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Question about single phase to three phase conversion

by Bararontok
Tags: conversion, phase, single
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Bararontok
#19
Apr13-12, 10:04 AM
P: 298
There are some circuit diagrams here for the open delta converter:

http://www.elec-toolbox.com/images/fig1-8.gif

http://2.bp.blogspot.com/_vfmOyxDCru..._Schematic.png

http://img.photobucket.com/albums/v2...Kconverter.gif

Sources:

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

http://www.circuitlab.org/2011/12/sc...er-miller.html

http://www.practicalmachinist.com/vb...ingdom-102960/

Are these the correct schematics for such converters?

****

Additionally is this diagram for 3-phase to 1-phase 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 3-phase power supply, the output inductor can be connected to a rectifier which will then be connected to an inverter to produce a 1-phase output.
Averagesupernova
#20
Apr13-12, 12:22 PM
P: 2,497
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.
Bararontok
#21
Apr13-12, 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 3-phase supply to avoid overloading and enable all the power from the 3 phases to be converted into a combined 1-phase output but it is correct that a pair of wires can be connected to a single pair of terminals from a 3-phase circuit and 1/3 of the power can be used as the 1-phase output. The 3-phase to 1-phase converter circuit diagram was made by the thread originator purely for curiosity's sake and not for any purpose.
Averagesupernova
#22
Apr13-12, 04:35 PM
P: 2,497
My point is that you cannot just throw 3 windings, one from each phase, on the same core. They will fight each other.
jim hardy
#23
Apr13-12, 05:27 PM
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Quote Quote by Bararontok View Post

Are these the correct schematics for such converters?

****
The first one is what i was describing.

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 step-up transformer. Works quite well for running motors. i've done it myself. Draw the phasor diagram and you'll see it.
NascentOxygen
#24
Apr13-12, 07:59 PM
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Quote Quote by Bararontok View Post
Additionally is this diagram for 3-phase to 1-phase 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 3-phase power supply, the output inductor can be connected to a rectifier which will then be connected to an inverter to produce a 1-phase output.
It represents in principle a method to convert from 3ϕ to single phase: you rectify the 3ϕ and use that DC to power an inverter. But I would need to examine his "transformer" arrangement very closely (at a low testing voltage) before anything else, as it is unusual. You might consider using an AC→DC→AC inverter like this if your need was for a single phase frequency that was different from that of the mains 3ϕ, or if it were essential that you must load all 3 phases equally (this could be a power authority condition). Otherwise, why not just connect your single phase motor (or its associated transformer) between any two of the phases from your 3ϕ supply?
SunnyBoyNY
#25
Apr13-12, 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 H-bridge. 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 back-to-back 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://techno-fandom.org/~hobbit/car...g/800/m112.jpg

Single phase inverter (bottom part): http://solar.smps.us/grid-tie-inverter-schematic.png

And here are two back-to-back VSC systems: http://ars.sciencedirect.com/content...006550-gr1.jpg

Source: power electronics engineer
Bararontok
#26
Apr14-12, 06:18 AM
P: 298
As a potential system for 1-phase to 3-phase 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:

NascentOxygen
#27
Apr14-12, 07:39 AM
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Quote Quote by Bararontok View Post
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?
This is a thought experiment, is it? You want to take one of the phases from 3ɸ and mess with its angle? Sure you can do that. Simply adding a capacitor to the phase from the mains won't do it, but if you use a transformer-inductor and capacitor as illustrated in earlier schemes, you could change the angle. It probably wouldn't have any practical use, as far as I can see -- I presume 120 is the all round optimum for rotating machinery.
Bararontok
#28
Apr14-12, 10:10 AM
P: 298
Quote Quote by NascentOxygen View Post
This is a thought experiment, is it? You want to take one of the phases from 3ɸ and mess with its angle? Sure you can do that. Simply adding a capacitor to the phase from the mains won't do it, but if you use a transformer-inductor and capacitor as illustrated in earlier schemes, you could change the angle. It probably wouldn't have any practical use, as far as I can see -- I presume 120 is the all round optimum for rotating machinery.
No, the intention was to use the device to convert 1-phase to 3-phase current. The supply would be split into 3 sets of terminals, 1 with no capacitor and the other 2 with capacitors of different ratings to give each one a different phase angle.
SunnyBoyNY
#29
Apr14-12, 10:15 AM
P: 72
Quote Quote by Bararontok View Post
No, the intention was to use the device to convert 1-phase to 3-phase current. The supply would be split into 3 sets of terminals, 1 with no capacitor and the other 2 with capacitors of different ratings to give each one a different phase angle.
Do not forget that virtually all three phase machines - PM, induction motors and VRM have their torque dependent on the current.

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 full-bridge rectifier to get 1ph->DC->3ph ac variable speed?
Bararontok
#30
Apr14-12, 11:14 AM
P: 298
Quote Quote by SunnyBoyNY View Post
Do not forget that virtually all three phase machines - PM, induction motors and VRM have their torque dependent on the current.

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 full-bridge rectifier to get 1ph->DC->3ph ac variable speed?
But each of the two capacitors will have differing values, is that not enough to produce three phase angles?
SunnyBoyNY
#31
Apr14-12, 11:20 AM
P: 72
Quote Quote by Bararontok View Post
But each of the two capacitors will have differing values, is that not enough to produce three phase angles?
First, if the two capacitors are connected to a DC link (which is a voltage source) then they are slaved to it. Voltage on them would not have any phase shift. There would need to be an inductive element between the voltage source and these capacitors to produce phase shift.

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 non-zero 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.
Bararontok
#32
Apr15-12, 02:22 AM
P: 298
Quote Quote by SunnyBoyNY View Post
First, if the two capacitors are connected to a DC link (which is a voltage source) then they are slaved to it. Voltage on them would not have any phase shift. There would need to be an inductive element between the voltage source and these capacitors to produce phase shift.

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 non-zero 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.
The source is a 1-phase AC source, not a DC source.
NascentOxygen
#33
Apr15-12, 06:08 AM
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Quote Quote by Bararontok View Post
No, the intention was to use the device to convert 1-phase to 3-phase current. The supply would be split into 3 sets of terminals, 1 with no capacitor and the other 2 with capacitors of different ratings to give each one a different phase angle.
A capacitor in series with the mains is a dodgy arrangement that allows a motor to see a phase with a leading angle, but that won't be anywhere near 120 as far as I can see, though I'm no expert. It relies on a characteristic of the motor winding. If you use a different motor, you'll need a different capacitor or performance will suffer even more.

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 thimble-sized 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.
Bararontok
#34
Apr15-12, 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.
NascentOxygen
#35
Apr15-12, 07:24 AM
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Quote Quote by Bararontok View Post
The capacitors of the design are in parallel to the load.
If the mains see the capacitors in parallel with the load then those capacitors can do nothing more than PF correction. The 1ɸ mains has to see them in series with something to achieve a phase lead.
SunnyBoyNY
#36
Apr15-12, 09:26 AM
P: 72
Quote Quote by Bararontok View Post
The source is a 1-phase AC source, not a DC source.
I assume you will rectify the single phase AC source to DC and put an energy buffer to the link as 1 phase connection is not capable of delivering continuous power as the three phases are.

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 1-phase connection with unity power factor, the available power is:

Pin = V*sin(wt)*I*sin(wt)=VI*sin^2(wt)=VI/2*(1-cos(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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