Overload protection of asynchronous motors. Motor overload protection with thermal relay. Undervoltage protection

Protection of electric motors.

Types of damage and abnormal modes of ED operation.

Damage to electric motors. In the windings of electric motors, earth faults of one phase of the stator, short circuits between turns and multiphase short-circuits can occur. Ground faults and multi-phase faults can also occur at motor leads, cables, couplings and funnels. Short circuits in electric motors are accompanied by the passage of large currents that destroy the insulation and copper of the windings, the steel of the rotor and stator. To protect electric motors from multiphase short-circuits, there is a current cut-off or longitudinal differential protection acting on tripping.

Single-phase earth faults in the stator windings of electric motors with a voltage of 3-10 kV are less dangerous than a short circuit, since 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, protection against earth faults 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, at an earth fault current of more than 5 A, the protection acts on tripping.

Protection against turn circuits on electric motors is not installed. Elimination of damage of this type is carried out by other protection of electric motors, since turn faults in most cases are accompanied by a ground fault or go into a multiphase short circuit.

Electric motors with voltages up to 600 V are protected against short circuits of all types (including single-phase ones) using fuses or high-speed electromagnetic releases of circuit breakers.

Abnormal modes of operation. The main type of abnormal operation for electric motors is their overload with currents greater than the rated one. Permissible overload time of electric motors, with, is determined by the following expression:

Rice. 6.1. 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 overloading of the mechanism (for example, blockage of a mill or crusher with coal, clogging of a fan or pieces of slag in the ash removal pump, etc.) and its malfunction (for example, damage to bearings, etc.). Currents significantly exceeding the rated ones pass when starting and self-starting electric motors. This is due to a decrease in the resistance of the electric motor with a decrease in its speed. Dependence of the electric motor current I from rotation frequency NS at a constant voltage across its terminals is shown in Fig. 6.1. The current is of greatest importance when the rotor of the electric motor is at a standstill; this current, called starting current, is several times higher than the rated value of the electric motor current. Overload protection can act on a signal, unloading a machine, or shutting down an electric motor. After switching off the short-circuit, 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 rotational speed of the electric motor and the voltage at its terminals. A decrease in the speed of only 10-25% leads to a decrease in the resistance of the electric motor to a minimum value corresponding to the starting current. Restoring the normal operation of the electric motor after switching off the short circuit is called self-starting, and the currents passing through it are called self-starting currents.

On all asynchronous electric motors, self-starting can be carried out without the risk of damage, and therefore their protection must be detuned from the self-starting mode. 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 self-starting of all running electric motors, some of them have to be turned off. For this, special undervoltage protection is used, which turns off irrelevant 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 work. In this case, the rotor speed decreases slightly, and the windings of the two undamaged phases are overloaded with a current 1.5-2 times higher than the rated one. Motor protection against two-phase operation is only used on electric motors protected by fuses, if two-phase operation may cause damage to the electric 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, blowing 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 energy 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) with two paired electric motors, which are connected to one switch. In this case, all protections are common for both electric motors, with the exception of zero sequence current protection, which is provided for each electric motor and is performed using current relays connected to the zero sequence CT installed on each cable.

Protection of asynchronous EM from phase-to-phase short-circuits, overloads and ground faults.

For protection against multiphase short circuits of electric motors up to 5000 kW, the maximum current cutoff is usually used. The most simple overcurrent cut-off 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, shown in Fig. 6.2 and 6.3. Cutoff 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. Cut-off operating current is selected according to the following expression:

where k cx is the circuit coefficient equal to 1 for the circuit in Fig. 6.3 and v3 for the circuit in fig. 6.2; I starting - starting current of the electric motor.

If the pickup current of the relay is detuned from the inrush current, the cutoff is usually reliably detuned and from. current, which the electric motor sends to the section with an external short circuit.

Knowing the rated current of the electric motor I nom and frequency of starting current k n specified in the catalogs, you can calculate the starting current using the following expression:

Rice. 6.2 Motor protection circuit with overcurrent cut-off with one instantaneous overcurrent 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 electric motor of the feed pump, 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 detune from this peak, the cut-off operating current is selected taking into account the safety factor: k n =1,8 for RT-40 type relays acting through an intermediate relay; k n = 2 for relays of types IT-82, IT-84 (RT-82, RT-84), as well as for direct-acting relays.

Rice. 6.3. Electric motor protection circuit against short circuits and overload with two RT-84 relays: a- current circuits, b- operational direct current circuits.

Rice. 6 4. Oscillogram of the starting current of the electric motor.

current cutoff of electric motors 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 scheme is its lower sensitivity compared to the cutoff made according to the scheme in Fig. 6.3, to a two-phase short circuit between one of the phases in which the CT is installed and the phase without CT. This takes place, since the tripping current of the cut-off made according to the single-relay circuit, according to (6.1), is v3 times higher than in the two-relay circuit. Therefore, on electric motors with a capacity of 2000-5000 kW, the current cutoff to increase the sensitivity is performed by two-relay. A two-relay cut-off circuit should also be used on electric motors with a power of up to 2000 kW, if the sensitivity coefficient of a single-relay circuit at a two-phase short circuit at the motor terminals is less than two.

On electric motors with a power of 5000 kW and more, longitudinal differential protection is installed, which provides a higher sensitivity to short-circuit at the terminals and in the windings of the electric motors. This protection is carried out in a two-phase or three-phase version with a relay of the RNT-565 type (similar to the protection of generators). It is recommended to take the operating current 2 I No.

Since the protection in the two-phase version does not react to double earth faults, one of which occurs in the motor winding on a phase V , in which there is no CT, a special protection against double short 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, dredging pumps, etc.), as a rule, with an effect on the signal or unloading of the mechanism. So, for example, on electric motors of mine mills, the protection can act to turn off the electric motor of the mechanism supplying coal, thereby preventing the mill from blocking up with coal.

The overload protection should switch off the electric motor on which it is installed only if the cause of the overload cannot be eliminated without stopping the electric motor. The use of overload protection with tripping action is also advisable in unmanned installations.

Overload protection operation current is taken equal to:

where k n = 1.1-1.2.

In this case, the overload protection relays will be able to operate from the inrush current, therefore the protection time delay is taken 10-20 s according to the condition of detuning from the start time of the electric motor. Overload protection is performed using an inductive element of the IT-80 (RT-80) relay (see Fig. 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 protection of which against overload should not act on shutdown, it is advisable to use a relay with two pairs of contacts of the IT-84 (RT-84) type, providing a separate action of the cutoff and the induction element.

For a number of electric motors (smoke exhausters, blowing fans, mills), the turn-around time of which is 30-35 s, the overload protection circuit with the RT-84 relay is supplemented with a time relay of the EV-144 type, which comes into effect after the current relay contact closes. In this case, the protection time delay can be increased up to 36 s. Recently, for overload protection of auxiliary electric motors, a protection circuit has been used with one current relay of the RT-40 type and one time relay of the EV-144 type, and for electric motors with a starting 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 disconnected, the self-starting of the electric motors takes place, connected to the section or bus system, on which during the short circuit there was a decrease in voltage. Self-starting currents, several times higher than the rated ones, pass through the supply lines (or transformers) for their own needs. As a result, the voltage on the buses of auxiliary needs, and, consequently, on the electric motors, decreases so much that the torque on the shaft of the electric motor may be insufficient for its reversal. Self-starting of electric motors may not occur if the bus voltage is below 55-65% I No. In order to ensure self-starting of the most critical electric motors, an undervoltage protection is installed, which turns off non-critical electric motors, the absence of which for some time will not affect the production process. At the same time, the total self-starting current decreases and the voltage on the auxiliary buses increases, due to which self-starting of critical electric motors is ensured.

In some cases, in the event of a prolonged voltage failure, the undervoltage protection also switches off the critical electric motors. This is necessary, in particular, for starting the ATS circuit of electric motors, as well as for the production technology. So, for example, in the event that all smoke exhausters stop, it is necessary to turn off the mill and blowing fans and dust feeders; in case of stoppage of blowing fans - mill fans and dust feeders. Disconnection of critical electric motors by undervoltage protection is also carried out in cases where their self-starting is unacceptable for safety reasons or because of the danger of damage to the driven mechanisms.

The most simple undervoltage protection can be performed with one voltage relay connected to the phase-to-phase voltage. However, such an implementation of protection is unreliable, since in case of breaks in the voltage circuits, a false shutdown of electric motors is possible. Therefore, a single-relay protection circuit is used only when using a direct-action relay. To prevent false operation of protection in case of voltage circuit failure, special voltage relay switching circuits are used. One of such schemes for four electric motors, developed at Tyazhpromelektroproekt, is shown in Fig. 6.5. Direct undervoltage relay KVT1-KVT4 included for phase-to-phase voltages ab and bc. To increase the reliability of protection, these relays are powered separately from devices and meters, which 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 dull, but through a breakdown fuse FV, which eliminates the possibility of single-phase short-circuit 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 - a circuit breaker with a delayed thermal release. Between phases A and WITH a capacitor C with a capacity of about 30 μF is included, the purpose of which is indicated below.

Rice. 6 5. Undervoltage protection circuit with a direct-acting relay, type RNV

In case of damage in voltage circuits, the considered protection will behave as follows. The short circuit of one of the phases to ground, as noted above, does not lead to the tripping of the circuit breakers, since the voltage circuits do not have a solid ground. With two-phase short-circuit of phases V and WITH only the circuit breaker will trip SF2 phase WITH... Voltage relay KVT1 and KVT2 remain connected to the normal voltage and therefore do not start. Relay KVT3 and KVT4, triggered by a short circuit in voltage circuits, after opening the circuit breaker SF2 will pull up again, since they will be energized from the phase A through capacitor WITH. With short-circuit phases AB or AS the circuit breaker will open SF1, in-phase A. After disconnecting the short-circuit relay KVT1 and KVT2 will pull up again under the action of the voltage from the phase WITH, coming through capacitor C. Relay KVT3 and KVT4 will not start. Relays will behave similarly in case of phase failure A and WITH... Thus, the considered protection scheme does not work falsely with the most probable damage to the voltage circuits. False operation of protection is possible only in case of improbable damage to voltage circuits - 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 a constant operating current, undervoltage protection is performed for each section of the auxiliary busbars according to the diagram shown in Fig. 6.6. Time relay circuit KT1, acting to turn off non-responsible electric motors, 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 is blown. Relay trigger voltage KV1 accepted about 70% U No.

Rice. 6.6. Undervoltage protection circuit for constant operating current: a- AC voltage circuits; b- operational chains I - to turn off irrelevant engines; II- to turn off critical engines.

The delay of the protection time for turning off non-responsible electric motors is detuned from the cutoffs of the electric motors and is set equal to 0.5-1.5 s. The time delay for turning off critical electric motors is taken 10-15 s, so that the protection does not act on their shutdown in case of voltage drops caused by short-circuit and self-starting of electric motors. As the operating experience shows, in a number of cases, self-starting of electric motors lasts 20-25 s with a decrease in the voltage on the auxiliary buses to 60-70% U nom . At the same time, if you do not take additional measures, the undervoltage protection (relay KV1), having a pickup setting (0.6-0.7) U nom , could modify and turn off critical electric motors. To prevent this in the timing relay coil KT2, acting on the shutdown of critical electric motors, the contact is turned on KV2.1 fourth voltage relay KV2. This undervoltage relay has a pickup 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 KT2, relay KV2 not installed.

Recently, power plants have used a different protection scheme, shown in Fig. 6.7. This circuit uses three starting relays: 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 chains

In normal mode, when the phase-to-phase voltages are symmetrical, the normally open contact KV1.1 in the winding circuit of the time relay CT1 and KT2 closed, and the closing KV1.2 open in the signaling circuit. Relay break contacts K.V2.1 and KV3.1 at the same time are open. With a decrease in voltage on all phases, the contact KV1.1 will remain closed and act in turn: the first stage of undervoltage protection, which is carried out by means of a relay KV2(pick-up setting 0.7 U nom) and KT1; the second - using a relay KV3(pick-up setting 0.5 U nom) and KT2. In case of violation of one or two phases of the voltage circuits, the relay is activated KV1, the closing contact of which KV1.2 a signal about a malfunction of the voltage circuits is given. When each stage of protection is triggered, a plus is supplied to the busbars SHMN1 and SHMN2 respectively, where it comes from on the circuit for shutting down electric motors. Protection action is signaled by indicator relays KN1 and KH2, having parallel windings.

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

Emergency operating modes of electric motors

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

Phase loss (phase loss) occurs in the event of a fuse blown in a phase, wire breakage, contact failure. In this case, a redistribution of currents occurs, increased currents begin to flow through the windings of the electric motor, it is not excluded that the mechanism stops and the electric machine breaks down. The most sensitive to half-phase modes are electric motors of small and medium power, i.e., which are most often used in industry and agriculture.

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

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

Types of electrical devices for the protection of electric motors

To protect electrical equipment from emergency modes, circuit breakers, fuses, built-in temperature protection devices, phase-sensitive protection and other devices are serially produced.

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

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

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

Overload protection They are divided into two types: direct-acting protection, which reacts to overcurrent, and indirect protection, which responds to over-temperature. Thermal relays are the most common type of overcurrent protection used to protect electric motors from overloads (including jamming). They are produced in the TRN, TRP, RTT, RTL series. Three-phase thermal relays PTT and RTL also protect against phase loss.

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

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

The protection of indirect action includes built-in temperature protection UVTZ, which responds not to the current value, but to the temperature of the electric motor winding, regardless of the reason that caused the heating. Currently, for these purposes, modern electronic and microprocessor-based thermal relays are increasingly used, responding to changes in the resistance of thermistors built into the stator winding of the electric motor.

The procedure for choosing the type of protection for electric motors

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

the most critical electrical receivers, the failure of which can lead to great damage, subject to systematic contamination, or operating at elevated temperatures, as well as with abruptly variable loads (crushers, sawmills, forage grinders) should be protected with built-in temperature protection and circuit breakers or fuses.

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

It is recommended to protect the protection of electric motors of average power (more than 1.1 kW) operating without maintenance personnel with phase-sensitive devices.

Thermal relays, phase-sensitive protection, and built-in temperature protection work reliably at low overloads and long-term operating modes. The choice of the preferred apparatus in this case must be made taking into account economic indicators. At variable loads with a period of load fluctuations commensurate with the constant heating of the motor, the thermal relays do not work reliably and the built-in temperature protection or phase-sensitive protection should be used. Under random loads, protective devices are more reliable, acting as a function of temperature, rather than current.

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

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

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

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

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

The scope of application of various protection devices depends on the number of electrical equipment failures, the amount of technological damage during shutdown, the cost of purchasing protection equipment. A feasibility study is required to select the preferred option.

Overload protection of an electric motor 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, they can all be divided into levels.

External short circuit protection level. 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 item of protection that must be installed in accordance with the safety rules on the territory of the Russian Federation.
The relay of protection of the electric motor against overloads will help to avoid a variety of critical damage during operation, as well as possible damage. These devices also belong to the external protection level.
An internal protection layer prevents possible overheating of engine parts. For this, external switches are sometimes used, and sometimes an overload relay.

Causes of hardware failures

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

A low level of electrical voltage or, conversely, a too high level of supply can cause failure.
A breakdown is possible due to the fact that the frequency of the current supply will change too quickly and often.
Incorrect installation of the unit or its components can also be dangerous.
Temperature rise to critical value or higher.
Too little cooling also leads to breakdowns.
The increased ambient temperature has a strong negative effect.
Few know that low pressure or engine installations well above sea level, which causes low pressure, also have a negative impact.
Naturally, it is necessary to protect the electric motor from overloads that may occur due to power failures.
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 be used to manually open or close the circuit. The fuse is the protection of the electric motor against overcurrent.

It is worth noting that the design of the emergency switch provides for a special casing that protects personnel from accidental contact with the device terminals, as well as the contacts themselves from oxidation.

With regard to the fuse, this device must be able to distinguish between overcurrent and short-circuit occurrence in the circuit. This is very important, as short-term overcurrent is acceptable. However, the motor overload current protection should trip immediately if this parameter continues to rise.

Short circuit fuses

There is a type of fuse that is designed to protect the unit against short circuit (short circuit). However, it is worth noting here that the fast-acting fuse may fail if a short-term overload occurs when the device is started, 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 protection itself, the quick-blow fuse can withstand a current that will exceed its nominal by 500% if the difference lasts no more than a quarter of a second.

Time delay fuses

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 is the time-delay fuse. 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 stronger increase in the parameter is possible, but for a shorter period before the fuse is triggered. However, more often than not, an interval of 10 seconds is enough to both start the engine and prevent the fuse from blowing. Overload protection, short circuit protection, as well as other type of electric motor, 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 the fuse is the segment during which its fusible element (wire) melts. When the wire is completely melted, the circuit is opened. If we talk about the dependence of the trip 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, the faster the wire melts, which means that the time for disconnecting the circuit is reduced.

Magnetic and thermal devices

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

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

Overload relay

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

Such a device is able to withstand a short-term current drop during engine start without breaking the circuit, which is most important.
Opening of the circuit occurs when the current increases to the value when there is a threat of damage to the protected device.
After the overload has been removed, the relay can reset automatically or it can be manually reset.

It should be noted that the overload protection of the electric motor using 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, within 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.

Fuse protection and relays

Nowadays, it is quite common to combine two means of protection - fuses and relays. This combination works as follows. The fuse must protect the motor against short circuits, and therefore 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 drawback that relays are introduced into the system, which react to weaker, but still dangerous current fluctuations. The most important thing in this case is to adjust the fuse so that it trips before any damage occurs.

Outdoor protective equipment

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

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 outage, such equipment also has several other advantages.

An outdoor device can generate and signal a malfunction even before it interferes with the operation of the machine.
Carries out diagnostics of those problems that have already arisen.
Allows you to check the relay during maintenance.

Based on the foregoing, it can be argued that there is a wide variety of devices for protecting an 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 overload of the electric motor occurs in the following cases:

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

Overloads are stable and short-term.

Only persistent overloads are dangerous for the electric motor.

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

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

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

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

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

t = T a-1 / k 2 -1

where t -permissible overload duration, sec;

T- heating time constant, sec;

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

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

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

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

on electric motors of mechanisms that are not damaged by technological overloads (for example, electric motors of circulation, feed pumps, etc.) and do not have severe starting or self-starting conditions, overload protection is not installed.
on electric motors subject to technological overloads (for example, electric motors of mills, crushers, dredging pumps, etc.), as well as on electric motors, which self-starting is not ensured, overload protection must be installed.
overload protection is performed with a shutdown action in the event that self-starting of the electric motor is not ensured or the technological overload cannot be removed from the mechanism without stopping the electric motor.
overload protection of the electric motor is performed with an effect on the unloading of the mechanism or a signal if the technological overload can be removed from the mechanism automatically or manually by personnel without stopping the mechanism and the electric motors are under the supervision of personnel.
on electric motors of mechanisms that can have both an overload that can be eliminated during the operation of the mechanism, and an overload, the elimination of which is impossible without stopping the mechanism, it is advisable to provide for the action of protection against overcurrents with a shorter time delay for unloading the mechanism (if possible) and a longer time delay for turning off the electric motor ... Responsible electric motors for auxiliary needs of power plants are under constant supervision of the personnel on duty, therefore, their protection from overload is carried out mainly with an effect on the signal.

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

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

Motor protection has three levels:

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

External overload protection , i.e. protection against overloads of the pump motor, and, therefore, prevention of damage and malfunctions in the operation of the electric motor. This is overcurrent protection.

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

Possible Engine Failure Conditions

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

Poor quality of power supply:

High voltage

Undervoltage

Unbalanced voltage / current (surges)

Frequency change

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

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

Insufficient cooling

High ambient temperature

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

High fluid temperature

Too high viscosity of the working fluid

Frequent switching on / off of the electric motor

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

A sharp rise in temperature:

Locked rotor

Phase loss

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

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

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

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

Fast-blow fuses

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

Time delay fuses

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

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

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

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

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

What is a circuit breaker and how does it work?

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

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

Thermal circuit breakers

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

Magnetic circuit breakers

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

Circuit breaker operating range

The circuit breakers differ in the level of the trip current. This means that you should always 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 electric motor, they allow to withstand temporary overloads without breaking the circuit.

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

Are set to the initial position automatically or manually after overload elimination.

IEC and NEMA standardize overload relay trip classes.

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

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

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

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

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

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

Modern outdoor motor protection relays

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

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

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

Diagnoses malfunctions that have occurred

Allows you to check the operation of the relay during maintenance

Monitors bearing temperature and vibration

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

For example, an electric motor can be protected against:

Overload

Rotor blocking

Jamming

Frequent restarts

Open phase

Short to ground

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

Low current

Overload warning

External overload relay setting

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

Calculation example

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

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

Calculations for 60 Hz

The voltage gain is determined by the following equations:

Calculation of the actual full load current (I):

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

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

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

I for "triangle":

I for "star":

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

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

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

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

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

At high ambient temperatures.

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

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

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

TP designation

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

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

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

In pump motors, the most common TP designations are:

TP 111: Gradual overload protection

TP 211: Protection against both fast and gradual overload.

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

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

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

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

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

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

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

Thermal protection devices built into the terminal box

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

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

Thermal circuit breaker built into the windings

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

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

Indoor installation

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

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

The principle of operation of the thermal circuit breaker

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

Connection

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

TP notation on the graph

Protection according to IEC 60034-11:

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

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

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

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

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

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

The principle of operation of the thermistor

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

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

Compared to PTO thermistors have the following advantages:

Faster response due to lower volume and weight

Better contact with the motor winding

Sensors are installed on each phase

Provides protection when the rotor is blocked

TP designation for motor with PTC

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

Compound

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

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

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

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

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