During the operation of asynchronous electric motors, like any other electrical equipment, malfunctions can occur - malfunctions, often leading to emergency operation, damage to the engine. its premature failure.
Before proceeding to the methods of protecting electric motors, it is worth considering the main and most common causes occurrence of emergency operation of asynchronous electric motors:
- Single-phase and interphase short circuits - in the cable, terminal box of the electric motor, in the stator winding (to the housing, interturn short circuits).
Short circuits are the most dangerous type of malfunction in the electric motor, because it is accompanied by the occurrence of very high currents, leading to overheating and burning of the stator windings.
A common cause of thermal overload of an electric motor, leading to abnormal operation, is the loss of one of the supply phases. This leads to a significant increase in current (twice the rated current) in the stator windings of the other two phases.
The result of the thermal overload of the electric motor is overheating and destruction of the insulation of the stator windings, leading to the short circuit of the windings and the failure of the electric motor.
The protection of electric motors from current overloads consists in the timely de-energization of the electric motor when high currents appear in its power circuit or control circuit, i.e. in the event of short circuits.
To protect electric motors from short circuits, fuse-links, electromagnetic relays, automatic switches with an electromagnetic release are used, selected in such a way that they withstand large starting overcurrents, but immediately work when short-circuit currents occur.
To protect electric motors from thermal overloads, a thermal relay is included in the electric motor connection circuit, which has control circuit contacts - voltage is applied to the magnetic starter coil through them.
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.
Simple and in a reliable way protection of the electric motor against phase failure will be the addition of an additional magnetic starter to the circuit for its connection:
Turning on the circuit breaker 1 closes the power supply circuit of the coil of the magnetic starter 2 (the operating voltage of this coil should be ~ 380 V) and closes the power contacts 3 of this starter, through which (only one contact is used) power is supplied to the coil of the magnetic starter 4.
By turning on the "Start" button 6 through the "Stop" button 8, the power circuit of the coil 4 of the second magnetic starter is closed (its operating voltage can be either 380 or 220 V), its power contacts 5 are closed and voltage is applied to the motor.
When the "Start" button 6 is released, the voltage from the power contacts 3 will go through the normally open block contact 7, ensuring the continuity of the power supply circuit of the magnetic starter coil.
As can be seen from this motor protection circuit, if for some reason one of the phases is missing, the voltage will not be supplied to the motor, which will prevent it from thermal overloads and premature failure.
Soft start of electric motors
Life of an electrician. Three-phase motor protection.
Motor overload protection
Protection of electric motors.
Types of damage and abnormal operating modes of ED.
Damage to electric motors. In the windings of electric motors, ground faults of one stator phase, short circuits between turns and multi-phase short circuits can occur. Earth faults and multi-phase faults can also occur at motor terminals, cables, couplings and funnels. Short circuits in electric motors are accompanied by the passage of high currents that destroy the insulation and copper of the windings, the steel of the rotor and stator. To protect electric motors from multi-phase short circuits, current cutoff or longitudinal differential protection is used, acting on shutdown.
Single-phase earth faults in the stator windings of electric motors with a voltage of 3-10 kV are less dangerous compared to short circuits, as they are accompanied by the passage of currents of 5-20 A, determined by the capacitive current of the network. Considering the relatively low cost of electric motors with a power of less than 2000 kW, earth fault protection is installed on them at an earth fault current of more than 10 A, and on electric motors with a power of more than 2000 kW - with an earth fault current of more than 5 A, the protection acts to turn off.
Protection against winding circuits on electric motors is not installed. Elimination of this type of damage is carried out by other motor protection systems, since coil faults in most cases are accompanied by a ground fault or turn into a multi-phase short circuit.
Electric motors with voltage up to 600 V are protected from short circuits of all types (including single-phase ones) using fuses or high-speed electromagnetic releases of automatic switches.
abnormal modes of operation. The main type of abnormal operation for electric motors is their overload with currents greater than the nominal one. Permissible overload time of electric motors, With, is determined by the following expression:
Rice. 6.1. The dependence of the electric motor current on the rotor speed.
where k - the multiplicity of the electric motor current in relation to the nominal; A - coefficient depending on the type and version of the electric motor: A == 250 - for closed electric motors with a large mass and dimensions, A = 150 - for open electric motors.
Overloading of electric motors can occur due to overload of the mechanism (for example, blockage of mill or crusher with coal, clogging of the fan with dust or pieces of slag from the ash removal pump, etc.) and its malfunction (for example, damage to bearings, etc.). Currents significantly exceeding the rated ones pass during start-up and self-starting of electric motors. This is due to a decrease in the resistance of the electric motor with a decrease in its speed. Motor current dependence I from rotational speed P at a constant voltage at its terminals is shown in fig. 6.1. The current is at its highest when the motor rotor is stopped; this current, called the starting current, is several times higher than the rated current of the electric motor. The overload protection can act on a signal, unload the machine or cut off the motor. After the short circuit is turned off, the voltage at the terminals of the electric motor is restored and the frequency of its rotation begins to increase. In this case, large currents pass through the windings of the electric motor, the values of which are determined by the frequency of rotation of the electric motor and the voltage at its terminals. Reducing the rotational speed by only 10-25% leads to a decrease in the resistance of the electric motor to a minimum value corresponding to the starting current. The restoration of the normal operation of the electric motor after a short circuit is turned off is called self-starting, and the currents passing in this case are called self-starting currents.
All asynchronous motors can be self-started without danger of damage and must therefore be protected against self-starting. The uninterrupted operation of thermal power plants depends on the possibility and duration of self-starting of asynchronous electric motors of the main mechanisms of their own needs. If, due to a large voltage drop, it is impossible to ensure the self-starting of all operating electric motors, some of them have to be turned off. For this, special undervoltage protection is used, which turns off irresponsible electric motors when the voltage at their terminals drops to 60-70% of the nominal. In the event of a break in one of the phases of the stator winding, the electric motor continues to operate. In this case, the rotor speed decreases somewhat, and the windings of two undamaged phases are overloaded with a current 1.5-2 times higher than the nominal one. Motor protection against two-phase operation is only used on motors protected by fuses, if two-phase operation can lead to damage to the motor.
At powerful thermal power plants, two-speed asynchronous electric motors with a voltage of 6 kV are widely used as a drive for smoke exhausters, draft fans and circulation pumps. These electric motors are made with two independent stator windings, each of which is connected through a separate switch, and both stator windings cannot be turned on at the same time, for which a special interlock is provided in the control circuits. The use of such electric motors allows you to save electricity by changing their speed depending on the load of the unit. On such electric motors, two sets of relay protection are installed.
In operation, electric drive circuits are also used, providing for the rotation of a mechanism (for example, a ball mill) by two paired electric motors that are connected to one switch. In this case, all protections are common for both motors, with the exception of the zero sequence current protection, which is provided for each electric motor and is carried out using current relays connected to the zero sequence CT installed on each cable.
Protection of asynchronous motors from phase-to-phase short circuits, overloads and earth faults.
For protection against multi-phase short circuits of electric motors up to 5000 kW, maximum current cutoff is usually used. The most simple current cutoff can be performed with direct acting relays built into the circuit breaker drive. With an indirect relay, one of the two schemes for connecting the CT and the relay is used, shown in fig. 6.2 and 6.3. The cut-off is performed with independent current relays. The use of current relays with a dependent characteristic (Fig. 6 3) makes it possible to provide protection against short circuit and overload using the same relays. The cutoff operation current is selected - according to the following expression:
where k cx - circuit coefficient equal to 1 for the circuit in fig. 6.3 and v3 for the circuit in fig. 6.2; I start - the starting current of the electric motor.
If the relay operating current is detuned from the inrush current, the cutoff is usually reliably detuned and from. current that the electric motor sends to the section during an external short circuit.
Knowing the rated current of the motor I nom and multiplicity of starting current k n specified in the catalogs, you can calculate the starting current using the following expression:
Rice. 6.2 Scheme of electric motor protection by current cut-off with one instantaneous current relay: a- current circuits, b- operational direct current circuits
As can be seen from the oscillogram shown in Fig. 6.4, which shows the starting current of the feed pump motor, at the first moment of starting, a short-term peak of the magnetizing current appears, which exceeds the starting current of the electric motor. To deviate from this peak, the cutoff operation current is selected taking into account the reliability factor: k n =1,8 for RT-40 type relays operating through an intermediate relay; k n = 2 for relay types IT-82, IT-84 (RT-82, RT-84), as well as for direct action relays.
Rice. 6.3. Electric motor protection circuit against short circuits and overload with two RT-84 type relays: a- current circuits, b- operational direct current circuits.
T
Rice. 6 4. Oscillogram of the starting current of the electric motor.
the current cutoff of electric motors with a power of up to 2000 kW should be performed, as a rule, according to the simplest and cheapest single-relay circuit (see Fig. 6.2). However, the disadvantage of this circuit is the lower sensitivity compared to the cutoff made according to the circuit in Fig. 6.3, to two-phase short circuits between one of the phases on which a CT is installed and a phase without a CT. This takes place, since the cutoff actuation current made according to a single-relay circuit, according to (6.1), is v3 times greater than in a two-relay circuit. Therefore, on electric motors with a power of 2000-5000 kW, the current cutoff is performed by two relays to increase sensitivity. A two-relay cut-off circuit should also be used on electric motors up to 2000 kW, if the sensitivity coefficient of a single-relay circuit for a two-phase short circuit at the motor outputs is less than two.
On electric motors with a power of 5000 kW or more, longitudinal differential protection is installed, which provides a higher sensitivity to short circuits at the terminals and in the windings of electric motors. This protection is carried out in a two-phase or three-phase version with a relay type RNT-565 (similar to the protection of generators). Tripping current is recommended to take 2 I nom.
Since the two-phase protection does not respond to double earth faults, one of which occurs in the motor winding on the phase V , in which there is no CT, a special protection against double circuits without time delay is additionally installed.
OVERLOAD PROTECTION
Overload protection is installed only on electric motors subject to technological overloads (mill fans, smoke exhausters, mills, crushers, haulage pumps, etc.), usually with an effect on a signal or unloading mechanism. So, for example, on the electric motors of shaft mills, protection can act to turn off the electric motor of the coal supply mechanism, thereby preventing blockage of the mill with coal.
The overload protection should switch off the motor on which it is installed only if the cause of the overload cannot be eliminated without stopping the motor. The use of overload protection with tripping action is also useful in unmanned installations.
The overload protection trip current is assumed to be:
where k n = 1.1-1.2.
In this case, the overload protection relay will be able to operate from the inrush current, so the protection time delay is assumed to be 10-20 s according to the condition of detuning from the motor start time. Overload protection is performed using an inductive element of the IT-80 (RT-80) type relay (see Figure 6.3). If the electric motor must be switched off during overloads, relays of the IT-82 (RT-82) type are used in the protection circuit. On electric motors, the overload protection of which should not act to trip, it is advisable to use a relay with two pairs of contacts of the IT-84 (RT-84) type, which provide a separate cut-off and induction element.
For a number of electric motors (smoke exhausters, draft fans, mills), the turnaround time of which is 30-35 s, the overload protection circuit with the RT-84 relay is supplemented by the EV-144 time relay, which comes into action after the current relay contact closes. In this case, the protection time delay can be increased up to 36 s. V Lately for overload protection of auxiliary electric motors, a protection circuit with one current relay of the RT-40 type and one time relay of the EV-144 type is used, and for electric motors with a start time of more than 20 s - a time relay of the VL-34 type (with a scale of 1-100 s ).
Undervoltage protection.
After the short circuit is turned off, the electric motors connected to the section or busbar system, on which the voltage decrease occurred during the short circuit, self-start. Self-starting currents, several times higher than the nominal ones, pass through the supply lines (or transformers) of their own needs. As a result, the voltage on the auxiliary buses, and, consequently, on the electric motors, decreases so much that the torque on the motor shaft may not be sufficient to turn it around. Self-starting of electric motors may not occur if the busbar voltage is below 55-65% I nom. In order to ensure self-starting of the most critical electric motors, undervoltage protection is installed, which turns off non-essential electric motors, the absence of which will not affect the production process for some time. At the same time, the total self-starting current decreases and the voltage on the auxiliary buses increases, which ensures self-starting of critical electric motors.
In some cases, during a long absence of voltage, the undervoltage protection also switches off critical electric motors. This is necessary, in particular, to start the AVR circuit of electric motors, as well as according to the production technology. So, for example, in the event of a stoppage of all smoke exhausters, it is necessary to turn off the mill and blast fans and dust feeders; in case of stop of blowers - mill fans and dust feeders. Shutdown of critical electric motors by undervoltage protection is also carried out in cases where their self-starting is unacceptable due to safety conditions or because of the danger of damage to the driven mechanisms.
The simplest undervoltage protection can be performed with one voltage relay connected to phase-to-phase voltage. However, this implementation of protection is unreliable, since in the event of breaks in the voltage circuits, a false shutdown of the electric motors is possible. Therefore, a single-relay protection circuit is used only when using a direct-acting relay. To prevent false protection operation in the event of a voltage circuit failure, special circuits for switching on a voltage relay are used. One of such schemes for four electric motors, developed at Tyazhpromelectroproekt, is shown in Fig. 6.5. Direct operated undervoltage relay KVT1-KVT4 connected to phase-to-phase voltages ab and bc. To increase the reliability of protection, these relays are powered separately from devices and meters that are connected to voltage circuits through a three-phase circuit breaker SF3 with instantaneous electromagnetic release (two phases of the circuit breaker are used).
Phase V voltage circuits are not grounded deafly, but through a breakdown fuse fv, It eliminates the possibility of single-phase short circuits in voltage circuits and also increases the reliability of protection. In phase A protection installed single-phase circuit breaker SFI with an electromagnetic instantaneous release, and in phase WITH - circuit breaker with delayed thermal release. Between phases A and WITH a capacitor C with a capacity of about 30 uF is included, the purpose of which is indicated below.
Rice. 6 5. Undervoltage protection circuit with direct acting relay type RNV
In case of damage in the voltage circuits, the protection in question will behave as follows. The short circuit of one of the phases to the ground, as noted above, does not lead to the tripping of the circuit breakers, since the voltage circuits do not have a dead ground. With a two-phase short circuit of the phases V and WITH only the circuit breaker will turn off SF2 phases WITH. Voltage relay KVT1 and KVT2 remain connected to normal voltage and therefore do not start. Relay KVT3 and KVT4, triggered by a short circuit in the voltage circuits, after the circuit breaker is turned off SF2 pull up again, as they will be energized from the phase A through a capacitor WITH. With short circuit phases AB or AC the circuit breaker will turn off SF1, installed in phase A. After switching off the short circuit relay KVT1 and KVT2 pull up again under the action of voltage from the phase WITH, coming through the capacitor C. Relay KVT3 and KVT4 won't start. Relays will behave similarly in the event of a phase failure. A and WITH. Thus, the protection scheme under consideration does not work falsely with the most probable damage to voltage circuits. False operation of the protection is possible only in case of unlikely damage to the voltage circuits - a three-phase short circuit or when the circuit breakers are turned off SF1 and SF2. Voltage circuit failure signaling is carried out by relay contacts KV1.1, KV2.1, KV3.1 and contacts of circuit breakers SF1.1, SF2.1, SF3.1.
In installations with direct operating current, undervoltage protection is performed for each section of the auxiliary busbars according to the diagram shown in fig. 6.6. In the timing relay circuit CT1, acting to turn off non-responsible electric motors, the contacts of three minimum voltage relays are connected in series KV1. Thanks to this switching on of the relay, false operation of the protection is prevented when any fuse in the voltage transformer circuits blows. Relay actuation voltage KV1 about 70% accepted U nom.
Rice. 6.6. Undervoltage protection circuit at direct operating current: a- alternating voltage circuits; b- operational circuits I- to turn off irresponsible engines; II- to turn off critical engines.
The protection time delay for switching off non-responsible electric motors is adjusted from the cut-offs of the electric motors and is set equal to 0.5-1.5 s. The time delay for turning off critical electric motors is assumed to be 10-15 s, so that the protection does not act to turn them off during voltage drops caused by short circuits and self-starting of electric motors. As operating experience shows, in some cases, self-starting of electric motors lasts 20-25 s with a decrease in voltage on the auxiliary buses to 60-70% U nom . At the same time, if no additional measures are taken, the undervoltage protection (relay KV1), having a trip setting (0.6-0.7) U nom , could modify and disable critical electric motors. To prevent this in the winding circuit of the time relay CT2, acting on the shutdown of critical electric motors, the contact is turned on KV2.1 fourth voltage relay KV2. This minimum voltage relay has a trip setting of the order of (0.4-0.5) U nom and reliably returns during self-start. Relay KV2 will keep its contact closed for a long time only when the voltage is completely removed from the auxiliary buses. In cases where the duration of self-starting is less than the time delay of the relay CT2, relay KV2 not installed.
Recently, power plants have used a different protection scheme, shown in Fig. 6.7. Three start relays are used in this circuit: negative sequence voltage relay KV1 type RNF-1M and undervoltage relay KV2 and KV3 type RN-54/160.
Rice. 6.7. Undervoltage protection circuit with positive sequence voltage relay: a- voltage circuits; b- operational circuits
In normal mode, when the phase-to-phase voltages are symmetrical, the NC contact KV1.1 in the winding circuit of the protection time relay CT1 and CT2 closed, and closing KV1.2 in the alarm circuit is open. Relay break contacts K.V2.1 and KV3.1 while open. When the voltage drops on all phases, the contact KV1.1 will remain closed and act in turn: the first stage of undervoltage protection, which is carried out using a relay KV2(operating setting 0.7 U nom) and CT1; the second - using a relay KV3(operating setting 0.5 U nom) and CT2. In the event of a violation of one or two phases of the voltage circuits, the relay is activated KV1, whose closing contact KV1.2 a signal is given about a malfunction of the voltage circuits. When each protection stage is triggered, a plus is supplied to the tires SHMN1 and SHMN2 respectively, from where it comes to the shutdown circuits of electric motors. The protection action is signaled by indicating relays KH1 and KH2, having parallel windings.
Probably everyone knows that various devices operate on the basis of electric motors. But for what protection of electric motors is needed, only a small part of users are aware. It turns out that they can break as a result of various unforeseen situations.
High-quality protective devices are used to avoid problems with high repair costs, unpleasant downtimes and additional material losses. Next, we will understand their device and capabilities.
How is motor protection created?
We will gradually consider the main motor protection devices and the features of their operation. But now let's talk about three levels of protection:
- External protection version for short circuit protection. Usually refers to different types or is presented in the form of a relay. They have an official status and are required to be installed in accordance with safety standards in the territory of the Russian Federation.
- The external version of motor overload protection helps prevent dangerous damage or critical failures in the process.
- The built-in type of protection will save in case of noticeable overheating. And this will protect against critical damage or failures during operation. In this case, external type switches are required; sometimes a relay is used to reset.
What causes an electric motor to fail?
During operation, sometimes unforeseen situations appear that stop the operation of the engine. Because of this, it is recommended to provide reliable motor protection in advance.
You can see the photo of various types of motor protection to get an idea of how it looks.
Consider cases of failure of electric motors in which serious damage can be avoided with the help of protection:
- Insufficient level of electrical supply;
- High level of voltage supply;
- Rapid change in the frequency of current supply;
- Improper installation of the electric motor or storage of its main elements;
- Increase in temperature and exceeding the permissible value;
- Insufficient cooling supply;
- Elevated temperature level environment;
- Reduced barometric pressure if the engine is operated at elevated altitude based on sea level;
- Increased temperature of the working fluid;
- Unacceptable viscosity of the working fluid;
- The engine often turns off and on;
- Rotor blocking;
- Unexpected phase break.
In order for the protection of electric motors against overload to cope with the listed problems and be able to protect the main elements of the device, it is necessary to use the option based on automatic shutdown.
A fusible version of the fuse is often used for this, as it is simple and capable of many functions:
The fuse-switch version is represented by an emergency switch and a fuse connected on the basis of a common housing. The switch allows you to open or close the network using a mechanical method, and the fuse creates high-quality motor protection based on the effects of electric current. However, the switch is mainly used for the process after-sales service when it is necessary to stop the transfer of current.
Fusible versions of fuses based on fast acting are considered excellent short circuit protectors. But short overloads can lead to breakage of fuses of this type. Because of this, it is recommended to use them on the basis of the effect of a slight transient voltage.
Fuses based on delay trip are able to protect against overload or various short circuits. Typically, they are able to withstand a 5-fold increase in voltage for 10-15 seconds.
Important: Automatic versions of circuit breakers differ in the level of current to operate. Because of this, it is better to use a circuit breaker capable of withstanding the maximum current in the event of a short circuit appearing on the basis of this system.
Thermal relay
V various devices a thermal relay is used to protect the motor from overloads under the influence of current or overheating of working elements. It is created using metal plates that have different coefficients of expansion under the influence of heat. Usually it is offered in conjunction with magnetic starters and automatic protection.
Automatic engine protection
Motor protection circuit breakers help to protect the winding from the occurrence of a short circuit, protect against load or breakage of any of the phases. They are always used as the first line of defense in the motor power supply network. Then a magnetic starter is used, if necessary, it is supplemented with a thermal relay.
What are the criteria for choosing a suitable machine:
- It is necessary to take into account the magnitude of the operating current of the electric motor;
- The number of windings used;
- The ability of the machine to cope with the current as a result of a short circuit. Regular versions operate up to 6 kA, and the best ones up to 50 kA. It is worth considering the response speed for selective ones less than 1 second, normal ones less than 0.1 seconds, high-speed ones about 0.005 seconds;
- Dimensions, since most of the machines can be connected with a bus based on a fixed type;
- Type of circuit release - usually thermal or electromagnetic method is used.
Universal protection blocks
Various universal motor protection units help protect the motor by cutting off voltage or blocking the ability to start.
They work in such cases:
- Voltage problems, characterized by surges in the network, phase breaks, violation of phase rotation or sticking, phase or linear voltage imbalance;
- mechanical congestion;
- Lack of torque for the ED shaft;
- dangerous operational characteristics body insulation;
- If a ground fault occurs.
Although undervoltage protection may be organized in other ways, we have considered the main ones. Now you have an idea about why it is necessary to protect the electric motor, and how this is done using various methods.
Motor protection photo
An analysis of the operating modes of an asynchronous motor shows that under production conditions there can be a variety of emergency situations that entail different consequences for the motor. The means of protection do not have sufficient universality to turn off the engine in all cases, regardless of the cause and nature of the emergency mode, in the event of any dangerous situation for it. Each emergency mode has its own characteristics. Currently used protective devices have disadvantages and advantages, which are manifested in certain conditions. The economic side of the issue should also be taken into account. The choice of means of protection should be based on a technical and economic calculation, in which it is necessary to take into account the cost of the protective device itself, the costs of its operation, and the amount of damage caused by an engine accident. It should be borne in mind that the reliability of the protection also depends on the characteristics of the working machine and its mode of operation. Thermal protection has the greatest versatility. But it is more expensive than other means of protection, and more complex in design. Therefore, its use is justified in cases where other types of protection either cannot provide reliable operation, or the protected installation imposes increased requirements on the reliability of the protection, for example, due to large damage in the event of an engine failure.
The type of protective device should be chosen when designing a process unit, taking into account all the features of its operation. Operating personnel must receive a complete necessary equipment. However, in some cases, when re-equipping or rebuilding a production line
It is up to the operating personnel to decide for themselves which type of protection is appropriate in a particular case. To do this, it is necessary to analyze the possible emergency modes of the installation and select the required protective device. In this brochure, we will not discuss in detail the methodology for selecting motor overload protection. We will limit ourselves to only some general recommendations that may be useful for the operating personnel of rural electrical installations.
First of all, it is necessary to establish the emergency modes characteristic of a given installation. Some of them are possible in all installations, and others only in some. Phase loss overloads are independent of the driven machine and can occur in all installations. Thermal relays and built-in temperature protection perform quite satisfactorily protective functions in this type of emergency mode. The use of special phase loss protection in addition to overload protection must be justified. In most cases, it is not required. Thermal relays and temperature protection are sufficient. It is necessary to systematically check their condition and adjust. Only in cases where an engine failure could lead to great damage, you can use a special overload protection when a phase is lost.
Thermal relays are not effective enough as a means of protection against overloads during alternating (with large fluctuations in loads), with intermittent and short-term operating modes. In these cases, the built-in temperature protection is more effective. In the case of machines with heavy starting, the built-in temperature protection should also be preferred.
Of the available variety of protection means for an asynchronous motor, only two devices have found wide application: thermal relays and built-in temperature protection. These two devices are competing in the design of electric drives of agricultural machines. To select the type of protection, a feasibility study is carried out using the reduced cost method. Without dwelling on the exact calculation by this method, we will consider the application of its main provisions to select the most advantageous protection option.
Preference should be given to the option that will have the lowest costs for the purchase, installation and operation of the devices in question. In this case, the damage that the production incurs from insufficient reliability of the protection action must be taken into account. The costs given to one year of use are determined by the formula
where K is the cost of the motor and protective device, including the cost of their transportation and installation;
ke - coefficient taking into account deductions for depreciation, equipment renewal, repairs;
E - operating costs (cost of maintenance of protective equipment, consumed electricity, etc.);
Y - the damage that the production bears due to the failure or incorrect action of the protection.
The amount of damage is made up of two terms
where Um is the technological damage caused by an engine failure (the cost of underdelivered or damaged products);
Kd - the cost of replacing a failed engine and protective device, including the costs of dismantling the old and installing new equipment;
p0 is the probability of failure (incorrect action) of the protection, which led to an engine failure.
Operating costs are much less than the other components of the reduced costs, so they can be neglected in further calculations. The cost of a motor with built-in protection and built-in protection equipment is more than the cost of a conventional motor and a thermal relay. But the first of the considered defenses is more perfect. It works effectively in almost all emergency situations, so the damage from its incorrect action will be less. The cost of more expensive protection will be justified only if the damage is reduced by an amount greater than the additional cost of more advanced protection.
The amount of technological damage depends on the nature technological process and equipment downtime. In some cases, it may be ignored. This applies primarily to separately operating plants, whose downtime during the elimination of an accident does not have a noticeable effect on the entire production. As production is saturated with mechanization and electrification, the level of requirements for the reliability of equipment operation increases. Downtime due to faulty electrical equipment leads to great damage, and in some cases becomes unacceptable. Using some average data, it is possible to determine the scope of economically justified application of more complex protection devices.
The value of the probability of protection failure p0 depends on the design and manufacturing quality of the equipment, as well as on the nature of the emergency mode in which the engine may find itself. As shown above, under some emergency conditions, thermal relays do not provide reliable shutdown of the engine. In this case, the built-in temperature protection is better. The experience of using this protection shows that the value of the probability of failure of this protection pb can be taken equal to 0.02. This means that there is a chance that out of 100 such devices, two may not work, resulting in an engine failure.
Using formulas (40) and (41), we determine at what value of the probability of failures of thermal relays ptr the reduced costs will be the same. This will make it possible to assess the scope of a particular device. Neglecting operating costs, we can write
where indexes vz and tr respectively mean built-in protection and thermal relay. From here we get
In order to represent the order of the required level of reliability of the operation of a thermal relay, consider an example.
Let us determine the maximum allowable value of ptr of the thermal relay TRN-10 with bimetallic elements complete with the A02-42-4CX engine by comparing with the option of using the A02-42-4SHTZ engine with built-in temperature protection UVTZ, for which we take pvz = 0.02. Technological damage is assumed to be zero. The cost of a motor with a thermal relay, including the cost of transportation and installation, is 116 rubles, and for the version with UVTZ protection - 151 rubles. The cost of replacing a failed A02-42-4CX engine and a TRN-10 thermal relay, taking into account the costs of dismantling the old equipment and installing a new one, is 131 rubles, and for the option with UVTZ protection - 170 rubles. In accordance with existing standards, we accept ke = 0.32. After substituting these data into equation (43), we obtain
The obtained values characterize the permissible failure probabilities, above which the use of thermal relays is economically unprofitable. Similar figures are obtained for other low power engines. To determine the feasibility of using the considered protection means, it is necessary to compare the permissible failure probabilities with the actual ones.
The lack of sufficient data on the actual values does not allow us to accurately determine the area of effective application of the considered protective devices by direct use of the stated method of technical and economic calculation. However, using the results of the analysis of the operating modes of the asynchronous motor and protective devices, as well as some data that indirectly characterize the indicators of the required reliability, it is possible to outline the areas of preferential use of one or another type of protective device.
The actual level of reliability of the protection operation depends not only on the principle of its operation and the quality of the equipment manufacturing, but also on the level of operation of the electrical equipment. Where maintenance of electrical equipment is established, despite some shortcomings of thermal relays, the accident rate of electric motors is low. The practice of advanced farms shows that with a well-established maintenance electrical installations, the annual percentage of failure of electric motors protected by thermal relays can be reduced to 5% or less.
However, it should be noted that such a conclusion is valid only when considering the overall picture. When considering certain specific conditions, preference should be given to other protection devices. Based on the analysis of the operating modes of the electric drive, it is possible to indicate a number of installations for which the probability of failures of thermal relays will be high due to shortcomings in the principle of their operation.
1. Electric drives of machines with a sharply variable load (feed grinders, crushers, pneumatic conveyors for loading silage, etc.). With large load fluctuations, thermal relays cannot "simulate" the thermal state of the motor, so the level of actual failures of thermal relays in such installations will be high.
2. Electric motors operating according to the "triangle" scheme. Their peculiarity lies in the fact that when one of the phases of the supply line breaks, the current in the remaining linear wires and phases increases unequally. In the most loaded phase, the current grows faster than in linear wires.
3. Electric motors of installations operating at an increased frequency of emergency situations leading to engine shutdown (for example, manure conveyors).
4. Electric motors of installations, the downtime of which causes great technological damage.
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.
Engine protection has three levels:
External installation short circuit protection . External protection devices are usually fuses different types or a short circuit protection relay. Protective devices of this type are obligatory and officially approved, they are installed in accordance with safety regulations.
External overload protection , i.e. protection against overloads of the pump motor, and, consequently, the prevention of damage and malfunctions of the electric motor. This is current protection.
Built-in motor protection with overheating protection to avoid damage and malfunction of the 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, there may be various faults. Therefore, it is very important to foresee the possibility of failure and its causes and to 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
Gradual increase in temperature and its exit beyond the permissible limit:
Insufficient cooling
High ambient temperature
Reduced atmospheric pressure (working at high altitude)
High fluid temperature
Too high viscosity of the working fluid
Frequent switching on / off of the electric motor
Load moment of inertia too high (different for each pump)
Rapid rise in temperature:
Rotor lock
Phase failure
To protect the network from 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 turn off the mains power. The fuse is the simplest device that performs two functions. As a rule, fuses are interconnected using an emergency switch, which can disconnect the motor from the mains. In the following pages, we will look at three types of fuses in terms of their principle of operation and applications: fuse switch, fast blow fuses, and slow blow fuses.
A fuse switch is an emergency switch and a fuse combined in a single housing. A circuit breaker can be used to open and close the circuit manually, while a fuse protects the motor from overcurrent. Switches are usually used in connection with service work, when it is necessary to interrupt the supply of current.
The emergency switch has a separate casing. This cover protects personnel from accidental contact with electrical terminals and also protects the switch from oxidation. Some emergency switches are equipped with built-in fuses, other emergency switches 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 current overloads are quite acceptable. But with a further increase in current, the protection device should operate immediately. It is very important to immediately prevent short circuits. A fuse switch is an example of a device used for overcurrent protection. Properly selected fuses in the circuit breaker open the circuit during current overloads.
Fast acting fuses
Fast acting fuses provide excellent short circuit protection. However, short-term overloads, such as motor starting current, can break these types of fuses. Therefore, fast-acting fuses are best used in networks that are not subject to significant transient currents. Typically, these fuses will carry about 500% of their rated current for one-fourth of a second. After this time, the fuse insert melts and the circuit opens. Thus, in circuits where the inrush current often exceeds 500% of the fuse's rated current, fast acting fuses are not recommended.
Fuses with delayed blowing
This type of fuse provides both overload and short circuit protection. As a rule, they allow a 5-fold increase in the rated current for 10 seconds, and even higher currents for a shorter time. This is usually sufficient to keep the motor running and the fuse not open. On the other hand, if overloads occur that last longer than the melting time of the fusible element, the circuit will also open.
The operating time of a fuse is the time it takes for the fusible element (wire) to melt before the circuit opens. For fuses, the operating time is inversely proportional to the current value - this means that the greater the current overload, the shorter the period of time for breaking the circuit.
In general, we can say that the pump motors have a very short acceleration time: less than 1 second. Therefore, for motors, time-delay fuses with a rated current corresponding to the full load current of the motor are suitable.
The illustration on the right shows the principle of forming the fuse operating time characteristic. The x-axis shows the relationship between the actual current and the full load current: if the motor draws full load current or less, the fuse does not open. But at 10 times the full load current, the fuse will open almost instantly (0.01 s). The response time is plotted on the y-axis.
During start-up, a sufficiently large current passes through the induction motor. In very rare cases, this leads to a shutdown by relays or fuses. To reduce the starting current, use various methods starting the electric motor.
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 a predetermined overcurrent value. If the circuit breaker is used within its operating range, opening and closing does not cause any damage to it. Immediately after the occurrence of an overload, you can easily resume the operation of the circuit breaker - it is simply reset to its original position.
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 large currents that occur when starting a motor and protect the motor from failures such as a 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 trip limit. Compared to thermal circuit breakers, magnetic circuit breakers have a more precisely defined operating time. The table shows the characteristics of two types of circuit breakers.
Operating range of circuit breaker
Circuit breakers differ in the level of operating current. This means that you should always choose 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 motor, they can withstand temporary overloads without breaking the circuit.
They open the motor circuit if the current exceeds the maximum permissible value and there is a threat of damage to the motor.
Are established in a starting position automatically or manually after elimination of an overload.
IEC and NEMA standardize overload relay trip classes.
As a rule, overload relays respond to overload conditions according to their tripping characteristics. For any standard (NEMA or IEC), the division of products into classes determines how long it takes the relay to open when overloaded. The most common classes are 10, 20 and 30. The numerical designation reflects the time required for the relay to operate. A class 10 overload relay trips in 10 seconds or less at 600% full load current, a class 20 relay trips in 20 seconds or less, and a class 30 relay trips in 30 seconds or less.
The slope of the response characteristic depends on the protection class of the motor. IEC motors are usually adapted to a particular application. This means that the overload relay can handle excess current very close to the relay's maximum capacity. Class 10 is the most common class for IEC motors. NEMA motors have a larger internal capacitor, so class 20 is more commonly used.
The class 10 relay is usually used for pump motors, as the acceleration time of motors is about 0.1-1 second. Many high inertia industrial loads require a class 20 relay to operate.
Fuses serve to protect the installation from damage that can be caused by a short circuit. Therefore, fuses must have 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 motor, but to a 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 a short circuit.
Modern external motor protection relays
Advanced external motor protection systems also provide protection against overvoltage, phase imbalance, limit the number of on / off, and eliminate vibration. In addition, they allow you to monitor the temperature of the stator and bearings via a temperature sensor (PT100), measure the insulation resistance and record the ambient temperature. In addition, 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 the thermal protection device.
External motor protection relays are designed to protect three-phase electric motors in case of a threat of damage to the motor 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 motor protection in various situations:
Gives a signal before a malfunction occurs as a result of the whole process
Diagnoses problems that occur
Allows you to check the operation of the relay during maintenance
Monitors temperature and vibration in bearings
You can connect an overload relay to central system building management for continuous monitoring and operational troubleshooting. If an external protection relay is installed in the overload relay, the period of forced downtime due to process interruption due to a breakdown is reduced. This is achieved by quickly detecting a fault and preventing damage to the motor.
For example, an electric motor can be protected from:
Overload
Rotor locks
Jamming
Frequent restarts
open phase
Ground shorts
Overheating (via motor signal via PT100 sensor or thermistors)
small current
Overload warning
External overload relay setting
The full load current at a certain voltage indicated on the rating plate is the guideline for setting the overload relay. Since in networks different countries different voltages are present, pump motors can be used at both 50 Hz and 60 Hz over a wide voltage range. For this reason, the motor rating plate indicates the current range. If we know the voltage, we can calculate the exact current carrying capacity.
Calculation example
Knowing the exact voltage for the installation, it is possible to calculate the full load current at 254 / 440 Y V, 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 connection at minimum voltages)
(Current values for delta and star connection at maximum voltages)
Now, using the first formula, you can calculate the full load current:
I for "triangle":
I for "star":
The values for full load current correspond to the allowable 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 given on the rating plate.
However, if the motors are designed with a load factor that is then indicated on the nameplate, e.g. 1.15, the current setting 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 rating plate.
Why do you need built-in motor protection if the motor is already equipped with an overload relay and fuses? In some cases, the overload relay does not register an overload of the motor. For example, in situations:
When the motor is closed (not cool enough) and slowly warms up to dangerous temperatures.
At high temperature environment.
When the external motor protection is set to too high trip current or not set correctly.
When the motor is restarted several times within a short period of time and the starting current heats up the motor, which can eventually damage it.
The level of protection that internal protection can provide is specified in IEC 60034-11.
TP designation
TP is an abbreviation for "thermal protection" - thermal protection. Exists different types thermal protection, which are indicated by the code TP (TPxxx). Code includes:
Type of thermal overload for which the thermal protection was 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 rapid and gradual overload.
Designation | Technical load and its variants (1st digit) | Number of levels and functional area (2nd digit) | |
TR 111 | Slow only (constant overload) | 1 level when off | |
TR 112 | |||
TR 121 | |||
TR 122 | |||
TR 211 | Slow and fast (constant overload, blocking) | 1 level when off | |
TR 212 | |||
TR 221 TR 222 | 2 levels for alarm and shutdown | ||
TR 311 TR 321 | Only fast (block) | 1 level when off | |
Image of the allowable temperature level when exposed to high temperature on the electric motor. Category 2 allows higher temperatures than category 1.
All Grundfos single-phase motors are equipped with motor current and temperature protection in accordance with IEC 60034-11. The type of motor protection TP 211 means that it responds to both gradual and rapid temperature increases.
Resetting the data in the device and returning to the initial position is carried out automatically. Three-phase Grundfos MG motors from 3.0 kW are equipped as standard with a PTC temperature sensor.
These motors have been tested and approved as TP 211 motors and respond to both slow and fast temperature rises. Other motors used for Grundfos pumps (MMG models D and E, Siemens, etc.) may be classified as TP 211, but they are usually TP 111.
The data on the rating plate must always be observed. Information about the type of protection for a specific motor can be found on the rating plate - marking with the letter TP (thermal protection) according to IEC 60034-11. As a rule, internal protection can be provided by two types of protection devices: Thermal protection devices or thermistors.
Thermal protection devices built into the terminal box
Thermal protection devices, or thermostats, use a snap action disc-type bimetal circuit breaker to open and close a circuit when a certain temperature is reached. Thermal protection devices are also called "klixons" (after the brand name from Texas Instruments). As soon as the bimetallic disk reaches the set temperature, it opens or closes a group of contacts in the connected control circuit. 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 should not be changed. The discs are hermetically sealed and are located on the terminal block.
The thermostat can supply voltage to the circuit alarm- 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 the thermostats are located on the outer surface of the ends of the coil, they respond to the temperature at the location. For three-phase motors, thermostats are considered unstable protection under braking conditions or other conditions of rapid temperature change. In single-phase motors, thermostats are used to protect against a blocked rotor.
Thermal circuit breaker built into the windings
Thermal protection devices 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, a thermal protection device is installed directly in the main circuit so that it acts as a winding protection device. Klixon and Thermik 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, however this results in a longer response time. The switches must be connected to an external control system. In this way, the motor is protected from gradual overload. For thermal circuit breakers, a relay - amplifier 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 characteristics. TN usually lies in the range of 150-160 °C.
Connection
Connection of a three-phase electric motor with built-in thermal switch and overload relay.
TP designation on the chart
IEC 60034-11 protection:
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 motor windings and protect it in case of locked rotor, prolonged overload and high ambient temperature. Thermal protection is provided by monitoring the temperature of the motor windings using PTC sensors. If the temperature of the windings exceeds the switch-off temperature, the resistance of the sensor changes according to the change in temperature.
As a result of this change, the internal relays de-energize the control circuit of the external contactor. The electric motor cools down, and the acceptable temperature of the electric motor winding is restored, the sensor resistance decreases to its original level. At this point, the control module will automatically reset unless it has been previously configured to reset and restart manually.
If the thermistors are installed at the ends of the coil by themselves, 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 installed in series and a solid state electronic switch in a closed 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 range of temperatures, 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 control relay. The relay opens the control circuit of the entire mechanism to disable the protected equipment. When the temperature of the winding is restored to an acceptable value, the control unit can be reset manually.
All Grundfos motors from 3 kW and above are equipped with thermistors. The Positive Temperature Coefficient (PTC) thermistor system is considered to be fault-tolerant because if the sensor fails or the sensor wire is disconnected, infinite resistance occurs and the system operates in the same way as when the temperature rises - the control relay is de-energized.
The principle of operation of the thermistor
Critical resistance/temperature values for motor protection sensors are defined in DIN 44081/DIN 44082.
The DIN curve shows the resistance in thermistor sensors as a function of temperature.
Compared to PTO, thermistors have the following advantages:
Faster response due to smaller volume and weight
Better contact with the motor winding
Sensors are installed on each phase
Provides protection in the event of a blocked rotor
TP designation for motor with PTC
Motor protection TP 211 is only realized when PTC thermistors are fully installed at the ends of the windings at the factory. The TP 111 protection is only realized when self installation at the place of operation. The motor must be tested and approved for the TP 211 marking. If the PTC thermistor motor has TP 111 protection, it must be equipped with an overload relay to prevent the effects of jamming.
Compound
The figures on the right show the connection diagrams of a three-phase electric motor equipped with PTC thermistors with Siemens releases. To implement protection against both gradual and rapid overload, we recommend the following connection options for motors equipped with PTC sensors with protection TP 211 and TP 111.
If a thermistor motor is marked TP 111, this means that the motor is only protected against gradual overload. In order to protect the motor from rapid overload, the motor must be equipped with an overload relay. The overload relay must be connected in series with the PTC relay.
Protection of the TP 211 motor is only ensured if the PTC thermistor is completely integrated in the windings. The protection of the TP 111 is realized only with self-connection.
The thermistors are designed according to 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. Siemens thermistors.
note: It is very important that the built-in PTC device be connected in series with the overload relay. Repeated switching on of the overload relay can cause the winding to burn out 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
