Turbine compressor rotor
As is known, three-phase asynchronous electric (el) motors with a squirrel-cage rotor are connected in a star or delta circuit, depending on the line voltage for which each winding is designed.
When starting especially powerful electric power. motors connected in a delta circuit exhibit increased starting currents, which in overloaded networks create a temporary voltage drop below the permissible limit.
This phenomenon is due to the design features of asynchronous electrical systems. engines in which the massive rotor has a fairly large inertia, and when it spins up, the motor operates in overload mode. Starting an electric motor becomes more difficult if there is a load with a large mass on the shaft - rotors of turbine compressors, centrifugal pumps or mechanisms of various machine tools.
Method for reducing starting currents of an electric motor
To reduce current overloads and voltage drops in the network, a special method is used to connect three-phase electricity. engine, which switches from star to delta as the speed increases.
Connection of motor windings: star (left) and triangle (right) When connecting star-connected windings of a motor designed for triangle connection to a three-phase network, the voltage applied to each winding is 70% less than the nominal value. Accordingly, the current when starting the electric the engine will be smaller, but it should be remembered that the starting torque will also be smaller.
Therefore, star-delta switching cannot be used for electric motors that initially have a non-inertial load on the shaft, such as the weight of a winch load or the resistance of a piston compressor.
Switching modes on an electric motor mounted on a piston compressor is unacceptable. To operate as part of such units, which have a large load at the time of start-up, special three-phase electric power is used. motors with wound rotor, in which starting currents are regulated using rheostats.
Star-delta switching can only be used for electric motors that have a freely rotating load on the shaft - fans, centrifugal pumps, shafts of machine tools, centrifuges and other similar equipment.
Centrifugal pump with asynchronous electric motor Implementation of changing motor winding connection modes
It is obvious that in order to start a three-phase electric motor in star mode with subsequent switching to a delta connection of the windings, it is necessary to use several three-phase contactors in the starter.
Set of contactors in the starter for star-delta switching In this case, it is necessary to block the simultaneous operation of these contactors, and a short-term switching delay must be provided so that the star connection is guaranteed to turn off before the triangle turns on, otherwise a three-phase short circuit will occur.
Therefore, the time relay (RT), which is used in the circuit to set the switching interval, must also provide a delay of 50-100 ms so that a short circuit does not occur.
Methods for implementing switching delay
Mode switching time diagram There are several principles for implementing delay using:
Manual mode switch Classic scheme
This system is quite simple, unpretentious and reliable, but has a significant drawback, which will be described below and requires the use of a bulky and obsolete time relay.
This RF provides a delayed shutdown due to the magnetized core, which takes some time to demagnetize.
Electromagnetic time delay relay It is necessary to mentally walk through the current flow paths to understand the operation of this circuit.
Classic mode switching scheme with current and time relays After turning on the three-phase automatic switch AV, the starter is ready for operation. Through the normally closed contacts of the “Stop” button and the contact of the “Start” button closed by the operator, current flows through the coil of the KM contactor. The power contacts of the KM are kept in the on state by “self-retaining”, thanks to the BKM contact.
In the fragment of the above diagram, the red arrow indicates the bypass contact The KM relay is necessary to enable the engine to be turned off with the “Stop” button. The pulse from the “Start” button also passes through the normally closed BKM1 and RV, starting the contactor KM2, the main contacts of which supply voltage to the star-type connection of the windings - the rotor is spun up.
Since at the moment of starting KM2 the contact BKM2 opens, then KM1, which ensures that the delta connection of the windings is turned on, cannot work in any way.
Contactors providing star (KM2) and delta (KM1) connection Starting current overloads. the engine is forced to almost instantly operate the RT connected to the circuits of current transformers TT1, TT2. In this case, the control circuit of the KM2 coil is shunted by the RT contact, blocking the operation of the RF.
Simultaneously with the launch of KM2, with the help of its additional normally open contact BKM2, a time relay is started, the contacts of which are switched, but KM1 does not operate, since BKM2 is open in the circuit of the KM1 coil.
Turning on the time relay - green arrow, switching contacts - red arrows As the speed increases, the starting currents decrease and the PT contact in the KM2 control circuit opens. Simultaneously with the disconnection of the power contacts that provide power to the star connection of the windings, BKM2 is closed in the KM1 control circuit and BKM2 is opened in the RV power circuit.
But, since the PB turns off with a delay, this time is enough for its normally open contact in the KM1 circuit to remain closed, due to which KM1 self-pickup occurs, connecting the windings in a triangle.
Normally open self-retaining contact KM1 Disadvantage of the classical scheme
If, due to incorrect calculation of the load on the shaft, it cannot gain momentum, then the current relay in this case will not allow the circuit to switch to delta mode. Long-term operation of electric An asynchronous motor in this mode of starting overload is extremely undesirable; the windings will overheat.
Overheated motor windings Therefore, to prevent the consequences of an unexpected increase in load when starting a three-phase electric. engine (worn bearing or foreign objects entering the fan, contamination of the pump impeller), you should also connect a thermal relay to the electrical power supply circuit. engine after the KM contactor (not indicated on the diagram) and install the temperature sensor on the casing.
Appearance and main components of the thermal relay If a timer (modern RV) is used to switch modes, which occurs within a set time interval, then when the motor windings are turned on in a triangle, the rated speed is set, provided that the load on the shaft meets the technical conditions of the electric motor.
Switching modes using a modern time relay CRM-2T The operation of the timer itself is quite simple - first the star contactor is turned on, and after the adjustable time has elapsed, this contactor is turned off, and with some also adjustable delay the delta contactor is turned on.
Correct specifications for using switching winding connections.
When starting any three-phase electric. The most important condition must be met - the load resistance torque must always be less than the starting torque, otherwise the electric motor simply will not start, and its windings will overheat and burn out, even if the star starting mode is used, at which the voltage is lower than the nominal one.
Even if there is a freely rotating load on the shaft, the starting torque when connected by a star may not be enough and the electric current may not be sufficient. the engine will not reach the speed at which it should switch to delta mode, since the resistance of the medium in which the mechanisms of the units rotate (fan blades or pump impeller) will increase as the rotation speed increases.
In this case, if the current relay is excluded from the circuit, and mode switching is carried out according to the timer setting, then at the moment of transition to the triangle, the same current surges will be observed of almost the same duration as when starting from a stationary rotor state.
Comparative characteristics of direct and transient engine starts with a load on the shaft Obviously, such a star-delta connection will not give any positive results if the starting torque is incorrectly calculated. But at the moment the contactor providing the star connection is turned off, at insufficient engine speed, due to self-induction, there will be a surge of increased voltage into the network, which can damage other equipment.
Therefore, when using star-delta switching, it is necessary to ensure the feasibility of such a connection of a three-phase asynchronous electric power supply. engine and recheck the load calculations.
In addition to the rheostatic and direct methods of starting asynchronous motors, there is another common method - switching from star to delta.
The star-delta switching method is used in motors that are designed to operate with delta windings connected. This method is carried out in three stages. At the beginning, the motor is started by connecting the windings in a star, at this stage the motor accelerates. Then they switch to the working triangle connection diagram, and when switching, you need to take into account a couple of nuances. Firstly, you need to correctly calculate the switching time, because if you close the contacts too early, the electric arc will not have time to go out, and a short circuit may also occur. If the switching takes too long, this can lead to a loss of motor speed, and as a result, an increase in current inrush. In general, you need to clearly adjust the switching time. At the third stage, when the stator winding is already delta-connected, the engine enters steady-state operating mode.
The meaning of this method is that when the stator windings are connected with a star, the phase voltage in them decreases by 1.73 times. The phase current that flows in the stator windings decreases by the same amount. When the stator windings are connected by a triangle, the phase voltage is equal to the linear voltage, and the phase current is 1.73 times less than the linear one. It turns out that by connecting the windings with a star, we reduce the linear current by 3 times.
To avoid getting confused by the numbers, let's look at an example.
Let's say that the working circuit of the winding of an asynchronous motor is a triangle, and the linear voltage of the supply network is 380 V. The resistance of the stator winding is Z = 20 Ohms. By connecting the windings at the moment of star starting, we will reduce the voltage and current in the phases.
The current in the phases is equal to the linear current and is equal to
After accelerating the engine, we switch from star to delta and get different values of voltages and currents.
As you can see, the linear current when connected by a triangle is 3 times greater than the linear current when connected by a star.
This method of starting an asynchronous motor is used in cases where there is a small load or when the engine is idling. This is due to the fact that when the phase voltage decreases by 1.73 times, according to the formula for the starting torque provided below, the torque decreases three times, and this is not enough to start with a load on the shaft.
Where m is the number of phases, U is the phase voltage of the stator winding, f is the frequency of the supply network current, r1, r2, x1, x2 are the parameters of the equivalent circuit of an asynchronous motor, p is the number of pole pairs.
It follows that regulation of the rotation speed of asynchronous electric motors can be carried out:
changing the frequency of the supply current;
changing the number of poles of the stator winding;
introducing additional resistances into the rotor winding circuit.
The first two methods are used to regulate the rotation speed of electric motors with a squirrel-cage rotor, and the last one is used for electric motors with a wound rotor (with slip rings).
Regulating the rotation speed by changing the frequency of the supply current is used very rarely, since this method is applicable only when the electric motor is powered from a separate generator. In this case, to regulate the speed, it is necessary to change the rotation speed of the supply generator in the same proportion as the speed of the controlled electric motor should change. If the electric motor is powered from a three-phase current network, then it is impossible to regulate its speed by changing the frequency. In practice, speed control by changing frequency is used only in... AC rowing electric installations, in which powerful rowing electric motors are powered by separate generators and therefore the frequency of the supply current can be adjusted arbitrarily.
Most often in practice, the second method is used, which makes it possible to quite simply carry out stepwise control of the rotation speed of asynchronous electric motors with a squirrel-cage rotor. If it is possible to change the number of pole pairs of the stator winding [see. formula (80) ] then, therefore, it is possible to step-by-step control the speed of rotation of the electric motor, since the number of pole pairs can be equal to 1, 2, 3, etc. Electric motors that allow switching the number of pole pairs must have either several in the stator slots independent windings, or one winding with a special switching device. The domestic industry produces two-, three- and four-speed electric motors, used mainly in marine transport and on some cranes. When the numbers of poles differ significantly from each other, two-speed axis electric motors are manufactured with two independent windings. One, for example, can be performed on 2 R= 2, and the second by 2 R= 8 poles. Then, when the first winding is connected to the network, the magnetic field of the stator will rotate at a speed n 1 = 60·50 / 1 = 3000 about /min, and when connecting the second winding to the network - at a speed n 1 = 60·50 / 4 = 750 about /min. The rotation speed of the rotor will change accordingly. n 2 = n 1 (1-s).
Often, one winding is placed in the stator slots of a two-speed electric motor, but it is made in such a way that it can be turned on in a triangle if necessary (Fig. 49, A) and a double star (Fig. 49, b). When such a winding is connected with a triangle, the number of poles is 2 R = 2A, and when turned on by a double star 2 R = A(Where A- any integer), i.e., when moving from a triangle to a double star, the number of pairs of poles of the stator winding is halved, and the speed of the electric motor doubles.
Regulation by switching the number of pole pairs is used only for an electric motor with a squirrel-cage rotor, because electric motors with a wound rotor have one
temporarily, when switching the stator winding, it is necessary to switch the rotor winding, which complicates the design of the electric motor and the switching device. This method of speed control is highly economical, but it is not without its drawbacks. In particular, speed control does not occur smoothly, but in jumps; a rather complex switching device is required, especially when the number of speeds is more than two; when moving from one speed to another, the stator circuit breaks, and current and torque shocks are inevitable; the power factor at lower speeds is lower than at higher speeds due to increased magnetic flux dissipation.
Speed control by introducing additional resistances into the rotor circuit is possible only with electric motors with a wound rotor. According to equation (97), when different active resistances are introduced into the rotor circuit, the rigidity of the characteristics changes (Fig. 50), i.e., under the same load, the speed of the electric motor will be different. Obviously, the higher the value of additional resistance, the softer the artificial characteristic and the lower the speed of the electric motor.
Let's say the electric motor is running at a steady speed. n 1 on natural characteristics A at the point 1 , developing some torque M 1 = M c . When introducing some resistance into the rotor circuit R 1 the electric motor will switch to operation according to the characteristic b, whose equation
Since at the moment the resistance is turned on, the speed of the electric motor will practically not change, the transition from the characteristic A for characterization b will happen horizontally 1 -2 , and the torque of the electric motor will decrease to M 2 , which is less than the moment of resistance of the mechanism M , therefore, the speed of the electric motor will decrease and the slip will increase. As slip increases, the torque, according to expression (92), increases until the torque of the electric motor again becomes equal to the moment of resistance of the mechanism, after which equilibrium of the moments will occur and the motor will rotate at a new steady speed n 3 (dot 3 ).
If necessary, additional resistance can be included R 2 . Then the speed of the electric motor will decrease to the value n 5 . When the resistances are turned off, the speed of the electric motor will increase, and the transition from one characteristic to another occurs in the reverse order, as shown in Fig. 50.
The latter method allows you to obtain a wide range of speeds, but is extremely uneconomical, since with an increase in the active resistance of the rotor circuit, energy losses in the electric motor increase, which means its efficiency decreases. The control rheostats themselves, especially for powerful electric motors, turn out to be bulky and emit a lot of heat.
It must also be borne in mind that most electric motors are now self-ventilated.
As a result, when the rotation speed decreases, cooling deteriorates and the electric motor cannot develop the rated torque.
For a long time, unregulated electric drives based on AM have been used in industry, but recently there has been a need forspeed regulation of asynchronous motors.
The rotor speed is
In this case, the synchronous rotation speed depends on the voltage frequency and the number of pole pairs
Based on this, we can conclude that the speed of the blood pressure can be adjusted by changing the slip, frequency and number of pole pairs.
Let's look at the main adjustment methods.
Speed control by changing active resistance in the rotor circuit
This speed control method is applicable inmotors with wound rotor. In this case, a rheostat is connected to the rotor winding circuit, which can gradually increase the resistance. As resistance increases, engine slip increases and speed decreases. This ensures that the speed is adjusted downwards from the natural characteristic.
The disadvantage of this method is that it is uneconomical, since as slip increases, losses in the rotor circuit increase, therefore, the engine efficiency decreases. Plus, the mechanical characteristics of the engine become flatter and softer, due to which a small change in the load torque on the shaft causes a large change in the rotation speed.
Speed control in this way is not effective, but despite this it is used in motors with a wound rotor.
Regulating motor speed by changing the supply voltage
This control method can be implemented by connecting an autotransformer to the circuit, in front of the stator, after the supply wires. At the same time, if you reduce the voltage at the output of the autotransformer, the engine will operate at a reduced voltage. This will lead to a decrease in engine speed, at a constant load torque, as well as a decrease in the overload capacity of the engine. This is due to the fact that when the supply voltage decreases, the maximum motor torque decreases by a factor of square. In addition, this torque decreases faster than the current in the rotor circuit, which means that losses also increase, with subsequent heating of the motor.
The method of regulation by changing the voltage is only possible downward from the natural characteristic, since it is impossible to increase the voltage above the nominal one, because this can lead to large losses in the engine, overheating and failure.
In addition to the autotransformer, you can use a thyristor voltage regulator.
Speed control by changing power frequency
With this control method, a frequency converter (FC) is connected to the motor. Most often this is a thyristor frequency converter. Speed control is carried out by changing the voltage frequency f, since in this case it affects the synchronous speed of rotation of the motor.
As the voltage frequency decreases, the overload capacity of the motor will drop; to prevent this, it is necessary to increase the voltage U 1 . The value by which you need to increase depends on the drive. If regulation is carried out with a constant load torque on the shaft, then the voltage must be changed in proportion to the change in frequency (as the speed decreases). When increasing the speed, this should not be done, the voltage should remain at the rated value, otherwise it may cause damage to the engine.
If speed control is carried out with constant engine power (for example, in metal-cutting machines), then the change in voltage U 1 must be made proportional to the square root of the change in frequency f 1.
When regulating installations with a fan characteristic, it is necessary to change the supplied voltage U 1 in proportion to the square of the change in frequency f 1.
Regulation by changing the frequency is the most acceptable option for asynchronous motors, since it provides speed control over a wide range, without significant losses and reducing the overload capabilities of the motor.
Regulation of blood pressure speed by changing the number of pole pairs
This control method is possible only in multi-speed asynchronous motors with a squirrel-cage rotor, since the number of poles of this rotor is always equal to the number of stator poles.
In accordance with the formula discussed above, the speed of the motor can be adjusted by changing the number of pole pairs. Moreover, the speed change occurs in steps, since the number of poles takes only certain values - 1,2,3,4,5.
Changing the number of poles is achieved by switching the coil groups of the stator winding. In this case, the coils are connected using various connection schemes, for example “star-star” or “star-double star”. The first connection diagram gives a change in the number of poles in a ratio of 2:1. This ensures constant engine power during switching. The second circuit changes the number of poles in the same ratio, but at the same time provides constant motor torque.
The use of this control method is justified by maintaining efficiency and power factor during switching. The downside is the more complex and enlarged design of the engine, as well as an increase in its cost.
Hello. With my review I will continue the series of reviews of components for the “smart home”. And today I’ll tell you about the electric motor rotation direction switch from ITEAD. The switch connects to your home Wi-Fi network, and you can control it via the Internet from anywhere in the world. In the review, I will test its operation and express my thoughts on improving and expanding the capabilities of the switch. If you are interested, welcome to cat.
The switch is supplied in an antistatic bag:
Its brief characteristics from the website of the manufacturer ITEAD, who is also the seller:
OverviewThis WiFi switch supports to control 7-32V DC or 125-250V AC motor’s clockwise/anticlockwise running. The switch adopts PSA 1-channel wifi module to realize motor clockwise/anticlockwise running control. Reversible status will be synchronously feedback to your phone! Input voltage: usb 5V or DC 7-32V.
The power supply switch uses a pulsed DC-BC converter:
Therefore, to power the switch, it is possible to apply a constant voltage of 7 to 32 Volts to the input:
Or the switch can be powered with 5 volts from micro USB:
Let's turn the board over and look at it from below:
I can’t help but notice that the flux from the relays and power contacts is poorly washed off.
A matrix of seven Darlington transistors, a linear regulator with a low voltage drop, and an unnamed microcircuit are installed here:
Let's connect a DC motor to the switch for testing:
You can connect motors with power from 7 to 32 volts. Power is connected according to the connection diagram:
The main thing is to keep the color of the wires, otherwise it won’t work)))
We supply power, in our case 7.5V and now it’s time to connect the switch to the smartphone application:
I described in detail how to install and configure the application in my review. Since the release of that review, the application has only gotten better and acquired a Russian-language interface.
Open the application and select add device. Adding devices has become even easier and is now done in four simple steps.
Step one. Press the button on the switch and hold it pressed for five seconds:
Step two. Select a Wi-Fi network and enter its password. If you have already used this application, you will no longer have to enter anything:
In the third step, the application searches for and connects the switch:
The fourth and final step is to give the switch a name:
Switch connected:
We go into the switch management and we are asked to update the firmware on it:
Click on settings and update the firmware:
Notice how the switch settings menu has changed after the firmware update:
Now here it is possible to select actions after turning off the power to the switch. There are three options. After power is restored, the motor continues to rotate in the same direction, the motor stops, or the motor starts rotating in the other direction.
It is also possible to set countdown timers:
Single or repeating timers:
Cyclic timers:
Manual control of changing the direction of rotation occurs by pressing this button on the screen:
The key is on - the engine rotates in one direction, off - it rotates in the other direction.
It is also possible to control the direction of rotation by briefly pressing the button on the switch itself. The LEDs on the relays indicate their operation:
The LED next to the button indicates connection to the network. When Wi-Fi is connected, it lights up. The connection is fast enough. 2-3 seconds. Until the LED lights up, remote control is impossible.
I illustrated the operation of the switch with a short video:
You can also connect 125-250 Volt AC motors to the switch. Only powering the switch itself needs to be done separately. As I wrote, there are two options for connecting the power supply:
And now let’s talk about how ITEAD could improve its product, which would undoubtedly expand its scope of application.
First, and most significant. The switch does not have a STOP button. Stopping the process requires the use of limit switches that momentarily interrupt the power supply to the switch. But sometimes the process does not need to be completed... And here a problem arises. Although, if the power to the switch is interrupted, it is possible to turn off two relays at once to stop the engine. You saw this in the switch settings. I would also like to be able to automatically turn off the engine when the load on it increases, in contrast to normal. But this will require complication of the scheme. But I am sure that such a function would be in demand.
Second. There are very few seconds in the timer settings. Sometimes a minute is too much.
And third. Manual control in the application is very uninformative. When changing the direction of rotation, the switch button shows the on or off state. I would like to see the rotation control buttons in the form of arrows, for greater clarity.
Well, in general, the switch is a very useful thing in process automation. And with the above modifications, there would be no price for it at all. In the meantime, the possibility and scope of its application are somewhat limited.
Thank you for your attention.
The product was provided for writing a review by the store. The review was published in accordance with clause 18 of the Site Rules.
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