Three-phase motor in single-phase network. Wiring plan for three-phase motor

There are situations in life when it is necessary to include some industrial equipment in the usual home power supply network. There is also a problem with the number of wires. For machines intended for operation at enterprises, there are usually three conclusions, and sometimes four. What to do with them, where to connect them? Those who tried to try different options, were convinced that the engines just do not want to spin. Is a single-phase connection of a three-phase motor possible? Yes, you can achieve rotation. Unfortunately, in this case the power drop is almost inevitable, but in some situations this is the only way out.

The voltages of the three-phase network and their ratio

In order to understand how to connect a three-phase motor to an ordinary outlet, you need to understand how the voltage in the industrial network correlates. Well-known values of voltages - 220 and 380 volts. Previously, there were 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 calculate.

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

Three-phase motor unit

Not all types of industrial motors can operate from one phase. The most common of these are "workhorses", making up the majority of electric machines in any enterprise - asynchronous machines with a capacity of 1 - 1.5 kVA. How does this three-phase motor work in a three-phase network, for which it is designed?

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

The windings are located at an angle of 120 °, so the rotating field created by each phase sequentially pushes each magnetized side of the rotor.

A triangle or a star?

A three-phase motor in a three-phase network can be switched in two ways - with or without a neutral wire. The first method is called a "star", in this case each of the windings is under phase voltage (between phase and zero) equal to 220 V under our conditions. The connection scheme for a three-phase "triangle" motor assumes a series connection of three windings and a linear (380 V) Voltage to the switching nodes. In the second case, the engine will give out about a half times the power.

How do I turn the motor on in the opposite direction?

The control of the three-phase motor can assume the need to change the direction of rotation to the opposite, that is, the reverse. To achieve this, you just need to swap two wires out of three.

For the convenience of changing the circuit in the motor terminal box, bridges are provided, usually made of copper. To turn on the "star" gently connect the three output wires of the windings together. "Triangle" is a little more difficult, but with it will cope with any electrician of medium qualification.

Phase-shifting capacitances

So, sometimes the question arises as to how to connect a three-phase motor to a normal home socket. If you just try to connect two wires to the plug, it will not spin. In order for the matter to go, you need to simulate the phase by shifting the applied voltage to some angle (preferably 120 °). This effect can be achieved by applying a phase-shifting element. Theoretically, this can be inductance, and even resistance, but more often a three-phase motor in a single-phase network is switched on using electric capacitors (capacitors), denoted on schemes with the Latin letter C.

As for the applications of throttles, it is difficult because of the difficulty in determining their value (if not indicated on the body of the device). To measure the value of L, you need a special instrument or a circuit assembled for this purpose. In addition, the choice of available chokes is usually limited. However, any phase-shifting element can be selected experimentally, but this is a troublesome business.

What happens when the engine is turned on? One of the junction points is fed zero, the other - the phase, and the third - a certain voltage shifted by some angle relative to the phase. It is also clear to the layman that the engine's operation will not be complete with respect to the mechanical power on the shaft, but in some cases the fact of rotation itself suffices. However, even at start-up, there may be some problems, for example, the lack of an initial torque that can move the rotor from its place. What to do in this case?

Starting capacitor

At the moment of starting the shaft, additional efforts are required to overcome the forces of inertia and friction of rest. To increase the torque, an additional capacitor must be installed, which is connected to the circuit only at the moment of start, and then disconnected. For these purposes, the best option is to use the closing button without fixing the position. The connection scheme of the three-phase motor with the starting capacitor is given below, it is simple and understandable. When the voltage is applied, press the "Start" button and the starting capacitor will create an additional phase shift. After the engine is untwisted to the required speed, the button can (and even need) to release, and only the working capacity will remain in the circuit.

Calculation of capacitance values

So, we found out that in order to include a three-phase motor in a single-phase network, an additional connection scheme is required, which, in addition to the start button, includes two capacitors. Their value needs to be known, otherwise the system will not work. First, we will determine the amount of electrical capacity necessary to make the rotor move from its place. With parallel inclusion, it is the sum:

C = Cm + Cp, where:

With st - start additional shut off after the take-up capacity;

C p is a working capacitor providing rotation.

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

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

U - voltage, when connected with a "star" - 220 V, and if the "triangle", then 380 V;

P is the power of a three-phase motor, sometimes in the event of a loss of the plate is determined by eye.

So, the dependencies of the required working power are calculated by the formulas:

With p = Cp = 2800 I n / U - for the "star";

With p = 4800 I n / U - for the "triangle";

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

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

Why do I need a fit

The calculation method given above is approximate. First, the nominal value indicated on the body of the electrical capacitance may differ significantly from the actual value. Secondly, paper capacitors (generally speaking, not cheap) are often used in the past, and they, like all other objects, are subject to aging, which leads to an even greater deviation from this parameter. Thirdly, the current that will be consumed by the engine depends on the magnitude of the mechanical load on the shaft, and therefore it can only be estimated experimentally. How to do it?

It will take a little patience. As a result, a rather large set of capacitors connected in parallel and in series can be obtained. The main thing is - after the end of the work, everything must be properly fixed, so that the soldered ends do not fall off from the vibrations emanating from the motor. And then it will not be superfluous to analyze the result again and, perhaps, to simplify the design.

Drawing up the battery capacity

If the master does not have special electrolytic ticks, which allows measuring the current without breaking the circuits, then connect the ammeter in series to each wire that enters the three-phase motor. In a single-phase network, the total value will flow, and the selection of capacitors should aim at the most uniform loading of the windings. It should be remembered that with a serial connection, the total capacity decreases according to the law:

1 / С = 1 / С1 + 1 / С2 ... and so on, and at parallel - on the contrary, it adds up.

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

Discharge resistor

The scheme of a three-phase motor, included between one phase and a neutral wire, is sometimes supplemented by resistance. It serves to ensure that the starting capacitor does not accumulate a charge remaining after the machine has already been turned off. This energy can cause an electric shock, not dangerous, but extremely unpleasant. In order to protect themselves, it is necessary to connect a resistor in parallel with the starting capacitance (for electricians this is called "shunting"). The magnitude of its resistance is large - from half a megohm to a megohm, and its size is small, so it's pretty and half a wattage. However, if the user is not afraid of being "pinched", then without this part it is quite possible to manage.

Use of electrolytes

As already mentioned, film or paper electrical capacitances are expensive, and they are not so easy to acquire as we would like. One-phase connection of a three-phase motor can be made using inexpensive and accessible electrolytic capacitors. At the same time, they too will not be cheap, since they must withstand 300 V DC. For safety, they should be shunted by semiconductor diodes (D 245 or D 248, for example), but it's worth remembering that when these devices break through, an alternating voltage will fall on the electrolyte, and it will first heat up very much and then explode loudly and effectively. Therefore, without extreme necessity, it is better to use paper-type capacitors that work under voltage even though they are constant, even variable. Some masters fully admit the use of electrolytes in the starting circuits. Due to the short-term action of alternating voltage on them, they may not be able to explode. It is better not to experiment.

If there are no capacitors

Where do ordinary citizens who do not have access to the best-selling electric and electronic parts acquire them? On flea markets and flea markets. There they lie carefully carved by someone's (usually elderly) hands from old washing machines, televisions and other out-of-home appliances and building household and industrial equipment. Many people are asking for these Soviet-made products: sellers know that if the item is needed, they will buy it, and if not, they will not take it for free. It happens that just the most necessary (in this case, the condenser) is just not there. And what should I do? No problem! The resistors will come down, but they need powerful, preferably ceramic and vitrified ones. Of course, the ideal resistance (active) phase does not shift, but in this world there is nothing ideal, and in our case it is good. Each physical body has its own inductance, electrical power and resistivity, whether it's a tiny speck of dust or a huge mountain. The inclusion of a three-phase motor in an outlet becomes possible if, on the above diagrams, the capacitor is replaced by a resistance whose value is calculated by the formula:

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

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

U is our faithful 220 volts.

Which engines are suitable?

Before buying a motor for a considerable amount of money, which the diligent owner intends to use as a drive for a grinding wheel, a circular saw, a drilling machine, or some other useful home appliance, it will not hurt to think about its applicability for these purposes. Not every three-phase motor in a single-phase network can work at all. For example, the MA series (it has a short-circuited rotor with a double cage) should be excluded so that it does not have to drag home a considerable and useless weight. In general, it is best to first experiment or invite an experienced person, an electrician, for example, and consult with him before buying. It is quite suitable asynchronous motor three-phase series UAD, APN, AO2, AO and, of course, A. These indices are indicated on the factory plates.