3 Phase to single phase

[nathanaelf]

Hi,
I am about to move my wood shop into a building that has a 240v 3 phase service. I don't have any 3 phase tools yet so I need to figure out how to run all of my single phase 240v tools. The existing sub panels in the place that seem to be single phase (they have double pole breakers and two hots and a ground) are just running off two of the 3 phase legs. This contradicts all of the advise I've gotten. Everyone I talk to says I need a transformer to bring the voltage down. Are transformers not needed for 240V 3 phase systems?
If I am supposed to run off of two of the 3 legs won't each leg be out of phase with each other and be 240v each instead of 120v each?
Will this work with a 240v single phase induction motor? I don't want to blow up my table saw.
Thanks,
Nate

 

[crutschow]

Just measure the voltage across the two phases. If it's the correct voltage then it should be OK to run your tools. If not then you may need a transformer or a different connection to the 3 phase.

 

[BrownOut]

Everyone I talk to says I need a transformer to bring the voltage down.

Be careful who you talk to Heh! You should be good to run with two phases for your 240V tools. Use phase to neutral for any 110-120V tools or appliances.

 

[Reloadron]

Just my take but you may want to ask someone if the three phase power @ 240 VAC is configured as Wye or Delta. Now in either case the phase to phase voltage between any two phases will be 240 VAC. However in for example Delta configuration there is no refrence to neutral or a neutral line. In the case of a 240 VAC WYE configuration the phase to phase voltage is 240 VAC, however, in a WYE configuration there is a neutral. But, the voltage from any phase to neutral is not 120 VAC (half the phase to phase voltage) but would be the square root of 3 divided by the phase to phase voltage or 240 VAC / 1.7230 = 139.29 volts. Anyway, before all else I would find out the three phase configuration the building has. I believe newer buildings have WYE but not sure. Where I am I work with 480 VAC three phase Delta. This Link is a good read on the subject.

I believe BrownOut's statement "Use phase to neutral for any 110-120V tools or appliances". He was thinking about the 240 Volt mains that enter a residence in the US where either line to neutral is 120 VAC.

Ron

 

[nathanaelf]

Thanks for the replies,
So if all three phases are 120v from phase to ground I have a delta configuration?
And if I get anything other than 120v from phase to ground I have wye?
And in the the case of wye I'd need to make sure I use the 120v phases for all 120v and 240v single phase tools?

 

[Reloadron]

No, if you have 240 VAC 3 phase the phase to phase voltage will be 240 VAC, that is true of a WYE or DELTA configuration. Both WYE and DELTA are used for 3 phase power distribution in buildings and factories. In a 3 phase delta configuration there is no neutral leg, therefore in 3 phase delta there is no reference to a neutral or ground. If you do measure from any phase to ground you will see a voltage but it is an undefined voltage. Now a 3 phase wye configuration is referenced to a neutral leg. there is a true and defined neutral. The voltage from any phase to neutral is the square root of 3 divided into the phase voltage. So in the case of 240 VAC 3 phase wye that would be 240 / 1.7320 = 138.56 VAC. There is no 120 VAC. The only way you would get 120 VAC is from a 208 VAC 3 phase wye configuration where the phase to phase is 208 VAC and any phase to neutral (neutral exist in a wye configuration) to ground. In this case we have 208 / 1.7320 = 120.

There are two ways to derive 120 VAC from a 240 VAC wye configuration. The first is the use of a transformer with a 240 VAC primary connected phase to phase or using a three phase transformer with a phase center tapped and that is an unusual setup. The latter would only be seen in a case where maybe 480 VAC came in at an entry and then a transformer in the facility. The transformer secondary would have one phase center tapped.

Looking back at your initial post there is something strange about this part:

I am about to move my wood shop into a building that has a 240v 3 phase service. I don't have any 3 phase tools yet so I need to figure out how to run all of my single phase 240v tools. The existing sub panels in the place that seem to be single phase (they have double pole breakers and two hots and a ground) are just running off two of the 3 phase legs.

Now in any residence in the US the entry power is generally 240 VAC split phase. The power from outside is the secondary of a transformer with a center tap. The center tap being neutral (ground). From either line to ground is 120 VAC and line to line is 240 VAC. If you open the circuit breaker panel there are two columns of breakers. If you measure between any two breakers that are side by side in a column you will measure 240 VAC. If you measure either breaker individually to ground you will see 120 VAC. Therefore if I want to power a 240 VAC device like my air conditioner I come off two breakers side by side in either column and get my 240 VAC. When this is done the breakers are "ganged" and tied together so if mechanically one breaker trips it takes the other with it mechanically. This is not 2 phase or poly phase but split phase power.

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications.

This link explains split phase power as in residential and light commercial applications. The link I posted earlier explains 3 phase power in wye and delta configurations.

What you describe looking back at your original post sounds more like split single phase power rather than three phase power. With the exception of your mention of 3 phase 240 VAC. Thus I suggest you find out exactly what is coming into the building.

Ron

 

[MRCecil]

The existing sub panels in the place that seem to be single phase (they have double pole breakers and two hots and a ground) are just running off two of the 3 phase legs.

Making the assumption you live in the US and from your description, the service supply could very well be from an open delta transformer on the pole (cheaper for the power company). That would give you, from T1 to T2 240V, from T2 to T3 240V but from T3 to T1 277V known as a "wild leg" in the trade. I would recommend that you have a qualified, licensed electrician check your system or get your power company to identify what exactly what type of transformer, on site, so you know for sure how to proceed.

If you have access to a meter, you can measure the voltage across all three legs at the main breaker if you feel comfortable about doing so, but be very careful. Also, is there any marking tape around the service conductors, and if so what color is the tape? Can you take a picture of the open panel and post it?

 

[nathanaelf]

Thanks,
I am planning to take some pictures of the transformer on the street and the main panel inside to post. I have this close up of the meter.

 

[MrAl]

Hi,


Yes be very careful when measuring the line voltages because these are definitely deadly, and being in the basement it's easy to have the feet grounded and accidentally touch something and this is how the heart gets stopped in many electrical shock accidents (arms through body through legs through feet to floor or ground).
You also need a meter that is definitely made to measure these kinds of voltages, keeping in mind you might encounter 440vac if it happens to have that in there also.
There's the "One Hand Behind Your Back Rule" too, for measuring voltages...this is where you connect the black lead to say neutral with a clip lead all with one hand behind your back and then take the red lead and probe with that one hand only. This prevents accidental hand to arm to body to arm to hand electrocution.

I would think if you measure 120vac then you have 120vac, if you measure 208vac then you have 208vac, if you measure 240vac then you have that, etc., using a meter set to read ac volts. There are a number of ways to extract power too, either line to line or line to neutral (Y system), so you'll get a different reading line to line than you will line to neutral.

If you know someone who works with this stuff you could ask them to take a look too.

 

[nathanaelf]

the lines in the top left of the picture go to my building

 

[nathanaelf]

the weather head

 

[nathanaelf]

the main panel is on the bottom, the one on top goes to two 3phase panels across the shop.

 

[nathanaelf]

If you zoom in on the bottom panel you can see that one of the main lines in is marked with red tape. the other two are supplying the single phase panel in the next picture.

 

[nathanaelf]

single phase panel

 

[blueroomelectronics]

I would suggest calling a qualified Electrician.

 

[BrownOut]

In the "single phase" box, measure the voltage between the wire with the red tape and the one with the white tape. That is most likely your 120V. Also, you should have 120V from the blue tape and white tape. If so, then any single pole breaker should give you 120V and any two pole breaker should be your 240 or 208V. You can make these voltage measurements safely, but pay attention to what you're doing, and please be careful.

 

[nathanaelf]

Is "single phase" the appropriate term?

Is there any way to tell from the pictures whether it's delta or wye?
just for future knowledge.

thanks!

 

[BrownOut]

Is "single phase" the appropriate term?

Not really. There are actually two phases and neutral. But we know what you're talking about.

Is there any way to tell from the pictures whether it's delta or wye?
just for future knowledge.

See the uninsultaed wire in your picture of the weather head? That is your neutral, and suggests a wye scheme. Also, note the wire with the white tape, and how it's connected to the frame of your box and not the power rails (there is a strip where all the neutral returns connect together). That is your neutral.

 

[tcmtech]

By looking at your pictures it would appear that you have the common style of 120/240/240 delta connected power system.
That is you have the primary power available as the 120/240 volt single phase source being supplied by the big 75 KVA transformer with a second 25 KVA 240 volt transformer supplying one leg of the three phase with the third phase leg being supplied as a 240 volt phantom phase.

This is a very common design for places that have most of their electrical load coming from 120 or 240 volt single phase equipment or other systems and only a limited amount of equipment that needs a full three phase power supply.