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Question about single phase to three phase conversion 
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#19
Apr1312, 10:04 AM

P: 298

There are some circuit diagrams here for the open delta converter:
http://www.electoolbox.com/images/fig18.gif http://2.bp.blogspot.com/_vfmOyxDCru..._Schematic.png http://img.photobucket.com/albums/v2...Kconverter.gif Sources: http://www.electoolbox.com/usefulinfo/xfmr3ph.htm http://www.circuitlab.org/2011/12/sc...ermiller.html http://www.practicalmachinist.com/vb...ingdom102960/ Are these the correct schematics for such converters? **** Additionally is this diagram for 3phase to 1phase conversion correct: The 3 power phases can be connected to inductors which will be placed in proximity to a larger inductor that is the same size as the sum of the volume of the 3 smaller inductors. The power will then be transferred through electromagnetic radiation and the inductors will thus serve as a transformer with all the energy being fed to one inductor. Because of the overlap of currents coming from the 3phase power supply, the output inductor can be connected to a rectifier which will then be connected to an inverter to produce a 1phase output. 


#20
Apr1312, 12:22 PM

P: 2,508

Bararontok, where did you get that schematic? I would like to see the smoke pour out of that transformer when hooked up. Incidentally, why is it needed to 'convert' from 3 phase to single phase? Just hook on to a pair of wires from a 3 phase source and you have your single phase. Unless you want to provide more power on the generated single phase than one phase of the original 3 can provide, I see no reason to have any kind of convertor.



#21
Apr1312, 01:01 PM

P: 298

Perhaps it is possible that the transformer can be designed with a wattage rating that is higher than the input power of the 3phase supply to avoid overloading and enable all the power from the 3 phases to be converted into a combined 1phase output but it is correct that a pair of wires can be connected to a single pair of terminals from a 3phase circuit and 1/3 of the power can be used as the 1phase output. The 3phase to 1phase converter circuit diagram was made by the thread originator purely for curiosity's sake and not for any purpose.



#22
Apr1312, 04:35 PM

P: 2,508

My point is that you cannot just throw 3 windings, one from each phase, on the same core. They will fight each other.



#23
Apr1312, 05:27 PM

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The rest are more complex devices that start with single phase and make a third phase by shifting angle with capacitors. Note the first one starts with three phases and ends with xame three phases. Point being, two vectors define three points and if those points are vertices of an equilateral triangle they are same geometry as three phase phasors. Second one appears to employ a motor , and that scheme actually works fairly well. My neighbor has one in his home workshop. It actually uses the motor as an induction generator for part of each cycle to make third phase. Third one is similar to second but with improvement of a stepup transformer. Works quite well for running motors. i've done it myself. Draw the phasor diagram and you'll see it. 


#24
Apr1312, 07:59 PM

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#25
Apr1312, 08:27 PM

P: 72

There are a few ways to convert power from a single phase AC to three phase AC. Nowadays, the most popular way is to use a single phase inverter/rectifier that converts the AC voltage and current waveforms into DC. The DC link voltage is usually regulated by the rectifier. A typical rectifier that is capable of bidirectional power flow is a simple Hbridge. A full bridge is also needed to obtain high power factor  the drawn current is also sinusoidal.
Now, we have the DC link to which we connect a three phase inverter. The simplest topology are three phase legs. The middle of each leg is connected to one phase of the utility via an inductor. It is not possible to connect two voltage sources without a magnetic element such as inductor. The three phase currents are regulated via current control algorithms by controlling the duty ratios of the three legs (PWM scheme). This topic has been rather mature for the past ten years. To obtain maximum efficiency and power factor, each phased current is a scaled version of its phase voltage. This way there is no reactive current in the system. These two systems are called backtoback AC/DC/AC system and it is a voltage sourced system. To convert power directly from AC to AC you would use a cycloconverter. These guys can handle high currents and voltages as they are mostly based on SCRs and triacs  current sourced converters. A picture of a three phase inverter is here: http://technofandom.org/~hobbit/car...g/800/m112.jpg Single phase inverter (bottom part): http://solar.smps.us/gridtieinverterschematic.png And here are two backtoback VSC systems: http://ars.sciencedirect.com/content...006550gr1.jpg Source: power electronics engineer 


#26
Apr1412, 06:18 AM

P: 298

As a potential system for 1phase to 3phase conversion, would it also be possible to use two capacitors to change the phase angle of the two other phase terminals where C2>C1 so that the third phase terminal will be out of phase to a greater degree than the second terminal? The diagram is below:



#27
Apr1412, 07:39 AM

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#28
Apr1412, 10:10 AM

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#29
Apr1412, 10:15 AM

P: 72

For example, if a PM machine has sinusoidal windings, you need to excite those windings with current that is exactly in the phase with the back EMF. Since the stator winding inductance is constant and your capacitors values are constant, the intended phase shift would work for one frequency only. Why don't you buy a flux drive and hook it up to a fullbridge rectifier to get 1ph>DC>3ph ac variable speed? 


#30
Apr1412, 11:14 AM

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#31
Apr1412, 11:20 AM

P: 72

Also, the phase shift would appear only at a certain frequency  so you would need to pull current at a certain frequency to have the two cap voltages phase shifted 120d to each other. At zero frequency the caps would resonate with their phase inductors. Provided nonzero resistance, it's a second order damped system. When the initial ringing dies out, both caps will have the same DC voltage as the source. 


#32
Apr1512, 02:22 AM

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#33
Apr1512, 06:08 AM

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You won't generate 120° lead just by adding a series capacitor to your single phase supply. It may be possible to generate 120° lead and lag using a more complex passive network, carefully designed, but it will fall apart as soon as you try to draw anything but miniscule current from it. You may be able to power a thimblesized miniature model 3ɸ motor as a demonstration, but nothing of any use, is my thinking. If it was as simple as you picture it, then there would be no need for the big, heavy duty complex circuits that are employed to do the task. 


#34
Apr1512, 06:47 AM

P: 298

The capacitors of the design are in parallel to the load. Though it may be correct that the capacitor would have to have a different value for different motors but the device can have a wattage rating label placed on it to ensure that the supply is not overloaded since other types of power supplies already have these labels anyway.



#35
Apr1512, 07:24 AM

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#36
Apr1512, 09:26 AM

P: 72

Simply, if you motor takes 3 kW, then it is 3 kW continuous. Torque is dependent on the rotor/stator flux linkage and stator current. With sinusoidal windings the equations will look like this: [itex] Va = X*sin(ωt)\\ Vb = X*sin(ωt+\frac{2 Pi}{3})\\ Vc = X*sin(ωt\frac{2 Pi}{3})\\ [/itex] Current are: [itex] Ia = Y*sin(ωt)\\ Ib = Y*sin(ωt+\frac{2 Pi}{3})\\ Ic = Y*sin(ωt\frac{2 Pi}{3})\\ [/itex] Thus the power to the rotor is: [itex] Pr = Va*Ia+Vb*Ib+Vc*Ic = \frac{3}{2}XY [/itex] As you can see the rotor power is not dependent on phase. On the other hand, provided that you want to pull power from a 1phase connection with unity power factor, the available power is: Pin = V*sin(wt)*I*sin(wt)=VI*sin^2(wt)=VI/2*(1cos(2wt)). Therefore you need an energy buffer. I would strongly suggest you do some reading on the topic. Excellent books are: Principles of Power Electronics by Kassakian, Schlecht, Vergese from MIT Fundamentals of Power Electronics by Erickson and Maksimovic from Colorado Ac Electric Machines and Their Control by Torrey from Union 


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