Star and delta connection for 380. Connecting a three-phase motor to a single-phase network

Switching the motor from star to delta is used to protect electrical circuits from overloads. Mostly powerful three-phase asynchronous motors from 30-50 kW, and high-speed ones ~3000 rpm, sometimes 1500 rpm, are switched from star to delta.

It is known that when the electric motor starts, its current increases up to 7 times. A squirrel-cage induction motor resembles a transformer with a short-circuited secondary winding.

If the engine is connected in a star, then a voltage of 220 Volts is supplied to each of its windings, and if the engine is connected in a triangle, then a voltage of 380 Volts is supplied to each of its windings. Here Ohm’s law “I=U/R” comes into play; the higher the voltage, the higher the current, but the resistance does not change.

Simply put, when connected to a delta (380), the current will be higher than when connected to a star (220).

When the electric motor accelerates and reaches full speed, the picture completely changes. The fact is that the engine has power that does not depend on whether it is connected to a star or a delta. Engine power depends largely on the iron and wire cross-section. Another law of electrical engineering “W=I*U” applies here.

Power is equal to current multiplied by voltage, that is, the higher the voltage, the lower the current. When connected to a delta (380), the current will be lower than to a star (220).

Let's get to practice

In the engine, the ends of the windings are brought out to the “terminal block” in such a way that, depending on how you place the jumpers, you will get a star or delta connection as shown in the figure. Such a diagram is usually drawn on the lid.

In order to switch from star to delta, we will use magnetic starter contacts instead of jumpers.

Let's look at the power section diagram, shown in thick lines.

Comments and reviews

Star-Triangle: 133 comments

1. Grumm

   Triangle error!
   But that's okay...
   How is phasing carried out (adjusted)?

2. Electrician

   The contents of the article are not true.
   When switching the motor from star to delta, the supply voltage will change accordingly from 380/220 to 220/127.
   The delta-connected engine is switched on at 220/127 V.
   If you turn it on 380/220 it will burn out.

   1. Roman

      We are talking about a 380/660 Y/A engine. You are not right.

      1. Roman

         Confused - correct: 380/660 A/Y

3. admin Post author

   According to the diagram, the motor will rotate in one direction; if you swap the phases on the starter P1, the rotation of the motor will change. The most important thing in this circuit is not to confuse the connection of the magnetic starter P2; its contacts act as JUMPERS for connection in a TRIANGLE.

4. admin Post author

   The contents of the article are completely true. When the motor is connected in a triangle, a voltage of 380 Volts is supplied to each winding, and if the motor is connected to a star, a voltage of 220 Volts is supplied to each winding. According to the scheme, we temporarily supply a reduced voltage of 220V for 10-15 seconds in order to reduce the starting current and reduce the engine jerk at the moment of starting. After this, the engine switches on to normal operation.

5. admin Post author

   Yes, it is worth pointing out that the motor voltage must correspond to the mains voltage; at this voltage it must operate connected in a triangle.

   By the way, I spotted this circuit on Japanese equipment.

6. admin Post author

   Electrician, where did you get the 220/127 from? If the network voltage is 380/220, then this means that when the engine is turned on in a triangle, each of its windings operates at 380 volts, and when the engine is turned on in a star, then 220 volts are supplied to the windings.

7. Eugene

   Guys, a similar scheme is already used in practice. Called “warm start” in pump rooms
   stations, etc. in high-rise construction.

   1. Eugene

      Excuse me, what launch? warm? Why not hot? This starting method is called “combined” in pumping stations. There is a “direct” start (star or triangle).
      However, it is more common today in high-rise construction (when using utility water supply stations - and this is important) to start frequency or frequency-network.
      Now about the subject. This star-delta start provides smoother acceleration on powerful engines to minimize network drawdown.
      However, as everyone knows, with a star we have a “shortage” in power.
      Not fatal during transition. The triangle has maximum power. By the way, this method is used when using powerful pumps at fire extinguishing stations.
      The only thing in the diagram that does not correspond to reality (practice) is the connection in the motor terminal box itself
      spruce.
      An example is Grundfoss pumps. The connection is very simple - U1-W2. V1-U2. W1-V2

      1. Alexander

         Not a lack of power, but of torque. The motor torque depends on the square of the voltage and when connected to a triangle, the torque is almost 3 times higher. A star circuit when starting an engine is used to reduce starting currents.

8. Dmitriy

   The diagram is absolutely correct, and everything is described correctly.

9. Megavolt

   Has anyone noticed that the RT and P3 relays are connected bypassing the “Start” button?
   They will work as soon as you connect the circuit to the network.

10. admin Post author

    Megavolt, you are right, thanks for the comment. They need to be connected on the other side of the start button or through an additional normally open contact P1

11. admin Post author

    The diagram has been corrected. If you click on the diagram you can see the old diagram.

    In the diagram at the top on the left, dotted lines show the possibility of connecting starter coils and time relays for 220 and 380 Volts. This common wire is connected to phase 380 volts, or to zero 220V. It is not advisable to connect along the dotted line to both phase and zero at the same time; it may result in a “short one”.

12. Michael

    Thanks for the diagram. Please, if possible, provide a diagram when the starter coils are designed for different voltages. For example, P2 is 220V and P3 is 380V. The STOP button in this case for some reason does not work. Thank you.

13. admin Post author

    If the starter coils are for different voltages, then instead of connecting to a common wire, the 220V coils are connected to zero, and the 380V coils to the phase. The rest of the scheme is unchanged.

14. Michael

    The Stop button does not work in this version. Installed a two-contact Stop button. I am breaking the two phases.

15. admin Post author

    And this button opens exactly two phases. We have two-contact buttons: one contact opens the circuit, the other closes it, turning on the signal lights.
    How it doesn't work, doesn't turn on or off.

16. Impedance

    Thank you admina for the brief, correct explanation of the operating principle of this scheme!!!

17. Bach

    There are starters together with a temporary relay. They can be easily removed and connected

18. Eugene
19. admin Post author

    Evgeniy, Ohm’s law is valid for active loads.
    Ohm's law is preserved, only on a rotating motor, in addition to the active resistance of the windings, inductive resistance appears. And with an inductive load, as the voltage increases, the inductive reactance increases, and accordingly the current decreases

    Yes, for reliable operation of the circuit, you should use a 660/380 motor if the network voltage is 380/220

20. Pamir

    Why was no one confused by the statement that “When connected to a delta (380), the current will be lower than to a star (220),” which directly contradicts what was written a few paragraphs above.
    Why are you afraid, one might ask, the power in the star and in the triangle are equal, then what is the point of switching to the triangle if the engine in the star also operates at rated power?
    admin, inductive (reactive) resistance depends only on frequency and not on voltage. And Ohm's law also works in this case; the higher the voltage, the greater the current.

21. admin Post author

    The circuit reduces the starting current, the engine turns on for a short time, during star starting. The jerk that the engine makes when starting is also reduced, this is especially true if the engine is under load.
    And in a triangle, less current is more power when the engine is running.

    The engine power does not depend on whether the engine is connected to a star or a delta. Engine power depends largely on the load

22. Pamir

    The power that the engine can develop is written on the nameplate, and it is determined by the engine parameters and the connection method, and only the currently consumed power depends on the load and it cannot exceed the declared one.
    When connected to a star, a lower voltage is applied to the motor windings (not linear 380 but phase 220), and accordingly lower starting and operating current (Ohm's law). From here it is clear that in a star, the power that the engine is capable of developing will be less than the rated one.
    Admin, you are confusing sources (generators, transformers) with loads. For a generator or transformer, the power will be the same for any type of connection, and the phase current in a delta is less than in a star. For the load, engine type, everything will be as I described above.

    1. Eugene

       “if you look through a telescope”... or better yet, at the engine nameplate, you can see... what? right... answers to questions... and they are written in the form In=...
       Example - P=1.5 kW. then I(380)=1500/380*1.732=2.3 (Simplified, without coefficients)
       For I(220)=1500/220=6.8.
       Ohm's Law is great. U=IxR. To put it simply, Voltage is directly proportional to Current.
       Accordingly, power is directly proportional to... voltage... and current.... Output - less voltage (or current, which is proportional) on the winding - less power. And here the point arises... DO NOT OVERLOAD THE NETWORK. BUT we are losing power.
       Well, and, as a result, the customer’s question: “why is the passport data 3 cubic meters per hour, but this shit pumps only 1 cubic meter?”

23. Kostantin

    switching from star to triangle ensures a smooth start. When you press the start button, the windings are switched to star (for our voltage 380\220) and in star it works at 660, after a certain time the windings switch to triangle and already operate at a rated voltage of 380 volts .

24. EVgen

    Engine AIR132 M2 11 Kw/3000 rpm. Is it possible to connect such a star-delta motor?

25. admin Post author

    EVgen, yes if it is 660/380

26. Dmitriy

    Good afternoon
    I’m a beginner, help me figure this out: “If the motor is connected in a star, then a voltage of 220 Volts is supplied to each of its windings, and if the motor is connected in a triangle, then a voltage of 380 Volts is supplied to each of its windings.”
    As I heard, when connecting the windings “star” - 380 V, and “triangle” - 220 V.
    Maybe I misunderstood something, or there was a typo in the article?

27. admin Post author

    Dmitry, Everything is written correctly in the article about the voltage on the motor windings. You have heard about phase-to-phase voltage in a network.
    If there is 380V between the phases in the network and the motor is connected “in a star”, then a voltage of 220V will be supplied to each motor winding.

    We take a 660/380 motor, in such an engine each winding is designed for 380 Volts, that is, it must be connected in a triangle.
    And at the moment of starting we connect it to the star and supply a reduced voltage of 220V to the windings. Accordingly, the starting current will be less.
    And when the engine accelerates, switch it to triangle.

28. vital
29. admin Post author
30. Yuri

    Interesting read.
    Switching from star to delta is used a) to reduce inrush currents; b) to increase the power factor of the electric motor and its load level. In the first case, for a 380/220 V network, it is necessary to take an electric motor whose voltage is written on the passport as 660/380 V. In the second case, the torque on the motor shaft, in addition to what has been said, should not exceed 30%. As for the diagram, it must be brought in accordance with GOST for designations, but a mixture of current and long-unused designations is given.

31. vik

    Hello everyone! I’ll say right away that for me the concepts of phase and linear current are elusive. In general, I will be grateful to anyone who can explain whether this circuit is suitable for (and what options I have) connecting the AIR90L2U3 electric motor (3 kW, approx. 3000 rpm, 380v). The network is three-phase - four wires enter the house. On the shield, the neutral is connected to the ground loop.
    Thank you in advance.

32. vik

    To forestall questions regarding 220/380 and 380/660, I’ll say right away that the nameplate simply says 380v. (without fractions)

33. admin Post author

    vik, the engine is low-power and can be connected without this circuit.
    Just through one starter and start-stop buttons.

34. vik

    thanks, there are three wires under the cover, does that mean it's just a star? I still need reverse.

35. admin Post author

    vik, If there are three wires under the cover, it means a star.
    To reverse, you need to swap two phases. They install two starters with simultaneous activation blocking (required electrical and additionally mechanical).

    An article with diagrams about connecting motors is currently being prepared and will appear on the website soon.

36. vik

    admin, please tell me whether the TRN-10U3 thermal relay is suitable for my engine (and how necessary it is)?
    Thank you.

37. admin Post author

    vik, What brand of thermal relay is not important, the main thing is what current.
    If a separate machine is installed on the engine, then there is no special need for a thermal relay, since the machine already has thermal protection.
    But protection is never superfluous, so it is better to install a thermal relay.

38. vik

    How do you know what current it is at? There is a stamp (TRN-10U3) stamped on one side of the contact, and the number 10 on the other.
    Or is the current regulated by a smooth regulator?
    Thank you.

    1. admin Post author

       It's probably 10 amps. The regulator can be used to smoothly adjust the current. Try installing it, it will work often, so it won’t work.

39. vik

    I have a reversible MP with three normally open contacts and one normally closed. I don't understand how to connect it. If normally closed contacts are used for blocking (to duplicate the mechanical one), then how to fix three power ones? It turns out that if you release the “start” button, the engine will stop rotating, right?

40. admin Post author

    vik, there are not enough contacts, there should be four normally open and one normally closed contacts.

    The coil of the second starter is connected through a normally closed contact for blocking.

    One normally open contact is used to block the “Start” button, and three power contacts.

    Additional contacts must be installed on the starters.

41. vik

    admin, thanks for your help. You won't be able to add contacts. I see the solution as follows: convert the main section of the starter into four normally open ones, reverse by holding the button (this completely covers my needs). The only lock remains mechanical. How critical is this?
    Thanks again.

42. vik

    Yes, there are still a couple of normally closed contacts on the second starter. Will it be useful if it opens the main section while holding the reverse button?

43. vik

    And another question: on the one hand, somewhere it was said that from a safety point of view it is better to isolate the engine from a metal structure, and in the circuit the neutral is grounded to the metal case in which it is assembled. Which is more appropriate?
    Thank you.

44. admin Post author

    vik, the mechanical lock is not very reliable, over time it can break and have to be removed. Well, if there is no other way out, you can do it this way.

    It has never been possible to isolate an engine from a metal structure. This structure and the engine itself must be grounded.
    The neutral is grounded to the metal body just for safety. In the event of a breakdown of the insulation on the housing, a short circuit will occur and the machine will turn off the engine.

45. vik

    admin, thank you very much for your help.
    The device I'm trying to build is a garden shredder. 99% of the time the engine will run in one direction. The reverse will only turn on if the crushed mass is wound around the cutting unit, so holding the button will be even preferable.
    I don’t think that this device (if it works out) will be used by anyone other than me. Well, I will try to refrain from pressing two buttons at the same time, so there is hope that the load on the mechanical lock will not be very shocking.
    Thanks again.

46. Andrey

    HELLO, I WANT TO KNOW WHETHER THIS DIAGRAM WILL SUIT IN MY CASE: 130 KW INDUCTION MOTOR, 5 VELECTRON STARTERS, “CLAPPERS”, I THINK THEY WILL WITHstand.

47. admin Post author

    Andrey, yes, if the voltage matches.

48. Completely confused...

    It's different on all sites. There is an engine (vacuum, water cooling), on the nameplate 380 volts, 5.5 kW. The terminal block on it is connected into a triangle.
    http://s018.radikal.ru/i516/1203/44/1f6335630318.jpg

    If I connect 380 will it be correct, or will it be correct to switch the terminals to star?

    Thank you in advance!

49. admin Post author

    Usually they write 380/220 or 660/380. If only 380 is written, then it is correct to connect it to a star.

    It’s safer to try connecting it to a star, see how it works, whether it produces the required power, and measure the current.
    If something goes wrong, you can switch to a triangle.

50. vik

    2admin:
    Good afternoon, I want to connect this device to protect against phase loss:
    http://www.kriwan.com/en/Protection_and_Controls-Products–25,productID__182.htm
    What is not clear is that the contacts that break the circuit (M2, M1) do not ring. This is fine? Perhaps they will close when voltage is applied?
    Thank you.

51. admin Post author

    vik, the contacts are probably open; if you apply voltage, they should close.
    It should turn off when at least one phase fails, but here all three phases are missing.

52. vik

    Logical, thanks.

53. Glory

    Here's a question. An asynchronous motor connected by a star (three terminals) must be connected to a single-phase network, there is a starting circuit with a resistance or capacitance, and the capacitance is starting and working, or only starting or only working. If the tank is only working, will the engine start with the button or not? If you use nichrome at start-up, the engine starts and the resistance is discarded. The question is, is it possible to use nichrome in one circuit for acceleration, and a capacitance (working) to increase engine power in operation? If yes, what is the scheme? I hope I didn't confuse you too much. Thank you very much in advance!

54. admin Post author
55. Glory

    admin
    Thanks, I'll try, but I don't want to disassemble the engine to add a fourth wire.

56. vik

    2admin:
    good afternoon, I bought a used three-phase electric motor on the market as 1.5 kW (the nameplate is illegible), I went online, and it looks like it is 0.75 kW. I was going to use it in a device that had a 1.1 kW single-phase power supply. How critical is the difference and what can you come up with? Can I connect it in a triangle?
    Thank you very much in advance.

57. vik

    2admin:
    Still waiting for your answer...

58. admin Post author

    vik, well, if you already bought it, then put the difference is not very critical. It will simply produce less power.
    For example, if you put it on a pump, a 0.75 kW motor will pump a smaller volume of water per unit of time than a 1.5 kW motor. And it will get hotter.
    You should not connect it to a triangle; it may burn out.

59. vik
60. vik

    2admin:
    Christ is Risen!
    I apologize in advance for bothering you on such a day - when connecting to a star, is it necessary to connect the common point to the engine housing or only the neutral?

61. admin Post author

    vik, when connecting to a star, you don’t have to connect the common point to anything at all. And connect the zero to the motor housing, and in another place the motor is still connected to ground. This is how we usually do it.
    If you wish, you can connect the middle point to the body.

62. vik

    Thank you.

63. Dimon

    Good afternoon! I’m finishing university right now, I have a special question in my diploma, the regulation of asynchronous motors by changing the winding connection diagrams from star to delta, it is necessary to calculate losses at different loads and connection diagrams. engine 4a315s6 110 kW, 380/660. can anyone help???

64. admin Post author

    Dimon, the engine engages in a star only when starting for just a few seconds. Then he switches to a triangle.

    It even became interesting that if the engine is switched to a star at low loads, and to a triangle when the load increases.
    Can this reduce losses?
    I think not, otherwise such schemes would be used everywhere.

65. PASS

    Can you please tell me if a three-phase 220V motor is connected to 380V, will it not burn out? and how to do it right
    admin writes:
    31 Jan 2012 at 20:08

    Vitalya, Such an engine needs to be connected only in a star, but if connected in a triangle it will burn out.

    I'm pissed off!!! It will burn anyway! Admin, where did you study?!
    Three-phase voltage 380V (linear!) and three-phase voltage 220V (linear!) are different values!!!
    Three-phase 220V motors are easier to connect via a converter. The simplest is a three-phase motor connected to a single-phase 220V network.

    1. Eugene

       Excuse me, where did you see 220V three-phase?) In the house? Sorry, the interphase is 380 with a line of 220...
       No, well, if 127 V is considered linear, then yeah.
       So, the Admin is not so wrong as he did not ask for the full parameters. What did Vitalya mean? 220/380? Or 127/220?

       1. admin Post author

          Eugene,
          Line voltage is the voltage between phases. A Phase voltage is the voltage between phase and zero.
          Although I agree it is necessary to clarify what kind of engine it is.

          And it often happens that the motor has only three terminals in a star or triangle; it is soldered inside. and is designed for only one voltage, for example, 380V or 220V

          The 220/380 motor for a 220/380 voltage network is connected to a star. And for a 220/127 network in a triangle.

          I haven’t come across 127/220 engines, and why would such an engine be everywhere in the 220/380 network.

66. admin Post author

    PASS, and three-phase voltage 380V (linear!) and three-phase voltage 220V (phase!) are almost the same values.
    If the engine is 220/127. The easiest way is to rewind it.

67. PASS

    It clearly says “3-phase 220V motor.” I have three of these and they work perfectly from the converter motor. And there’s no need for the extra hassle of rewinding!
    And I MYSELF know the difference between phase and line voltages.

68. DIMA

    SHEMA RABOTAET MALAKA

69. Plague

    “Switching a motor from star to delta is used to protect electrical circuits from overloads.” Hmmm...Actually, all the fuss is due to the increased starting torque, which can hardly be called smooth, “warm” and fluffy. That is, we deliberately overload the engine with current for a short time along the triangle and after gaining speed we switch to long-term star mode.

70. admin Post author

    The plague, if you need a smooth start, switch from star to delta, but you need starting torque, then vice versa.
    In practice, I have not encountered switching circuits from triangle to star; the circuit from star to triangle is more often used.

71. Don Migeli

    Why engine

72. Don Migeli

    380/220 660/380 - does this mean if it is a triangle then the first value of the fraction, and if it is a star then the second?

    Why can only 660/380 be connected to a star-delta circuit?

73. admin Post author

    Don Migeli,
    The less voltage in the fraction is phase and the greater is linear.

    Because the electric motor only switches on to low voltage for a few seconds at the moment of starting, and after starting it switches to normal operation.

    For a 220/380 engine, the usual connection diagram is star; if you connect it in a triangle, it will burn out.
    And for a 380/660 engine, the usual circuit is a triangle.
    This is at a network voltage of 220/380

74. Don Migeli

    Thanks for the answer, can you help me choose a cable? What is the basis for starting from the current on the nameplate or is a calculation needed?

    1. admin Post author

       Don Migeli, from nameplate current or power

75. Don Migeli

    if 22 kW, 46.2 A - how is it that each phase has 46A or 46 should be divided into 3 phases, can you be more specific?

    1. admin Post author

       Don Migeli, 46A on each phase.

76. Don Migeli
77. Andron

    Good afternoon. Tell me how can I find out which winding connection of the motor is “star” or “delta”?? There are three wires coming out of it, but how is it connected? I want to start it, but I don’t know which capacitor to install??

78. Nick

    on the nameplate 220/380 the triangle is only 220. The star 380 can be 220 with a reduction in torque. It all depends on what you want to get, high torque or limit the starting current. Do not burn the engines.

79. Sergey

    Good afternoon, I have the same problem: the engine nameplate says 380/660, but when switching from star to triangle, the automatic switches out instantly. The motor worked fine after rewinding and before rewinding, is it possible that it was rewinded incorrectly and how can I check this?

    1. admin Post author

       Maybe it was rewound 220/380, but this is more difficult, it’s easier to count the number of turns on a burnt-out motor and rewind the same number.
       It is necessary to measure the current in the star and compare it with the current on the nameplate to see if it is much different.

80. Sergey

    I tried to start it without load, the circuit works fine, the currents are below the nominal value. I changed the size of the pulley to reduce the load, now it does not knock out and the currents are normal. Thank you very much for your help.

81. Sergey

    Compressor with 7.5 kW motor.
    The line drops a lot and the engine does not fully accelerate.
    I propose to change the diameter of the engine pulley, increase the cross-section of the cable from the meter to the compressor, and connect it to a star.
    Will these measures be enough, and what else can be done?

    1. admin Post author

       Sergey, First of all, increase the cable cross-section.

82. Sergey

    That's where I thought I'd start.
    But there is also interest in why they installed three thousand units for the compressor.
    Usually we used to see compressors with motors of 900 or one and a half thousand, but this???

    1. admin Post author

       Maybe his blood pressure is higher?

83. Arthur

    The old 75 kV motor was started from a star to a delta, but for some reason the new one indicated the connection with a triangle D-D. Is it possible to start it like the old motor?

    1. admin Post author

       Yes you can

84. Alexander

    Help me figure it out, we bought an engine on the cheap with the overall dimensions of an AIR 180M but there are 6 ends inside, there is no sign. How to figure out its connection diagram, triangle or star, and how many revolutions it will give us and what power?

A three-phase electric motor is an electrical machine designed to operate in alternating current. Such a motor consists of a stator and a rotor. The stator has three windings shifted by one hundred and twenty degrees. When three-phase voltage appears in the winding circuit, magnetic fluxes are formed at the poles and the rotor rotates. Electric motors are either synchronous or asynchronous. Three-phase ones are widely used in industry and in everyday life. Such motors can be single-speed, in which case the motor windings are connected in a star or delta pattern, and multi-speed. The latter units are switchable, in which case there is a transition from one connection scheme to another.

Three-phase electric motors are divided according to winding connection diagrams. There are two connection schemes - star and delta connections. Connecting the motor windings according to the “star” type is a connection of the ends of the motor windings at one point (zero node): an additional terminal is obtained - zero. The free ends are connected to the phases of the 380 V electric current network. Externally, this connection resembles a three-pointed star. The photo shows the following diagram: a “star” and “delta” connection. Connecting the windings of an electric motor according to the “delta” type is a winding: the end of the first is connected to the beginning of the second winding, the end of the second to the beginning of the third, and the end of the third to the beginning of the first. Three-phase voltage is supplied to the winding connection nodes. With this connection of the windings, there is no zero terminal. Outwardly, it resembles a triangle.

Star and delta connections are equally common and do not differ significantly. To connect the windings in a star type (when the engine is operating in rated mode), the line voltage must be greater than when connected in a delta type. Therefore, the characteristics of a three-phase motor are indicated as follows: 220/380 V or 127/220 V. If necessary, the rated windings must be connected as a star, and the rated voltage of the motor will be 380/660 V (delta type).

It should be noted that a combined star and delta connection is often used. This is done in order to start the electric motor more smoothly. When starting, a star connection is used, and then a special relay is used to switch to delta, thus reducing the starting current. Such circuits are recommended for starting high-power electric motors that require large starting current. It is important to remember that in this case the starting current exceeds the rated current by seven times.

There are other combinations when connecting electric motors, for example, a star and delta connection can be replaced by a double, triple star, as well as other connection options. Such methods are used for multi-speed (two-, four-, etc.) electric motors.

The windings of generators, transformers, electric motors and other electrical receivers, when connected to a three-phase network, are connected in two ways: star or delta. These connection diagrams are very different from each other and carry different current loads. Therefore, there is a need to understand the question of how star and delta connections are made - what is the difference?

What are schemes?

Connecting the windings with a star is their connection at one point, which is called the zero point or neutral. It is designated by the letter "O".

A delta connection diagram is a series connection of the ends of the working windings, in which the beginning of one winding is connected to the end of another.

The difference is obvious. But what is the purpose of these types of connections, why are star-delta connections used in different electrical installations, what is the effectiveness of both. Many questions arise on this topic, and we need to sort them out.

Let's start with the fact that when starting the same electric motor, the current, which is called starting, has a high value, which exceeds its rated value by six or eight times. If it is a low-power unit, then the protection can withstand such a current, but if it is a high-power electric motor, then no protective blocks will withstand it. And this will certainly cause a voltage “sag” and failure of fuses or circuit breakers. The engine itself will begin to rotate at a low speed, different from the nameplate speed. That is, there are many problems with starting current.

Therefore, it just needs to be reduced. There are several ways to do this:

• install one of the following devices in the electric motor connection system: transformer, inductor, rheostat;
• the connection diagram of the rotor windings changes.

It is the second option that is used in production as the simplest and most effective. The star-delta circuit is simply converted. That is, when starting the engine, its windings are connected in a star pattern, then as soon as the motor picks up speed, it switches to a triangle. The process of switching star to delta is done automatically.

It is recommended that in electric motors where two connection options are used simultaneously - star-delta - to connect the windings according to the star circuit, that is, to their common connection point, connect the neutral from the power supply. Why is this necessary? The thing is that when working with this connection option, there is a high probability of asymmetry in the amplitudes of different phases. It is the neutral that will compensate for this asymmetry, which usually appears due to the fact that the stator windings may have different inductive reactance.

Advantages of the two schemes

The star scheme has quite serious advantages:

• smooth start of the electric motor;
• its rated power will correspond to the passport data;
• the engine will operate normally both under short-term high loads and long-term low overloads;
• During operation, the motor housing will not overheat.

As for the triangle circuit, its main advantage is that the electric motor achieves maximum power during its operation. But at the same time, it is recommended to strictly adhere to the operating modes that are described in the motor passport. Testing of electric motors connected in a delta configuration has shown that its power is three times greater than those connected in a star configuration.

If we talk about generators that supply current to the power supply network, then the star and delta connection schemes are exactly the same in their technical parameters. That is, the voltage output by the triangle will be greater, although not three times, but not less than 1.73 times. In fact, it turns out that the generator voltage at star, equal to 220 volts, is converted into 380 volts if you switch from one option to another. But it should be noted that the power of the unit itself remains unchanged, because everything obeys Ohm’s law, in which voltage and current are in inverse proportion. That is, an increase in voltage by 1.73 times reduces the current by exactly the same amount.

Hence the conclusion: if all six ends of the windings are located in the terminal box of the generator, then it will be possible to obtain voltages of two ratings that differ from each other by a factor of 1.73.

Drawing conclusions

Why are delta and star connections found in all modern high-power electric motors today? From all of the above, it becomes clear that the main requirement of the situation is to reduce the current load that occurs during the startup of the unit itself.

If you write down the formulas for such a connection, they will look like this:

Uph=Ul/1.73=380/1.73=220, where Uph is the voltage on the phases, Ul is on the supply line. This is a star connection.

After the electric unit accelerates, that is, its rotation speed corresponds to the passport data, a transition to a triangle from a star will occur. From here the phase voltage will become equal to the linear voltage.

Each stator of a three-phase electric motor has three coil groups (windings) - one for each phase, and each coil group has 2 terminals - the beginning and end of the winding, i.e. There are only 6 pins that are signed as follows:

• C1 (U1) is the beginning of the first winding, C4 (U2) is the end of the first winding.
• C2 (V1) is the beginning of the second winding, C5 (V2) is the end of the second winding.
• C3 (W1) is the beginning of the third winding, C6 (W2) is the end of the third winding.

Conventionally, in the diagrams, each winding is depicted as follows:

The beginnings and ends of the windings are brought out into the terminal box of the electric motor in the following order:

The main winding connection diagrams are triangle (denoted by Δ) and star (denoted by Y), which we will analyze in this article.

Note: In the terminal box of some electric motors you can only see three outputs- this means that the motor windings are already connected inside its stator. As a rule, the windings inside the stator are connected when repairing an electric motor (if the factory windings are burned out). In such motors, the windings are usually connected in a star configuration and are designed for connection to a 380 Volt network. To connect such a motor, you simply need to supply three phases to its three outputs.

1. Connection diagram of electric motor windings according to the “triangle” diagram

To connect the windings of an electric motor according to the “triangle” diagram, it is necessary: ​​connect the end of the first winding (C4/U2) to the beginning of the second (C2/V1), the end of the second (C5/V2) to the beginning of the third (C3/W1), and the end of the third windings (C6/W2) - with the beginning of the first (C1/U1).

Voltage is applied to terminals “A”, “B” and “C”.

In the terminal box of the electric motor, the connection of the windings according to the “delta” diagram has the following form:

A, B, C—connection points for the power cable.

1. Connection diagram of electric motor windings according to the “star” scheme

To connect the windings of an electric motor in a star configuration, it is necessary to connect the ends of the windings (C4/U2, C5/V2 and C6/W2) to a common point, while voltage is applied to the beginning of the windings (C1/U1, C2/V1 and C3/W1 ).

Conventionally, this is depicted in the diagram as follows:

In the terminal box of the electric motor, the star connection of the windings has the following form:

1. Definition of winding terminals

Sometimes situations arise when, after removing the cover from the terminal box of an electric motor, you are horrified to discover the following picture:

In this case, the winding terminals are not labeled, what should I do? Don't panic, this issue can be completely resolved.

The first thing to do is to divide the leads into pairs, each pair should have leads related to one winding, this is very easy to do, we will need a tester or a two-pole voltage indicator.

If using a tester, set its switch to the resistance measurement position (underlined by a red line); when using a bipolar voltage indicator, before use, it is necessary to touch the live parts under voltage for 5-10 seconds to charge it and check its functionality.

Next, you need to take any one terminal of the winding, conditionally take it as the beginning of the first winding and accordingly sign it “U1”, then touch the “U1” terminal we signed with one tester or voltage indicator probe, and touch with the second probe any other terminal from the remaining five unsigned ends. If, having touched the second terminal with the second probe, the tester readings have not changed (the tester shows one) or in the case of the voltage indicator - not a single light comes on - we leave this end and touch the other terminal of the remaining four ends with the second probe, and touch the ends with the second probe to until the tester readings change, or, in the case of a voltage indicator, until the “Test” light comes on. Having found the second terminal of our winding in this way, we accept it conditionally as the end of the first winding and sign it “U2” accordingly.

We proceed in the same way with the remaining four pins, also dividing them into pairs and signing them respectively as V1, V2 and W1, W2. You can see how this is done in the video below.

Now that all the pins are divided into pairs, it is necessary to determine the actual beginnings and ends of the windings. This can be done in two ways:

The first and simplest method is the selection method, which can be used for electric motors with a power of up to 5 kW. To do this, we take our conditional ends of the windings (U2, V2 and W2) and connect them, and briefly, preferably no more than 30 seconds, apply three-phase voltage to the conditional beginnings (U1, V1 and W1):

If the engine starts and runs normally, then the beginnings and ends of the windings are determined correctly; if the engine hums a lot and does not develop the proper speed, then there is an error somewhere. In this case, you just need to swap any two terminals of one winding, for example U1 with U2, and start again:

If the problem persists, return U1 and U2 to their places and swap the following two pins - V1 with V2:

If the engine works normally, the pins are identified correctly, the work is finished, if not, return V1 and V2 to their places and swap the remaining pins W1 with W2.

Second method: We connect the second and third windings in series, i.e. we connect together the end of the second winding with the beginning of the third (terminals V2 with W1), and apply to the first winding to terminals U1 and U2 reduced variable voltage(no more than 42 Volts). In this case, voltage should also appear at terminals V1 and W2:

If voltage does not appear, then the second and third windings are connected incorrectly, in fact, two beginnings (V1 with W1) or two ends (V2 with W2) are connected together, in this case we just need to change the inscriptions on the second or third winding, for example V1 with V2. Then check the first winding in a similar way, connecting it in series with the second, and applying voltage to the third. This method is presented in the following video:

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The design of a three-phase electric motor is an electrical machine that requires three-phase alternating current networks for normal operation. The main parts of such a device are the stator and the rotor. The stator is equipped with three windings shifted between each other by 120 degrees. When three-phase voltage appears in the windings, magnetic fluxes are formed at their poles. Due to these flows, the engine rotor begins to rotate.

In industrial production and in everyday life, three-phase asynchronous motors are widely used. They can be single-speed, when the motor windings are connected by star and delta, or multi-speed, with the ability to switch from one circuit to another.

Star and delta connection of windings

All three-phase electric motors have windings connected in a star or delta configuration.

When connecting the windings in a star circuit, their ends are connected at one point in the zero node. Therefore, we get one more additional zero output. The other ends of the windings are connected to the phases of the 380 V network.

The delta connection consists of a series connection of windings. The end of the first winding is connected to the starting end of the second winding, and so on. Ultimately, the end of the third winding will connect to the beginning of the first winding. Three-phase voltage is supplied to each connection node. The triangle connection is distinguished by the absence of a neutral wire.

Both types of compounds have received approximately the same distribution and do not have significant distinctive features.

There is also a combined connection when both options are used. This method is used quite often; its goal is a smooth start of the electric motor, which cannot always be achieved with conventional connections. At the moment of direct start-up, the windings are in the star position. Next, a relay is used that provides switching to the triangle position. Due to this, the starting current decreases. The combined circuit is most often used when starting high-power electric motors. Such motors also require a significantly higher starting current, approximately seven times the rated value.

Electric motors can be connected in other ways when a double or triple star is used. These connections are used for motors with two or more variable speeds.

Starting a three-phase electric motor with star-delta switching

This method is used to reduce the starting current, which can be approximately 5-7 times the rated current of the electric motor. Units with too much power have such a starting current that the fuses easily blow, the circuit breakers turn off and, in general, the voltage drops significantly. With such a decrease in voltage, the incandescence of the lamps decreases, the torque of other electric motors decreases, and the contactors spontaneously turn off. Therefore, different methods are used to reduce the inrush current.

Common to all methods is the need to reduce the voltage in the stator windings during direct start-up. To reduce the starting current, the stator circuit can be supplemented with a choke, rheostat or automatic transformer during starting.

The most widespread is switching the winding from a star to a triangle position. In the star position, the voltage becomes 1.73 times less than the rated value, therefore the current will be less than at full voltage. During start-up, the motor speed increases, the current decreases and the windings switch to the delta position.

Such switching is allowed in electric motors that have a lightweight starting mode, since the starting torque is reduced by approximately two times. This method is used to switch those engines that can be structurally connected into a triangle. They must have windings capable of operating at .

When to switch from triangle to star

When it is necessary to make a star and delta connection of the electric motor windings, you should remember that it is possible to switch from one type to another. The main option is the star-delta switching circuit. However, if necessary, the reverse option is also possible.

Everyone knows that electric motors that are not fully loaded have a decrease in power factor. Therefore, it is advisable to replace such engines with devices with lower power. However, if replacement is impossible and there is a large power reserve, a delta-star switch is performed. The current in the stator circuit should not exceed the nominal value, otherwise the electric motor will overheat.