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# Thread: Rotary Phase Converter (Create 3-phase power from a single phase source)

1. Size is relative. This is a generic instruction booklet. There's a chart that shows what size idler motor is recommended for the load. Basically, since were only using two legs of the three-phase you have only 2/3's of the load capacity. You would need to multiply the load by 1.5 and pick the nearest larger HP motor available as the idler. For example, for a 7-1/2 HP load you would need 7.5 * 1.5 = 11.25 HP idler. Since that size does not exist, go to the nearest larger motor available which would be 15 HP.

The link following is a good reference. http://www.sprecherschuh.com/downloa...chart_v206.pdf

2. ## Clarification on USA power

Originally Posted by old kodger
Hi folks,
I'm in Australia and I downloaded David's converter plans, unaware that in the States you have what is called high single phase.
Now my comprehension of that, from an Aussie point of view, is that you actually have two 120 volt phases and neutral return giving you 220 volts across the two phases.
In Australia we have 220volt single phase BUT it's only one hot line and one neutral return and the neutral is bonded to earth.

I've been in discussion with David over this and he was not aware of this situation, when this was made clear, he agreed that his design would not work in areas with this configuration of grid supply, and suggested that I post this information on the forum as a "heads up" to others contemplating building this design.

It's a bit of a sod that this exists because I've got several three phase bits of equipment and only a single phase supply (we recently moved house).

Best of Luck to all,

Rob.

Clarification

Actually here in the States we do not refer to this as two phases, its just single phase... Its 208-240 VAC depending on the transformer providing the service to the customer... Both Residential and Industrial the single phase is provided with a center tap off the transformer, this leads to 120 vac legs, that are hot with respect to neutral. Neutral is also earth ground at both the transformer pole and at the circuit breaker load center... But the hot legs are always opposite polarity to each other so they have a maximum voltage of 208-240 vac when connected across the two hot legs of the transformer secondary output... The neutral wire or center tap, carry's current based on the loading in each hot leg, if one hot leg is carrying more current than the other, this will cause a flow of current in the neutral or center tap wire.

Sincerely,
Ron

3. Originally Posted by Mcgyver28117
Clarification

Actually here in the States we do not refer to this as two phases, its just single phase... Its 208-240 VAC depending on the transformer providing the service to the customer... Both Residential and Industrial the single phase is provided with a center tap off the transformer, this leads to 120 vac legs, that are hot with respect to neutral. Neutral is also earth ground at both the transformer pole and at the circuit breaker load center... But the hot legs are always opposite polarity to each other so they have a maximum voltage of 208-240 vac when connected across the two hot legs of the transformer secondary output... The neutral wire or center tap, carry's current based on the loading in each hot leg, if one hot leg is carrying more current than the other, this will cause a flow of current in the neutral or center tap wire.

Sincerely,
Ron
Ron,

Thanks for that clarification. I'm curious though; is our 208/240 actually 2-phase? I've had some folks refer to it as 2-phase but I haven't been able to confirm that it is. It would depend on how the transformer was wound wouldn't it?

4. Well it kinda goes like this, people become familiar with 3 phase and associate it with 3 wires, then they later see 2 wires and say hey it's 2 phase... but that is not how it works, nor what it means... with 3 phase their are 3 sine waves 120 degrees apart, if one has a 3 channel oscilloscope, this can be see as the some wave between legs a to b, leg b to c and legs c to a... with only 2 wires, we have only one sine wave or one phase, the center tap only provides a voltage divide by 2 function, stiil just one sine wave, because if we have an equal numbers of devices on the leg a to center tap and leg b to center tap, we have no current flow through the center tap... It's just one phase...

Ron

5. Two phase? Well it is a bit more complicated than that. And it is embedded in tradition.

Three phase is not three phase because it uses three wires. It is three phase because the sine waves have THREE different phase angles. So the three wires are a consequence of that, not a defining factor. In fact, if there was no fourth conductor (wire or otherwise) that defined the ground level that you are measuring the three phases from, then you could not measure those three phase angles. They must be measured from some reference point. So a three phase system, by necessity has four conductors.

OK, so what is two phase. Simply stated it is a system with two AC Voltages that, when measured from a ground conductor, have TWO different phase angles. Four phase would have four such AC Voltages, five phase would have five, six phase would have six, etc. Simply stated, that is what is meant by a number of phases. Please note that I did not say that the phase angles measured between the various phases must be the same. They do NOT need to be the same. Thus, a three phase system normally has equal phase angles of 120, 120, and 120. But other combinations that add up to 360 degrees would also be three phases: example: 100, 120, and 140. That example would not be common, but it could not be called two phase or four phase or any other number of phases other than three. Other examples would be possible: 10, 40, 310 or 25, 35, 300 or any combination of three numbers that add up to 360. All would be three phase systems.

But two phase? What is two phase? From a general point of view, two phase is simply any system that has two AC Voltages that have different phase angles when measured from a stated ground reference. HOWEVER there is tradition. Somewhere in our past, there was a system that used TWO phases that were separated by 90 and 270 degrees. This probably came about because the mechanical generator that was used had two coils that were oriented 90 degrees to each other. That is the simplest way to arrange two coils on a rotor and it is certainly how any designer would do it. That IS a two phase system. No arguement, but I did use the preposition "a" not "the". Since it was the only two phase system at that time, it was simply known as "two phase" just as 120, 120, 120 three phase was called simply three phase.

Then along came distribution to the homes and small shops and offices. The vast majority of these did not need three phase so only one phase of the three phases that are generated and distributed nationwide is carried to a local group of homes. The three phase power comes to your neighborhood, but your block only gets ONE of those three. So it was called SINGLE or ONE phase. Sounds logical. Here comes the rub. That SINGLE phase is distributed with a SINGLE transformer on a pole near you at a 230 VAC level. Someone must have thought that this was kind of high and wanted to lower it, perhaps for safety. So they came up with the idea of a center tap on that pole transformer. The single 230 VAC phase became TWO 115 VAC lines that were 180 degrees apart. So, is that split phase still a SINGLE phase or is it now two phases? The only difference is in terminology and the history of the system in the US. Since there was a prior two phase system (the 90, 270 one) some wise or perhaps not so wise engineer choose to call it a split phase system. The 115-0-115 Volt split phase system that we use in the US is certainly two phases by the basic definition of phases. But by common usage, it is not called that. It is commonly referred to as single phase and not so commonly as split phase.

I agree that there is no limit to the confusion that this causes. Frankly, the terminology is not consistent.

6. Originally Posted by Paul Alciatore
......TWO 115 VAC lines that were 180 degrees apart....
Excellent explanation Paul. So then, if our friend in Australia (and most other former British Crown Colony countries) has a only a single hot and a neutral (earth ground), my phase converter doesn't have a change to succeed.

7. Originally Posted by nhengineer
Excellent explanation Paul. So then, if our friend in Australia (and most other former British Crown Colony countries) has a only a single hot and a neutral (earth ground), my phase converter doesn't have a change to succeed.
I would say that it probably does but! they will require a buck or boost transformer with a center tap secondary connect the primary to the 1 hot and the neutral ground bond the center tap to the neutral ground then they will have split phase which then would be like having US mains your idler rotary phase converter would do the rest

8. hi Guys,
All of the comments here hold water. What has NOT been considered is cost. Sadly, and I have discussed this with David, is that a transformer, with 10-15kw capability transforming from 240volts to 480volts center tapped, to give "two phases" 180 degrees apart, is of the order of \$2000, then you have to get a three phase motor as a idler. Here (Aus) if a motor runs, it's a going concern and serious money is asked for it. By comparison, I checked ebay before I posted, and the best price I could find for a new 3phase 10kva generator is \$2990. A second hand old 25kva one, not running "where is as is" is \$1300.
So you can see that whilst Davids plans are possible here, they are not fiscally viable.

Regards,
Rob.

9. Originally Posted by Frank S
I would say that it probably does but! they will require a buck or boost transformer with a center tap secondary connect the primary to the 1 hot and the neutral ground bond the center tap to the neutral ground then they will have split phase which then would be like having US mains your idler rotary phase converter would do the rest
Acme (my usual source) buck/boost seem to only be available as 120/240 X 12/24 which is not at all what I want. https://www.hubbell-acmeelectric.com...ase-buck-boost Any suggestions? I'd like to make my converter universally acceptable.

10. Hi, Forgot to mention, ALL three phase motors here are 240 volts per leg.
Rob.

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