There are situations in life when you need to include some kind of industrial equipment in a regular home power supply network. Immediately there is a problem with the number of wires. Machines designed for operation at enterprises have, as a rule, three conclusions, and sometimes four. What to do with them, where to connect them? Those who tried to try various options were convinced that the motors just did not want to spin. Is a single-phase connection of a three-phase motor possible at all? Yes, you can achieve rotation. Unfortunately, in this case, a power drop of almost two times is inevitable, but in some situations this is the only way out.
Three-phase network voltages and their ratio
In order to understand how to connect a three-phase motor to a conventional outlet, you should understand how the voltages in the industrial network relate. The well-known voltage values ​​are 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 to be easier to count round numbers.
Most industrial electrical equipment is designed to be connected to a three-phase AC network . The voltage of each phase with respect to the neutral wire is 220 volts, just like in a home outlet. 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 diagram of stresses. The length of the longest side will be equal to the length of the thigh times 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 on a single phase. The most common of these are “workhorses”, which make up the majority of electric machines in any enterprise - asynchronous machines with a capacity 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 supporter of the theory of a three-phase supply network, which in our time has become dominant. Three-phase asynchronous motor works by the principle of induction of currents from stator windings to closed rotor conductors. As a result of their flow through short-circuited windings, a magnetic field arises in each of them, interacting with the stator lines of force. This produces a torque leading to a circular motion of the axis of the engine.
The windings are located at an angle of 120 °, so the rotating field created by each of the phases successively pushes each magnetized side of the rotor.
A triangle or a star?
A three-phase motor in a three-phase network can be switched on in two ways - with or without neutral wire. The first method is called a “star”, in this case each of the windings is under a phase voltage (between phase and zero), which is equal to 220 V. voltage on switching nodes. In the second case, the engine will produce about one and a half times more power.
How to turn on the motor in the opposite direction?
Control of a three-phase motor may imply the need to change the direction of rotation to the opposite, that is, reverse. To achieve this, you just need to swap two of the three wires.
For the convenience of changing the circuit, jumpers are made in the terminal box of the engine, usually made of copper. To turn on the “star”, gently connect the three output wires of the windings together. The “Triangle” turns out to be a little more complicated, but any average-skilled electrician can handle it.
Phase shifting tanks
So, sometimes the question arises of 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 go, you need to simulate the phase by shifting the applied voltage by some angle (preferably 120 °). This effect can be achieved by applying a phase-shifting element. Theoretically, this can be both inductance and even resistance, but most often a three-phase motor in a single-phase network is switched on using electric capacitors (capacitors), indicated on the circuits with the Latin letter C.
As for the applications 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 is required. In addition, the choice of available chokes is usually limited. However, experimentally, any phase-shifting element can be selected, but this is a troublesome business.
What happens when you turn on the engine? Zero is applied to one of the connection points, a phase is supplied to the other, and a certain voltage is shifted to the third, shifted by a certain angle relative to the phase. It is clear to the layman that the engine will not be fully functional in terms of mechanical power on the shaft, but in some cases the fact of rotation is sufficient. However, even at start up some problems may arise, for example, the lack of an initial moment capable of moving the rotor from its place. What to do in this case?
Starting capacitor
At the time of starting, the shaft requires additional efforts to overcome the forces of inertia and static friction. To increase the torque, an additional capacitor should be installed, connected to the circuit only at the time of start, and then disconnected. For these purposes, the best option is to use a locking button without fixing the position. The connection diagram of a three-phase motor with a starting capacitor is shown below, it is simple and understandable. At the moment of voltage supply, press the “Start” button, and the starting capacitor will create an additional phase shift. After the engine spins up to the desired speed, the button can (and even need) to be released, and only the working capacity will remain in the circuit.
Calculation of capacity
So, we found out that in order to turn on a three-phase motor in a single-phase network, an additional connection diagram is required, which, in addition to the start button, includes two capacitors. You need to know their value, otherwise the system will not work. To begin with, we determine the amount of electric capacitance needed to make the rotor move. When connected in parallel, it is the sum of:
= + , where:
With st - the starting additional disconnectable after take-off capacity;
With p - 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 factory). This parameter can also be determined using a simple formula:
I n = P / (3 x U), where:
U - voltage, when connected by a "star" - 220 V, and if it is a "triangle", then 380 V;
P - power of a three-phase motor, it is sometimes determined in case of loss of the plate by eye.
So, the dependences of the required operating 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 2-3 times more than the working one. The unit of measure is microfarads.
There is a very simple way to calculate the capacitance: C = P / 10, but this formula gives the order of the number rather than its value. However, you have to tinker anyway.
Why fit?
The calculation method above is an estimate. Firstly, the nominal value indicated on the body of the electric capacitance may differ significantly from the actual one. Secondly, paper capacitors (generally speaking, an expensive thing) are often used, and they, like all other items, are subject to aging, which leads to an even greater deviation from this 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 estimated experimentally. How to do it?
It takes a little patience here. As a result, a rather large set of capacitors connected in parallel and in series can be obtained . The main thing is to fix everything well after the work is completed so that the soldered ends do not fall off from the vibrations emanating from the motor. And then it will not be superfluous to once again analyze the result and, possibly, simplify the design.
Capacity Battery Compilation
If the master does not have special electrolytic clamps at his disposal, which allows you to measure current without opening the circuits, you should connect the ammeter in series to each wire that enters the three-phase motor. The total value will flow in a single-phase network, and the selection of capacitors should strive for 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 / = 1 / 1 + 1 / 2 ... and so on, and with parallel - on the contrary, it develops.
Also, you must not forget about such an important parameter as the voltage for which the capacitor is designed. It should be not less than the nominal value of the network, but better with a margin.
Discharge resistor
The circuit of a three-phase motor connected between one phase and a neutral wire is sometimes supplemented by resistance. It serves to ensure that the starting capacitor does not accumulate charge remaining after the machine is already turned off. This energy can cause an electric shock, not dangerous, but extremely unpleasant. In order to protect yourself, you should connect a resistor in parallel with the starting capacitance (for electricians, this is called “bypass”). The magnitude of its resistance is large - from half a megohm to a megohm, and in size it is small, so half a watt power is enough. However, if the user is not afraid to be “pinched”, then without this detail it is quite possible to do.
The use of electrolytes
As already noted, film or paper electric containers are expensive, and to acquire them is not as easy as we would like. It is possible to make a single-phase connection of a three-phase motor using inexpensive and affordable electrolytic capacitors. At the same time, they will not be really cheap either, since they must withstand 300 volts of direct current. For safety, they should be shunted with semiconductor diodes (D 245 or D 248, for example), but it is not out of place to remember that when these devices are pierced, an alternating voltage will reach the electrolyte, and it will heat up first, and then explode, loudly and effectively. Therefore, unless absolutely necessary, it is better to use paper-type capacitors that operate under constant voltage, even variable voltage. Some masters quite allow the use of electrolytes in starting circuits. Due to the short-term exposure to alternating voltage, they may not have time to explode. 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 get them? At flea markets and flea markets. There they lie, carefully soldered by someone else's (usually elderly) hands from old washing machines, televisions, and other household and industrial equipment that has become obsolete. They are asking 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 (in this case, the capacitor) just does not exist. And what to do? No problem! Resistors also come down, only powerful, preferably ceramic and vitrified ones are needed. 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 resistance, whether it be a tiny speck of dust or a huge mountain. The inclusion of a three-phase motor in a socket becomes possible if the capacitor is replaced with a resistance in the above schemes, the nominal value of which is calculated by the formula:
R = (0.86 x U) / kI, where:
kI - current value at three-phase connection, A;
U - our faithful 220 Volts.
Which engines are suitable?
Before you buy for a considerable amount of money a motor that a zealous owner is going to use as a drive for an grinding wheel, circular saw, drill press or 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 squirrel-cage rotor with a double cage) should be excluded so that you do not have to drag a considerable and useless weight home. In general, it is best to first experiment or invite an experienced person, an electrician, for example, and consult with him before buying. A three-phase asynchronous motor of the UAD, APN, AO2, AO series and, of course, A is quite suitable. These indices are indicated on the name plates.