3 phase

sorry to ask such a basic question. button its been a while since I took motors and generators in class. Im just trying to get everything right in my head. on a 3 phase 480v motor i remember that each phase is 120 degrees out of phase, but what is the voltage on each individual phase

whatever the supply voltage is from line to line..."normally" in the US it can be EITHER 208/220 0r 480, in your case it should be 480 or within 10% of that more or less. IF you were to check ONE leg of a 480v system to ground you may get a reading around 277v...depending on if you get a reading at all.

dont know what i was thinking

we used to use a 3 phase 120v supply for some of our weapon test sets. I had to think about it for a second for some reason i was thinking all three legs were used to create 480 v, but I got to remembering that when i checked each leg with my dmm they were all 120, the 3 phase is just for a smoother signal since you have 3 60 hz signals 120 deggrees out of phase with each other. your rms voltage will be higher with 3 phase then single phase. right?

Either of these configurations can be used for 3 phase motors, the reason to use 3 phase is because of lower current usage. The less current used the less you have to pay for KW usage to the utility company. Simple math P=ExI

I suggest you do a web search on wye delta and read up on the reasons for using either.

Let me state something else, WYE configuartions can have a center tap that is used as a neutral. When you measure voltage with a DMM from line to neutral you will read 120 but line to line will be 240 (depending). I kind of think that what you may have thought of as 120v 3 phase was a WYE configuration with a neutral where the line to neutral (maybe ground also) voltage was 120 but line to line would be 240. Yes the math with a WYE configuration is slightly different than Delta and the low voltage will be different than line to ground voltage on WYE. RMS voltage will always be .707 of the line to reference, which may be ground, neutral or another line.

You will have to excuse me for making a short reply. I can offer more tomorrow.

I will be sure to add WYE/DELTA info on my website.

Here's Ron's picture...

beerchug

-Eric

In a balanced three phase system, the line-to-line voltage will be 1.732 times the line-to-neutral voltage. Thus, a system that has 120V line-to-neutral (commonly used for lighting and general commercial distribution) will have 208V phase-to-phase.

A nominal 230V phase-to-phase system will have around 133V phase-to-neutral and is not really suitable for use as a single phase distribution service since the voltage is too high.

Similarly, a 460V service will have 277V phase-to-neutral if properly balanced.

A 460V wye configured power source will use three 277v transformer secondaries with one side of their coils connected together to form a center neutral. This arrangement tends to enforce balance in the system and any single phase load hung from phase to neutral will not pull the system out of balance. Excess load will clear the fuse or breaker.

A 460V delta configured power source uses three 460V transformer secondaries with outside end pairs connected together to form the three phases. There is no connection to neutral so there is no forced balance. Loads hung from one phase to neutral or ground will pull the system completely out of balance and there will be very little or no voltage on that phase to ground to provide current for the single phase load. There are delta systems out there with one leg of the delta system deliberately grounded. I hate to see that due to significantly increased shock hazard and needlessly high phase to ground insulation stress in equipment. An exception to this is hospitals where running one leg grounded is done to improve the sensitivity of GFI equipment and reduce shock hazard and electrical noise problems, I'm told.

While delta systems are being promoted as a way to keep your facility running with a single ground fault, I don't particularly care for the hazards involved and will not start up VFD and softstart equipment on them without an isolation transformer utilizing a wye secondary. I got hit with 700VDC some years ago due to a grounded leg delta system even with disconnects open on an old analog DC drive. Once is enough for that, I assure you!

3Phase 480V

To determine the voltage (mathematically) for any given leg in a Delta configuration, simply divide the total 3P voltage by the square root of 3.

For example: 480V / ã3 = 277V

Of course, a multimeter placed on any one line to ground or any line to line will yield the actual voltage.

Ben

Just to clear things up a bit, a 240V DELTA transformer will read 120V from each leg to ground. I haven't seen any 240Y130 usage, not to say there isn't any.

How to get it right in your head

This may be oversimplification, but it seems to me there are a few people confused about three phase voltages. Just to put the theory right in your head think of it this way.

Think of single phase 120 volts as a 12 inch hand on a clock pointing straight up to noon. The distance of 12 inches we will picture as 120 volts. Now picture 3 phase as three hands. One at noon, one at 4, and one at 8. Each phase to the center (N) is 12 inches, or 120 volts. If you measured the distance between the end of each one, you would see it measures 20.8 inches, or 208 volts. This is vector addition. If two voltages of 120 are headed in opposite directions (180 degrees), you would get 240 volts (a single phase 3 wire edison, like in your house). Since in a three phase system Phase A and B are only 120 degrees apart, they added up to 208. Draw it out on paper and you will see what I mean. Three lines 27.7 (277 volts) inches long 120 degrees apart will give you a measurement of 48 inches (480 volts) from the tip of each phase.

WYE 240/130 is more common than you think. Next time you goto the county or state fair (or any festival with a carnival present) talk to the show elelctrician. Mobile generators can be setup to use a 3 phase WYE 240/130 setup BUT when I was doing that the actual voltages measured more like 230 line to line and 130 neutral to line under NO LOAD conditions. LOAD conditions I set to make the genset lose a few volts which would set it at 125/225 then with maximum line loss (sometimes long runs are made) then you ended up with 120/220.

BTW most items I know of that are rated say for 120volts have a 1- to 15% or better over/under voltage capability which technically means that 130v on a 115/120 device is acceptable.

GNewson, I'm sorry but a 240 three phase transformer wired delta or wye will not give you 120V to ground but, if balance is somehow enforced, 138V to ground. You must have only 208V phase to phase to get 120V to ground or neutral.

Of course, you could play some clever tricks like rsdoran mentions above but, these are tricks, if you please. Clever too, I might add!

Using line drop to get 120V on a 240V generator!!!!! Mercy!!!!!
And here I thought the only things I would learn here would be good things!!!!

Nobody can claim us sparkies are adaptable when cornered!!!! (SNICKER!!)

CENTER-TAP GROUNDED DELTA

Nominal Voltages
Phase-Phase Phase-Neutral
240 120 (phases A & C)
208 (phase B)

Used for commercial or industrial facilities with primarily three-phase loads. The high leg (phase B) must be identified, and is not usable for phase-to-neutral loads


GROUNDED WYE Nominal Voltages Phase-Phase Phase-Neutral
208 120
480 277
600 347

This is the most common system for large commercial office buildings at 208 volts, or industrial facilities at 480 volts with 277 volt lighting. All three phases can supply phase-to-neutral loads.

I have to disagree with that Dick, sorry. Delta at 60hz wired with a neutral will offer 240v line to line and 120 line to neutral.

A Double Delta @60hz will offer a better single phase option with line to neutral @120v and line to line @ 240v
http://www.friesen.com/manuals/PTO_Connection_Diagrams.asp

Y'all give me a few days please. I have decided to make this my first project for my new site. I will give as much detailed information as I can develop. Will also allow me to put the formulas back in my head.

Vetteboy,

I like the way you think! That is an elegant simplification!

rsdoran said:

I have to disagree with that Dick, sorry. Delta at 60hz wired with a neutral will offer 240v line to line and 120 line to neutral

Click to expand...

OK, now I'm REALLY confused. After looking at Dick's Center-Tap Grounded Delta picture, I thought I now saw how those "red leg" systems are generated.

Then you (Ron) say "No"...

I never knew that the "red leg" (or "devil leg", or "high leg", or whatever you want to call it) was 208V to neutral. I always thought it was 240V to neutral. I've never measured it, I just know not to use it for single-phase loads (and ***** about not having nearly enough single-phase circuits).

We have that type of supply at our facility, so I whipped out my meter. Sure enough, you get 240V phase to phase (A-B, A-C, B-C), 120V phase to neutral on A and C, and 208V phase to neutral on B. Actually, it's closer to 215V, but still a heck of a lot lower that 240V...

I always thought this was a "trick" the power companies used to provide 3-Phase in remote areas that previously didn't have it. Just throw a transformer at it...

So, Ron, can you elaborate on why it doesn't work that way?

Just wondering...

beerchug

-Eric