Different 3-phase question

Hey folks.

I was wondering about running 3 phase power for backup power, such as a genset.

Not knowing much about the subject it seems that if it was wired for 120/208 one could run 120v lighting and small appliances off each of the 120v legs.

My question is can single phase motors be run off of the 3-phase circuit?

It would seem that in the single phase situation the voltage spike would be 180 degrees alternating between the two hot legs.

As I see it the three-phase power would be 120 degrees between any two of the three legs.

Prolly way over my head on this but would appreciate any info.

Thanks,

Brad
 
Your second sentence makes a little sense then it is downhill from there.You don't need three phase unl;ess you need to run a three phase motor. Just worry about sizing the generator to the wattage required. If you don't have anything 3 phase now then why would you need this?
 
A 208 Y 120 Volt Three Phase Four Wire service, be it from the Utility or your own Genset, can supply three legs (A, B, C) of single phase 120 VAC Line to Neutral power PLUS can be used to power a 208 volt three phase motor or other such 208 volt three phase loads.

SO TO ANSWER YOUR QUESTION YES a single phase motor (say 120 volt) can be ran off one leg of a 208 Y 120 Volt genset. It would be fed from any one of the legs (A or B or C) and Neutral, thats 120 volt single phase AC.

HOWEVER if your homes main distribution panel is the typical 120/240 volt Single Phase Three Wire, and you wanted to feed it via a Transfer Switch by a genset, you would ONLY be using two of the gensets three avaialble legs of 120 volt mind you !! PLUS it would only be 208 NOT 240 Line to Line. "Some" 240 volt equipment may not like that idea so well lol !!!!

NOTE if a genset had say 5 a KVA rating, you still only have total 5 KVA available, regardless if you were dividing the load up among the three legs.

NOTE while some 240 volt motors (depends on motor design and load etc) can operate at the lesser 208 volts, Id prefer a genset that supplies the same voltage as my utility provides and/or the load requires.

NOTE many commercial and light industrial services are indeed 208 Y 120 Volt Three Phase Four Wire, but thats NOT a typical household service

I guess what Im saying is if you have a 208 Y 120 Volt Three Phase Four Wire main service panel at your home, then a similar three phase genset and transfer switch would be practical HOWEVER if your serive is only 120/240 volt single phase three wire, then the backup genset should be similar.

YES its true if you have a 208 Y 120 Volt Three Phase Four Wire genset you can serve three legs of 120 volt (with respect to Neutral) versus only 2 legs if it were 120/240 single phase genset BUT WHAT ARE YOUR REAL NEEDS???

If you want to use 208 Y 120 three phase genset power for backup as your posting asks, HOW DO YOU INTEND TO USE AND DISTRIBUTE IT if everything you have is either 120 volt single phase or 240 volt (as typical for a home) ??????????

Unless you have some 208 volt three phase loads you need back up power for, I dont see why you want a 208 volt three phase genset (although yes it can deliver three legs of 120). Makes more sense to me to have a genset thats the SAME AS YOUR UTILITY FEED and use a Transfer Switch

ONE FINAL THING there is such thing as a 120/240 Volt Three Phase Four Wire center tapped Delta Red Leg Service (utility or as genset) but thats NOT for a typical household service. It can supply full true 240 Volt Three Phase Power,,,,,,,,,,,Three legs of full 240 volt Single Phase Power,,,,,,,Two legs of 120 VAC Single Phase off the center tapped transformer. Theres two legs of 120 volts single phase with respect to neutral, but on the red or wild high leg its 208 single phase with respect to neutral ITS NOT FOR THE TIMID OR AMATEUR.

BOTTOM LINE FOR ROOKIES 208 DOES NOT EQUAL 240 they are differently derived systems.

Any questions??????

John T long retired Electrical Engineer but a guy retains these such basics
 
Brad,

Generally speaking, power companies in America don't and won't run 3 phase to residential areas. The rest of the world? But here at least, if you happen to stumble across what seem to be good deals on 208 or 230 3 phase motors or appliances, walk away, they will do you no good. In theory your appliances are balanced on the customer side of the main, and they are done so on the service of two 120 single phases and nuetral, 240 across them delivered to your main. You don't have three phase and smaller generators that you would purchase from say a Menards or Lowes for residential aren't setup for it. You don't have access to it, don't need it, wouldn't want to deal with the expense of it, nor would the power guys.

Related, unrelated, there is a four or sixteen square block area (I forget which) in NYC that is DC voltage though in tribute to Thomas Edison. That would be a pain in the rear for readily available appliances and stuff when one fails. Not like you can run right down to Best Buy at 5:00 in the evening and purchase a brand spanking new...

The long and short of it Brad, is do yourself a favor and not bother...

Mark
 
220/230 volt 3 phase generator
Any line to neutral=110/115 volt - light bulb drill etc
Any line to line=220/230 volt - welder,stove etc
All 3 lines = 3 phase power to motor etc
Not all generators have a neutral coming out to the box meaning you may not be able to utilize 110/115 volt.
Dave
 
Dave, (with all due respect NOT to fight or argue ONLY to correct to others for the record)

NOT NECESSARILY, HERES THE REAL DEAL.....Youre close but no cigar as they say

WHILE ITS INDEED TRUE "ANY" LEG IN A 208 Y 120 Volt Three Phase Four Wire Service has:

120 VAC Single Phase to Neutral

BUTTTTTTTTT if its a 120/240 Volt Three Phase Four Wire Center Tapped DELTA

THEN ONLY TWOOOOOO LEGS HAVE 120 TO NEUTRAL

THE OTHER THIRD RED/HIGH LEG IS 208 TO NEUTRAL

In my over 40 year Electrical Ebgineering career there were times I specified 208 Y 120 Volt Three Phase Four Wire Services and other times I specified 120/240 Volt Three Phase Four Wire DELTA (and often the one leg was center tapped so 120 was available on TWO but NOT 3 legs, the odd one is 208 to Neutral NOT 120!!!

A DELTA IS NOT A Y AND 208 DOES NOT EQUAL 240

120 times the square root of 3 equals 208 NOTTTTTTTTTTTTTTTTTT 240.

There are 240 volt three phase gensets (could be pure 240 DELTA with three legs of 240 single phase or 240 three phase) and if you center tap one leg you get 120 but the other (red/high) leg to Neutral is 120 x 1.732 = 208 NOT 240

and

Theres 208 Y 120 Volt three phase gensets with ALL THREE legs of 120 to Neutral or 208 (120 x 1.732) three phase

This may be confusiung to lay persons but its pretty common engineering practice HOPE THIS HELPS AND DONT CONFUSE YALL Im here to help if I can NOT stir things up lol

God Bless

John T Longggggg retired Electrical Engineer
 
Hi John no offence taken, it has been a couple of decades since I worked in a rewind shop, so your wealth of wisdom will prevail here.
My intention was to point out to Brad that not just any 3 phase unit would serve his needs.
Much like some of the old 220 volt polyphase units did not have an available neutral for 110 use.
Dave
 
Hey guys,

Didn't mean to cause trouble but a machine is for sale at a good price and I was curious.

It is military surplus diesel powered 15kw and according to the id plate can be wired for either 120/208 V 400HZ 3-phase or 240/416 V 400HZ 3-phase.

Also states that it is type 1 class 1 mode 2 size 15.

Would this indicate that there are three hots and a neutral?

Thanks for the informative responses,

Brad
 
Good point, since as many non electrical gents arent aware three phase can be wayyyyyyy different as it may be 208 Y 120 (3 legs of 120) or 120/240 Center tapped Delta (2 of 120, 1 of 208) and they just ARE NOT the same.......

Fun chattin with ya take care now

John T
 
NO TROUBLE Brad, just a good question and a FUN discussion.

IF ITS 400 HZ AS SOME MILITARY EQUIPMENT THAT JUST AINT GONNA WORK FOR YOUR TYPICAL USA 60 HZ LOADS

The mathmetical imperitave that 120 x the square root of three is 208 and since you state 120/208 three phase or 240/416 YEP ITS THREE PHASE Y

John T
 
Brad, you have the answer about that generator.

I use a 3 phase PTO generator for my emergency power here, and for powering a couple of very large stationary tools that I bought after I ran inadequate wire to the outbuilding. Works great for both the 3 phase tools and the house, electronics included.

You do need to know what to connect to what however. My generator I got cheap from a country store whose "electrician" hooked up his cash registers wrong and fried them. Not once, but twice. A $3 Harbor Freight volt meter would have solved the problem if his "electrician" had a clue, but I didn"t tell him.

If you have a cheap opportunity for a different generator, come back and ask. Just "hots and a neutral" isn"t enough to go by. That"s how I came by my generator cheap.
 
Brad the 400 cycle units are used primarily for aircraft ground power,
You could use it for incandescent light bulbs or heater elements that don't care about frequency but thats about it.
Dave
 
Yea there was a news artical about four years ago on that DC power in NYC. The last was somewhere in the Wall st. area. In the late 60s my grandparents lived up on 86th between 2nd and 3rd ave. The brown stone was all 117volt DC! Needed an AC/DC radio and a little TV. Thank you tom ed! The guy who really knew what to do was Tesla! That guy was way smarter than Edison!By the way ..In south philadelphia,a small area of Vineland,NJ and somewhere in ST.Louis they have 2 Phase. That is sure weird stuff with four or five wires! If someone tells ya there ain't such thing he is a real dope. Jeffcat
 
Thanks guys!

Guess I will rule that machine out.

Sometimes this site feels like college.

Well perhaps a college way out in the country.... Where the profs wear bib overhauls.... And the classroom is right next to a hayfield..... Which was just fertilized.

I love learning and this discussion may get me to crack a math book just so I don't sound like such a dodo.

Thanks,

Brad
 
John T when you say Y do you mean wye? I just want to make sure I am reading that correct. I forgot that gens could be wired in wye or delta.

BTW, at our tech college all three legs are 110 to neutral. We don't have a hot leg. How would that work? What would it read phase to phase?

To the world...when hooking up single phase equipment to three phase power make sure you use the two lower legs. There is "always" one that is hotter than the other two and you should leave that one out. This is extra imporant when working with machines with transformers (ie welders)
 
Yes Y means wyeeeeeeeeeeeeee configuration versus the other DELTA configuration.

YOUR QUESTION: BTW, at our tech college all three legs are 110 to neutral. We don't have a hot leg. How would that work? What would it read phase to phase?

ANSWER: if you have three legs ALLLLL of which read 120 to Neutral thats a 208 Y 120 Volt Three Phase Four Wire configuration THERE IS NOOOOO RED OR HIGH 208 VOLT LEG TO "NEUTRAL". On that system its 208 (120 x square root of 3) line to line but only 120 ANY line to Neutral.

NOTE if the system is a 120/240 Volt Three Phase Four Wire Cenetr Tapped (only one winding tapped) DELTA known as a Red Leg or High Leg system,,,,,,,, Its 120 volts to Neutral on ONLY TWO of the legs (the 2 legs on the center tapped transformer) BUT ITS 208 ON THE HIGH/RED LEG TO NEUTRAL......

YOUR STATEMENT: To the world...when hooking up single phase equipment to three phase power make sure you use the two lower legs. There is "always" one that is hotter than the other two and you should leave that one out. This is extra imporant when working with machines with transformers (ie welders)

CLOSE BUT NO CIGAR: Its NOT "always" theres one "hotter"...... On the 208 Y 120 system ALLLLLL legs are the same and alllllll are 120 to Neutral and its 208 any line to any other line. HOWEVER on the 120/240 volt three phase 4 wire Delta yesssssssssssssss one leg is "hotter" as its 208 to neutral while the other 2 are 120 to Neutral

NOTE its also common to use a 240 or 480 volt STRAIGHT NON CENTER TAPPED DELTA. Thats called Three Phase THREE Wire DELTA. 240 line to any other line, you can use it to feed 240 single phase loads or 240 three phase loads

Got it now????????

best wishes

John T
 
I don't know about that John T. I know you have a lot of real world experience, but I also trust my formal education. What I was trying to say is one leg is high, but still the same voltage. Our professor said two will be 208 and one will be 218 (or something like that). He said you should then tap off the 208 legs if you need single phase power.

Assuming the red leg is L2 and 208 volts to ground and L1 and L3 read 110 to ground. What would you read from L1 to L2?
 
Yep I have 40 + years of real world experience. I was an AC Power Distribution Design Engineer and have a BSEE as part of my "formal education" as you call it from good old Purdue University a top notch Engineering School, so not to brag but YEP I DO KNOW SOMETHING ABOUT THAT LOL

To answer your question:

In a 120/240 Volt Three Phase Four Wire center Tapped DELTA, IF L2 is the Red/High Leg then:

Its 240 Volts L1 to L2
Its 240 Volts L2 to L3
Its 240 Volts L3 to L1
Its 120 Volts L1 to Neutral
Its 120 volts L3 to Neutral
Its 208 Volts L2 to Neutral (Red/High Leg)

Its the transformer between L1 & L3 thats center tapped with its center grounded and that point is the Neutral

In a 208 Y 120 Volt Three Phase Four Wire System

Its 120 L1 to Neutral
Its 120 L2 to Neutral
Its 120 L3 to Neutral
Its 208 L1 to L2
Its 208 L2 to L3
Its 208 L3 to L1

So thats about as plain as I can describe it to a lay person sure hope this helps. If some "Professor" says different then Hes wrong and Im right is all I can tell you. NOTE sure there can be some slight different voltages (204, 206, 208, 210, 212 etc etc) depending on resistances and loads and the tap settings etc. etc. Im unsure that "Professor" understands the difference in a Delta versus Y configuration???

NOTE do NOTTTTTTTT feel bad if you cant understand this and sorry I can NOT put in a paragraph here what takes volumes of books to explain and years of engineering practice and experience to fully comprehend. I try my best to help and educate the fine gents here but I cant make them understand volumes of theory and years of expreience here in a few sentences BUT BY GOLLY I DO TRY MY BEST.

God bless you for your good thoughtful questions and sparky chat and I sincerely hope I helped you understand all this, sorry if I couldnt.......

John T BSEE, JD
 
"Assuming the red leg is L2 and 208 volts to ground and L1 and L3 read 110 to ground. What would you read from L1 to L2?"

It is a fixed math calculation. A matter of summing the vectors (phasors), sines, cosines. Your set of numbers do not fit the math. IF you actually had 110 for L1 to N and L3 to Neutral, then the math would yield 190.5 for high-leg, not 208. And if high leg were 208, then L1/L3 to neutral would be 120.
In either case, L1 to L2 to L3 & L3 to L1 would be equal at 220, or 240.
 
These guys are just having trouble with the MATH as you correctly point out. The figures I keep using are typical and nominal (based on math such as the 208 derives from 120 x square root of 3 etc) and science, but that dont mean any given voltmeter is gonna yield those exact results. The laws of physics and math must apply and the Y and Delta configurations I keep quoting them are correct based on vector sums and pretty basic trig etc. Also a non electrical person has a tough time with Y versus Delta when they start throwing out figures they heard about somewhere down at the pool hall.

But hey God Bless them all, they have some good questions, and I still try my best to help and will keep doing so and thanks to you also

John T
 
Hey Brad, grab it.

When I mentioned that typical small generators sold in say Menards or Lowes aren't setup for 3 phase, I was going to mention military generators that are setup to do either single or three phase. When I was in the Army, we had a wide variety of gen sets that were capable of doing both, starting with the 10K gasoline sets, and of course our 15K, 30K, and 60K diesel sets did as well...single phase were L1 and L2 to nuetral for 110, or 220 across L1 and L2. That is all that you will need for residential. That will get you 110, 110, 220 single phase. L3 and L4 in conjunction with L1 and L2 will get you three phase across L1 and L2, L2 and L3, and L3 and L4...which you will not need.

My mistake Brad. I thought that you were talking about motors and commercial grade appliances. If I had known that you were looking at a 15K military Gen set, probably diesel, I would have told you to jump all over it and here is why. Those old diesel military gen sets needed to run somewhere between 60 and 80 percent draw to keep from tearing them up, which meant that we had to tie in load banks across the loads. In tactical maintenance, which I was, we generally used 30K and 60K gen sets because we at times had that much stuff hooked up to them, but at times not...and on the latter occasions, may have fired up huge gen sets in the field, and had only say like 10K draw on them of equipment shelters in being maintained. On those occasions, we would have to dial in the load banks across our coils to increase the draw to as I recall a minimum of 60 percent (36K) a typical maximum of 80 percent (48K) on a 60K gen set, or 18K to 24K on a 30K gen set.

Brad, from what you are saying as I understand it, you have the opportunity to purchase an old 15K military gen set, I assume diesel because thats what we used, and if that;s the case and its in good shape, jump all over it. With a 15K,you won't have to worry about a load bank to make up the difference in load draw, and those old Continental diesels ran a real consistent speed once you dialed them up to the correct speed (cycles/hertz) and locked the dial in. Whereas the smaller 10K gasoline units used governors that had tendencies to float nominal speeds all over the place, which gen sets are known for, dirty power. With our diesels, we would dial them in, walk away and come back in an hour after the engines were warmed up, fine tune the speeds, relock the dials, and they would run error free all day and night. Out in the field, we always rotated two gen sets. One day one, next day the other, and so on.

Brad, you have a chance to buy and old diesel 15K in good shape? Grab it. It will do single and three phase, will give you 110, 110, 220 single phase. Grab it, grab it, grab it...or I will. You won't find anything out on the market new that will keep up with it without paying mega $$$. Grab it.

Good luck Brad.

Mark
 
Military Mark-IN Please read the posts.
The unit Brad was considering was a 400 cycle generator, you do not want to be plugging in anything other than an aircraft or a light bulb into 400 cycle power.
I have worked on countless 400 hz units you would have to run the engine at less than 300 RPM to produce 60 hz.
Dave
 
And by the way Brad, 15K, 30K, 60K diesel military gen sets...5 wires. Hots on L1, L2, L3, L4, and a nuetral. You will NOT use five. You WILL USE ONLY 3...L1, L2, and Nuetral. L1 to Nuetral...110, L2 to Nuetral...110, across L1 and L2...220. Actually, I should be saying 120/240 since thats what they are supposed to be.

My first exposure to three phase out in the field was because I was repair for microwave line of site, troposcatter, and satellite radio repair. There was this site where they had several comm vans hooked up to a 60K gen set, and someone hooked the five wire cables to the vans incorrectly across its coils. They got the color codes wrong at the coils, which meant they were wrong at the recepticals at the shelters. They had nuetral on a hot coil, hot on Nuetral, and at the equipment it got worse because someone tied ground to Nuetral on one van, which they didn't ground like they were supposed to, so when they fired it up everything that was ground was hot, and since it wasn't grounded and was separated from ground by the big rubber tires, the soldiers that walked up and grabbed the rails while standing in wet mud, became the fuses to ground and were getting knocked on their butts. Then someone did the same, but drove a ground rod like they should have, but when they hooked it up, shut down the 60k gen set because they were running a hot leg directly to ground. I got sent out because the entire site was down, like 20 or so communication vans off line and that was a big deal, and that site was the hub to all of the remote sites. That was my introduction to three phase...tracing cable connections through a huge load center back to the coils on a 60K gen set in the dark of night, me, a generator mechanic, a flashlite, a volt meter and some serious brass standing over our shoulders yelling at us in what turned out to be their operators mistakes.

Good luck.

Mark
 
Don't rule the machine out. Buy it. Its capable of both single and three phase. Buy it, use coils L1 and L2 for single phase.

Mark
 
NO PROBLEM, great questions, its been a fun and informative discussion and hopefully someone learned a thing or two, yours truly included...

John T
 
(quoted from post at 05:38:55 04/23/13) NO PROBLEM, great questions, its been a fun and informative discussion and hopefully someone learned a thing or two, yours truly included...

John T

One of our trouble shooting problems last night involved a delta wired high leg transformer. They had four DMMS and you had to say what the expected reading was on each DMM.
 
(quoted from post at 09:31:25 04/23/13)
(quoted from post at 05:38:55 04/23/13) NO PROBLEM, great questions, its been a fun and informative discussion and hopefully someone learned a thing or two, yours truly included...

John T

One of our trouble shooting problems last night involved a delta wired high leg transformer. They had four DMMS and you had to say what the expected reading was on each DMM.
ope they let you see where the meters were connected! :)
 
It should have been near or in mathmatecial proportion to

240 L1 L2
240 L2 L3
240 L3 L1 (any outher corner to any other corner)

120 L1 N
120 L3 N (The Center tapped winding/transformer is from L 1 to L3)

Of course, a shorted winding, very non proportional loads, wiring, resistive problems and a whole other sort of problems may cause differences, but you already know that

Good Luck with it

John T
 

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