Hi all! Today I will show you how to get from a conventional single-phase 220 V network - three-phase, and without special costs. But first, I’ll tell you about my problem preceding the search for such a solution.
I had a Soviet powerful table saw (2 kW), which was connected to a three-phase network. My attempts to power it from a single-phase network, as is usually accepted, did not seem possible: there was a strong power drawdown, the starting capacitors were heated, the engine itself was heated.
Luckily, at one time I spent due time searching for a solution on the Internet. Where I came across one video where one guy made a kind of splitter with a powerful electric motor. Then he launched this three-phase network around the perimeter of his garage and connected all other devices requiring three-phase voltage to it. Before starting work, he came to the garage, started the distributing engine, and it worked until he left. In principle, I liked the solution.
I decided to repeat and make my splitter. As an engine, I took an old Soviet one with 3.5 kW of power, with windings turned on by a star.
Scheme
The whole circuit consists of only a few elements: a general mains switch, a start button, a 100 microfarad capacitor and a powerful motor itself.
How does everything work? First, we supply single-phase power to the distributing motor, connect the capacitor with the start button, thereby starting it. As soon as the motor spins up to desired rpm, the capacitor can be turned off. Now you can connect a load to the output of the phase splitter, in my case a desktop circular and a few more three-phase loads.
The body of the device - the frame is made of L-shaped corners, all equipment is fixed on a piece of OSB sheet. From above, the handles for carrying the entire structure have been redone, and a three-pin socket is connected to the output.
After connecting the saw through such a device, a significant improvement in work turned out, nothing gets hot, there is enough power and not only for the saw. Nothing growls, does not buzz, as it was before.
It is only desirable to take a distributing motor more powerful than consumers by at least 1 kW, then there will be no noticeable power drawdown under a sharp load.
Whoever says anything about not pure sine or it will not work, I advise you not to listen to them. The voltage sine is clean and broken exactly at 120 degrees, as a result, the connected equipment receives high-quality voltage, which is why it does not heat up.
The second half of the readers who will talk about the 21st century and the great availability frequency converters three-phase voltage, I can say that my output is several times cheaper, since the old motor is quite easy to find. You can even take unsuitable for the load, with weak and almost broken bearings.
My phase splitter in idle mode does not consume so much: 200 - 400 W somewhere, the power of the connected tools grows many times over, compared to the usual connection scheme through starting capacitors.
In conclusion, I want to justify my choice of this solution: reliability, incredible simplicity, low costs, high power.
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Three-phase electric motors in everyday life and amateur practice actuate a variety of mechanisms - circular saw, electric planer, fan, drilling machine, pump. The most commonly used are three-phase asynchronous motors with a squirrel-cage rotor. Unfortunately, a three-phase network in everyday life is an extremely rare phenomenon, therefore, to power them from a conventional electrical network amateurs use:
♦ phase-shifting capacitor, which does not allow to fully realize the power and starting characteristics of the engine;
♦ trinistor "phase-shifting" devices, which further reduce the power on the motor shaft;
♦ various other capacitive or inductive-capacitive phase-shifting circuits.
But the best thing is to get a three-phase voltage from a single-phase one using an electric motor that acts as a generator. Consider circuits that allow, having a single-phase alternating voltage, to obtain two missing phases.
Note.
Any electrical machine is reversible: a generator can serve as an engine, and vice versa.
Rotor conventional asynchronous motor after an accidental disconnection of one of the windings, it continues to rotate, and there is an EMF between the terminals of the disconnected winding. This phenomenon makes it possible to use a three-phase asynchronous electric motor to convert a single-phase voltage into a three-phase one.
Scheme No. 1. For example, a conventional three-phase asynchronous electric motor with a squirrel-cage rotor was used for this by S. Gurov (village of Ilyinka, Rostov region). This engine, like the generator, has: a rotor; three stator windings shifted in space by an angle of 120°.
We apply single-phase voltage to one of the windings. The motor rotor will not be able to start rotating on its own. He needs some way to give the initial impetus. Further, it will rotate due to the interaction with the magnetic field of one stator winding.
Conclusion.
The magnetic flux of the rotating rotor will induce the induction EMF in the other two stator windings, i.e. the missing phases will be restored.
The rotor can be made to rotate, for example, using a device with a starting capacitor. By the way, its capacitance does not have to be large, since the rotor of an asynchronous converter is set in motion without a mechanical load on the shaft.
One of the disadvantages of such a converter is unequal phase voltages, which leads to a decrease in the efficiency of the converter itself and the motor-load.
If you supplement the device with an autotransformer of the appropriate power, turning it on, as shown in Fig. 1, it is possible to achieve an approximate equality of the phase voltages by switching the taps. The stator of a defective electric motor with a power of 17 kW was used as the magnetic circuit of the autotransformer. Winding - 400 turns of enameled wire with a cross section of 4-6 mm 2 with taps after every 40 turns.
Rice. 1. circuit diagram converter
It is better to use "low-speed" motors (up to 1000 rpm) as converter motors.
They are very easy to start, the ratio of starting current to operating current is much less than that of motors with a speed of 3000 rpm, and therefore, the load on the network is “softer”.
Rule.
The power of the motor used as a converter must be greater than that of the electric drive connected to it. Always start the converter first, and then connect the three-phase current consumers to it. Turn off the unit in reverse order.
For example, if the converter is a 4 kW motor, the load power should not exceed 3 kW. The 4 kW converter discussed above and manufactured by S. Gurov , has been used in his personal household for several years. A sawmill, a grain mill, a grinder work from it.
Schemes No. 2-4. Under the influence magnetic field stator in short-circuited rotor winding induction motor currents flow, turning the rotor into an electromagnet with pronounced poles, inducing a sinusoidal voltage in the stator windings, including those not connected to the network.
The phase shift between sinusoids in different windings depends only on the location of the latter on the stator and in a three-phase motor is exactly 120 °.
Note.
The main condition for the transformation of an asynchronous electric motor into a phase number converter is a rotating rotor.
Therefore, it should be untwisted beforehand, for example, using a conventional phase-shifting capacitor.
The capacitance of the capacitor is calculated by the formula:
C=k*I f /U network
where k \u003d 2800 if the motor windings are connected by a star; k \u003d 4800 if the motor windings are connected in a triangle; I f - rated phase current of the electric motor, A; U ce ti - voltage of a single-phase network, V.
You can use capacitors MBGO, MBGP, MBGT K42-4 for an operating voltage of at least 600 V or MBGCH K42-19 for a voltage of at least 250 V.
Note.
The capacitor is needed only to start the motor-generator, then its circuit is broken, and the rotor continues to rotate, so the capacitance of the phase-shifting capacitor does not affect the quality of the generated three-phase voltage.
A three-phase load can be connected to the stator windings. If it is not there, the energy of the supply network is spent only on overcoming friction in the rotor bearings (not counting the usual losses in copper and iron), so the efficiency of the converter is quite high.
As a converter of the number of phases, the author of the schemes V. Kleimenov tested several different electric motors. Those of them, the windings of which are connected by a star, with an output from a common point (neutral) were connected according to the circuit shown in Fig. 2. In the case of connecting the windings with a star without a neutral or a triangle, the circuits shown, respectively, in fig. 3 and fig. 4.
Rice. 2. Scheme of the converter, the motor windings in which are connected by a star, with a conclusion from a common point (neutral)
Rice. 3. Converter circuitmotor windings in which are connected by a star without a neutral
Rice. 4. Converter circuit; motor windings in which are connected by a triangle
In all cases, the engine, launched by pressing the button SB 1 and holding it for 15 C,until the rotor speed reaches the rated speed. Then close the switchSA1, and the button was released.
Schemes No. 5. Usually, the ends of the windings of an asynchronous three-phase electric motor are brought out to a three- or six-terminal block. If the block is three-terminal, then the phase stator windings are connected by a star or a triangle. If it is six-terminal, the phase windings are not connected to each other (Ya. Shatalov, Irba, Krasnoyarsk Territory).
In the latter case, it is important to connect them correctly. When turned on by a star, the winding terminals of the same name (beginning or end) should be combined into a zero point. In order to connect the windings with a triangle, you must:
♦ connect the end of the first winding with the beginning of the second one;
♦ the end of the second - with the beginning of the third;
♦ the end of the third - with the beginning of the first.
But what if the terminals of the motor windings are not marked?
Then proceed as follows. Three windings are determined with an ohmmeter, conventionally designating them I, II and III. To find the beginning and end of each of them, any two are connected in series and an alternating voltage of 6-36 V is applied to them. A voltmeter is connected to the third winding alternating current(Fig. 5).
Rice. 5. Wiring diagram of a voltmeter to determine the windings
Availability AC voltage indicates that the windings I and II are connected in accordance, and the absence of voltage is opposite. In the latter case, the conclusions of one of the windings should be reversed. After that, the beginning and end of the windings I and II are marked (the outputs of the windings I and II of the same name in Fig. 5 are marked with dots). To determine the beginning and end of the winding III, the windings are interchanged, for example, II and III, and the measurements are repeated according to the method described above.
