windings of the other two phases.

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

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, that is, when short circuits occur. 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 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 - through them, voltage is applied to the coil of the magnetic starter.

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 an electric motor from phase loss is to add an additional magnetic starter to its connection diagram:

Fig. 3 Wiring diagram for additional magnetic starter

Turning on the circuit breaker 1 leads to the closure of the power supply circuit of the coil of the magnetic starter 2 (the operating voltage of this coil should be ~ 380 V) and the closure of 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 engine. 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 absent, the voltage will not flow to the motor, which will prevent it from thermal overloads and premature failure.

Reliable and uninterrupted operation of electric motors is ensured, first of all, by their proper selection in terms of rated power, operating mode and design. Of no less importance is the observance of the necessary requirements and rules when drawing up an electrical diagram, choosing control gear, wires and cables, installing and operating an electric drive.

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

Emergency operating modes of electric motors

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

Emergency modes include:

1) multiphase (three- and two-phase) and single-phase short circuits in the motor windings; multiphase 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 the boxes of resistances); phase short circuits to the frame or neutral wire inside the motor or in the 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 insulation overlap. Short-circuit currents sometimes reach values that are tens and hundreds of times higher than the values of the normal mode currents, and their thermal effect and dynamic forces to which live 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: when the working mechanism is overloaded for technological reasons, especially severe conditions of starting the engine under load or stalling, prolonged decrease in the mains voltage, the loss of one of the phases of the external power circuit or a wire break in motor winding, mechanical damage in the motor or working mechanism, as well as thermal overloads when the cooling conditions of the motor deteriorate. Thermal overloads primarily cause 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 of asynchronous electric motors

In order to protect the electric motor from damage in case of violation of normal operating conditions, as well as to promptly disconnect the faulty motor from the network, preventing or thereby limiting the development of an accident, protective measures are provided. The main and most effective means is the electrical protection of motors, carried out in accordance with the "Rules for Electrical Installations" (PUE). Depending on the nature of possible damage and abnormal operating modes, there are several main most common types of electrical protection for induction motors.

Protection of induction motors against short circuits

Short-circuit protection turns 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) act almost instantly, that is, without a time delay.

Overload protection of asynchronous motors

Overload protection protects the motor against impermissible overheating, in particular, even with relatively small but long-term thermal overloads. Overload protection should only be used for electric motors of those working mechanisms, in which abnormal increases in load are possible due to disturbances in the working process.

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

Fig. 6 Winding shop

Protection of asynchronous electric motors against undervoltage or undervoltage

Protection against undervoltage or undervoltage (zero protection) is performed using one or more electromagnetic devices, acts to turn off the motor in case of a power interruption or when the mains voltage drops below the set value and protects the motor from spontaneous switching on after the elimination of the power interruption or restoration of normal mains voltage.

Special protection against operation on two phases protects the motor from overheating, as well as from "overturning", that is, stopping under current due to a decrease in the torque developed by the motor, in the event of an open circuit in one of the phases of the main circuit. Protection acts on engine shutdown. Both thermal and electromagnetic relays are used as protection devices. In the latter case, the protection may not have a time delay.

Fig. 7

Other types of electrical protection of asynchronous motors

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

Electrical devices used to protect electric motors

Electrical protection devices can implement one or several types of protection at once. For example, some circuit breakers provide short-circuit and overload protection. Some of the protection devices, for example fuses, are single-acting devices and require replacement or recharging after each operation, while 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-resetting and manual resetting.

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 procedure (presence or absence of permanent maintenance personnel). on a construction site, in a workshop, etc., identifying the most frequently recurring violations of the normal operation of engines and technological equipment. You should always strive to ensure that protection is as simple and reliable as possible in operation.

Each motor, regardless of its power and voltage, must be protected against short circuits. The following circumstances should be borne in mind here. On the one hand, the protection must be offset 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 circuits, short circuits between phases near the zero point of the stator winding, short circuits to the case 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). It is very difficult to simultaneously fulfill these conflicting requirements with simple and cheap protections. Therefore, the protection system for low-voltage asynchronous motors is built under the deliberate assumption that with some of the above-mentioned damages in the motor, the latter is not turned off by the protection immediately, but only during the development of these damages, after the current consumed by the motor from the network has significantly increased.

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

State Unitary Enterprise PPZ "Blagovarsky"

State Unitary Enterprise "Plemptitsezavod Blagovarskiy" is the legal successor 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 pedigree poultry plant with the assignment of the functions of a selection and genetic center for duck breeding. The Blagovarsky breeding farm is located near the village of Yazykovo, Blagovarsky district of the Republic of Bashkortostan.

The total 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 head of laying duck.

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

The structure of the plant includes:

1. Workshop for parental herd of ducks: it has 30 buildings with the number of poultry places for 110 thousand heads.

2. Workshop for rearing young stock: it has 6 buildings with the number of poultry places for 54 thousand heads.

3. Hatcheries: 3 workshops with a total capacity of 695,520 pcs. eggs per tab.

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

5. A 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 vehicles 27.

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

Ducks created by breeders of the Blagovarsky Plemptitsezavod breeders have become widespread in the Russian Federation, they are successfully bred in both Krasnodar and Primorsky regions. The use of breeding ducks of the breeding farm in the structure of the total population of ducks in Russia is about 80%.


	Workplace	Type of work	Work execution technology	Signature of the hands.	Note
		Installation work.	Disassembly and assembly of 3-phase asynchronous motors.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Cabling.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Cabling.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Grain crusher assembly, water heater installation.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Lighting system installation.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.
08.07.12-09.07.12	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Planned work.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Installation of a diesel power plant.


	Workplace	Type of work	Work execution technology	Signature of the hands.	Note
11.07.12-15.07.12	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	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 circuit breakers.
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"
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Planned work.	Cleaning and cleaning from 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 start-up of AVM.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Disassembly and assembly of 3-phase asynchronous motors.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Lighting system installation.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Maintenance of transformers.
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Replacement, dismantling and maintenance of the ventilation system "Climate-47"
	Blagovarsky district, State Unitary Enterprise "PPZ Blagovarsky"	Installation work.	Replacement of circuit breakers.

Start of practice 06/26/12 End of practice 08/04/12

CONCLUSION

As a result of passing industrial operational practice in the State Unitary Enterprise PPZ "Blagovarsky", I studied the structure of the enterprise, the diagram of the enterprise's power supply network, as well as collected material

There is practically no equipment in operation where electric is not used. This type of electromechanical actuator is widely used in various configurations. From a constructive point of view, an electric motor is a simple equipment, quite understandable and simple. However, the operation of the electric motor is accompanied by significant loads of a different nature. That is why, in practice, motor protection relays are used, the functionality of which is also versatile. The degree of efficiency for which the protection of an electric motor is designed, as a rule, is determined by the circuit solutions for the implementation of relays and control sensors.

With regard to insignificant service motors, an instantaneous relay with an inverse-dependent response time to phase overcurrents is used for automatic shutdown.

Motor protection circuit against overcurrent and ground faults: 1, 2, 3 - current transformers; 4, 5, 6 - current cut-off devices; F1, F2, F3 - linear phases; 7 - ground

Phase sequence relays are usually set to 3.5 to 4 times the motor operating current, allowing for a sufficient time delay to prevent tripping when the motor starts.

For service motors of high importance, current relays with inversely dependent operating times are generally not used. The reason for this is that the circuit breaker is activated directly in the motor circuit.

Overheating of stator windings

Critical condition mainly due to continuous overload, rotor braking or stator current imbalance. For complete protection, in this case, the three-phase motor must be equipped with overload monitoring elements on each phase.

Here, to protect insignificant service motors, overload protection or direct tripping to disconnect from the power source in the event of an overload is usually used.

If the rated motor power exceeds 1000 kW, an inverse current time relay is generally used instead of a single RTD relay.

Thermistors of the limiting temperature for the stator of the motor: 1 - tinned part of the conductor 7-10 mm; 2 - length size 510 - 530 mm; 3 - thermistor length 12 mm; 4 - thermistor diameter 3 mm; Arc connections 200 mm long

For important motors, automatic shutdown is optional. A thermal relay is used as the main protector against overheating of the stator windings.

Rotor overheating factor (phase)

Rotor overheating protection is often found in wound (phase) rotor motors. The increase in rotor current is reflected in the stator current, which requires activation of the stator overcurrent protection.

The current setting of the stator protection relay as a whole is equal to the full load current increased by 1.6 times. This value is quite enough to determine the phase rotor overheating and enable the blocking.

Undervoltage protection

The electric motor draws excessive current when operating under voltage below the established norm. Therefore, undervoltage or overvoltage protection must be provided by overload sensors or temperature sensitive elements.

To avoid overheating, the motor must be de-energized for 40-50 minutes even in case of small overloads exceeding 10-15% of the standard.

The classic version of thermal monitoring of the stator winding: T - temperature sensors embedded directly among the winding conductors

A protective relay should be used to monitor the heating of the motor rotor due to negative sequence currents occurring in the stator due to imbalance in the supply voltage.

Imbalance and Phase Failure

An unbalanced three-phase supply also causes negative sequence current to flow in the stator windings of the motor. This condition causes overheating of the stator and rotor (phase) windings.

The imbalance condition transmitted to the motor for a short time must be controlled and maintained at such a level to avoid the occurrence of a continuous unbalance condition.

It is preferable to supply the phase-to-phase fault monitoring relay from the positive phase, and for protection against earth faults, use an instantaneous differential relay connected to the circuit of the current transformer circuit.

Unintended phase reversal

In some cases, phase reversal is seen as dangerous for the motor. For example, this condition can negatively affect the operation of elevator equipment, cranes, lifts, and some types of public transport.

Here it is imperative to provide protection against phase reversal - a specialized relay. The operation of the phase reversal relay is based on an electromagnetic principle. The device contains a disk motor driven by a magnetic system.

Board and diagram of the phase reversal device: 1 - automatic switch or fuse-link; 2 - overload protection; 3 - current phase; 4 - phase reverse; 5 - electric motor

If the correct phase sequence is noted, the disc generates torque in a positive direction. Therefore, the auxiliary contact is held in the closed position.

When phase reversal is detected, the disc torque is reversed. Consequently, the auxiliary contact switches to the open position.

This switching system is used for protection, in particular for controlling a circuit breaker.