Single-phase to three-phase converter. One phase to three phase converter
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Three-phase electric motors in everyday life and in amateur practice drive the most various 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, so to power them from an ordinary electrical network amateurs use:
♦ phase-shifting capacitor, which does not allow the full realization of 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 way is to obtain three-phase voltage from single-phase using an electric motor that acts as a generator. Let's consider circuits that allow, having a single-phase alternating voltage, to obtain two missing phases.
Note.
Any electric machine is reversible: a generator can serve as a motor, and vice versa.
Conventional rotor asynchronous electric 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 single-phase voltage into three-phase.
Scheme No. 1. For example, a conventional three-phase asynchronous electric motor with a squirrel-cage rotor was used for this by S. Gurov (Ilyinka village, Rostov region). This engine, like the generator, has: a rotor; three stator windings, shifted in space by an angle of 120°.
Let's apply single-phase voltage to one of the windings. The engine rotor will not be able to start rotating on its own. He needs to be given an initial impetus in some way. Next, it will rotate due to interaction with the magnetic field of one stator winding.
Conclusion.
The magnetic flux of the rotating rotor will induce an induced 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 capacity does not have to be large, since the rotor of an asynchronous converter is driven without 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 load motor.
If you supplement the device with an autotransformer of appropriate power, turning it on as shown in Fig. 1, you can achieve approximate equality of phase voltages by switching taps. The stator of a faulty 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. Schematic diagram of the converter
It is better to use “low-speed” motors (up to 1000 rpm) as electric motors for converters.
They start very easily, the ratio of starting current to operating current is much lower than that of engines with a rotation 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. The converter should always be started first, and then three-phase current consumers should be connected 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. It powers a sawmill, a grinder, and a grinding machine.
Schemes No. 2-4. Under the influence magnetic field stator, currents flow in the short-circuited rotor winding of an asynchronous motor, turning the rotor into an electromagnet with salient 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 turning an asynchronous electric motor into a phase number converter is a rotating rotor.
Therefore, it should be pre-unwinded, for example, using a conventional phase-shifting capacitor.
The capacitance of the capacitor is calculated using the formula:
C=k*I f /U network
where k = 2800 if the motor windings are star connected; k = 4800 if the motor windings are connected by a triangle; I f - rated phase current of the electric motor, A; U ce ti - single-phase network voltage, V.
You can use capacitors MBGO, MBGP, MBGT K42-4 on operating voltage not less than 600 V or MBGCH K42-19 for a voltage of not less than 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 capacity 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.
The author of the circuits, V. Kleimenov, tested several different electric motors as phase number converters. Those of them, the windings of which are connected by a star, with the output from a common point (neutral) were connected according to the diagram 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. Diagram of a converter in which the motor windings are connected by a star, with output from a common point (neutral)
Rice. 3. Converter circuitthe windings of the motor in which are connected by a star without a neutral
Rice. 4. Converter circuit; the windings of the motor in which are connected by a delta
In all cases the engine, started by pressing the button S.B. 1 and holding it for 15 C,until the rotor speed reaches the rated speed. Then the switch was closedS.A.1, and the button was released.
Schemes No. 5. Typically, the ends of the windings of an asynchronous three-phase electric motor are connected to a three- or six-terminal block. If the block is three-terminal, it means that the phase stator windings are connected in a star or triangle. If it is six-terminal, the phase windings are not connected to each other (Ya. Shatalov, Irba village, Krasnoyarsk Territory).
In the latter case, it is important to connect them correctly. When switched on by a star, the terminals of the windings 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 to the beginning of the second;
♦ 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. An ohmmeter is used to determine three windings, 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. An alternating current voltmeter is connected to the third winding (Fig. 5).
Rice. 5. Connection diagram for a voltmeter to determine the windings
The presence of alternating voltage indicates that windings I and II are switched on in agreement, and the absence of voltage indicates that windings are switched on in opposition. In the latter case, the terminals of one of the windings should be swapped. After this, mark the beginning and end of windings I and II (the same terminals of windings I and II in Fig. 5 are marked with dots). To determine the beginning and end of winding III, the windings are swapped, for example, II and III, and the measurements are repeated using the method described above.
In a private house, apartment, country house, that is, in domestic conditions, the most common standard single-phase voltage is 220 Volts, which is obtained by connecting the consumer to one phase and a neutral conductor. This voltage is called phase voltage; its generator is mainly a 6 kV/380 V power transformer installed on distribution substation, feeding this consumer. Sometimes, especially in a private home, it becomes necessary to start and operate an asynchronous three-phase motor designed for 380 Volts. There are schemes that make it possible to connect this motor to a single-phase 220 V network, but in this case the power of the electrical asynchronous machine is greatly lost. Accordingly, the question arises of how to get 380 Volts from 220 at home, for efficient work electric motor.
What is important to know
In a three-phase network, all three phases have a shift of 120 degrees. If it were necessary to convert three-phase 220 Volts to 380V, or single-phase 220V to the same, but with a voltage of 380V, then this can be done very simply using a conventional step-up transformer. In this problem, it is necessary not just to increase the voltage, but to obtain a full-fledged three-phase network from a single-phase one.
There are three main ways in which this manipulation can be done:
- using an electronic converter (inverter);
- by connecting two additional phases;
- due to the use of a three-phase transformer, but the power is still reduced.
Before converting the mains voltage, you need to consider whether it is possible to connect the motor to a standard single-phase network without loss of power. First you need to look at the plate on the engine itself, some of them are designed for both of these voltages, as shown in the first photo. You just need a capacitor for starting.
The second plate shows that the machine is designed exclusively for star connection of windings and a voltage of 380 Volts:
You can, of course, disassemble the engine and find the ends of the windings, but this is already problematic. Let us dwell in more detail on the creation of a high-quality three-phase network of 380 V from 220.
Methods for obtaining 380 V from 220
Voltage transformer
This device is more widely known as an inverter, and it consists of several blocks. To begin with, the device rectifies this single-phase voltage, and then inverts it into an alternating voltage of a given frequency. In this case, there can be any number of phases shifted by a certain degree, but optimally for the operation of generally accepted standard electrical equipment it is three and, accordingly, their shift is 120 degrees. Making such a complex device at home is very problematic, so it is recommended to simply buy it; besides, the market for these products is very developed.
Here circuit diagram inverter:
And this is what it looks like in the factory case:
Often, these devices not only convert single-phase to three-phase voltage, but also protect electric motors from overloads, short circuits and overheating.
Three phase method
This method must be agreed with Energonadzor or the supplier company electrical energy, since this requires connecting two additional phases from the panel, which are on each floor of apartment buildings.
Here more question It’s not about how to convert single-phase voltage, but how to connect it, and for this you only need a three-phase extension cord, and if everything is done legally, then a meter.
Three-phase transformer
To turn 220 Volts into 380 Volts, you need a three-phase transformer of the required power for the voltage of one of the windings 220 V and the other 380 V. Most often, they already have windings connected in a star or triangle. After that, the voltage from the network is connected directly to two phases of the winding on the lower side, and to the third terminal through a capacitor. The capacitance of the capacitor is calculated from the ratio of 7 μF for every 100 W of power. The rated voltage of the capacitor must be at least 400 Volts. Such a device cannot be connected without a load. In this case, there will still be a decrease in both engine power and efficiency. If the converter is made using an electric motor rather than a transformer, then the output will have a three-phase voltage, but its value will be the same as in the network, namely 220 V.
Hi all! Today I will show you how to get three-phase from a regular single-phase 220 V network, and without special costs. But first, I’ll tell you about my problem that preceded the search for such a solution.
I had a powerful Soviet desktop a circular saw(2 kW), which was connected to a three-phase network. My attempts to power it from a single-phase network, as is usually customary, were not possible: there was a strong power drawdown, the starting capacitors got hot, and the engine itself got hot.
Fortunately, at one time I spent due time searching for a solution on the Internet. Where I came across a video where one guy made a kind of splitter using a powerful electric motor. Next, he launched this around the perimeter of his garage. three-phase network and connected to it all other devices requiring three-phase voltage. Before starting work, he came to the garage, started the dispensing engine and it worked until he left. In principle, I liked the solution.
I decided to repeat it and make my own splitter. As an engine, I took an old Soviet one with 3.5 kW of power, with star-connected windings.
Scheme
The whole scheme consists of just a few elements: general power switch, a start button, a 100 uF capacitor and a powerful motor itself.
How does it all work? First, we supply single-phase power to the distributing motor, connect the capacitor with the start button, thereby starting it. Once the motor has spun up to the desired speed, the capacitor can be turned off. Now you can connect a load to the output of the phase splitter, in my case a tabletop circular and several more three-phase loads.
The device body - the frame is made of L-shaped corners, all equipment is fixed to a piece OSB sheet. Handles for carrying the entire structure have been redesigned on top, and a three-pin socket is connected to the output.
After connecting the saw through such a device, there was a significant improvement in operation, nothing gets hot, there is enough power, and not only for the saw. Nothing growls or buzzes like it did before.
It is only advisable to take the distributing motor at least 1 kW more powerful than the consumers, then there will not be a noticeable drop in power under sudden load.
No matter who says anything about the sine not being pure or it will give nothing, I advise you not to listen to them. The voltage sine wave is clean and split exactly 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 speak in the 21st century and 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 one that is 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 increases significantly compared to the usual scheme connections via starting capacitors.
In conclusion, I would like to justify my choice of this solution: reliability, incredible simplicity, low costs, high power.
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Attention! Assembly of the device requires skills in the field of power electronics and involves contact with high voltage, which can be life-threatening for both the engineer and the users of the device. Make sure you have the required qualifications.
D5- an operational amplifier designed to operate with a single-supply 12V supply, with a high input impedance and the ability to connect a load of 2 kOhm or less to the output. K544UD1, KR544UD1 are well suited.
D6- integrated voltage stabilizer (KREN) for 12V.
VT5- Low-power high-voltage transistor at 600 volts. It only works when the circuit is turned on. So no power is dissipated during operation.
VD9- Zener diode 15V.
C11- 1000uF 25V.
R25- 300kOhm 0.5W
D1- Integrated pulse-width modulating (PWM) controllers. This is 1156EU3 or its imported analogue UC3823.
Addition from 02/27/2013 Foreign controller manufacturer Texas Instruments gave us a surprisingly pleasant surprise. UC3823A and UC3823B microcircuits appeared. These controllers have slightly different pin functions than the UC3823. They will not work in circuits for UC3823. Pin 11 has now acquired completely different functions. In order to use controllers with letter indices A and B in the described circuit, you need to double the resistors R22, exclude resistors R17 and R18, hang (not connect anywhere) legs 16 and 11 of all three microcircuits. As for Russian analogues, readers write to us that the wiring is different in different batches of microcircuits (which is especially nice), although we have not yet seen a new wiring.
D3- Half bridge drivers. IR2184
R7, R6- 10 kOhm resistors. C3, C4- 100nF capacitors.
R10, R11- Resistors of 20 kOhm. C5, C6- Electrolytic capacitors 30 µF, 25 volts.
R8- 20kOhm, R9- tuning resistor 15 kOhm
R1, R2- 10 kOhm trimmers
R3- 10 kOhm
C2, R5- a resistor and a capacitor that set the operating frequency of the PWM controllers. We select them so that the frequency is about 50 kHz. The selection should start with a 1 nF capacitor and a 100 kOhm resistor.
R4- These resistors in different arms are different. The fact is that to obtain a sinusoidal voltage with a phase shift of 120 degrees. a phase-shifting circuit is used. In addition to shifting, it also weakens the signal. Each link attenuates the signal by 2.7 times. So we select a resistor in the lower arm in the range from 10 kOhm to 100 kOhm so that the PWM controller minimum value sinusoidal voltage (from the output of the operational amplifier) was closed, with a slight increase it began to produce short pulses, and when the maximum was reached it was practically open. The resistor of the middle arm will be 9 times larger, the resistor of the upper arm will be 81 times larger.
After selecting these resistors, the gain can be more accurately adjusted using trimming resistors R1.
R17- 300 kOhm, R18- 30 kOhm
C8- 100nF. These can be low voltage capacitors. There is no high voltage on them, although they are located in the high-voltage part.
R22- 0.23 Ohm. 5W.
VD11- Schottky diodes. Schottky diodes are selected to provide minimal on-state voltage drop across the diode.
R23, R24- 20 Ohm. 1W.
L1- choke 10mH (1E-02 H), for current 5A, C12- 1uF, 400V.
L2 - several turns of thin wire on top of inductor L1. If inductor L1 has X turns, then coil L2 should have [ X] / [60 ]
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