Select motor overload protection. Motor protection: main types, connection diagrams and principle of operation. Instructions on how to install with your own hands Causes of overheating

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There is practically no equipment in operation where electric would not be used. This type of electromechanical drives of various configurations is used everywhere. 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 the electric motor is designed, as a rule, is determined by the circuit solutions for the implementation of relays and control sensors.

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

The phase sequence relays are usually set to 3.5-4 times the motor operating current, with enough time delay to prevent tripping when the motor starts.

For service motors of high importance, current relays with inverse time tripping are generally not used. The reason for this is the involved circuit breaker directly in the motor circuit.

Overheating of the stator windings

A critical condition, mainly due to continuous overload, stalled rotor or stator current imbalance. For complete protection, in this case, a three-phase motor must be equipped with overload control elements on each phase.

Here, to protect minor service motors, overload protection or direct tripping from the power source in case of overload is usually used.

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

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

Rotor overheating factor (phase)

Protection against overheating of the rotor is often found in engines with a wound (phase) rotor. An increase in the rotor current is reflected in the stator current, which requires the inclusion of protection against overcurrent of the stator.

The setting of the stator protection relay for current in general is a value equal to the full load current increased by 1.6 times. This value is quite enough to determine the overheating of the phase rotor and turn on the lock.

Undervoltage protection

The motor draws excessive current when operating under voltage below the specified limit. Therefore, protection against undervoltage or overvoltage must be provided by overload sensors or temperature sensing elements.

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

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

Imbalance and phase failure

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

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

It is preferable to power the phase-to-phase monitoring relay from the positive phase, and to protect 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 a dangerous phenomenon for the motor. For example, such a condition may adversely affect the operation of elevator equipment, cranes, lifts, and some types of public transport.

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

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

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

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

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.

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Protection of the electric motor against overload today is one of the main tasks that must be solved in order to successfully operate this device. These types of engines are used quite widely, and therefore many ways have been invented to protect them from various negative effects.

Protection levels

There is a wide variety of devices to protect this equipment, however, all of them can be divided into levels.

• External level of short circuit protection. Most often, various types of relays are used here. These devices and the level of protection are at the official level. In other words, this is a mandatory subject of protection, which must be installed in accordance with the safety rules on the territory of the Russian Federation.
• The motor overload protection relay will help to avoid a variety of critical damage during operation, as well as possible damage. These devices also belong to the external level of protection.
• The inner layer of protection prevents possible overheating of engine parts. For this, external switches are sometimes used, and sometimes overload relays.

Reasons for hardware failures

Today, there is a wide variety of problems due to which the performance of an electric motor can be impaired if it is not equipped with protection devices.

1. A low level of electrical voltage or, conversely, too high a supply level can cause failure.
2. Damage is possible due to the fact that the frequency of the current supply will change too quickly and often.
3. Incorrect installation of the unit or its components can also be dangerous.
4. Rise in temperature to a critical value or higher.
5. Too little cooling also leads to breakdowns.
6. The increased ambient temperature has a strong negative effect.
7. Few people know that low pressure or installing the engine much above sea level, which causes low pressure, also has a negative effect.
8. Naturally, it is necessary to protect the motor from overloads that may occur due to power failures.
9. Frequent switching on and off of the device is a negative defect that also needs to be eliminated with the help of protection devices.

Fuses

The full name of the protective equipment is a fusible safety switch. This device combines both a circuit breaker and a fuse, which are located in one housing. The switch can also open or close the circuit manually. The fuse is the protection of the electric motor against overcurrent.

It should be noted that the design of the emergency switch provides for the presence of a special casing that protects personnel from accidental contact with the terminals of the device, as well as the contacts themselves from oxidation.

As for the fuse, this device must be able to distinguish between an overcurrent and a short circuit in the circuit. This is very important, as short-term overcurrent is quite acceptable. However, the motor overload current protection should trip immediately if this parameter continues to increase.

Short circuit fuses

There is a type of fuse that is designed to protect the unit from a short circuit (short circuit). However, it is worth noting here that the fast-acting fuse may fail if a short-term overload occurs during the start-up of the device, that is, an increase in the starting current. For this reason, such devices are usually used in networks where such a jump is not possible. As for the motor overload protector itself, the fast blow fuse can withstand a current that will exceed its rated current by 500% if the difference lasts no more than a quarter of a second.

Fuses delayed

The development of technology has led to the fact that it was possible to create a device for protection against both overload and short circuit at the same time. This tool was a fuse with a delay. The peculiarity is that it is able to withstand a 5-fold increase in current if it lasts no more than 10 seconds. An even larger increase in the parameter is possible, but for a shorter period before the fuse blows. However, most often an interval of 10 seconds is enough to start the engine, and so that the fuse does not work. Protection against overloads, against short circuit, as well as another type of electric motor by such a device is considered one of the most reliable.

It is also worth noting here how the response time of this protection device is determined. The response time of a fuse is the length of time for which its fusible element (wire) melts. When the wire is completely melted, the circuit opens. If we talk about the dependence of the disconnection time on overload for these types of protective equipment, then they are inversely proportional. In other words, the current protection of the electric motor against overloads works like this - the higher the current strength, the faster the wire melts, which means that the time for disconnecting the circuit is reduced.

Magnetic and thermal devices

To date, automatic thermal-type devices are considered the most reliable and economical devices for protecting an electric motor from thermal overloads. These devices are also capable of withstanding large current amplitudes that may occur during instrument start-up. In addition, thermal fuses protect against problems such as a locked rotor, for example.

Protection of asynchronous electric motors against overload can be carried out using automatic magnetic switches. They are highly reliable, accurate and economical. Its peculiarity lies in the fact that the temperature limit of its operation is not affected by changes in the ambient temperature, which is very important in some operating conditions. They also differ from thermal themes, they have a more precisely defined response time.

Overload relay

The functions of this device are quite simple, however, and quite important.

1. Such a device is able to withstand a short-term current surge during engine start without breaking the circuit, which is most important.
2. Opening of the circuit occurs if the current increases to the value when there is a threat of breakage of the protected device.
3. After the overload is removed, the relay can reset automatically or can be reset manually.

It should be noted that the current protection of the electric motor against overloads with the help of a relay is carried out in accordance with the response characteristic. In other words - depending on the class of the device. The most common are classes 10, 20 and 30. The first group are relays that operate in the event of an overload, for 10 seconds and if the numerical value of the current exceeds 600% of the nominal. The second group is triggered after 20 seconds or less, the third, respectively, after 30 seconds or less.

Fused Protectors and Relays

At present, it is quite common to combine two means of protection - fuses and relays. This combination works as follows. The fuse must protect the motor from a short circuit, and therefore it must have a sufficiently large capacity. Because of this, it cannot protect the device from lower, but still dangerous, currents. It is to eliminate this shortcoming that relays are introduced into the system that respond to weaker, but still dangerous current fluctuations. The most important thing in this case is to adjust the fuse in such a way that it will work before damage to any element occurs.

Outdoor protection

Nowadays, advanced external motor protection systems are quite often used. They can protect the device from overvoltage, phase imbalance, are able to eliminate vibrations or limit the number of on and off. In addition, such tools have a built-in thermal sensor that helps control the temperature of the bearings and stator. Another feature of such a device is that it is able to perceive and process a digital signal that creates a temperature sensor.

The main purpose of external protective equipment is to maintain the performance of three-phase motors. In addition to being able to protect the motor during a power failure, such equipment also has several other advantages.

• An outdoor device can generate and signal a fault before it affects the operation of the machine.
• Diagnoses problems that have already arisen.
• Allows you to test the relay during maintenance.

Based on the foregoing, it can be argued that there is a wide variety of devices for protecting the electric motor from overload. In addition, each of them is able to protect the device from certain negative influences, and therefore it is advisable to combine them.

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.

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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.

Motor overload occurs

With prolonged start-up and self-start,

when overloading the driven mechanisms,

When the voltage drops at the motor outputs.

in the event of a phase break.

For the electric motor, only stable overloads are dangerous. Overcurrents caused by starting or self-starting of the electric motor are short-term and self-destruct when the normal speed is reached.

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 itself. The main danger of overcurrents is the accompanying increase in the temperature of individual parts, and first of all, the windings. Temperature rise accelerates wear of the winding insulation and reduces the life of the motor. The overload capacity of the electric motor is determined by the characteristic of the relationship between the overcurrent and the allowable time of its passage:

where t- allowable overload duration, s;

BUT - coefficient depending on the type of motor insulation, as well as the frequency and nature of overcurrents; for conventional engines BUT= 150-250;

TO - overcurrent ratio, i.e. ratio of motor current I d to I nom.

Type of overload characteristic at constant heating time T = 300 s is shown in fig. 20.2.

When deciding on the installation of relay protection from overload and the nature of its action, they are guided by the operating conditions of the electric motor, bearing in mind the possibility of a stable overload of its drive mechanism:

a. On electric motors of mechanisms that are not subject to technological overloads (for example, electric motors of circulation, feed pumps, etc.) and do not have severe starting or self-starting conditions, overload protection may not be installed. However, its installation is advisable on the engines of objects that do not have permanent maintenance personnel, given the danger of overloading the engine with a reduced supply voltage or open-phase mode;

Rice. 20.2. Characteristics of the dependence of the allowable overload duration on the multiplicity of the overload current

b. On electric motors subject to technological overloads (for example, electric motors of mills, crushers, pumps, etc.), as well as on electric motors, the self-starting of which is not provided, an overload protection relay must be installed;

in. Overload protection is performed with a shutdown action if 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;

G. Electric 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 the personnel without stopping the mechanism, and the electric motors are under the supervision of the personnel;

d. 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 the action of a relay protection from overcurrents with a shorter time delay for turning off the electric motor; in those cases when critical auxiliary electric motors of power plants are under constant supervision of the personnel on duty, their protection against overload can be performed with an action 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, it makes it possible to most fully use the overload characteristic of the electric motor, taking into account the previous load and ambient temperature. The best characteristic of the RZ from overcurrents would be one that passed slightly below the overload characteristic (dashed curve in Fig. 20.2).

20.4. Overload 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 Q allocated in the resistance of its heating element. Thermal relays are made on the principle of using the difference in the coefficient of linear expansion of various metals under the influence of heating. The basis of such a thermal relay is a bimetallic plate consisting of metals soldered over the entire surface a and b with very different linear expansion coefficients. When heated, the plate bends towards the metal with a lower coefficient of expansion and closes the relay contacts .

The heating of the plate is carried out by the heating element when current passes through it.

Thermal relays are difficult to maintain and adjust, have different characteristics of individual relay instances, often do not correspond to the thermal characteristics of electric motors and are dependent on ambient temperature, which leads to a mismatch between the thermal characteristics of the relay and the electric motor. Therefore, thermal relays are used in rare cases, usually in magnetic starters and 0.4 kV automatic machines.

20.5. Overload protection with current relays. To protect electric motors from overload, overcurrent relays are usually used using relays with limited dependent characteristics of the RT-80 type or overcurrent relays with independent current relays and time relays.

The advantages of MTZ in comparison with thermal ones are their simpler operation and easier selection and adjustment of the characteristics of relay protection. However, overcurrent protection does not allow using the overload capabilities of electric motors due to their insufficient time of action at low current ratios.

Overcurrent protection with an independent time delay in a single-relay design is usually used on all asynchronous electric motors for auxiliary needs of power plants, and at industrial enterprises - for all synchronous (when it is combined with relay protection from asynchronous mode) and asynchronous electric motors that drive critical mechanisms, as well as for non-responsible asynchronous electric motors with a start-up time of more than 12 ... 13 s.

IDMT overload relays are better matched to the thermal characteristics of the motor, however, and they underutilize the overload capacity of motors in the low current region.

Overload protection with a dependent time delay characteristic can be implemented on a PT-80 type relay or a digital relay.

The overload protection trip current is set from the detuning condition from I nom electric motor:

where to ots– detuning factor, taken equal to 1.05.

Overload protection time t3 P should be such that it is longer than the start time of the motor t start , while motors involved in self-starting have longer self-starting times.

The starting time of asynchronous motors is usually 8 ... 15 s. Therefore, the characteristic of a relay with a dependent characteristic must have a time of at least 12 ... 15 s at the starting current. On relay protection from overload with an independent characteristic, the time delay is assumed to be 14 ... 20 s.

20.6. Overload protection with thermal time delay characteristic on a digital relay. In a digital motor protection relay, for example, type MiCOM P220 has a thermal model of the motor from the positive and negative sequence components of the current consumed by the motor in such a way as to take into account the thermal effect of the current in the stator and rotor. The negative sequence component of the currents flowing in the stator induces currents of significant amplitude in the rotor, which create a significant temperature increase in the rotor winding. The result of the addition carried out MiCOM P220 is the equivalent thermal current I e sq. , showing the temperature rise caused by the motor current. Current I e sq. calculated according to the dependency:

(20.7)

K e– the coefficient of amplification of the effect of the negative sequence current takes into account the increased effect of the negative sequence current in comparison with the positive sequence on motor heating. In the absence of the necessary data, it is assumed to be 4 for domestic engines and 6 for foreign ones.

Additional relay functions MiCOM P220 associated with motor thermal overload are the following .

· Prohibition of disconnection from thermal overload when starting the engine.

· Thermal overload alarm.

· Inhibit start.

· Long start.

Rotor jamming.

Jamming of the engine rotor can occur when the engine is started or during its operation.

The function of the rotor jamming with the engine running is entered automatically when it is successfully turned after the specified time delay has elapsed.

In Sepam 2000 digital relays protection of the engine against prolonged start-up and jamming of the rotor is made differently. The first protection trips and turns off the motor if the motor current from the beginning of the starting process exceeds the value 3 I nom within the specified time t 1 = 2t start. The start of the start is detected at the moment when the absorbed current rises from 0 to 5% of the rated current. The second protection is activated if the start is completed, the motor is running normally, and in steady state, the motor current suddenly reaches a value of more than 3 I nom and kept for a specified time t 2 = 3-4s.

Asymmetry. Protection of the motor against overload by negative sequence currents protects the motor from voltage supply with reverse phase sequence, from phase failure, from operation with long-term voltage unbalance.

When voltage is applied to the motor with reverse phase sequence, the motor starts to rotate in the opposite direction, the driven mechanism may be stuck or rotate with a resistance torque that differs from the forward rotation torque. Thus, the magnitude of the negative sequence current of the motor can fluctuate over a wide range. In the event of a phase failure, the motor reduces the torque by 2 times and, to compensate, the current increases by 1.5 ... 2 times.

With asymmetry of the supply voltages, the negative sequence current can have a different value up to the smallest values. The appearance of the negative sequence current most of all affects the heating of the motor rotor, where it induces currents of double frequency. Thus, it is advisable to have protection against I 2 that would shut down the engine to prevent it from overheating.

Protection has 2 levels:

step I o br > with independent time delay. The trip current is assumed to be (0.2 ... 0.25) I nom engine. The time delay must ensure the disconnection of asymmetrical short circuits in the adjacent network, for which it must be one step higher than the protection of the supply transformer:

(20.8)

step I arr >> IDMT can be used to increase the sensitivity of the protection if the real thermal characteristics of the motor in terms of negative sequence current are known.

Loss of load. The function allows you to detect the decoupling of the engine from the mechanism driven by it due to a broken coupling, conveyor belt, water leakage from the pump, etc. to reduce the operating current of the motor.

Minimum current setting:

where I xx - no-load current of the engine with the mechanism is determined during testing.

Motor undercurrent time delay tI < is determined based on the technological features of the mechanism - possible short-term load shedding, in the absence of such considerations, it is taken equal to:

Time delay for inhibition of automatic undercurrent motor t ban delays the input of automation when starting the engine, if the load is connected to the engine after it has been turned, or is determined based on the technology of applying the load to the engine, if the load is connected to the engine constantly. The setting must be equal to the motor turnaround time plus the required margin:

Number of engine starts. In the absence of specific engine data, the following general considerations can be used:

− According to the PTE, domestic engines are required to provide 2 starts from a cold state and 1 start from a hot state.

− The motor cooling time constant is 40 min.

− The following settings can be made in the start counting automation:

Setting for the time during which the starts are counted: T reading = 30 minutes.

Number of hot starts -1. Number of cold starts - 2.

Time setting during which restart is prohibited T ban= 5 minutes. Do not use the minimum time between starts.

Self-start resolution time. Self-starting of engines at power plants should be ensured with a power interruption time of 2.5 s. According to these data, a calculation check is made to ensure self-starting during a power failure of engines at power plants.

Thus, for power plants, we can take T self-locking = 2.5 s

For other conditions, you should determine the time for which a power outage is possible, for example, the duration of the ATS, perform a calculated self-start check, and if it is provided during such a power outage, set the specified time on the device. If self-starting is not ensured at any power interruption, or if it is disabled, the "self-starting enable" function is not enabled.

test questions

1. What protection should asynchronous motors have in accordance with the PUE?

2. What kind of protection should synchronous motors have in accordance with the PUE?

3. How is the protection and protection settings against phase-to-phase motors selected?

4. How is motor overload protection implemented and setpoints selected?

5. How is protection implemented and motor undervoltage protection settings selected?

6. What are the protection features of synchronous motors?