I would, but I'm not sure it is my business to tell. If you really want to know, I'll ask the customer if they are OK with it. As for what the machine does, a piece of granite comes off a conveyor from a cutter and this thing grabs it and starts grinding and polishing and washing. There are multiple arms and actuators and sensors. It looks like a lot of the motive power comes from compressed air.
3 Phase from Single Phase 120/208
- Thread starter CoolWill
- Start date
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
This might work if the phase converter used the neutral. Do most do this?
I've installed 5 or 6 rotary or static phase converters and have never seen one with a neutral. Not saying it doesn't exist, but I don't think it is standard.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I didn't think so.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Most phase converters don't use the neutral, but IMHO that is irrelevant to the point I was making.
Ideally a phase converter takes single phase input and synthesizes the third leg. If you plot all the voltages on a phase diagram, then you've been given 2 initial points (the input single phase) and the phase converter synthesizes a third point that lies on the vertex of an equilateral triangle with the other two points. This is exactly equal to finding the midpoint between the two initial points, making a 90 degree angle, and going 0.866x the distance between the two initial points.
The phase converter doesn't know anything about the source 3 phase system, and doesn't care. It just takes 2 input legs and synthesizes the third, and the set of three (ideally) lie on an equilateral triangle drawn on a phase diagram.
If the input legs are referenced to ground, that defines the ground reference of the derived 3 phase system.
If you start with ordinary split 120/240V single phase, the derived leg will 'look' like the high leg of a 3 phase delta. The two initial legs are at 120V to ground and opposite phase, so the derived third leg ends up 208V to ground and 90 degrees out of phase.
If you start with 120/208V single phase split off a 208/120V three phase system, the derived leg will _by coincidence_ 'look' like the original 3rd leg of the three phase system. Not because the phase converter is connected to neutral, but because the initial two legs are 120V to ground/supply neutral and 120 degrees apart in phase; take those two points and derive an equilateral triangle, and the third leg ends up 120V to original supply neutral and 120 degrees out of phase from the other two. (I just realized that there is a _huge_ exception to this!)
If the synthesized rotation created by the phase converter matches the phase rotation of the original system, then my above analysis holds. But the phase converter doesn't know the phase rotation of the input system; it can't. So it could just as easily derive the third leg with the _opposite_ rotation. Draw this on the phase diagram and the derived third leg would _not_ coincide with the original supply third leg. If I did my math right, then in this case the 3rd leg to supply neutral voltage is about 300V.
-Jonathan
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
If the neutral is not brought into the phase converter it will only see a single voltage of 208V instead of its nameplate 240V. There will be an internal point which the converter uses to generate a voltage of 1.73X the midpoint of the input feed. In this case it will be 1.732 X 104V resulting in high leg of 180V.
The original L-N 120V never comes into play.
The original L-N 120V never comes into play.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Winnie's point is that if you connect L1 and L2 to the phase converter, it creates an L4 of fixed voltage relationship to L1-L2. And if L1-L2 come from a wye system, the N of that wye-system has a fixed voltage relationship to L1-L2. The result is that L1, L2, L4 and N all have a fixed votlage relationship to each other.
So if L1-L2 is 208V, and L4 completes a 208V delta 3P3W system, then the L4-N voltage will either be 120V, or 240V, depending on a polarity choice. I think if you get 240V L4-N wired one way, you could swap L1-L2 and get 120V L4-N.
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I agree that the original L-N is never intentionally or explicitly used. I agree that 180V high leg is relative to the midpoint between L1 and L2.
The midpoint between L1 and L2 on a 120/208V single phase system just happens to by 60V to the system neutral. Thus the derived third leg (which the phase converter is making 180V relative to the midpoint between L1 and L2) will _by coincidence_ be 120V to the original system neutral. (or 240V to the original system neutral if the derived rotation is in the opposite direction, I guess I did get my math wrong)
-Jonathan
Crazy update: All of this got ran up the chain and back down again. So the digital phase converter plus transformer combo is what is going to happen. I'm already doing a lot of work there, so I've got the job. But that isn't the crazy part.
They had already pulled that generator out of the weeds and pressure washed it preparing to do the lash up. Turns out it is an 18 kW. Now that the plans have changed, they said they were going to scrap it and asked me if I wanted it. So we loaded on a trailer and I brought home. I put a battery on it and tried to start it, but got nothing. A little troubleshooting and I found the starter relay was bad. Standard automotive type 30 amp relay. I put another one in and it fired up after a few cranks! I didn't let it run long because the diesel fuel is old and who knows what else. But it's pretty crazy to me to walk away with a free 18 kW generator that someone else thought was bad. They said one person who looked at it thought the starter was bad, and another thought the crank was seized. Who are these people?
Thanks for all the education and advice.
They had already pulled that generator out of the weeds and pressure washed it preparing to do the lash up. Turns out it is an 18 kW. Now that the plans have changed, they said they were going to scrap it and asked me if I wanted it. So we loaded on a trailer and I brought home. I put a battery on it and tried to start it, but got nothing. A little troubleshooting and I found the starter relay was bad. Standard automotive type 30 amp relay. I put another one in and it fired up after a few cranks! I didn't let it run long because the diesel fuel is old and who knows what else. But it's pretty crazy to me to walk away with a free 18 kW generator that someone else thought was bad. They said one person who looked at it thought the starter was bad, and another thought the crank was seized. Who are these people?
Thanks for all the education and advice.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
They usually are connected in delta configuration. The single phase supply which can have a neutral even if you don't bring it to the phase converter is one side of the delta, the derived phase (if conversion is perfect) is at normal high leg voltage to the neutral of the supply circuit.
If you use a 120/208 for the supply voltage you would have a 208 delta from the conversion. Not certain exactly what the voltage of the high leg would be as the neutral isn't at 180 degrees between the incoming ungrounded conductors.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
If you envision the neutral offset from the straight line between the two phases, that places it closer to the derived phase, which is why the voltage to neutral is the same as the two real phases.
This picture (not the text) illustrates what I'm trying to say. Notice how R1 and R2 do not form a straight line, but instead are bent toward the lone dot in the lower-right corner of the wye:
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I get what you are saying, I just did not lay it out before making previous comment is why I don't know what that voltage would be.
Wouldn't (or couldn't) your derived leg be above and to the left of the neutral though?
with 120/208 as the input, you end up with mostly a delta but the side the input power is connected to is not straight. R1 and R2 on that drawing is the input source coils, and at least with a roto phase converter there would be a full (208) delta imposed onto that image but another thing i don't really know is whether the source neutral would be inside that delta or outside, or maybe it could be either depending on other details. Probably is going to be inside though. Would still give you similar issues as trying to run line to neutral loads on high leg of what we are used to talking about. There is a definite voltage between high leg and neutral but no direct coil between those two points and assuming that voltage is nominal 120 you wouldn't want to connect a 120 volt load to it as it likely not all that stable of a voltage when loading conditions change.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Or it could end up farther from the derived phase, and then you end up with double the Lderived-N voltage.
Cheers, Wayne
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
I bet it has already been covered in this thread, the easiest way to derive full 120/208 would be a pair of 120:120 isolation transformers.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Yes, I believe so, depending on rotation, perhaps? It seems to be a matter of polarity.
Yes, a 208v delta, and the neutral would be in the center.
Again, it would be a 208Y/120v wye used as a 208v delta, electrically speaking.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Yes, this is pretty much what I've been saying.
If you take L1 and L2 from a 120/208V system and feed that to a rotary phase converter, the RPC will synthesize a delta based on those 2 points.
If the rotation matches, then the derived phase will happen to coincide with the original L3, but be unstable and not really useful for anything except driving a motor (which will tend to reinforce that third phase).
If the rotation doesn't match, then the synthesized delta won't overlap with the original wye, and the derived phase will be 240V from the original wye neutral.