8. Operating principle and reverse (change of direction of rotation) of a three-phase asynchronous motor.
The figure shows an electromagnetic circuit of an IM with a short-circuited rotor winding in section, including a stator (1), in the grooves of which there are three phase stator windings (2), represented by one turn. The beginnings of the phase windings are A, B, C, and the ends are respectively X, Y, Z. In the cylindrical rotor (3) of the engine, there are rods (4) of short-circuited windings, closed at the ends of the rotor by plates.
When a three-phase voltage is applied to the phase stator windings, stator currents iA, iB, iC flow in the turns of the stator winding, creating a rotating magnetic field with a rotational speed n1. This field crosses the rods of the short-circuited winding of the rotor and EMF is induced in them, the direction of which is determined by the rule of the right hand. The EMF in the rotor bars creates the rotor currents i2 and the rotor magnetic field, which rotates with the frequency of the stator magnetic field. The resulting AM magnetic field is equal to the sum of the stator and rotor magnetic fields. The conductors with current i2, located in the resulting magnetic field, are acted upon by electromagnetic forces, the direction of which is determined by the left-hand rule. The total amplification Fres applied to all rotor conductors forms the rotating electromagnetic moment M of the induction motor.
The rotating electromagnetic moment M, overcoming the resistance moment Mc on the shaft, forces the rotor to rotate with a frequency n2. The rotor rotates with acceleration if the moment M is greater than the moment of resistance Ms, or at a constant frequency if the moments are equal.
The rotor speed n2 is always less than the speed of the magnetic field of the machine n1, because only in this case there is a rotating electromagnetic torque. If the rotor speed is equal to the rotational speed of the stator MF, then the EM torque is zero (the rotor rods do not cross the motor MF, and the current is zero). The difference between the rotational frequencies of the stator and rotor MF in relative units is called the slip of the motor:
s \u003d n 1− n 2.n 1
Slip is measured in relative units or percentages with respect to n1. In the operating mode close to the rated motor slip is 0.01-0.06. Rotor speed n 2 \u003d n 1 (1− s).
Thus, a characteristic feature of an asynchronous machine is the presence of slip - the inequality of the frequencies of rotation of the magnetic field of the motor and rotor. Therefore, the machine is called asynchronous.
When an asynchronous machine is operating in a motor mode, the rotor speed is less than the rotational speed of the MP and 0< s < 1. в этом режиме обмотка статора питается от сети, а вал ротора передает механический момент на исполнительный орган механизма. Электрическая энергия преобразуется в механическую.
If the IM rotor is inhibited (s \u003d 1), this is a short-circuit mode. If the rotor speed coincides with the rotational speed of the MP, then the engine torque does not arise. This is the ideal idle mode.
To change the direction of rotation of the rotor (reverse the motor), you need to change the direction of rotation of the MP. To reverse the motor, it is necessary to change the phase sequence of the applied voltage, i.e., switch two phases.
9. Equivalent circuit and mechanical characteristics of a three-phase asynchronous motor.
Rн \u003d R "----- |
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Rн \u003d R "----- |
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E \u003d E " | ||||||||
In the scheme, an asynchronous machine with electromagnetic coupling of the stator and rotor circuits is replaced by an equivalent reduced equivalent circuit. In this case, the parameters of the rotor winding R2 and x2 are reduced to the stator winding, provided that E1 \u003d E2 ". E2", R2 ", x2" are the reduced parameters of the rotor.
included in the winding of a stationary rotor, that is, the machine has a resistive load.
The magnitude of this resistance is determined by the slip and, therefore, the mechanical load on the motor shaft. If the moment of resistance on the motor shaft is Мс \u003d 0, then slip s \u003d 0; in this case, the value of R n \u003d ∞ and I2 "\u003d 0, which corresponds to the work
engine idling.
In no-load mode, the stator current is equal to the magnetizing current I 1 \u003d I 0. The magnetic circuit of the machine is represented by a magnetizing circuit with the parameters x0, R0 - the inductive and active resistances of the stator winding magnetization. If the moment of resistance on the motor shaft exceeds its torque, then the rotor stops. In this case, the value of Rн \u003d 0, which corresponds to the short circuit mode.
The first circuit is called the T-shaped blood pressure equivalent circuit. It can be converted to a simpler form. For this purpose, the magnetizing circuit Z 0 \u003d R 0 + jx 0
take out to common clamps. So that in this case the magnetizing current I 0 does not change its value, resistances R1 and x1 are connected in series in this circuit. In the obtained L-shaped equivalent circuit, the resistances of the stator and rotor circuits are connected in series. They form a working circuit, parallel to which the magnetizing circuit is connected.
Current value in the working circuit of the equivalent circuit:
I "2 \u003d | Where U1 - phase |
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"1 - s 2 | √ (R 1 + | R "2 | ||||||||||||||
√ (R 1+ R 2+ R 2s | ) + (x 1 + x 2) | ) + (x 1 + x 2) |
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mains voltage.
The electromagnetic moment of the AM is created by the interaction of the current in the rotor winding with the rotating MF of the machine. The electromagnetic moment M is determined through the electromagnetic power:
P uh | 2 πn 1 | The angular frequency of rotation of the stator MP. |
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P e2 | m1 I2 "2 R" 2 | That is, the EM torque is proportional to the power of electrical |
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ω 1s | |||||||||||||||
ω 1s | |||||||||||||||
losses in the rotor winding. | |||||||||||||||
2 R 2 " |
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2 ω 1 [(R 1 + | ) + (x 1 + X 2 ") 2] |
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Taking in the equation the number of motor phases m1 \u003d 3; x1 + x2 "\u003d xk, we investigate it for an extremum. To do this, equate the derivative dM / ds to zero and obtain two extreme points. At these points, the moment Mk and slip sk are called critical and, respectively, equal:
± R "2 | ||||||||||
√ R1 2 + sк 2 | Where “+” for s\u003e 0, “-” for s< 0. |
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M to \u003d | 3U 1 2 | |||||||||
2 ω 1 (R 1 ± √ | ||||||||||
R1 2 + Xk 2 | ||||||||||
The dependence of the EM torque on slip M (s) or on the rotor speed M (n2) is called the mechanical characteristic of the IM.
If we divide M by Mk, we get a convenient form of writing the equation of the mechanical characteristics of the blood pressure:
2 Mk (1 + ask) | ||||||||||||||||
2ask | R2 " |
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2 Mk | 3 Uph 2 | R2 " | ||||||||||||||
2 ω 1x k | ||||||||||||||||
Figure: 1 Wiring diagram of a single-phase asynchronous motor with a starting capacitor.
Let's take as a basis the already connected single-phase asynchronous motor, with the direction of rotation clockwise (Fig. 1).
Figure 1
- points A, B conventionally indicate the beginning and end of the starting winding; for clarity, brown and green wires are connected to these points, respectively.
- points C, B conventionally indicate the beginning and end of the working winding; for clarity, red and blue wires are connected to these points, respectively.
- arrows indicate the direction of rotation of the rotor of the induction motor
Task.
Change the direction of rotation of a single-phase asynchronous motor in the opposite direction - counterclockwise. To do this, it is enough to reconnect one of the windings of a single-phase asynchronous motor - either working or starting.
Option number 1
We change the direction of rotation of a single-phase asynchronous motor by reconnecting the working winding.
Fig. 2 With this connection of the working winding, relative to Fig. 1, the single-phase induction motor will rotate in the opposite direction.
Option number 2
We change the direction of rotation of a single-phase asynchronous motor by reconnecting the starting winding.
Fig. 3 With this connection of the starting winding, relative to Fig. 1, the single-phase induction motor will rotate in the opposite direction.
Important note.
This method of changing the direction of rotation of a single-phase asynchronous motor is possible only if the motor has separate branches of the starting and working winding.
Fig. 4 With this connection of the motor windings, reverse is impossible.
In fig. 4 shows a fairly common version of a single-phase induction motor, in which the ends of the windings B and C, the green and red wires, respectively, are connected inside the housing. Such an engine has three leads, instead of four as in Fig. 4 brown, purple, blue wire.
UPD 03/09/2014Finally, it was possible to check in practice, not very correct, but still used method of changing the direction of rotation of an induction motor. For a single-phase asynchronous motor, which has only three leads, it is possible to make the rotor rotate in the opposite direction, it is enough to swap the working and starting windings. The principle of such an inclusion is shown in Fig. 5
Figure: Non-standard asynchronous motor reverse
Hello, dear readers and visitors of the site "Notes of an Electrician".
In the last article, we talked about, got acquainted with the diagram of its connection to the electrical network with a voltage of 220 (V), the designation and marking of the terminals.
In the same article, I promised to tell you in the near future how you can organize its reverse, i.e. control the direction of rotation of the motor remotely, and not using jumpers in the terminal box.
So let's get started.
In principle, there is nothing complicated. The principle of the control circuit is similar, with the exception of some details. Actually, I have not had to deal with the reverse circuit of single-phase motors before, and this circuit was implemented by me in practice for the first time.
The essence of the scheme is to change the direction of rotation of the shaft of a single-phase capacitor motor remotely using buttons (push-button station). Remember, in the previous article, we manually changed the position of two jumpers on the motor terminal block to change the direction of the working winding (U1-U2). Now you need to remove these jumpers, because their role in this scheme will be performed by normally open (n.d.) contacts of contactors.
Preparation of equipment for reversing a single-phase motor
First, we list all the electrical equipment that we need to purchase to organize the reverse of the AIRE 80S2 capacitor motor:
1. Circuit breaker
We use two-pole 16 (A), with characteristic "C" from IEK.
This button post has 3 buttons:
- forward button (black)
- back button (black)
- stop button (red)
Let's analyze the push-button post.
We can see that each button has 2 contacts:
- normally open contact (1-2), which closes when you press the button
- normally closed contact (3-4), which is closed until the button is pressed
Please note that in the photo, the outermost button on the left is upside down. If you connect the reverse circuit of a single-phase motor yourself, then be careful, the buttons in the push-button post may be inverted. Follow the labeling of the contacts (1-2) and (3-4).
3. Contactors
You also need to purchase two contactors. In my example, I use small-sized contactors KMI-11210 from IEK, which are mounted on a DIN rail. These contactors have 4 normally open (NO) contacts and are capable of switching loads up to 3 (kW) at 230 (V) alternating voltage. Here they are just right for us, tk. our tested single-phase motor AIRE 80S2 has a power of 2.2 (kW).
Instead of contactors, you can purchase, by the example of which I described their structure and principle of operation.
The coils of this contactor are designed for an alternating voltage of 220 (V), which will need to be taken into account when assembling a single-phase motor reverse control circuit.
Here, in fact, is my work.
I have already said in the last article that one of the readers of the "Notes of an Electrician" site named Vladimir asked me to help him with a power of 2.2 (kW) and draw up (invent) a reverse circuit for him. According to my sketches (including assembly sketches), Vladimir assembled the above scheme in. A little later, he wrote to me in the mail that he tested the scheme, everything works, no complaints.
If you have any questions on the materials of the site, then ask me them in the comments or at. Within 12-24 hours, and maybe faster, it all depends on my employment, I will answer you.
Now I will tell you how this circuit works.
The principle of operation of the reverse circuit of a single-phase motor
First of all, we turn on the supply machine.
1. Forward rotation
When the "forward" button is pressed, the coil of the contactor K1 receives power through the following circuit: phase - N.C. contact (3-4) of the "stop" button - n.z. contact (3-4) of the "back" button - no. contact (1-2) of the pressed forward button - the coil of the contactor K1 (A1-A2) - zero.
Contactor K1 is pulled up and closes all its normally open (n.o.) contacts:
- 1L1-2T1 (self-grabbing coil K1)
- 5L3-6T3 (simulates jumper U1-W2)
- 13NO-14NO (simulates jumper V1-U2)
You do not need to hold the forward button, because the coil of the contactor K1 stands on "self-catch" through its own N.O. contact (1L1-2T1).
The single-phase motor starts to rotate in the forward direction.
2. Reverse rotation
When you press the "back" button, the coil of the contactor K2 receives power through the following circuit: phase - NC. contact (3-4) of the stop button - n.z. contact (3-4) of the forward button - no. contact (1-2) of the pressed "back" button - the coil of the contactor K2 (A1-A2) - zero.
Contactor K2 picks up and closes its following normally open (n.o.) contacts:
- 1L1-2T1 (self-grabbing coil K2)
- 3L2-4T2 (phase to the motor in the power circuit)
- 5L3-6T3 (simulates jumper W2-U2)
- 13NO-14NO (imitates jumper U1-V1)
You do not need to hold the "back" button with your finger. the coil of the contactor K2 stands on "self-catching" through its own N.O. contact (1L1-2T1).
The single-phase motor starts to rotate in the opposite direction.
To stop the engine, press the stop button.
3. Blocking
The presented reverse circuit of a single-phase capacitor motor has a button lock, i.e. If, with the motor running in the forward direction, you mistakenly press the "back" button, then the contactor K1 will first turn off, and then the contactor K2 will work. And vice versa. Thus, we have a blocking from simultaneously two contactors K1 and K2.
You can apply other types of locks, but I limited myself to this one.
P.S. This concludes my article. If you liked my article, I will be very grateful if you share it on social networks. And also do not forget to subscribe to my new articles - it will be more interesting further.
Figure: 1 Wiring diagram of a single-phase asynchronous motor with a starting capacitor.
Let's take as a basis the already connected single-phase asynchronous motor, with the direction of rotation clockwise (Fig. 1).
Figure 1
- points A, B conventionally indicate the beginning and end of the starting winding; for clarity, brown and green wires are connected to these points, respectively.
- points C, B conventionally indicate the beginning and end of the working winding; for clarity, red and blue wires are connected to these points, respectively.
- arrows indicate the direction of rotation of the rotor of the induction motor
Task.
Change the direction of rotation of a single-phase asynchronous motor in the opposite direction - counterclockwise. To do this, it is enough to reconnect one of the windings of a single-phase asynchronous motor - either working or starting.
Option number 1
We change the direction of rotation of a single-phase asynchronous motor by reconnecting the working winding.
Fig. 2 With this connection of the working winding, relative to Fig. 1, the single-phase induction motor will rotate in the opposite direction.
Option number 2
We change the direction of rotation of a single-phase asynchronous motor by reconnecting the starting winding.
Fig. 3 With this connection of the starting winding, relative to Fig. 1, the single-phase induction motor will rotate in the opposite direction.
Important note.
This method of changing the direction of rotation of a single-phase asynchronous motor is possible only if the motor has separate branches of the starting and working winding.
Fig. 4 With this connection of the motor windings, reverse is impossible.
In fig. 4 shows a fairly common version of a single-phase induction motor, in which the ends of the windings B and C, the green and red wires, respectively, are connected inside the housing. Such an engine has three leads, instead of four as in Fig. 4 brown, purple, blue wire.
UPD 03/09/2014Finally, it was possible to check in practice, not very correct, but still used method of changing the direction of rotation of an induction motor. For a single-phase asynchronous motor, which has only three leads, it is possible to make the rotor rotate in the opposite direction, it is enough to swap the working and starting windings. The principle of such an inclusion is shown in Fig. 5
Figure: Non-standard asynchronous motor reverse
Before choosing a connection diagram for a single-phase asynchronous motor, it is important to determine whether to reverse. If, for full-fledged operation, you often need to change the direction of rotation of the rotor, then it is advisable to organize reversal using a push-button post. If one-way rotation is enough for you, then it will do without the possibility of switching. But what if, after connecting through it, you decide that the direction still needs to be changed?
Suppose that an asynchronous single-phase motor, already connected using a starting-charging capacity, initially rotates the shaft clockwise, as in the picture below.
Let's clarify important points:
- Point A marks the beginning of the starting winding, and point B marks its end. A brown wire is connected to the initial A terminal, and a green wire to the final terminal.
- Point C marks the beginning of the working winding, and point D marks its end. A red wire is connected to the initial contact, and a blue wire to the final contact.
- The direction of rotation of the rotor is indicated by arrows.
We set ourselves the task - to reverse a single-phase motor without opening its case so that the rotor begins to rotate in the other direction (in this example, against the movement of the clock hand). It can be solved in three ways. Let's consider them in more detail.
Option 1: reconnecting the working winding
To change the direction of rotation of the motor, you can only swap the beginning and end of the working (permanently on) winding, as shown in the figure. You might think that for this you have to open the case, take out the winding and turn it over. This is not necessary, because it is enough to work with contacts from the outside:
- There should be four wires coming out of the case. 2 of them correspond to the beginning of the working and starting windings, and 2 - to their ends. Determine which pair belongs to the working winding only.
- You will see that two lines are connected to this pair: phase and zero. With the engine off, reverse it by switching the phase from the initial winding contact to the final one, and zero - from the final to the initial one. Or vice versa.
As a result, we get a diagram where points C and D are interchanged. Now the rotor of the induction motor will rotate in the other direction.
Option 2: reconnecting the starting winding
The second way to organize the reverse of a 220 Volt asynchronous motor is to swap the beginning and end of the starting winding. This is done by analogy with the first option:
- From the four wires coming out of the motor box, find out which of them correspond to the wires of the starting winding.
- Initially, the end B of the starting winding was connected to the beginning of the working winding C, and the beginning of A was connected to the starting-charging capacitor. You can reverse a single-phase motor by connecting a capacitor to terminal B, and the beginning of C with the beginning of A.
After the steps described above, we get a diagram, as in the figure above: points A and B have changed places, which means the rotor began to turn in the opposite direction.
Option 3: changing the starting winding to the working one, and vice versa
It is possible to organize the reverse of a single-phase 220V motor in the ways described above, only on condition that the branches from both windings with all beginnings and ends come out of the case: A, B, C and D. But often there are motors in which the manufacturer intentionally left outside only 3 contacts. With this he secured the device from various "homemade products". But still there is a way out.
The figure above shows a diagram of such a "problem" motor. He has only three wires coming out of the case. They are marked with brown, blue and purple colors. The green and red lines corresponding to the end B of the starting and the beginning of the C of the working winding are interconnected internally. We cannot get access to them without disassembling the engine. Therefore, it is not possible to change the rotation of the rotor by one of the first two options.
In this case, proceed as follows:
- Remove the capacitor from the initial terminal A;
- Connect it to terminal D;
- From wires A and D, as well as the phases, they start up the branches (you can reverse it using a key).
Take a look at the picture above. Now, if you connect a phase to branch D, then the rotor rotates in one direction. If the phase wire is thrown to branch A, then the direction of rotation can be changed in the opposite direction. The reverse can be done by manually disconnecting and connecting the wires. Using a key will help make the job easier.
Important! The last version of the reversible connection scheme for an asynchronous single-phase motor is incorrect. It can only be used if the following conditions are met:
- The length of the starting and working windings is the same;
- Their cross-sectional area corresponds to each other;
- These wires are made from the same material.
All of these quantities affect resistance. It should be constant for the windings. If suddenly the length or thickness of the wires differ from each other, then after you organize the reverse, it turns out that the resistance of the working winding will be the same as it was before with the starting one, and vice versa. This can also be the reason that the motor cannot start.
