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Author Topic: Residential three phase power --------------------------------------  (Read 43453 times)
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Tim WA1HnyLR
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« Reply #25 on: August 18, 2009, 12:58:42 PM »

I wish I had three phase power here. There is only single phase up here on Henry Nyellar mountain. Three phase is about two miles from here. I have had the use of 3 phase power at my work station when I worked for Armstron transmitter. I used it to run Big Mac a pair of 4-1000s modulated by a pair. Originally this transmitter had a single phase 4Kv supply. Dale(VE3AAM and I worked together for a while. We spent a number of evenings removing the single phase power components from the McMartin BA 2.5K and sliding in the iron from a Gates 5P . More power!!!. At full strap the plate voltage was about 4800 voltys the way I configured the iron. The high power/low power switching was easy. The primaries of the plate transformer were in Delta for full strap mode and Wye for Cruizin' mode. Very smooth transition from one to the other. In closing: If you are lucky enough to have 3 phaze power in your residence make use of it. Time to put that big maul on the channel cawwmmmn ownnn
DeTim WA1HnyLR
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N0WEK
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« Reply #26 on: August 18, 2009, 02:33:22 PM »

Back in the 1970s a friend moved his wood shop from an industrial building with 3 phase into his garage. His table saw was a 5 hp Rockwell Unisaw, the radial arm saw and air compressor were both 3 phase as well.

I bought a small, $40 box from Northern Hydraulics, which at the time was one small shop in Burnsville, MN. It ran the table saw and radial arm just fine but wouldn't start the compressor on cold mornings. I finally wired the box to one 240 volt breaker and added a 1 HP spare motor that ran as long as the breaker was on, which improved starting. The usual drill was to come out in the morning, fire up the wood stove, switch on the breaker for the three phase, fire up the radial arm to provide more power for starting the air compressor, start the air compressor and shut off the radial arm saw. The compressor started fine by itself after the first run to warm up. We ran that way for several years until he moved back into a building that had three phase.

I don't know what was in the box but I'm sure it was just a few oil caps.
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W7TFO
WTF-OVER in 7 land Dennis
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« Reply #27 on: August 18, 2009, 05:49:42 PM »

Two-transformer distribution or open-delta is an approximation of three phase power.  One transformer is giving two legs & neutral, at 180 degrees phase.  The other transformer is tied across the leftover leg to give the remaining 120 degrees.  This is akin to the Scott connection.  It is easily identified by the presence of a 'wild leg', with a higher leg-to-neutral than the other two.  It cannot give equal voltages to neutral like real three-pig service.

This is widely used for motor loads, where the rotary inertia carries the deal OK.  Don't buy it for eventual use in power supplies, as it will not easily power a single-core transformer (such as a plate transformer) down the line due to the screwy phase relationship.  I will generate a host of unfriendly harmonics in the ripple, usually leading to rectifier and filter failure.
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KE6DF
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« Reply #28 on: August 18, 2009, 07:08:14 PM »

This discussion has now gotten me wondering which type of system I have.

We may have the wye configuration, but then, if I'm understanding this correctly, the single phase systems must run on 208 (for those that don't run on 120).

In that house, there is an electric range and a 5 ton AC.

It's possible the range might be running on 208 even though if it is a 240 volt model. The burners would not get as hot, but the oven would probably work because it's thermostatically controlled -- even if it's only getting 208. It would heat up slower though.

The AC, however, must either be a 208 volt single phase unit, or perhaps 3 phase. I assume it wouldn't work properly if it's desinged for 240 and only getting 208.

Looks like I need to do a little more checking.

I wish I had brought my DVM on this trip.

Or perhaps I've got one of those unbalanced systems, but I can't see my dad ordering that back in the '50's when he put in the three phase AC conpressor.
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N4LTA
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« Reply #29 on: August 18, 2009, 08:58:27 PM »

Have to disagree on the open delta bank being an approximation of three phase. It is true three phase just like any other three phase delta delivery. I am a power system engineer - been doing it for nearly 40 years.

If you have two solid voltage sources spaced 120 degrees apart delta connected - then the other point on the delta must be 120 degrees from the other two - there is nothing approximate about it - it looks like and is the exact thing as if you used three transformers - except in the capacity that is available from the two transformer rather than three.

The high leg is due to the nature of the tapped winging - not due to any phantom principal.

Draw a triangle to scale  - side 1 is a center tapped 240 volt winding going from a to b

From b to c is another leg - a 240 volts winding

The vector voltage from a to center tap or from B to center tap is 120 volts.

The vector voltage from C to a is 240 volts even though there is no winding -

the vector voltage from c to the center tap of the tapped winding is about 190 volts or the high leg.

All of these voltages are real and usable.

The open delta bank was a standard for many power companies for years. It was the standardization of the wye system at 208/120 and 480/277 volts that has slowly phased it out.
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« Reply #30 on: August 18, 2009, 09:42:08 PM »

If you do have Wye it's simple to figure out. You will have four wires to your breaker box. The voltage from leg to leg will be 208 volts. The voltage from any leg or phase to neutral will be 120 volts. It could be slightly higher or lower depending on the load. I have seen lows of 198 volts in the same configuration in factories. That was not good because our industrial battery chargers would not properly charge the fork lift batteries in their given time.  
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N4LTA
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« Reply #31 on: August 18, 2009, 10:11:23 PM »

With a wye system - 208/120  you will have approx 208 from any two phase wires and 120 volts from any phase wire to neutral. 208 volts was selected to give the standard 120 volts from phase to ground.'

In any grounded wye connected system the phase to neutral voltage is the phase to phase voltage divided by the square root of three. So 208 volts was selected as the standard phase to phase voltage.

Before the 208/120 system was common, the most often used system was the delta connected 240 volt system.

It consisted of three 240 volt transformer connected in a delta configuration. For motors and three phase loads only the voltage was 240 between any two phase wires. There was no voltage from phase to ground and there was no neutral. When 120 volts was needed , it was easy to center tap one of the three transformers and ground it which would give 120 volts from two of the phases to ground - BUT the other phase had the "high leg"  which caused problems.

The high leg was there no matter whether you used three transformers in a full delta or two in an open delta.
This configuration usually used a standard three phase panel and every third breaker space was blanked out.

Now , it is most common when only motor loads or balanced three phase loads are to be served, that a 3 wire 208 wye or 3 wire 480 volt wye system is used rather than a delta connection. The center of the wye is ungrounded and unconnecetd and no neutral is used. Since the center of the wye is not connected to ground  - thre is no voltage from line to ground.
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W7TFO
WTF-OVER in 7 land Dennis
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« Reply #32 on: August 19, 2009, 04:20:12 AM »

I still stand by the transmitter companies' statement:  Open Delta does not rectify to DC without problems.  If you used it anyway, it was at your own risk and you could expect to replace expensive parts soon and on your own.  RCA, Continental Electronics, Collins, Harris/Gates eTc.
 
                                      ...AND...

My neighbor has a two-pig open delta 240V service in his machine shop.  It will not play into a single-core 3 ph transformer, pops all the 200A fuses when connected.  I put a scope on it, and found the waveforms were just as I said:  180 degrees on the 120/240, and 120 degrees on the wild leg.  Not that I care, but that is the way Arizona Public Service set it up.  It is a moot point, as the local Co-op has taken over the entire grid out here and will be rebuilding all of it. 

I am already on with them, 480/277 Wye.  8.6c/kWh
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N4LTA
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« Reply #33 on: August 19, 2009, 09:14:01 AM »

No need arguing with you. You are obviously talking about something other than a two transformer wye connected primary, delta connected secondary bank.

The output is three phase - 120 degrees  - it would be impossible for it to be anything else -

And as I said  - the high leg is there in a two or three transformer bank - it is not phantom. - It's not to hard to prove - draw the triangle.



From a standard engineering text:

Open Delta Connection:

    The open delta transformer connection can be made with only two transformers instead of three (figure 1-8). This connection is often used when the amount of three phase power needed is not excessive, such as a small business. It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).
   
    Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).

    The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.



You and your scope are not looking at an open delta connection - or ..............your wrong?

This is pretty basic stuff - I'd would have no problem putting my stamp on it.



Hugh P. Bunn, PE, SC #9801

NC, SC, GA, VA,
Unlimited electrical contractor SC, NC, GA, TN



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W7TFO
WTF-OVER in 7 land Dennis
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« Reply #34 on: August 19, 2009, 01:22:18 PM »

Hi Hugh,

Obviously I am not looking at a correct setup for this guy.  That is why he called me over in the first place.  His service is wacky, and is already being re-engineered correctly by the growing electrical co-op.

My points were-

1:  If & when you ask for a 3-phase service, be sure what you are getting .  Just because the 'power company' sets it up and it works after a fashion, doesn't make it right.  If you know a P.E. have them look at the work order.

2:  Hams like us would most likely jump into the polyphase camp to run a vintage broadcast transmitter.  Hence my relation of the warning about power supplies.

There can be no argument with your statements, the physics of electricity is what is is.  The caveats contained in both your texts and mine relative to the various power distribution schemes are what I relate to as an 'approximation' in my previous post.

What I distilled out of it years ago was open delta setups are a cheap way to go, and Murphy's law finds them more often.

dg
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N4LTA
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« Reply #35 on: August 19, 2009, 03:09:19 PM »

I hope I didn't irritate you because I did not mean to.

There are a bunch of connection that generate a phase shift but not a true 120 degree shift and not true 3 phase. There are hundreds of stories of things worse than fuses blowing with them. There is no way you could run a three phase transmitter with one of these rigs effectively.

The best three phase system is the modern 208 or 480 wye system - as you said - but there are lots of old delta services still on the grid. They cause a lot of grief with the high leg and the confusion about motor voltage. Most 3 phase motors are still rated 230 volts  - they work fine on a 208 volt system but they cause lots of confusion - people ask for 230 volt three phase - and it usually isn't available - or needed.

I could tell you a lot of horror stories.

Pat
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