Delta connection diagram from 380 to 220. Connecting a three-phase motor to a single-phase network without loss of power. Schemes for connecting three-phase motors to a single-phase network

In various amateur electromechanical machines and devices, in most cases three-phase asynchronous motors with a squirrel cage rotor are used. Alas, a three-phase network in everyday life is a very rare phenomenon, therefore, to power them from an ordinary electrical network, amateurs use a phase-shifting capacitor, which does not allow the full power and starting properties of the motor to be realized.

Asynchronous three-phase electric motors, namely them, due to their widespread use, often have to be used, consist of a stationary stator and a moving rotor. Winding conductors are laid in the stator slots with an angular distance of 120 electrical degrees, the beginnings and ends of which (C1, C2, C3, C4, C5 and C6) are brought out into the junction box.

Delta connection (for 220 volts)

Star connection (for 380 volts)

Three-phase motor junction box with jumper positions for star connection

When a three-phase motor is turned on to a three-phase network, a current begins to flow through its windings at different times in turn, creating a rotating magnetic field that interacts with the rotor, forcing it to spin. When the motor is connected to a single-phase network, no torque capable of moving the rotor is created.

If you can connect the engine on the side to a three-phase network, then determining the power is not difficult. We place an ammeter at the break in one of the phases. Let's launch. We multiply the ammeter readings by the phase voltage.

In a good network it is 380. We get the power P=I*U. We subtract 10-12% for efficiency. You get the actually correct result.

There are mechanical instruments for measuring revolutions. Although it is also possible to determine by ear.

Among the various methods of connecting three-phase electric motors to a single-phase network, the most common is connecting the third contact through a phase-shifting capacitor.

Connecting a three-phase motor to a single-phase network

The rotational speed of a three-phase motor operating from a single-phase network remains almost the same as when it is connected to a three-phase network. Alas, this cannot be stated about power, the losses of which reach significant values. Clear values of power loss depend on the switching circuit, operating conditions of the motor, and the capacitance value of the phase-shifting capacitor. Approximately, a three-phase motor in a single-phase network loses within 30-50% of its own power.

Not many three-phase electric motors are ready to perform well in single-phase networks, but most of them cope with this task completely satisfactorily - except for power loss. Mainly, for operation in single-phase networks, asynchronous motors with a squirrel-cage rotor (A, AO2, AOL, APN, etc.) are used.

Asynchronous three-phase motors are designed for 2 rated network voltages - 220/127, 380/220, and so on. Electric motors with an operating voltage of windings of 380/220V (380V for star, 220 for delta) are more common. The highest voltage is for the "star", the lowest - for the "triangle". In the passport and on the motor plate, in addition to other characteristics, the operating voltage of the windings, their connection diagram and the likelihood of its change are indicated.

Three-phase motor labels

The designation on plate A states that the motor windings can be connected both as a “triangle” (at 220V) and a “star” (at 380V). When connecting a three-phase motor to a single-phase network, it is better to use a delta circuit, since in this case the motor will lose less power than when switched on as a star.

Plate B informs you that the motor windings are connected in a star configuration, and the junction box does not take into account the possibility of switching them to delta (there are no more than 3 terminals). In this case, all that remains is to either come to terms with a large loss of power by connecting the motor in a star configuration, or, having penetrated the electric motor winding, try to bring out the missing ends in order to connect the windings in a delta configuration.

If the operating voltage of the motor is 220/127V, then the motor can only be connected to a single-phase 220V network using a star circuit. When you turn on 220V in a delta circuit, the engine will burn out.

Beginnings and ends of windings (various options)

Probably the main difficulty in connecting a three-phase motor to a single-phase network is to understand the electrical wires going into the junction box or, in the absence of one, simply leading out of the motor.

The most common option is when the windings in an existing 380/220V motor are already connected in a delta circuit. In this case, you simply need to connect the current-carrying electrical wires and the working and starting capacitors to the motor terminals according to the connection diagram.

If the windings in the motor are connected by a “star”, and there is a possibility of changing it to a “triangle”, then such a case also cannot be classified as labor-intensive. You just need to change the winding connection circuit to a “triangle” one, using jumpers for this.

Determination of the beginnings and ends of the windings. The situation is more difficult if 6 wires are brought out into the junction box without indicating their belonging to a specific winding and marking the beginnings and ends. In this case, it comes down to solving two problems (Although before doing this, you should try to search the Internet for some documentation for the electric motor. It may describe what electrical wires of different colors refer to.):

identifying pairs of wires related to one winding;

finding the beginning and end of the windings.

The first problem is solved by “ringing” all the wires with a tester (measuring resistance). When there is no device, it is possible to solve it using a light bulb from a flashlight and batteries, connecting the existing electrical wires into the circuit alternately with the light bulb. If the latter lights up, it means that the two ends being tested belong to the same winding. This method identifies 3 pairs of wires (A, B and C in the figure below) related to 3 windings.

Determination of pairs of wires belonging to one winding

The second task is to determine the beginnings and ends of the windings; here it will be somewhat more complicated and you will need a battery and a pointer voltmeter. Digital is not suitable for this task due to inertia. The procedure for determining the ends and beginnings of the windings is shown in diagrams 1 and 2.

Finding the beginning and end of the windings

A battery is connected to the ends of one winding (for example, A), and a pointer voltmeter is connected to the ends of the other (for example, B). Now, when you break the contact of wires A with the battery, the voltmeter needle will swing in some direction. Then you need to connect a voltmeter to winding C and do the same operation with breaking the battery contacts. If necessary, changing the polarity of winding C (switching ends C1 and C2) it is necessary to ensure that the voltmeter needle swings in the same direction as in the case of winding B. Winding A is checked in the same way - with a battery connected to the winding C or B.

Ultimately, all manipulations should result in the following: when the battery contacts break with any of the windings, an electric potential of the same polarity should appear on the other two (the device arrow swings in one direction). Now all that remains is to mark the conclusions of the 1st bundle as the beginning (A1, B1, C1), and the conclusions of the other as the ends (A2, B2, C2) and connect them according to the desired pattern - “triangle” or “star” (when the motor voltage is 220 /127V).

Extracting missing ends. Probably the most difficult option is when the engine has a fusion of windings in a “star” configuration, and there is no ability to switch it to a “delta” (no more than 3 electrical wires are brought into the distribution box - the beginning of windings C1, C2, C3).

In this case, to turn on the motor according to the “triangle” circuit, you need to bring the missing ends of the windings C4, C5, C6 into the box.

Schemes for connecting a three-phase motor to a single-phase network

Triangle connection. In the case of a home network, based on the belief of obtaining greater output power, single-phase connection of three-phase motors in a delta circuit is considered more suitable. With all this, their power can reach 70% of the nominal. 2 contacts in the junction box are connected directly to the electrical wires of a single-phase network (220V), and the 3rd - through the working capacitor Cp to any of the first 2 contacts or the electrical wires of the network.

Ensuring launch. It is possible to start a three-phase motor without a load using a working capacitor (more details below), but if the electric motor has some kind of load, it either will not start or will begin to gain speed extremely slowly. Then, for a quick start, you need an auxiliary starting capacitor Sp (calculation of the capacitor capacity is described below). The starting capacitors are turned on only for the duration of the engine startup (2-3 seconds, until the speed reaches approximately 70% of the nominal), then the starting capacitor must be disconnected and discharged.

It is convenient to start a three-phase motor using a special switch, one pair of contacts of which closes when the button is pressed. When it is released, some contacts open, while others remain on - until the "stop" button is pressed.

Switch for starting electric motors

Reverse. The direction of rotation of the motor depends on which contact ("phase") the third phase winding is connected to.

The direction of rotation can be controlled by connecting the latter, through a capacitor, to a two-position switch connected by its two contacts to the first and 2nd windings. Depending on the position of the switch, the engine will rotate in one direction or the other.

The figure below shows a circuit with a starting and running capacitor and a reverse button, which allows for comfortable control of a three-phase motor.

Connection diagram for a three-phase motor to a single-phase network, with reverse and a button for connecting a starting capacitor

Star connection. A similar diagram for connecting a three-phase motor to a network with a voltage of 220V is used for electric motors whose windings are designed for a voltage of 220/127V.

Capacitors. The required capacity of working capacitors for operating a three-phase motor in a single-phase network depends on the connection circuit of the motor windings and other characteristics. For a star connection, the capacitance is calculated using the formula:

Cp = 2800 I/U

For a triangle connection:

Cp = 4800 I/U

Where Cp is the capacitance of the working capacitor in microfarads, I is the current in A, U is the network voltage in V. The current is calculated by the formula:

I = P/(1.73 U n cosph)

Where P is the electric motor power kW; n - engine efficiency; cosф - power factor, 1.73 - coefficient that determines the correspondence between linear and phase currents. The efficiency and power factor are indicated in the passport and on the motor plate. Traditionally, their value is located in the spectrum of 0.8-0.9.

In practice, the capacitance value of the working capacitor when connected in a triangle can be calculated using the simplified formula C = 70 Pn, where Pn is the rated power of the electric motor in kW. According to this formula, for every 100 W of electric motor power, you need about 7 μF of working capacitor capacity.

The correct selection of capacitor capacity is checked by the results of engine operation. If its value is greater than required under these operating conditions, the engine will overheat. If the capacity is less than required, the power output of the motor will become very low. It makes sense to look for a capacitor for a three-phase motor, starting with a small capacitance and gradually increasing its value to a rational one. If possible, it is much better to choose a capacitance by measuring the current in the electrical wires connected to the network and to the working capacitor, for example, with a current clamp. The current value should be closer. Measurements should be made in the mode in which the engine will operate.

When determining the starting capacity, we first proceed from the requirements for creating the required starting torque. Do not confuse the starting capacitance with the capacitance of the starting capacitor. In the above diagrams, the starting capacitance is equal to the sum of the capacitances of the working (Cp) and starting (Sp) capacitors.

If, due to operating conditions, the electric motor starts without load, then the starting capacitance is traditionally assumed to be the same as the working capacitance, in other words, a starting capacitor is not needed. In this case, the connection diagram is simplified and cheaper. To simplify this and generally reduce the cost of the circuit, it is possible to organize the possibility of disconnecting the load, for example, by making it possible to quickly and comfortably change the position of the motor to drop the belt drive, or by making the belt drive a pressing roller, for example, like the belt clutch of walk-behind tractors.

Starting under load requires the presence of an additional tank (Sp) that is connected temporarily to start the engine. An increase in the switchable capacitance leads to an increase in the starting torque, and at a certain specific value, the torque reaches its maximum value. A further increase in capacitance leads to the opposite effect: the starting torque begins to decrease.

Based on the condition of starting the engine under a load closest to the rated load, the starting capacitance must be 2-3 times greater than the working capacitance, that is, if the capacity of the working capacitor is 80 µF, then the capacitance of the starting capacitor must be 80-160 µF, which will provide the starting capacitance (sum of the capacitance of the working and starting capacitors) 160-240 µF. Although, if the engine has a small load when starting, the capacitance of the starting capacitor may be less or may not exist at all.

Starting capacitors operate for a short time (only a few seconds during the entire connection period). This makes it possible to use cheaper starting electrolytic capacitors, specially designed for this purpose, when starting the engine.

Note that for a motor connected to a single-phase network through a capacitor, operating in the absence of a load, the winding fed through the capacitor carries a current 20-30% higher than the rated one. Therefore, if the engine is used in an underloaded mode, the capacity of the working capacitor should be minimized. But then, if the engine was started without a starting capacitor, the latter may be required.

It is much better to use not 1 large capacitor, but several much smaller ones, partly due to the ability to select a good capacitance, connecting additional ones or disconnecting unnecessary ones, the latter are used as starting ones. The required number of microfarads is obtained by connecting several capacitors in parallel, based on the fact that the total capacitance in a parallel connection is calculated using the formula:

Determination of the beginning and end of the phase windings of an asynchronous electric motor

When operating or manufacturing this or that equipment, it often becomes necessary to connect an asynchronous three-phase motor to a regular 220 V network. This is quite realistic and not even particularly difficult, the main thing is to find a way out of the following possible situations if there is no suitable single-phase motor, and a three-phase one is lying without business, and also if there is three-phase equipment, but in the workshop there is only a single-phase network.

To begin with, it makes sense to recall the diagram for connecting a three-phase motor to a three-phase network.

Connection diagram for a 220 V three-phase electric motor according to the “Star” and “Triangle” circuits

For ease of understanding, the magnetic starter and other switching units are not shown. As can be seen from the diagram, each motor winding is powered by its own phase. In a single-phase network, as its name suggests, there is only one “phase”. But it is also enough to power a three-phase electric motor. Let's take a look at an asynchronous motor connected to 220 V.

How to connect a three-phase electric motor 380 V to 220 V through a capacitor according to the “Star” and “Triangle” circuit: diagram.

Here, one winding of a three-phase electric motor is directly connected to the network, the other two are connected in series, and voltage is supplied to their connection point through the phase-shifting capacitor C1. C2 is the starting button and is turned on by button B1 with self-return only at the moment of starting: as soon as the engine starts, it must be released.

Several questions immediately arise:

How effective is this scheme?
How to ensure engine reverse?
What capacities should capacitors have?

In order to make the motor rotate in the other direction, it is enough to “reverse” the phase arriving at the connection point of windings B and C (Triangle connection) or to winding B (Star circuit). The circuit, which allows you to change the direction of rotation of the rotor by simply clicking the SB2 switch, will look like this.

Reversing a 380 V three-phase motor operating on a single-phase network

It should be noted here that almost any three-phase motor is reversible, but you need to select the direction of rotation of the motor before starting it. It is impossible to reverse the electric motor while it is running! First you need to de-energize the electric motor, wait for it to stop completely, select the desired direction of rotation with the SB1 toggle switch, and only then apply voltage to the circuit and briefly press button B1.

Capacitances of phase-shifting and starting capacitors

To calculate the capacity of a phase-shifting capacitor, you need to use a simple formula:

C1 = 2800/(I/U) - for inclusion according to the “Star” circuit;
C1 = 4800/(I/U) - for switching on according to the “Triangle” scheme.

Here:

C1 is the capacity of the phase-shifting capacitor, μF;
I is the rated current of one motor winding, A;
U is the voltage of a single-phase network, V.

But what to do if the rated current of the windings is unknown? It can be easily calculated by knowing the motor power, which is usually printed on the device nameplate. To calculate we use the formula:

I = P/1.73*U*n*cosф, where:

I—current consumption, A;
U—mains voltage, V;
n - efficiency;
cosф - power factor.

The symbol * denotes the multiplication sign.

The capacity of the starting capacitor C2 is selected 1.5–2 times greater than the capacity of the phase-shifting one.

When calculating a phase-shifting capacitor, you need to keep in mind that an engine operating at less than full load may overheat at the design capacitor capacity. In this case, its denomination must be reduced.

Work efficiency

Unfortunately, a three-phase motor, when powered by one phase, will not be able to develop its rated power. Why? In normal mode, each of the motor windings develops a power of 33.3%. When the motor is turned on, for example, in a “triangle” mode, only one winding C operates in normal mode, and at the point of connection of windings B and C, with a correctly selected capacitor, the voltage will be 2 times lower than the supply voltage, which means the power of these windings will drop 4 times - i.e. only 8.325% each. Let's do a simple calculation and calculate the total power:

33,3 + 8,325 + 8,325 = 49.95%.

So, even theoretically, a three-phase motor connected to a single-phase network develops only half of its rated power, and in practice this figure is even less.

A way to increase the power developed by the motor

It turns out that it is possible to increase engine power, and significantly. To do this, you don’t even have to complicate the design, but just connect a three-phase motor according to the diagram below.

Asynchronous motor - 220 V connection using an improved circuit

Here windings A and B are already operating in nominal mode, and only winding C delivers a quarter of the power:

33,3 + 33,3 + 8,325 = 74.92%.

Not bad at all, isn't it? The only condition for this connection is that windings A and B must be turned on in antiphase (marked with dots). Reversing such a circuit is done in the usual way - by switching the polarity of the capacitor-winding C circuit.

One final note. In place of the phase-shifting and starting capacitor, only non-polar paper devices can operate, for example, MBGCH, which can withstand a voltage one and a half to two times higher than the supply voltage.

There are situations in life when you need to connect some industrial equipment to a regular home power supply network. A problem immediately arises with the number of wires. Machines intended for use in enterprises usually have three, but sometimes four, terminals. What to do with them, where to connect them? Those who tried to try various options were convinced that the motors simply did not want to spin. Is it even possible to connect a single-phase three-phase motor? Yes, you can achieve rotation. Unfortunately, in this case, the power drop is inevitable by almost half, but in some situations this is the only way out.

Voltages and their ratio

In order to understand how to connect a three-phase motor to a regular outlet, you need to understand how the voltages in the industrial network relate. The voltage values are well known - 220 and 380 Volts. Previously, there was still 127 V, but in the fifties this parameter was abandoned in favor of a higher one. Where did these “magic numbers” come from? Why not 100, or 200, or 300? It seems that round numbers are easier to count.

Most industrial electrical equipment is designed to be connected to a three-phase network. The voltage of each phase in relation to the neutral wire is 220 Volts, just like in a home socket. Where does 380 V come from? It is very simple, just consider an isosceles triangle with angles of 60, 30 and 30 degrees, which is a vector stress diagram. The length of the longest side will be equal to the length of the thigh multiplied by cos 30°. After some simple calculations, you can make sure that 220 x cos 30° = 380.

Three-phase motor device

Not all types of industrial motors can operate from a single phase. The most common of them are the “workhorses” that make up the majority of electrical machines in any enterprise - asynchronous machines with a power of 1 - 1.5 kVA. How does such a three-phase motor work in the three-phase network for which it is intended?

The inventor of this revolutionary device was the Russian scientist Mikhail Osipovich Dolivo-Dobrovolsky. This outstanding electrical engineer was a proponent of the theory of a three-phase power supply network, which has become dominant in our time. three-phase operates on the principle of induction of currents from the stator windings to closed rotor conductors. As a result of their flow through the short-circuited windings, a magnetic field arises in each of them, interacting with the stator power lines. This produces a torque that leads to circular motion of the motor axis.

The windings are angled 120° so that the rotating field generated by each phase pushes each magnetized side of the rotor in succession.

Triangle or star?

A three-phase motor in a three-phase network can be switched on in two ways - with or without a neutral wire. The first method is called “star”, in this case each of the windings is under (between phase and zero), equal in our conditions to 220 V. The connection diagram of a three-phase motor with a “triangle” involves connecting three windings in series and applying linear (380 V) voltage to switching nodes. In the second case, the engine will produce about one and a half times more power.

How to turn the motor in reverse?

Control of a three-phase motor may require changing the direction of rotation to the opposite, that is, reverse. To achieve this, you just need to swap two of the three wires.

To make it easier to change the circuit, jumpers are provided in the motor terminal box, usually made of copper. For star switching, gently connect the three output wires of the windings together. The “triangle” turns out to be a little more complicated, but any average qualified electrician can handle it.

Phase shifting tanks

So, sometimes the question arises about how to connect a three-phase motor to a regular home outlet. If you just try to connect two wires to the plug, it will not rotate. In order for things to work, you need to simulate the phase by shifting the supplied voltage by some angle (preferably 120°). This effect can be achieved by using a phase-shifting element. Theoretically, this could be inductance or even resistance, but most often a three-phase motor in a single-phase network is switched on using electrical circuits designated by the Latin letter C on the diagrams.

As for the use of chokes, it is difficult due to the difficulty of determining their value (if it is not indicated on the device body). To measure the value of L, a special device or a circuit assembled for this purpose is required. In addition, the choice of available chokes is usually limited. However, any phase-shifting element can be selected experimentally, but this is a troublesome task.

What happens when you turn on the engine? Zero is applied to one of the connection points, phase is applied to the other, and a certain voltage is applied to the third, shifted by a certain angle relative to the phase. It is clear to a non-specialist that the operation of the engine will not be complete in terms of mechanical power on the shaft, but in some cases the very fact of rotation is sufficient. However, already at startup, some problems may arise, for example, the lack of an initial torque capable of moving the rotor from its place. What to do in this case?

Start capacitor

At the moment of starting, the shaft requires additional efforts to overcome the forces of inertia and static friction. To increase the torque, you should install an additional capacitor, connected to the circuit only at the moment of start, and then turned off. For these purposes, the best option is to use a locking button without fixing the position. The connection diagram for a three-phase motor with a starting capacitor is shown below, it is simple and understandable. At the moment the voltage is applied, press the “Start” button, and it will create an additional phase shift. After the engine spins up to the required speed, the button can (and even should) be released, and only the working capacity will remain in the circuit.

Calculation of container sizes

So, we found out that in order to turn on a three-phase motor in a single-phase network, an additional connection circuit is required, which, in addition to the start button, includes two capacitors. You need to know their value, otherwise the system will not work. First, let's determine the amount of electrical capacitance required to make the rotor move. When connected in parallel, it is the sum:

C = C st + Wed, where:

C st - starting additional capacity that can be switched off after takeoff;

C p is a working capacitor that provides rotation.

We also need the value of the rated current I n (it is indicated on the plate attached to the engine at the manufacturer). This parameter can also be determined using a simple formula:

I n = P / (3 x U), where:

U - voltage, when connected as a “star” - 220 V, and if connected as a “triangle”, then 380 V;

P is the power of a three-phase motor; sometimes, if the plate is lost, it is determined by eye.

So, the dependencies of the required operating power are calculated using the formulas:

С р = Ср = 2800 I n / U - for “star”;

C p = 4800 I n / U - for a “triangle”;

The starting capacitor should be 2-3 times larger than the working capacitor. The unit of measurement is microfarads.

There is also a very simple way to calculate capacity: C = P /10, but this formula gives the order of the number rather than its value. However, in any case you will have to tinker.

Why adjustment is needed

The calculation method given above is approximate. Firstly, the nominal value indicated on the body of the electrical capacitance may differ significantly from the actual one. Secondly, paper capacitors (generally speaking, an expensive thing) are often used second-hand, and they, like any other items, are subject to aging, which leads to an even greater deviation from the specified parameter. Thirdly, the current that will be consumed by the motor depends on the magnitude of the mechanical load on the shaft, and therefore it can only be assessed experimentally. How to do this?

This requires a little patience. The result can be a rather voluminous set of capacitors. The main thing is to secure everything well after finishing the work so that the soldered ends do not fall off due to vibrations emanating from the motor. And then it would be a good idea to analyze the result again and, possibly, simplify the design.

Composing a battery of containers

If the master does not have at his disposal special electrolytic clamps that allow you to measure the current without opening the circuits, then you should connect an ammeter in series to each wire that enters the three-phase motor. In a single-phase network, the total value will flow, and by selecting capacitors one should strive for the most uniform loading of the windings. It should be remembered that when connected in series, the total capacitance decreases according to the law:

It is also necessary not to forget about such an important parameter as the voltage for which the capacitor is designed. It should be no less than the nominal value of the network, or better yet, with a margin.

Discharge resistor

The circuit of a three-phase motor connected between one phase and a neutral wire is sometimes supplemented with resistance. It serves to prevent the charge that remains on the starting capacitor from accumulating after the machine has already been turned off. This energy can cause an electric shock, which is not dangerous, but extremely unpleasant. In order to protect yourself, you should connect a resistor in parallel with the starting capacitance (electricians call this “bypassing”). The value of its resistance is large - from half a megohm to a megohm, and it is small in size, so half a watt of power is enough. However, if the user is not afraid of being “pinched,” then this detail can be completely dispensed with.

Using Electrolytes

As already noted, film or paper electrical containers are expensive, and purchasing them is not as easy as we would like. It is possible to make a single-phase connection to a three-phase motor using inexpensive and readily available electrolytic capacitors. At the same time, they won’t be very cheap either, since they must withstand 300 Volts of DC. For safety, they should be bypassed with semiconductor diodes (D 245 or D 248, for example), but it would be useful to remember that when these devices break through, alternating voltage will hit the electrolyte, and it will first heat up very much, and then explode, loudly and effectively. Therefore, unless absolutely necessary, it is better to use paper-type capacitors that operate under either constant or alternating voltage. Some craftsmen completely allow the use of electrolytes in starting circuits. Due to short-term exposure to alternating voltage, they may not have time to explode. It's better not to experiment.

If there are no capacitors

Where do ordinary citizens who do not have access to in-demand electrical and electronic parts purchase them? At flea markets and flea markets. There they lie, carefully soldered by someone’s (usually elderly) hands from old washing machines, televisions and other household and industrial equipment that are out of use and out of use. They ask a lot for these Soviet-made products: sellers know that if a part is needed, they will buy it, and if not, they will not take it for nothing. It happens that just the most necessary thing (in this case, a capacitor) is just not there. So what should we do? No problem! Resistors will also do, you just need powerful ones, preferably ceramic and vitrified ones. Of course, ideal resistance (active) does not shift the phase, but nothing is ideal in this world, and in our case this is good. Every physical body has its own inductance, electrical power and resistivity, whether it is a tiny speck of dust or a huge mountain. Connecting a three-phase motor to a power outlet becomes possible if in the above diagrams you replace the capacitor with a resistance, the value of which is calculated by the formula:

R = (0.86 x U) / kI, where:

kI - current value for three-phase connection, A;

U - our trusty 220 Volts.

What engines are suitable?

Before purchasing a motor for a lot of money, which a zealous owner intends to use as a drive for a grinding wheel, circular saw, drilling machine or any other useful household device, it would not hurt to think about its applicability for these purposes. Not every three-phase motor in a single-phase network will be able to operate at all. For example, the MA series (it has a squirrel-cage rotor with a double cage) should be excluded so that you do not have to carry considerable and useless weight home. In general, it is best to experiment first or invite an experienced person, an electrician, for example, and consult with him before purchasing. A three-phase asynchronous motor of the UAD, APN, AO2, AO and, of course, A series is quite suitable. These indices are indicated on the nameplates.

Three-phase asynchronous motors are deservedly the most popular in the world, due to the fact that they are very reliable, require minimal maintenance, are easy to manufacture and do not require any complex and expensive devices when connecting, unless adjustment of the rotation speed is required. Most of the machines in the world are driven by three-phase asynchronous motors; they also drive pumps and electric drives of various useful and necessary mechanisms.

But what about those who do not have a three-phase power supply in their personal household, and in most cases this is exactly the case. What if you want to install a stationary circular saw, electric jointer or lathe in your home workshop? I would like to please the readers of our portal that there is a way out of this difficult situation, and one that is quite simple to implement. In this article we intend to tell you how to connect a three-phase motor to a 220 V network.

Let us briefly consider the principle of operation of an asynchronous motor in its “native” three-phase 380 V networks. This will greatly help in later adapting the motor for operation in other, “non-native” conditions - single-phase 220 V networks.

Asynchronous motor device

Most of the three-phase motors produced in the world are squirrel-cage induction motors (SCMC), which do not have any electrical contact between the stator and the rotor. This is their main advantage, since brushes and commutators are the weakest point of any electric motor; they are subject to intense wear and require maintenance and periodic replacement.

Let's consider the ADKZ device. The engine is shown in cross-section in the figure.

The cast housing (7) houses the entire electric motor mechanism, which includes two main parts - a stationary stator and a movable rotor. The stator has a core (3), which is made of sheets of special electrical steel (an alloy of iron and silicon), which has good magnetic properties. The core is made of sheets due to the fact that under conditions of an alternating magnetic field, Foucault eddy currents can arise in the conductors, which we absolutely do not need in the stator. Additionally, each core sheet is coated on both sides with a special varnish to completely eliminate the flow of currents. We only need from the core its magnetic properties, and not the properties of an electric current conductor.

A winding (2) made of enameled copper wire is laid in the grooves of the core. To be precise, there are at least three windings in a three-phase asynchronous motor - one for each phase. Moreover, these windings are laid in the grooves of the core with a certain order - each is located so that it is at an angular distance of 120° to the other. The ends of the windings are led out into the terminal box (in the figure it is located at the bottom of the motor).

The rotor is placed inside the stator core and rotates freely on the shaft (1). To increase efficiency, they try to make the gap between the stator and rotor minimal - from half a millimeter to 3 mm. The rotor core (5) is also made of electrical steel and it also has grooves, but they are not intended for wire winding, but for short-circuited conductors, which are located in space so that they resemble a squirrel wheel (4), for which they received their Name.

The squirrel wheel consists of longitudinal conductors that are connected both mechanically and electrically to the end rings. Typically, the squirrel wheel is made by pouring molten aluminum into the grooves of the core, and at the same time, both rings and fan impellers (6) are molded as a monolith. In high-power ADKZ, copper rods welded with end copper rings are used as cell conductors.

What is three-phase current

In order to understand what forces make the ADKZ rotor rotate, we need to consider what a three-phase power supply system is, then everything will fall into place. We are all accustomed to the usual single-phase system, when the socket has only two or three contacts, one of which is (L), the second is a working zero (N), and the third is a protective zero (PE). The rms phase voltage in a single-phase system (the voltage between phase and zero) is 220 V. The voltage (and when a load is connected, the current) in single-phase networks varies according to a sinusoidal law.

From the above graph of the amplitude-time characteristic it is clear that the amplitude value of the voltage is not 220 V, but 310 V. So that readers do not have any “misunderstandings” or doubts, the authors consider it their duty to inform that 220 V is not the amplitude value, but the root mean square or current. It is equal to U=U max /√2=310/1.414≈220 V. Why is this done? For convenience of calculations only. Constant voltage is taken as the standard, based on its ability to produce some kind of work. We can say that a sinusoidal voltage with an amplitude value of 310 V in a certain period of time will produce the same work that a constant voltage of 220 V would do in the same period of time.

It must be said right away that almost all generated electrical energy in the world is three-phase. It’s just that single-phase energy is easier to manage in everyday life; most electricity consumers only need one phase to operate, and single-phase wiring is much cheaper. Therefore, one phase and neutral conductor are “pulled out” from a three-phase system and sent to consumers - apartments or houses. This is clearly visible in the entrance panels, where you can see how the wire goes from one phase to one apartment, from another to a second, from a third to a third. This is also clearly visible on the poles from which the lines go to private households.

Three-phase voltage, unlike single-phase, has not one phase wire, but three: phase A, phase B and phase C. Phases can also be designated L1, L2, L3. In addition to the phase wires, of course, there is also a working zero (N) and a protective zero (PE) common to all phases. Let's consider the amplitude-time characteristic of three-phase voltage.

It is clear from the graphs that three-phase voltage is a combination of three single-phase ones, with an amplitude of 310 V and an rms value of the phase (between phase and working zero) voltage of 220 V, and the phases are shifted relative to each other with an angular distance of 2 * π / 3 or 120 ° . The potential difference between the two phases is called linear voltage and is equal to 380 V, since the vector sum of the two voltages will be U l =2*U f *sin(60°)=2*220*√3/2=220* √3=220*1.73=380.6 V, Where U l– linear voltage between two phases, and U f– phase voltage between phase and zero.

Three-phase current is easy to generate, transmit to its destination and subsequently convert it into any desired type of energy. Including the mechanical energy of rotation of the ADKZ.

How does a three-phase asynchronous motor work?

If you apply an alternating three-phase voltage to the stator windings, currents will begin to flow through them. They, in turn, will cause magnetic fluxes, also varying according to a sinusoidal law and also shifted in phase by 2*π/3=120°. Considering that the stator windings are located in space at the same angular distance - 120°, a rotating magnetic field is formed inside the stator core.

This constantly changing field crosses the “squirrel wheel” of the rotor and causes an EMF (electromotive force) in it, which will also be proportional to the rate of change of the magnetic flux, which in mathematical language means the derivative of the magnetic flux with respect to time. Since the magnetic flux changes according to a sinusoidal law, this means that the EMF will change according to the cosine law, because (sin x)’= cos x. From the school mathematics course it is known that the cosine “leads” the sine by π/2=90°, that is, when the cosine reaches its maximum, the sine will reach it after π/2 - after a quarter of the period.

Under the influence of EMF, large currents will arise in the rotor, or more precisely, in the squirrel wheel, given that the conductors are short-circuited and have low electrical resistance. These currents form their own magnetic field, which spreads along the rotor core and begins to interact with the stator field. Opposite poles, as is known, attract, and like poles repel each other. The resulting forces create a torque causing the rotor to rotate.

The stator's magnetic field rotates at a certain frequency, which depends on the supply network and the number of pole pairs of the windings. The frequency is calculated using the following formula:

n 1 =f 1 *60/p, Where

f 1 – alternating current frequency.
p – number of pole pairs of stator windings.

Everything is clear with the frequency of alternating current - in our power supply networks it is 50 Hz. The number of pole pairs reflects how many pairs of poles there are on the winding or windings belonging to one phase. If one winding is connected to each phase, spaced 120° from the others, then the number of pole pairs will be equal to one. If two windings are connected to one phase, then the number of pole pairs will be equal to two, and so on. Accordingly, the angular distance between the windings changes. For example, when the number of pole pairs is two, the stator contains a winding of phase A, which occupies a sector of not 120°, but 60°. Then it is followed by the winding of phase B, occupying the same sector, and then phase C. Then the alternation is repeated. As the pole pairs increase, the sectors of the windings decrease accordingly. Such measures make it possible to reduce the rotation frequency of the magnetic field of the stator and, accordingly, the rotor.

Let's give an example. Let's say a three-phase motor has one pair of poles and is connected to a three-phase network with a frequency of 50 Hz. Then the stator magnetic field will rotate with a frequency n 1 =50*60/1=3000 rpm. If you increase the number of pole pairs, the rotation speed will decrease by the same amount. To increase the engine speed, you need to increase the frequency supplying the windings. To change the direction of rotation of the rotor, you need to swap two phases on the windings

It should be noted that the rotor speed always lags behind the rotation speed of the stator magnetic field, which is why the motor is called asynchronous. Why is this happening? Let's imagine that the rotor rotates at the same speed as the stator's magnetic field. Then the squirrel wheel will not “pierce” the alternating magnetic field, but it will be constant for the rotor. Accordingly, no EMF will be induced and currents will stop flowing, there will be no interaction of magnetic fluxes and the moment driving the rotor in motion will disappear. That is why the rotor is “in constant striving” to catch up with the stator, but it will never catch up, since the energy causing the motor shaft to rotate will disappear.

The difference in the rotation frequencies of the magnetic field of the stator and the rotor shaft is called the slip frequency, and it is calculated by the formula:

∆ n=n 1 -n 2, Where

n1 – rotation frequency of the stator magnetic field.
n2 – rotor speed.

Slip is the ratio of the sliding frequency to the rotation frequency of the stator magnetic field, it is calculated by the formula: S=∆n/n 1 =(n 1 —n 2)/n 1.

Methods for connecting windings of asynchronous motors

Most ADKZ has three windings, each of which corresponds to its own phase and has a beginning and an end. Winding designation systems may vary. In modern electric motors, a system has been adopted for designating windings U, V and W, and their terminals are designated by number 1 as the beginning of the winding and by number 2 as its end, that is, winding U has two terminals U1 and U2, winding V–V1 and V2, and winding W - W1 and W2.

However, asynchronous motors made during the Soviet era and having the old marking system are still in use. In them, the beginnings of the windings are designated C1, C2, C3, and the ends are C4, C5, C6. This means that the first winding has terminals C1 and C4, the second winding C2 and C5, and the third winding C3 and C6. The correspondence between the old and new notation systems is presented in the figure.

Let's consider how windings can be connected in an ADKZ.

Star connection

With this connection, all ends of the windings are combined at one point, and phases are connected to their beginnings. In the circuit diagram, this connection method really resembles a star, which is why it got its name.

When connected by a star, a phase voltage of 220 V is applied to each winding individually, and a linear voltage of 380 V is applied to two windings connected in series. The main advantage of this connection method is small starting currents, since the linear voltage is applied to two windings, and not to one. This allows the engine to start “softly,” but its power will be limited, since the currents flowing in the windings will be less than with another connection method.

Delta connection

With this connection, the windings are combined into a triangle, when the beginning of one winding is connected to the end of the next - and so on in a circle. If the linear voltage in a three-phase network is 380 V, then much larger currents will flow through the windings than with a star connection. Therefore, the power of the electric motor will be higher.

When connected by a delta at the moment of starting, the ADKZ consumes large starting currents, which can be 7-8 times higher than the rated ones and can cause network overload, so in practice, engineers have found a compromise - the engine starts and spins up to rated speed using a star circuit, and then automatic switching to triangle.

How to determine which circuit the motor windings are connected to?

Before connecting a three-phase motor to a single-phase 220 V network, it is necessary to find out what circuit the windings are connected to and at what operating voltage the ADKZ can operate. To do this, you need to study the plate with technical characteristics - the “nameplate”, which should be on each engine.

You can find out a lot of useful information on such a “nameplate”

The plate contains all the necessary information that will help connect the motor to a single-phase network. The presented nameplate shows that the engine has a power of 0.25 kW and a speed of 1370 rpm, which indicates the presence of two pairs of winding poles. The ∆/Y symbol means that the windings can be connected either by a triangle or a star, and the following indicator 220/380 V indicates that when connected by a triangle, the supply voltage should be 220 V, and when connected by a star - 380 V. If such Connect the motor to a 380 V network in a triangle, then its windings will burn out.

On the next nameplate you can see that such a motor can only be connected with a star and only to a 380 V network. Most likely, such an ADKZ will have only three terminals in the terminal box. Experienced electricians will be able to connect such a motor to a 220 V network, but to do this they will need to open the back cover to get to the winding terminals, then find the beginning and end of each winding and make the necessary switching. The task becomes much more complicated, so the authors do not recommend connecting such motors to a 220 V network, especially since most modern ADKZ can be connected in different ways.

Each motor has a terminal box, most often located on the top. This box has inputs for power cables, and on top it is closed with a lid that must be removed with a screwdriver.

As electricians and pathologists say: “An autopsy will tell.”

Under the cover you can see six terminals, each of which corresponds to either the beginning or the end of the winding. In addition, the terminals are connected by jumpers, and by their location you can determine by what scheme the windings are connected.

Opening the terminal box showed that the “patient” had obvious “star fever”

The photo of the “opened” box shows that the wires leading to the windings are labeled and the ends of all windings – V2, U2, W2 – are connected to one point by jumpers. This indicates that a star connection is taking place. At first glance, it may seem that the ends of the windings are located in the logical order V2, U2, W2, and the beginnings are “confused” - W1, V1, U1. However, this is done for a specific purpose. To do this, consider the ADKZ terminal box with connected windings according to a triangle diagram.

The figure shows that the position of the jumpers changes - the beginnings and ends of the windings are connected, and the terminals are located so that the same jumpers are used for reconnection. Then it becomes clear why the terminals are “mixed up” - this makes it easier to transfer jumpers. The photo shows that terminals W2 and U1 are connected by a piece of wire, but in the basic configuration of new engines there are always exactly three jumpers.

If, after “opening” the terminal box, a picture like the one in the photograph is revealed, this means that the motor is intended for a star and a three-phase 380 V network.

It is better for such an engine to return to its “native element” - in a three-phase alternating current circuit

Video: An excellent film about three-phase synchronous motors, which has not yet been painted

It is possible to connect a three-phase motor to a single-phase 220 V network, but you must be prepared to sacrifice a significant reduction in its power - in the best case, it will be 70% of the nameplate, but for most purposes this is quite acceptable.

The main connection problem is the creation of a rotating magnetic field, which induces an emf in the squirrel-cage rotor. This is easy to implement in three-phase networks. When generating three-phase electricity, an EMF is induced in the stator windings due to the fact that a magnetized rotor rotates inside the core, which is driven by the energy of falling water at a hydroelectric power station or a steam turbine at hydroelectric power stations and nuclear power plants. It creates a rotating magnetic field. In engines, the reverse transformation occurs - a changing magnetic field causes the rotor to rotate.

In single-phase networks, it is more difficult to obtain a rotating magnetic field - you need to resort to some “tricks”. To do this, you need to shift the phases in the windings relative to each other. Ideally, you need to make sure that the phases are shifted relative to each other by 120°, but in practice this is difficult to implement, since such devices have complex circuits, are quite expensive, and their manufacture and configuration require certain qualifications. Therefore, in most cases, simple circuits are used, while somewhat sacrificing power.

Phase shift using capacitors

An electric capacitor is known for its unique property of not passing direct current, but passing alternating current. The dependence of the currents flowing through the capacitor on the applied voltage is shown in the graph.

The current in the capacitor will always “lead” for a quarter of the period

As soon as a voltage increasing along a sinusoid is applied to the capacitor, it immediately “pounces” on it and begins to charge, since it was initially discharged. The current will be maximum at this moment, but as it charges, it will decrease and reach a minimum at the moment when the voltage reaches its peak.

As soon as the voltage decreases, the capacitor will react to this and will begin to discharge, but the current will flow in the opposite direction, as it discharges it will increase (with a minus sign) as long as the voltage decreases. By the time the voltage is zero, the current reaches its maximum.

When the voltage begins to increase with a minus sign, the capacitor is recharged and the current gradually approaches zero from its negative maximum. As the negative voltage decreases and it approaches zero, the capacitor discharges with an increase in the current through it. Next, the cycle repeats again.

The graph shows that during one period of alternating sinusoidal voltage, the capacitor is charged twice and discharged twice. The current flowing through the capacitor leads the voltage by a quarter of a period, that is - 2* π/4=π/2=90°. In this simple way you can obtain a phase shift in the windings of an asynchronous motor. A phase shift of 90° is not the ideal 120°, but it is quite sufficient for the necessary torque to appear on the rotor.

Phase shift can also be obtained by using an inductor. In this case, everything will happen the other way around - the voltage will lead the current by 90°. But in practice, more capacitive phase shift is used due to simpler implementation and lower losses.

Schemes for connecting three-phase motors to a single-phase network

There are many options for connecting ADKZ, but we will consider only the most commonly used and easiest to implement. As discussed earlier, to shift the phase, it is enough to connect a capacitor in parallel with any of the windings. The designation C p indicates that this is a working capacitor.

It should be noted that connecting the windings in a triangle is preferable, since more useful power can be “removed” from such an ADKZ than from a star. But there are motors designed to operate in networks with a voltage of 127/220 V. There must be information about this on the nameplate.

If readers come across such an engine, then this can be considered good luck, since it can be connected to a 220 V network using a star circuit, and this will ensure a smooth start and up to 90% of the nameplate rated power. The industry produces ADKZs specially designed for operation in 220 V networks, which can be called capacitor motors.

Whatever you call the engine, it’s still asynchronous with a squirrel-cage rotor

It should be noted that the nameplate indicates an operating voltage of 220 V and parameters of the operating capacitor of 90 μF (microfarad, 1 μF = 10 -6 F) and a voltage of 250 V. It is safe to say that this motor is actually three-phase, but adapted for single-phase voltage.

To facilitate the start-up of powerful ADSCs in 220 V networks, in addition to the working capacitor, they also use a starting capacitor, which is turned on for a short time. After the start and a set of rated speeds, the starting capacitor is turned off, and only the working capacitor supports rotor rotation.

The starting capacitor “gives a kick” when the engine starts

The starting capacitor is C p, connected in parallel to the working capacitor C p. It is known from electrical engineering that when connected in parallel, the capacitances of the capacitors add up. To “activate” it, use the SB push-button switch, held down for several seconds. The capacity of the starting capacitor is usually at least two and a half times higher than that of the working capacitor, and it can retain its charge for quite a long time. If you accidentally touch its terminals, you can get a fairly noticeable discharge through the body. In order to discharge C p, a resistor connected in parallel is used. Then, after disconnecting the starting capacitor from the network, it will be discharged through a resistor. It is selected with a sufficiently high resistance of 300 kOhm-1 mOhm and a power dissipation of at least 2 W.

Calculation of the capacity of the working and starting capacitor

For reliable start-up and stable operation of the ADKZ in 220 V networks, you should most accurately select the capacitances of the working and starting capacitors. If the capacitance C p is insufficient, insufficient torque will be created on the rotor to connect any mechanical load, and excess capacitance can lead to the flow of too high currents, which can result in an interturn short circuit of the windings, which can only be “treated” by very expensive rewinding.

What is calculated	Formula	What is needed for calculations
Capacitance of the working capacitor for connecting star windings – Cp, µF	Cр=2800I/U; I=P/(√3Uηcosϕ); Cр=(2800/√3)P/(U^2n* cosϕ)=1616.6P/(U^2n* cosϕ)	For everyone: I – current in amperes, A; U – network voltage, V; P – electric motor power; η – engine efficiency expressed in values from 0 to 1 (if it is indicated on the engine nameplate as a percentage, then this indicator must be divided by 100); cosϕ – power factor (cosine of the angle between the voltage and current vector), it is always indicated in the passport and on the nameplate.
Capacity of the starting capacitor for connecting star windings – Cp, µF	Cп=(2-3)Cр≈2.5Ср
Capacitance of the working capacitor for connecting the windings in a triangle – Cp, µF	Cр=4800I/U; I=P/(√3Uηcosϕ); Cр=(4800/√3)P/(U^2n* cosϕ)=2771.3P/(U^2n* cosϕ)
Capacity of the starting capacitor for connecting windings in a triangle – Cn, µF	Cп=(2-3)Cр≈2.5Ср

The formulas given in the table are quite sufficient to calculate the required capacitor capacity. Passports and nameplates may indicate efficiency or operating current. Depending on this, you can calculate the necessary parameters. In any case, that data will be enough. For the convenience of our readers, you can use a calculator that will quickly calculate the required working and starting capacity.

Households often require means of mechanization. A homemade machine, a water pump, equipment for a small business... you never know what you might need a good electric motor for! However, the problem is that industrial electric motors are designed to operate in a three-phase network (380 V).

While in residential buildings and apartments the network is single-phase, or 220 V. But there is a solution! Let's look at how to make an industrial motor work from a household network.

Differences between a single-phase motor and a three-phase motor

In a three-phase motor, rotation of the rotor causes a magnetic field, which is induced in the stator by the alternating voltage of each of the three phases relative to each other. This ensures engine efficiency. The engine speed remains the same with single-phase and three-phase connections, but the power with single-phase is significantly reduced.

In this case, we will receive no more than 70% of the rated power from the engine. To achieve the best possible result, the motor windings must be connected in a triangle. If the connection is made as a star, then the maximum power (even theoretically) will be no more than 50% of the rated one. To clarify the method of connecting the windings (if you find it difficult to distinguish a “star” from a “triangle”), it is recommended to view additional information.

Since a three-phase motor has three outputs, the neutral and phase wires are connected to two of them, and the third is connected through a capacitor. In this case, the direction of rotation will depend on how the capacitor is connected - to the zero or phase terminals.

Connection diagrams for three-phase 220 volt motors

If the engine is low-power (less than 1.5 kW), and the connection occurs without a load, then for successful operation it is enough to simply connect a capacitor to the circuit. For example, solder one terminal to the input of the neutral wire, and the other to the free end of the winding, or the third terminal of the triangle. If the direction of rotation is not satisfactory, then you simply need to attach the second terminal of the capacitor to the input of the phase wire.

To start a loaded or powerful engine, a more powerful “push” is required, which can be provided by an additional (starting) capacitor. It is soldered into the circuit parallel to the main one, but it does not work constantly, but only for a few seconds, while the engine starts. It is usually connected via a button or two-position toggle switch. To start, you need to press the button (turn on the toggle switch) until the engine starts and picks up speed. Then the button is released, breaking the network and turning off the capacity.

The engine can be made to operate in direct and reverse modes. To do this, a toggle switch is added to the connection diagram, which in one position connects the capacitor to the neutral wire, and in the other to the phase wire. In a reversible circuit, if the engine starts slowly or does not start at all, a starting capacitor can also be added. It is connected in the same way in parallel to the main one and is turned on with the “Start” button.

You can often hear the question: is it possible, in principle, to start a three-phase motor without a capacitor? Unfortunately, this cannot be done. This way you can only start a motor that was originally designed to work with a single-phase 220 V network.

Selection of capacitor capacity

The operating voltage of the capacitor must be at least 300 V. Capacitors of the BGT, MBChG, MBPG and MBGO brands are best suited for the circuit. All data (type, Urab, capacity) are indicated on the case.

To calculate the required capacity, use the formula:

for triangle connection C = (I/U)x4800;
for star connection C = (I/U)x2800.

Where C is the capacitance of the capacitor in microfarads (uF), I is the rated current in the windings (according to the passport), U is the supply voltage (220 V), and the numbers are coefficients for different types of winding connections.

As for starting capacitors, their capacitance must be selected through experiment. Usually it is 2-3 of the working nominal value.

Let's give an example of calculation

The connection is a triangle. The rated current consumed is 3 A. Substituting the values into the formula, we get C = (3/220) x 4800 = 65 μF. In this case, the capacitance of the starting capacitor must be selected within the range of 130-180 μF. However, there are no 65 µF capacitors on sale, so we assemble a set of 6 pieces. 10 µF each and add another one - 5 µF.

It should be taken into account that the calculation used data for rated power. If the engine runs underload, it will overheat. In this case, it is necessary to reduce the capacitance of the capacitors in order to reduce the current in the winding. But as the capacity decreases, the power that the engine can develop will also decrease.