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#130254 - 02/27/06 04:42 PM Single Phase to Three Phase Conversion  
rhpope  Offline
Member
Joined: Feb 2006
Posts: 18
North Carolina
Would it be possible to convert single phase power to three phase power by simulating two phase power with single phase power and then inputing it into a Scott Tee transformer (or some variation of this) to convert it to three phase.

This would start with the basis that when you build a rotary three phase converter, you are simulating two phase power on the input side when capacitance is used between one (or both) of the incoming single phase lines to the third phase (producing a phase shift to start the idler motor) on the idler motor and then the idler motor produces another phase shift to get essentially a high leg 240 Delta three phase.

It is my understanding that some of the smaller transformers (dry type distribution ones) have only two sets of coils in them that have three phase input and 2 phase output. By using this type of transformer, one could produce three phase.

Can anyone explain the difference between two phase 3 wire and two phase 4 wire? As in this post of the two phase schematics: https://www.electrical-contractor.net/ubb/Forum15/HTML/000028.html

The 3 wire schematic and how a RPC is hooked up is where I am getting my ideas from.

This may be impossible. Just throwing around some ideas. Any opinions out there on this subject?


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#130255 - 02/27/06 10:07 PM Re: Single Phase to Three Phase Conversion  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
Wow, that sounds like alot of effort! In this day of hi-power IGBTs, I would just go with a single phase rectifier feeding a 3 phase inverter or VFD.
Joe


#130256 - 03/03/06 07:52 AM Re: Single Phase to Three Phase Conversion  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Possible? Yes.

Practical? No.

In order to get polyphase power, you need to store energy from the single phase source and then release it 'delayed' appropriately. You could certainly do this with a capacitor setup, but the capacitance would need to change as the load changes, and with any sort of resistive load you wouldn't be able to get a full 90 degree phase shift.

Once you had your two phases, a suitable transformer arrangement could generate any number of phases.

You will get better results with the rotary phase converter, where the energy gets stored in the inertia of the rotor, and the phase angles are set by the physical positions of the converter coils.

Two phase three wire has two phase legs and a single common conductor. Two phase four wire has _four_ phase legs, arranged in pairs.

Remember that voltage is _always_ measured between two points, eg two wires. A single point does not have voltage except by reference to another point. In AC circuits, the voltage between these two points is always changing, and you can graph voltage versus time. With a single pair of wires, you don't have phase defined. But when you have more than a single pair of wires you can graph voltage versus time for both pairs, and then describe the 'phase angle' between the two graphs.

With two phase three wire, you have supply leg A and supply leg B, and the common. If you measure the voltage A to common, and graph it, and then measure B to common, and graph that, you will see a 90 degree phase difference. This is similar to three phase four wire 'wye' service, where you have supply legs A, B, and C, and a common, all with a 120 degree phase difference.

With two phase four wire, you have legs A and A', B and B'. You measure A to A' and then measure B to B', and again find a 90 degree phase difference.

-Jon


#130257 - 03/03/06 04:58 PM Re: Single Phase to Three Phase Conversion  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
An old perennial rears its head again.

From Hillybillysawmiller (2003) https://www.electrical-contractor.net/ubb/Forum7/000234.html
to
https://www.electrical-contractor.net/ubb/Forum7/HTML/000209.html

Alan

edited to fix 1st link.

[This message has been edited by Alan Belson (edited 03-03-2006).]

[This message has been edited by Alan Belson (edited 03-03-2006).]

[This message has been edited by Scott35 (edited 03-08-2006).]


Wood work but can't!

#130258 - 03/04/06 12:59 AM Re: Single Phase to Three Phase Conversion  
RODALCO  Offline
Member
Joined: Dec 2005
Posts: 854
Titirangi, Akld, New Zealand
rotary converter is easiest.

I use a 5 HP 3Ø 1500 RPM motor which I start up with caps, when up to speed, switch caps off with 3 sec. timer.
Use cap. around 150µF 400 Volts.
Feed from 16 Amps C curve MCB from 230 Volts 50 Hz supply.
Sometimes I have to give the 3Ø motor a push at the heavy pulley which is providing the inertia, if it doesn't start first time.

I then have acces to 3 x 230 Volts at the motor terminals which I use to drive a 3Ø 3000 RPM motor which drives an 80 Amps 12 Volts alternator for experiments or charging the car accu quickly when the kids have turned the car light on overnight at the odd occasion and drained the car accumulator.


The product of rotation, excitation and flux produces electricty.

#130259 - 03/17/06 02:39 PM Re: Single Phase to Three Phase Conversion  
FountainGuy  Offline
Member
Joined: Mar 2006
Posts: 30
Lawrenceville,GA, U.S.A
I have had two experiences using an "Add-A-Phase" unit. This is simply adding a unit (manufacture not mentioned) that consists of a motor to obtain that third leg, to create that beautiful/harmonic/energy-efficient three-phase power. Unfortunately, both of these experiences were ugly. The units worked, but the "Add-A-Phase" units created a lot of heat and the fountain systems was eventually rewired so that only the motor was on this ""Add-A-Phase" three-phase load.

Since the two nightmares, I have had several great experiences with variable frequency drives. This VFD allows single-phase power coming in the unit and three-phase power going out of it. There are so many VFD manufacturers out there. Any manufacturer would be glad to tell you what model to use for your application. They will also tell you what size conductors you will need going in and out of the unit. And if you're not comfortable with the on-board programming, I know that there are manufacturers that will pre-program the unit for you, to get the sale. If you need three phase power for a motor, then this is the way to go, VFDs include a motor starter and a motor protector.


#130260 - 03/18/06 11:54 AM Re: Single Phase to Three Phase Conversion  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
http://www.kilowattclassroom.com/Archive/VFDarticle.pdf

I found this useful in understanding how a VFD drive can run a 3-phase motor.
The diagrams show a dc supply, so the single-phase needs a rectification/smoothing stage?
Q.How much would one of these units cost, ballpark, for, say, a 5hp motor application, between single phase supply and motor starter?

Alan


Wood work but can't!

#130261 - 03/18/06 03:52 PM Re: Single Phase to Three Phase Conversion  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
http://www.driveswarehouse.com

This is one drive supplier in the USA. It looks like most of the drives above 3 HP have 3 phase input sections. The original poster doesn't infer a need for a VFD per se, but rather just an inverter.

Inverters, VFDs, and UPSs all hang out together in my head. I think of an input section that is either a single or 3 phase rectifier or perhaps, a 3rd rail voltage conditioner. Then, I think of a DC bus which may or may not have a battery string attached. Next, you will have a single or 3 phase inverter. It will either be controlled by a variable frequency driver or one that is phase locked to the alternate source. In the case of a UPS, should the alternate f drift out of tolerance, it will likely switch to an internal reference. (not good) Then, usually snubbers and chokes and output tranformers. If you have a UPS, you then hopefully have a full static bypass switch with mechanical bypassing for maintenance.

With new technologies, we get to learn new terms. For IGBTs, the most evil term is de-saturation or just "desat". It is used to describe the act of turning a 400 amp, 1200 volt dual IGBT into a $300 firecracker.
Joe


#130262 - 03/22/06 08:52 AM Re: Single Phase to Three Phase Conversion  
FountainGuy  Offline
Member
Joined: Mar 2006
Posts: 30
Lawrenceville,GA, U.S.A
I am in need of a 15 HP pump but I only have 120/240V 1-Phase service. The VFD size that I need is 30hp 3-phase, which will provide enough current for a 15hp 208vac 3 phase motor using single phase input. My cost for a Yaskawa brand VFD in a NEMA 1 enclosure is $2153. Another option is to go with a 15 HP single-phase pump, which I have only one price so far at $4,190, which is almost twice the price of a much-more-common 15 HP 3-phase pump. I'm still pursuing pump prices from other vendors. Notice that NEC 1-Phase motor charts only goes up to 10HP! Maybe I should go with the VFD. If the motor needs to be replaced down the road, a 15HP 3-PH motor would be a lot easier to find, and will cost much less. I have two identical systems involved, so this is a big decision for me.


#130263 - 03/22/06 08:13 PM Re: Single Phase to Three Phase Conversion  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 790
Chicago, Il.
FountainGuy,
Do you require a VFD, or just a 3 phase inverter or converter @ 60Hz?
Joe


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