What should be taken into account when making a three-phase connection to a private house. Operating principle of an asynchronous motor with connection diagrams 3-phase motor in a single-phase network

29.10.2023

Since the electricity consumption of household appliances is small, single-phase voltage is used to supply electricity to homes. All electrical appliances, including the electricity meter, are connected to the electrical network using two wires. An increase in power of devices connected to the electrical network over 10 kilowatts requires a reduction in the load on the wires. This is achieved using three-phase voltage. In this case, the electrical power used is distributed between three wires when connecting “star” and four wires when connecting “star with neutral wire”.

To account for the power consumed in three-phase networks, special three-phase electricity meters are used. When working with electrical loads of low power, a galvanic connection is used, and for high powers, a transformer connection of these electricity meters is used. If currents are more than 100 Amperes and maximum power is more than 60 kilowatts, current transformers are used.

The meter is installed in a special design. Usually this is an electrical panel designed to be attached with several screws to a platform with an electric meter. As a rule, it is necessary to fix only three to four screws, which does not require a long time and does not cause difficulties.

The installed electricity meter should not be operated in excess of the voltage and current values set for it.

The connection diagram for a 3-phase electricity meter is below.

Having securely secured the electric meter in the electrical panel and turned off the voltage, you can begin installation work on connecting the elements of the electrical circuit.

A power cable with four insulated conductors is used for connection. A circuit breaker is installed at the input, to which the power cable is connected. After the switch, phases A, B and C and 0 are respectively connected to terminals 1,3, 5 and 7 of the meter.

Connecting a digital electric meter is carried out with the obligatory observance of the phase sequence – A, B, C. Special devices allow you to determine the phasing. If the meter is incorrectly phased, it will display an error message. In this case, the connected wires change places.

The load is connected to terminals 2, 4 and 6, and 0 to the 8th terminal.

The grounding of the electric meter is connected to the electrical panel housing, which must be grounded.
If a three-phase network is used to connect both three-phase and single-phase loads, 0 must be grounded to avoid possible overvoltages. Overvoltages occur when the insulated 0 breaks.

The meter should be connected in strict accordance with the instruction manual, since the numbering of the terminals and the connection diagram itself may differ for different models, especially those released at different times. New models of electricity meters allow for remote data monitoring and therefore have a larger number of terminals.

Operation of the meter is allowed only after it has been checked and sealed by the relevant services.

Three-phase electric motors have higher efficiency than single-phase 220 volt ones. If you have a 380 Volt input in your house or garage, then be sure to buy a compressor or machine with a three-phase electric motor. This will ensure more stable and economical operation of the devices. To start the motor, you will not need various starting devices and windings, because a rotating magnetic field appears in the stator immediately after connecting to a 380-volt power supply.

Selecting a motor switching circuit

3-phase connection diagrams motors using magnetic starters I described in detail in previous articles: “” and ““.

It is also possible to connect a three-phase motor to a 220 Volt network using capacitors. But there will be a significant drop in the power and efficiency of its operation.

In the stator of an asynchronous motor at 380 V there are three separate windings, which are connected to each other in a triangle or star and 3 opposite phases are connected to the three beams or vertices.

You must consider that when connected with a star, the start will be smooth, but in order to achieve full power it is necessary to connect the motor with a triangle. In this case, the power will increase by 1.5 times, but the current when starting powerful or medium-sized motors will be very high, and can even damage the insulation of the windings.

Before connecting electric motor, read its characteristics in the passport and on the nameplate. This is especially important when connecting 3-phase electric motors made in Western Europe, which are designed to operate from a mains voltage of 400/690. An example of such a nameplate is in the picture below. Such motors are connected only in a “delta” configuration to our electrical network. But many installers connect them in the same way as domestic ones in a “star” and the electric motors burn out, especially quickly under load.

On practice all electric motors are domestically produced for 380 Volts they are connected by a star. Example in the picture. In very rare cases, in production, in order to squeeze out all the power, a combined star-delta connection circuit is used. You will learn about this in detail at the very end of the article.

Star-delta motor connection diagram

In some There are only 3 of our electric motors. the end of a stator with windings - this means that a star is already assembled inside the engine. All you have to do is connect 3 phases to them. And in order to assemble a star, both ends of each winding or 6 terminals are needed.

The ends of the windings in the diagrams are numbered from left to right. Numbers 4, 5 and 6 are connected to 3 phases A-B-C from the mains.

When a three-phase electric motor is connected by a star, the beginnings of its stator windings are connected together at one point, and 3 phases of 380 Volt power supply are connected to the ends of the windings.

When connected by a triangle The stator windings are connected to each other in series. In practice, it is necessary to connect the end of one winding to the beginning of the next. 3 power phases are connected to the three points connecting them to each other.

Star-delta connection

To connect the motor according to a rather rare star scheme at launch, with subsequent transfer for operation in operating mode to a triangle scheme. This way we can squeeze out maximum power, but it turns out to be a rather complex circuit without the possibility of reversing or changing the direction of rotation.

For the circuit to operate, 3 starters are required. The first K1 is connected to the power supply on one side, and on the other - the ends of the stator windings. Their origins are connected to K2 and K3. From starter K2, the beginning of the windings are connected respectively to other phases according to a triangle diagram. When K3 is turned on, all 3 phases are short-circuited with each other and a star operating circuit is obtained.

Attention, magnetic starters K2 and K3 should not be turned on at the same time, otherwise an emergency shutdown of the circuit breaker will occur due to the occurrence of an interphase short circuit. Therefore, an electrical interlock is made between them - when one of them is turned on, the block contacts open the control circuit of the other.

The scheme works as follows. When the starter K1 is turned on, the time relay turns on K3 and the engine starts according to the star circuit. After a predetermined interval sufficient for the engine to fully start, the time relay turns off the starter K3 and turns on K2. The motor switches to operating the windings in a triangle pattern.

Shutdown occurs starter K1. When you restart it, everything repeats again.

Related materials:

I also tried this option. Star connection. I start a 3 kilowatt engine using a 160 microfarad capacitor. And then I remove it from the network (if you do not remove it from the network, the capacitor begins to heat up). And the engine runs independently at quite good speeds. Is it possible to use it in this way? Is it not dangerous?

Novel:

Hello! There is a 1.5 kW Vesper Frequency Drive, which transforms from a single phase 220 volt network into 3 phases at the output with interphase 220V to power an asynchronous 1.1 kW. dv. 1500 rpm However, when the 220 volt network is turned off, it is necessary to power it from a direct current inverter, which uses the battery as a backup power source. The question is, is it possible to do this through an ABB changeover switch (i.e. manually switch to powering the Vesper from a direct current inverter) and will the direct current inverter not be damaged?

Experienced Electrician:

Roman, hello. To do this, you need to read the instructions or ask questions to the inverter manufacturer, namely, whether the inverter is capable of connecting to the load (or in other words, its overload capacity for a short time). If you don’t take risks, then it’s easier (when 220 volts disappear) to turn off the electric motor using an automatic switch or switch, turn on the power from the inverter with a changeover switch (thus powering the frequency switch) and then turn on the engine. Or make a scheme for uninterrupted operation - constantly supply mains voltage to the inverter, and take it from the inverter to the frequency converter. In the event of a power outage, the inverter remains in operation thanks to the battery and there is no interruption in the power supply.

Sergey:

Good afternoon. A single-phase motor from an old Soviet washing machine rotates in different directions every time it starts (there is no system). The engine has 4 terminals (2 thick, 2 thin. I connected it through a switch with a third outgoing contact. After starting, the engine runs stably (does not heat up). I can’t understand why it rotates in different directions.
1. Experienced Electrician:
  
  Sergey, hello. The thing is that a single-phase motor does not care where it rotates. The field is not circular (as in a three-phase network), but pulsating for 1/50 of a second at the “plus” phase relative to zero, and 1/50 for the “minus” phase. It's like spinning a battery a hundred times a second. Only after the engine has spun up does it maintain its rotation. An old washing machine may not have had a strict direction of rotation. If we assume this, then at the moment of launch on the “positive” half-wave of the sine wave it starts in one direction, and with a negative half-wave - in the other. It makes sense to try to set the current bias of the starting winding through the capacitor. The current in the starting winding will begin to lead the voltage and will set the rotation vector. As I understand it, you now have two wires (phase and neutral) going to the motor from the working winding. One of the wires of the starting winding is connected to the phase (conditionally, just actually tightly with one of the wires), and the second wire goes to zero through the third non-latching contact (also conditionally, in fact, to another of the network wires). So try installing a capacitor with a capacity of 5 to 20 µF between the wire and the non-locking contact and observe the result. In theory, you should rigidly set the direction of the magnetic field with this. In fact, this is a capacitor motor (single-phase asynchronous, all capacitor motors) and here only three points are possible: either the capacitor always works and then you need to select the capacitance, or it sets the rotation, or the start occurs without it, but in any direction.
Galina:

Hello
Sergey:

Good afternoon. I assembled the circuit, as you said, set the capacitor to 10 uF, the engine now starts steadily only in one direction. The direction of rotation can only be changed if the ends of the starting winding are swapped. Therefore, the theory worked flawlessly in practice. Thank you very much for the advice.
Galina:

Thanks for the answer, I bought a CNC milling machine in China, a 3-phase motor at 220, and here (I live in Argentina) the network is single-phase at 220, or 3-phase at 380
I consulted with local specialists - they say that I need to change the engine, but I really don’t want to. Help me with advice on how to connect the machine.
Galina:

Hello! Thank you very much for the information! A couple of days later the machine arrives. I’ll see what’s really there, and not just on paper, and I suppose I’ll still have questions for you. Thanks again!
Hello! Is this option possible: draw a 3-phase 380v line and install a step-down transformer to have 3-phase 220v? The machine has 4 motors, the main power is 5.5 kw. If this is possible, then what kind of solution is needed?
Yura:

Hello!
Please tell me - is it possible to power an asynchronous three-phase electric motor of 3.5 kW from 12-volt batteries? For example, using three household inverters 12-220 with a pure sine wave.
1. Experienced Electrician:
  
  Yuri, hello. Purely theoretically this is possible, but in practice you will encounter the fact that when starting, an asynchronous motor creates a large starting current and you will have to use an appropriate inverter. The second point is complete phasing (a frequency shift of three inverters by an angle of 120° relative to each other), which cannot be done unless provided by the manufacturer, therefore you will not be able to achieve manual synchronization at a frequency of 50 Hz (50 times per second). Plus the engine power is quite large. Based on this, I would recommend that you pay attention to the “battery-inverter-frequency converter” combination. The frequency converter is capable of producing the required synchronized phases of the voltage that will be at the input. Almost all engines have the ability to switch on 220 and 380 volts. Therefore, having received the desired voltage and received the desired connection diagram, you can use a frequency converter to make a smooth start, avoiding large starting currents.
  1. Yura:
    
    I don’t understand a little - my inverters are 1.5 kW, that is, do you recommend using a battery of batteries and one such inverter in conjunction with a frequency converter? how will he pull it out???
    or do you recommend using an inverter of appropriate power - 3.5 kW? then the need for a frequency converter is unclear...
    1. Experienced Electrician:
      
      I'll try to explain.
      1. Learn about three-phase current. Three phases are not three voltages at 220 volts. Each phase has a frequency of 50 hertz, that is, it changes its value from plus to minus 100 times per second. In order for an asynchronous motor to start working, it needs a circular field. In this field, three phases are shifted relative to each other by an angle of 120°. In other words, phase A reaches its peak, after 1/3 of the time this peak reaches phase B, after 2/3 of the time phase C, then the process repeats. If the change of peaks of the sine wave occurs chaotically, the engine will not start to rotate, it will simply hum. Therefore, either your inverters must be phased or there is no point in them.
      2. Study information about asynchronous motors. The starting current reaches values of 3-8 times the rated one. Therefore, if we take an approximate value of 5 amperes, then when starting the engine the current can be 15-40 amperes or 3.3 - 8.8 kW per phase. An inverter of less power will burn out immediately, which means you need to take the inverter at maximum power, even if it lasts only half a second or even less, and this will be an expensive pleasure.
      3. Study the information on the frequency converter. The frequency generator can provide both a smooth start and the conversion of one phase into three. A smooth start will allow you to avoid large starting currents (and the purchase of a heavy-duty inverter), and converting one phase into three will allow you to avoid the expensive procedure for phasing inverters (if they are not initially adapted to this, then you definitely cannot do this on your own and you will have to find a good electronics engineer ).
      
      I recommend getting a powerful inverter coupled with a frequency converter if you really need to get full power from your engine.

Valery:

Hello. Please tell me, is it possible to use this motor (imported) to be connected to our 220V network for a woodworking machine?
There are 4 options on the nameplate:
— 230, triangle, 1.5kw, 2820 /min., 5.7A, 81.3%
— 400, star, 1.5kw, 2800/min., 3.3A, 81.3%
— 265, triangle, 1.74kw, 3380/min, 5.7A, 84%
— 460, evezda, 1.74kw, 3380/min, 3.3A, 84%
Judging by this, this engine is very well suited for d.o. machine (according to option 1). There are probably 6 contacts in the box? Good (relatively) speed. 230V is confusing - how will it behave in a 220V network? Why is the maximum current according to options 1, 3?
Is it possible to use this motor for the machine and how to connect it to a 220V network?

Valery:

Thank you very much for everything. For your patience, re-explaining everything that has been repeated many times in other comments. I re-read all this, in some places more than once. I read a lot of information. on various sites for converting 3 ph.d. to the 220v network. (from the moment my assistants set fire to the electric motor of a small homemade machine). But I learned a lot more from you, features that I didn’t know about and hadn’t encountered before. Today, after using a search engine, I went to this site, re-read almost all the comments and was amazed at the usefulness and accessibility of the information.
Regarding my questions. Here's the thing. On my old machine (formerly, my father’s) there is the same old electric. dv. But it has lost power and is “beating” from the housing (probably the burnt winding is shorting). There is no tag, a classic triangle, no terminals - it was probably altered at some point. They offer me a new engine, Polish, it seems, with the given options on the tag. By the way, there is 50 Hz for each option. And after sending the comment, I carefully looked at all 4 options given and understood why the current is higher in the triangle.
I will take it and turn it on in 220 according to option 1 in a triangle through capacitors with 70% power. The gear ratio can be increased, but the machine could have more power.
Yes, besides the classic triangle and star, there are other options for connecting 380 to a 220 network. And there is (you know) a simpler way to determine the beginning of the windings using a battery and a switch.
Valery:

Today I received a photo of the email nameplate. dv. You're right. There are 3 and 4 options 60Hz. And now it is clear that it could not be otherwise and that at 50Hz - a maximum of 3000 rpm. Another question. How reliably and for a long time do electrolytic capacitors work with one turn on through a powerful diode as a working one? con.?
Alexander:

Hello, can you tell me how to attach a file with a photo to ask a question?
Sergey:

Good afternoon.
A little history. On a water heating boiler (a large industrial one - for heating an enterprise) I use two VILO circulation pumps with a German electric motor of 7.5 kW each. When we received both pumps, we connected them in a triangle. We worked for a week (everything was fine). The hot water boiler automation adjusters arrived and told us that the connection diagram for both engines should be switched to a “star” one. We worked for a week and one after another both engines burned out. Tell me, can reconnecting from delta to star be the cause of burnt-out German engines? Thank you.
Alexander:

Hello, Experienced Electrician) Tell me your opinion about this motor connection diagram, I came across it on one forum

“Partial counter star, with working capacitors in two windings”
Link to the diagram and diagram describing the operating principle of such a circuit - https://1drv.ms/f/s!AsqtKLfAMo-VgzgHOledCBOrSua9

It is said that this motor connection diagram was developed for a two-phase network and shows the best results when connected to 2 phases. But in a single-phase 220V network it is used because it has better characteristics than the classic ones: star and triangle.
What can you say about this option for connecting a three-phase motor to a 220V network? Has the right to life? I want to try it on a homemade lawn mower.
1. Experienced Electrician:
  
  Alexander, hello. Well, what can I tell you? Firstly, the literacy of both the presentation of the material and the literacy of the language of the article are incredibly impressive. Secondly, for some reason very few people know about this method. Thirdly, if this method were effective and better, it would have long been included in educational literature. Fourthly, there is no theoretical explanation of this method anywhere. Fifthly, there are proportions, but there are no formulas for calculating the capacitance (that is, conditionally, you can take 1000 μF or 0.1 μF as a reference point - the main thing is to maintain the proportions???). Sixth, the topic was not written by an electrician. Seventh, I personally can’t wrap my head around the first winding, which is connected backwards and through a capacitor - all this makes me think that someone came up with something and wants to pass something off as an invention that supposedly works better for two-phase networks. Theoretically, this can be allowed, but there is little theoretical data for reflection. In theory, if you somehow obtain one or the other half-wave from one or the other phase, but the circuit should then have a different form (when using two phases, it is definitely a star, but using a neutral wire and two capacitors to it or from him... and again, it turns out to be garbage. In general, experiment, and then write back - I’m interested in what happens, but I personally don’t want to conduct such experiments, well, or if they give me an engine and say - it can be killed, then I’ll experiment. I have already written about the selection of capacitors both in the comments and in the links to the article “Capacitor for a three-phase motor” on this site and on the site of the “hereditary master" - there is no need to thoughtlessly install a capacitor according to the formula. You need to take into account the motor load and select a capacitor according to the working current in a specific operating cycle.
  1. Alexander:
    
    Thanks for the answer.
    On the forum where I came across this, several people tried this scheme on their engines (including the person who posted it) and they say that they are very pleased with the results of its work. Regarding the competence of the person who proposed it, as I understand it, he seems to be on topic (and the moderator of that forum), the diagram is not his, as he said, he found it in some old books on engines. But that’s it, I have an engine suitable for experiments , I'll try it on it.
    Regarding the formulas, I just didn’t present all the entries from that thread, a lot of things are written there, I added more from the main one if you’re interested, look at the same link.
    1. Experienced Electrician:
      
      Alexander, experiment and write the result. I can say one thing - I am an inquisitive comrade, but I have not heard about such a scheme either from textbooks or from the lips of many authoritative senior comrades. My neighbor, an even more inquisitive electronics engineer with a focus on electricity, hasn’t heard either. One of these days I'll try to ask him.
      Competence is such a... questionable thing when it comes to the Internet. You never know who is sitting on the other side of the screen and what he is like, and whether he has the diploma he is talking about hanging on his wall, or whether he knows any of the subjects that are indicated on the diploma. I’m not trying to criticize the person at all, I’m just trying to say that you don’t always have to believe one hundred percent of the person on the other side of the screen. If something happens, you won’t be able to push him to the wall for harmful advice, and this gives rise to complete irresponsibility.
      There is another “dark” point - forums are often created in order to generate income and all means are good for this, as an option, to propose some kind of tricky topic, promote it, even if it is not entirely working, but unique, that is, only on his website. And “several” people, this could just be a moderator, talk to themselves under several nicknames to promote the topic. Again, I don’t criticize that particular person, but I’ve already seen this type of black PR on the forum.
      Now let's touch on old books and the Soviet Union. There were few fools in the USSR (among those who were involved in development) and if the scheme had proven itself, it would probably have been included in the textbooks I studied from, at least for mention and for general development that such an option was possible. And our teachers were not fools, and on electrical machines the guy generally gave a lot of interesting information beyond the curriculum, but he had never heard of this scheme.
      Conclusion, I don’t believe that this circuit is better (it’s possible that it’s better for two phases, but you still need to look at it and draw the “correct” circuit so that the effect of currents and their displacement is clear), although I admit that it works. There are plenty of such options, when someone has cleverly done something, but it works :) As a rule, the person himself does not understand what he has done and does not delve into the essence, but is trying hard to modernize something.
      Well, one more conclusion: if this scheme were really better, then it would at least be known, but I only learned about it from you, with all my insatiable curiosity.
      In general, I’m waiting for your opinions and results, and then you’ll see, I’ll conduct an experiment with my neighbor on a practical and theoretical basis.
  2. Alexander:
    
    Hello. You are right) I immediately connected it with a triangle in the workshop, although I didn’t mow it, and I can only evaluate the engine’s performance visually, by ear and by my own feelings) since I have nothing with which to measure the same currents on different circuits. I am far from a serious electrician, I can basically twist something together using a ready-made circuit with already known parts, ring it and check it with a 220-380 voltmeter). In the description of the circuit it was said that its advantage is in lower engine power losses and in its operating mode close to the nominal one. I will say that it was easier for me to brake the shaft on the engine using a triangle than using this diagram. Yes, and he rotated on it, I would say faster. It works for me on this engine and I liked how the engine itself works, so I didn’t bother collecting and stuffing two circuits one by one into one box and checking how it works. For now, I stuffed the capacitors into a temporary box to see how it would work (maybe I’ll have to add or remove something else), and then I thought I’d arrange the whole thing beautifully and compactly with some kind of protection. I’m wondering where I came across this diagram, people used it to connect low-power motors and no one wrote about connecting at least 1.5 or 2 kW. As far as I understand, for them you need a lot of capacitors (compared to a triangle), and there should also be ones for high voltage. I’m here and decided to ask around about this scheme, since I really hadn’t heard about it anywhere before and thought maybe the experts would tell me from the point of view of theory and science whether it should work or not.
    I can say for sure that the engine is spinning and, as for me, it’s very good, but what should happen with currents, voltages and what should lag behind or lead according to this scheme and I would like to hear from someone who knows. Maybe this scheme is just a scam? and it is no different from the same triangle (except for the extra wires and capacitors. At my house now there is no need for powerful motors, so I could try connecting them through capacitors according to this circuit and see how they would work. Previously, I had both a circular saw and a jointer , so they have engines of about 2.5 kW connected in a triangle, they stalled if you put a little more load on them, as if they had no more than a kilowatt. Now it’s just all this in the workshop, which has 380. I’ll mow it a couple or three more times, and If everything goes well, I’ll design my miracle mower correctly and post a photo, it might be useful to someone.
    
    Vladimir:
    
    Good evening, tell me how to change the direction of rotation of the shaft of a 380V synchronous electric motor connected from star to triangle.

Instructions

As a rule, to connect a three-phase electric motor, three wires and a supply voltage of 380 are used. There are only two wires in a 220 volt network, so in order for the engine to work, voltage must also be applied to the third wire. For this purpose, a capacitor is used, which is called a working capacitor.

The capacitor capacity depends on the engine power and is calculated by the formula:
C=66*P, where C is the capacitance of the capacitor, μF, P is the power of the electric motor, kW.

That is, for every 100 W of engine power it is necessary to select about 7 µF of capacitance. Thus, a 500-watt motor requires a capacitor with a capacity of 35 µF.

The required capacity can be assembled from several capacitors of smaller capacity by connecting them in parallel. Then the total capacity is calculated using the formula:
Ctotal = C1+C2+C3+…..+Cn

It is important to remember that the operating voltage of the capacitor should be 1.5 times the power supply to the electric motor. Therefore, with a supply voltage of 220 volts, the capacitor should be 400 volts. Capacitors can be used of the following types: KBG, MBGCh, BGT.

To connect the motor, two connection schemes are used - “triangle” and “star”.

If in a three-phase network the motor was connected according to a delta circuit, then we connect it to a single-phase network according to the same circuit with the addition of a capacitor.

The star connection of the motor is carried out according to the following diagram.

To operate electric motors with a power of up to 1.5 kW, the capacity of the working capacitor is sufficient. If you connect a higher power engine, then such an engine will accelerate very slowly. Therefore it is necessary to use a starting capacitor. It is connected in parallel with the run capacitor and is used only during engine acceleration. Then the capacitor is turned off. The capacitor capacity to start the engine must be 2-3 times greater than the operating capacity.

After starting the engine, determine the direction of rotation. Typically you want the motor to rotate clockwise. If the rotation occurs in the desired direction, you do not need to do anything. To change direction, it is necessary to remount the engine. Disconnect any two wires, swap them and reconnect. The direction of rotation will change to the opposite.

When performing electrical installation work, follow safety regulations and use personal protective equipment against electric shock.

Three-phase electric does not contain brushes that can wear out and require periodic replacement. It is less efficient than collector, but much more efficient than asynchronous single-phase. Its disadvantage is its significant dimensions.

Instructions

Find the nameplate on the three-phase electric motor. It shows two voltages, for example: 220/380 V. The engine can be powered by any of these voltages, it is only important to connect its windings correctly: for the lower of the indicated voltages - with a triangle, for the higher - with a star.

1.1. Selecting a three-phase motor for connection to a single-phase network.

Among the various methods of starting three-phase electric motors in a single-phase network, the simplest is based on connecting the third winding through a phase-shifting capacitor. The useful power developed by the engine in this case is 50...60% of its power in three-phase operation. Not all three-phase electric motors, however, work well when connected to a single-phase network. Among such electric motors we can highlight, for example, those with a double cage squirrel-cage rotor of the MA series. In this regard, when choosing three-phase electric motors for operation in a single-phase network, preference should be given to motors of the A, AO, AO2, APN, UAD, etc. series.

For normal operation of a capacitor-start electric motor, it is necessary that the capacitance of the capacitor used varies depending on the speed. In practice, this condition is quite difficult to fulfill, so two-stage motor control is used. When starting the engine, two capacitors are connected, and after acceleration, one capacitor is disconnected and only the working capacitor is left.

1.2. Calculation of parameters and elements of an electric motor.

If, for example, the electric motor’s data sheet indicates its supply voltage is 220/380, then the motor is connected to a single-phase network according to the diagram shown in Fig. 1

After turning on the batch switch P1, contacts P1.1 and P1.2 close, after which you must immediately press the “Acceleration” button. After gaining speed, the button is released. Reversing the electric motor is carried out by switching the phase on its winding with toggle switch SA1.

The capacity of the working capacitor Cp in the case of connecting the motor windings in a “triangle” is determined by the formula:

And in the case of connecting the motor windings in a “star”, it is determined by the formula:

The current consumed by the electric motor in the above formulas, with a known power of the electric motor, can be calculated from the following expression:

The capacity of the starting capacitor Sp is chosen 2..2.5 times greater than the capacity of the working capacitor. These capacitors must be designed for a voltage of 1.5 times the mains voltage. For a 220 V network, it is better to use capacitors such as MBGO, MBPG, MBGCh with an operating voltage of 500 V and higher. Subject to short-term switching on, electrolytic capacitors of the K50-3, EGC-M, KE-2 types with an operating voltage of at least 450 V can be used as starting capacitors. For greater reliability, electrolytic capacitors are connected in series, connecting their negative terminals together, and are shunted diodes (Fig. 2)

The total capacitance of the connected capacitors will be (C1+C2)/2.

In practice, the capacitance values of the working and starting capacitors are selected depending on the engine power according to the table. 1

Table 1. The value of the capacitances of the working and starting capacitors of a three-phase electric motor depending on its power when connected to a 220 V network.

It should be noted that in an electric motor with capacitor starting in no-load mode, a current flows through the winding fed through the capacitor by 20...30% higher than the rated one. In this regard, if the engine is often used in underloaded mode or idling, then in this case the capacitance of the capacitor C p should be reduced. It may happen that during an overload the electric motor stops, then to start it, the starting capacitor is connected again, removing the load altogether or reducing it to a minimum.

The capacity of the starting capacitor C p can be reduced when starting electric motors at idle or with a light load. To turn on, for example, an AO2 electric motor with a power of 2.2 kW at 1420 rpm, you can use a working capacitor with a capacity of 230 μF, and a starting capacitor - 150 μF. In this case, the electric motor starts confidently with a small load on the shaft.

1.3. Portable universal unit for starting three-phase electric motors with a power of about 0.5 kW from a 220 V network.

To start electric motors of various series, with a power of about 0.5 kW, from a single-phase network without reversing, you can assemble a portable universal starting unit (Fig. 3)

When you press the SB1 button, the magnetic starter KM1 is triggered (toggle switch SA1 is closed) and its contact system KM 1.1, KM 1.2 connects the electric motor M1 to the 220 V network. At the same time, the third contact group KM 1.3 closes the SB1 button. After complete acceleration of the engine, turn off the starting capacitor C1 using toggle switch SA1. The engine is stopped by pressing the SB2 button.

1.3.1. Details.

The device uses an electric motor A471A4 (AO2-21-4) with a power of 0.55 kW at 1420 rpm and a magnetic starter of the PML type, designed for alternating current voltage of 220 V. Buttons SB1 and SB2 are paired type PKE612. Toggle switch T2-1 is used as switch SA1. In the device, the constant resistor R1 is wire-wound, type PE-20, and the resistor R2 is type MLT-2. Capacitors C1 and C2 type MBGCh for a voltage of 400 V. Capacitor C2 is made up of parallel connected capacitors of 20 μF 400 V. Lamp HL1 type KM-24 and 100 mA.

The starting device is mounted in a metal case measuring 170x140x50 mm (Fig. 4)

Rice. 4 Appearance of the starting device and drawing of the panel, pos. 7.

On the top panel of the case there are “Start” and “Stop” buttons - a signal lamp and a toggle switch to turn off the starting capacitor. On the front panel of the device case there is a connector for connecting an electric motor.

To turn off the starting capacitor, you can use an additional relay K1, then there is no need for toggle switch SA1, and the capacitor will turn off automatically (Fig. 5)

When you press the SB1 button, relay K1 is triggered and contact pair K1.1 turns on the magnetic starter KM1, and K1.2 turns on the starting capacitor C. The magnetic starter KM1 is self-locking using its contact pair KM 1.1, and contacts KM 1.2 and KM 1.3 connect the electric motor to the network. The "Start" button is kept pressed until the engine fully accelerates, and then released. Relay K1 is de-energized and turns off the starting capacitor, which is discharged through resistor R2. At the same time, the magnetic starter KM 1 remains switched on and provides power to the electric motor in operating mode. To stop the electric motor, press the "Stop" button. In an improved starting device according to the diagram in Fig. 5, you can use a relay of the MKU-48 type or the like.

2. The use of electrolytic capacitors in electric motor starting circuits.

When connecting three-phase asynchronous electric motors to a single-phase network, as a rule, ordinary paper capacitors are used. Practice has shown that instead of bulky paper capacitors, you can use oxide (electrolytic) capacitors, which are smaller in size and more affordable to purchase. An equivalent replacement diagram for conventional paper is shown in Fig. 6

The positive half-wave of alternating current passes through the chain VD1, C2, and the negative half-wave VD2, C2. Based on this, it is possible to use oxide capacitors with a permissible voltage that is half that of conventional capacitors of the same capacity. For example, if in a circuit for a single-phase network with a voltage of 220 V a paper capacitor with a voltage of 400 V is used, then when replacing it, according to the above diagram, you can use an electrolytic capacitor with a voltage of 200 V. In the above diagram, the capacitances of both capacitors are the same and are selected in the same way as the method for selecting paper capacitors capacitors for the starting device.

2.1. Connecting a three-phase motor to a single-phase network using electrolytic capacitors.

The diagram for connecting a three-phase motor to a single-phase network using electrolytic capacitors is shown in Fig. 7.

In the above diagram, SA1 is the engine rotation direction switch, SB1 is the engine acceleration button, electrolytic capacitors C1 and C3 are used to start the engine, C2 and C4 are used during operation.

Selection of electrolytic capacitors in the circuit shown in Fig. 7 is best done using current clamps. Currents are measured at points A, B, C and equality of currents at these points is achieved by stepwise selection of capacitor capacitances. Measurements are carried out with the engine loaded in the mode in which it is expected to operate. Diodes VD1 and VD2 for a 220 V network are selected with a maximum permissible reverse voltage of at least 300 V. The maximum forward current of the diode depends on the engine power. For electric motors with a power of up to 1 kW, diodes D245, D245A, D246, D246A, D247 with a direct current of 10 A are suitable. With a higher motor power from 1 kW to 2 kW, you need to take more powerful diodes with the corresponding forward current, or put several less powerful diodes in parallel , installing them on radiators.

Please note that if the diode is overloaded, breakdown may occur and alternating current will flow through the electrolytic capacitor, which can lead to its heating and explosion.

3. Connection of powerful three-phase motors to a single-phase network.

The capacitor circuit for connecting three-phase motors to a single-phase network makes it possible to obtain no more than 60% of the rated power from the motor, while the power limit of the electrified device is limited to 1.2 kW. This is clearly not enough to operate an electric planer or electric saw, which should have a power of 1.5...2 kW. The problem in this case can be solved by using a higher power electric motor, for example, with a power of 3...4 kW. Motors of this type are designed for a voltage of 380 V, their windings are star-connected and the terminal box contains only 3 terminals. Connecting such a motor to a 220 V network leads to a reduction in the rated power of the motor by 3 times and by 40% when operating in a single-phase network. This reduction in power makes the engine unsuitable for operation, but can be used to spin the rotor idle or with minimal load. Practice shows that most electric motors confidently accelerate to rated speed, and in this case, starting currents do not exceed 20 A.

3.1. Refinement of a three-phase motor.

The easiest way to convert a powerful three-phase motor into operating mode is to convert it to a single-phase operating mode, while receiving 50% of the rated power. Switching the motor to single-phase mode requires slight modification. Open the terminal box and determine which side of the motor housing cover the winding terminals fit on. Unscrew the bolts securing the cover and remove it from the engine housing. Find the place where the three windings are connected to a common point and solder an additional conductor with a cross-section corresponding to the cross-section of the winding wire to the common point. The twist with a soldered conductor is insulated with electrical tape or a polyvinyl chloride tube, and the additional terminal is pulled into the terminal box. After this, the housing cover is replaced.

The electric motor switching circuit in this case will have the form shown in Fig. 8.

During engine acceleration, a star connection of the windings is used with the connection of a phase-shifting capacitor Sp. In operating mode, only one winding remains connected to the network, and the rotation of the rotor is supported by a pulsating magnetic field. After switching the windings, the capacitor Cn is discharged through the resistor Rр. The operation of the presented circuit was tested with an AIR-100S2Y3 type engine (4 kW, 2800 rpm) installed on a homemade woodworking machine and showed its effectiveness.

3.1.1. Details.

In the switching circuit of electric motor windings, a packet switch with an operating current of at least 16 A should be used as a switching device SA1, for example, a switch of type PP2-25/N3 (two-pole with neutral, for a current of 25 A). Switch SA2 can be of any type, but with a current of at least 16 A. If motor reversal is not required, then this switch SA2 can be excluded from the circuit.

A disadvantage of the proposed scheme for connecting a powerful three-phase electric motor to a single-phase network can be considered the sensitivity of the motor to overloads. If the load on the shaft reaches half the engine power, then the shaft rotation speed may decrease until it stops completely. In this case, the load is removed from the motor shaft. The switch is first moved to the “Acceleration” position, and then to the “Work” position and further work continues.

In order to improve the starting characteristics of motors, in addition to the starting and running capacitor, you can also use inductance, which improves the uniformity of phase loading. All this is written in the article Devices for starting a three-phase electric motor with low power losses

When writing the article, some of the materials from the book by V.M. Pestrikov were used. "Home electrician and more..."

Asynchronous three-phase motors are common in production and everyday life. The peculiarity is that they can be connected to both three-phase and single-phase networks. In the case of single-phase motors, this is impossible: they only operate when powered by 220V. What are the ways to connect a 380 Volt motor? Let's look at how to connect stator windings depending on the number of phases in the power supply using illustrations and a training video.

There are two basic schemes (video and diagrams in the next subsection of the article):

triangle,
star.

The advantage of a delta connection is that it operates at maximum power. But when the electric motor is turned on, high starting currents are produced in the windings, which are dangerous for equipment. When connected by a star, the motor starts smoothly, since the currents are low. But it will not be possible to achieve maximum power.

In connection with the above, motors when powered by 380 Volts are connected only by a star. Otherwise, high voltage when switched on by a delta can develop such inrush currents that the unit will fail. But under high load, the output power may not be enough. Then they resort to a trick: they start the engine with a star for safe inclusion, and then switch from this circuit to a delta for gaining high power.

Triangle and star

Before we look at these diagrams, let's agree:

The stator has 3 windings, each of which has 1 beginning and 1 end. They are brought out in the form of contacts. Therefore, for each winding there are 2 of them. We will designate: winding - O, end - K, beginning - N. In the diagram below there are 6 contacts, numbered from 1 to 6. For the first winding, the beginning is 1, the end is 4. According to the accepted notation, this is HO1 and KO4. For the second winding - NO2 and KO5, for the third - HO3 and KO6.
There are 3 phases in the 380 Volt electrical network: A, B and C. Let’s leave their symbols the same.

When connecting the windings of an electric motor with a star, first connect all the beginnings: HO1, HO2 and HO3. Then KO4, KO5 and KO6 are respectively supplied with power from A, B and C.

When connecting an asynchronous electric motor with a triangle, each beginning is connected to the end of the winding in series. The choice of the order of winding numbers is arbitrary. It may turn out: NO1-KO5-NO2-KO6-NO3-KO2.

Star and delta connections look like this: