3- to 1-phase?



Thanks for the information!
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On 26 Feb 2007 01:28:12 GMT, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

Right. The normal convention is to mark the High Leg with Orange tape or paint everywhere it shows up, and put it on the B phase - middle busbar. And put a big warning on the face of the panel, to try to rein in the clueless before they do something very stupid.
I've hit it a few times where a "Handyman" fixes the lights at a condo or commercial building, or adds a new circuit - and then the Management Company calls and says 'the bulbs don't last long...'
Yup - new breaker on the High Leg for the new circuit he added. "Well, there was a vacant spot, so I used it..."

Yup. Home Depot is even stocking a few variants of 3-phase 240V panels and breakers, if you want to put your hands on one to see. But most commercial gear is rated as Panelboards which gives you bolt-on breakers and a lot heavier construction. If it's mostly motor and equipment loads, you can even go larger than 42 breakers in the panel, and 400A main busses in a bolt-on panel are not too uncommon.
Sometimes you even see four-pole breakers - but the last one is a shunt trip coil for electric fryers and griddles at a restaurant, so the fire suppression system can kill the equipment power.

If it's a 3Ph Open Delta panel and you don't use up the B phase stabs as A-B or B-C for all the straight 240V 1Ph loads, you start running out of 120V (Ph-to-N) A and C phase spaces real fast.

You only have to worry about that on ranges and clothes dryers, because they DO have 120V loads on them. Or if it goes to a 4-wire 120/240V rated receptacle for portable items like welders and industrial floor buffers, because you never know what portable equipment they're liable to connect. If the receptacle has a neutral prong in it they're going to assume that it's 120V on each leg.
On new construction the clue for 120V connected loads would be there is a neutral wire and it is connected, but on existing structures (more than about 5 years old) that is NOT a reliable method - you used to run the oven light and the clock/timer from one phase to ground.

Delta service is more for industrial parks and small commercial spaces where they will need some 3-phase, but they are all smaller occupancies where 200A of 120/240V 3Ph is plenty, separate 120/240 1Ph and 277/480 3Ph services would be overkill. (And another monthly meter charge to pay.)
Getting supplied at 277/480V and running a local transformer in each unit to run the 120/240V loads is a PITA for small stores and shops, and it's more expensive on several fronts.
First, 480V switchgear is a LOT more expensive than 240V, and then you have to buy a step-down transformer and another breaker panel for each unit, and when you want to work on the 480V you have to turn off their lights and computers too.
And you have to turn it off to work on it - 240V isn't so bad (a few sparks and the main pops) but working on 480V hot is insanity...
Open Delta is very common on Condos or Apartments - they put up a big transformer (50 or 100 KVA) for all the single-phase loads in the building, and they hang a small second transformer (10 or 25 KVA) to get the third leg for one 3-phase service for the Elevator(s).
They can have one 200A 3-Phase service and panel for house loads, or on larger buildings they use a 400A or 600A 3Ph Standing Section, send a 3Ph breaker straight to the elevator machine, and send A and C phases past that to regular single-phase panels for house circuits.

More common on Condos where there's one service transformer for the whole building, and not much 240V load where people would complain about slow electric ovens and wimpy cooktops... (That's fine, most people prefer Natural Gas for that, the bills are a WHOLE lot lower.) They put in one three-phase House Meter for the elevator, three-phase disconnects, and then the different Modular Metering stacks for the units are rotated through the phases to balance the loads. First stack of meters is connected A-B, second B-C, third C-A.
Welcome to R.C.Metalworking - where if you aren't really careful you might learn something. ;-P
--<< Bruce >>--
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So how does that work? The fire system just shorts that 4th leg to ground or puts a load on it to cause the system to trip off the power?

Thanks! Lots of good new stuff to know! I never thought about why an apartment building would need 3 phase power but the fact they have elevators and that elevators would use it is something that seems obvious. I guess the air handling systems in larger buildings (like a hotel or office building) are probably all 3 phase as well.
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You might want to look at this: http://en.wikipedia.org/wiki/Scott-T_transformer
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This rather ingeneous system converts between two phase (quadrature) and three phase systems. What we are talking about here is single phase and three phase... Big difference...
Jerry
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snipped-for-privacy@gmail.com wrote:

Yeah, very neat. I had never heard of that.
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wrote:

I think that it converts electricity to two-phase (four wire), not single phase (two wire).
i
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wrote:

Yep. (Anyone for another single vs two phase thread???).BG
Don Young
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On 26 Feb 2007 21:00:47 GMT, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

No, the shunt trip is electrically isolated from the rest of the breaker. They are usually 24V or 120V coils, even when the breaker is switching 480V.
The fire system has one or two Form C (SPDT or DPDT) microswitches on the initiator arm for triggering alarms and external equipment. When the fire system discharges the switch closes and sends 120V to the shunt-trip coil - the breaker has two wires sticking out like a GFCI to connect to the trip circuit. The solenoid coil of the shunt trips the breaker the same way an overload would.
If the equipment in question has a motor starter, it's far easier to wire the Normally Closed circuit of that fire-system trigger switch to drop out the starter coil. Then you don't have to go buy a special shunt trip breaker.
(Just did a paint spray booth that way - the fire system kills the starter for the exhaust fan, and the exhaust fan starter dropping out opens the auxiliary contacts and the power to the air supply cutoff solenoid, which cuts air to the spray guns. The spray air is interlocked so they can't spray without turning the booth exhaust fan on first.)
Oh! There's another oddball breaker with wires sticking out, but you won't ever see it except at gasoline service stations, to power the turbine pumps in the storage tanks - Switched Neutral Breakers.

Large motors are inherently more efficient running on 3 Phase, because it's kick-kick-kick instead of kick-coast-kick-coast.
And with the constant starting and stopping of elevator pump motors, they live far longer on 3-phase - KISS, no starting capacitors or start switches.
It would make sense for Condos to send 3-Ph 208V up to the units, if only for the AC Condensing Unit compressor motors - since every little bit of added energy efficiency helps reduce overall demand. They even used to make special sub-panels just for this purpose that had one or two 3-Ph breaker slots at the top of the buss, and the rest were regular 1-Ph...
But some power utilities <coughL.A.DWPcough> have this absolute neurotic phobia against supplying any sort of 3-Ph power for end-use by a (Gasp!) residential customer.
--<< Bruce >>--
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Ah, so the fire control system has to simply send the correct voltage to cause it to trip. Simple enough.

It's even better than that. I just learned about this in the past year. I never would have guessed it.
With single phase, the torque goes up and down with the AC cycle as you say. But with 3 phase, the back and forth forces sum and the result is a constant torque. Not just smoother, but actually constant (at least in theory - in practice I think the motors are not perfectly linear so there is some small variation in the torque).
Here's a simple animated graphic from wikipedia that shows how the three vector forces sum to produce a constant rotating field - the end of the three arrows produces a constant rotating vector of fixed length (aka fixed torque):
http://en.wikipedia.org/wiki/Image:3phase-rmf-320x240-180fc.gif
This also works with only 2 phases 90 degrees apart. It produces the same constant torque either way. But to deliver 2 phase power would require 4 wires (2 for each phase). Or, if it was cut down to 3 wires by making one side of each phase a common, it produces a higher amount of current in the common wire. It will have to carry 1.414 times more current than the two phase wires. This means for power delivery, you would have to use either 4 wires of the same size, or 3 wires with a larger wire for the common. But with 3 phase, where each wire acts as a common between two phases, only 3 wires are needed and all 3 wires carry the exact same current load which gives the advantage of constant torque motors with only 3 equal sized wires.
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snip-----

Home shop types are known to buy surface grinders that are designed around three phase motors, then change them to single phase because they lack three phase service. Big mistake in many cases. The uneven power pulses translate into surface finish problems that are virtually impossible to eliminate.
Three phase motors are not only smoother, they are also much cheaper to build because of their simplicity.
Harold
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On Wed, 28 Feb 2007 05:18:00 GMT, with neither quill nor qualm,

Are there differences in this regard between static and dynamic 3-phase converters, 'Arry?
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Dunno, never having experienced either, but if I was to venture a guess, yeah, it's possible, although at a diminished rate due to small discrepancies in phase angle. Properly balanced, it's possible that each of them could perform equally to properly generated 3 phase. Bruce would be the best person to address this issue, at least from the electrical standpoint. I think that each grinder would have its own characteristics. No reason to not-----they already do have. Some are just created better than others.
Harold

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Yes,assuming the voltage and frequency matches the motor nameplate data. Measure the voltage between the two wires you have chosen to be sure.
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Happens all the time. But since you had to ask the question it indicates there might be a few more things you might need to know.
Tell us about your project.
Wes
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snipped-for-privacy@lycos.com wrote:

Thanks Wes and all.
I don't have a current project - just something I wondered about and thought would be good to know for reference. I'd been aware that in theory this was possible, but thought there might be a "gotcha" in practice. As I thought, the answer was right here. Long live RCM.
Jordan
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Jordan wrote:

Jordan as you seem to be posting from Australia I wonder if your mains is the same as that in the UK. My understanding of UK mains is the single phase is one 3 phase leg and neutral giving 240V between phase and neutral but the voltage between any 2 3 phase legs is 415V due to the 120 degree phase difference. As was mentioned by another check the voltage before connecting. AFAIK the UK setup is the same across Europe but the US seems to have many 3 phase supply arrangements from what others have said at various times on RCM.
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David Billington wrote:

That's right - 415/240V 3 and 1 phase in Oz.
Seems like all I need to do is check for the correct voltage (using RMS meter?) before connecting, and it'll work OK. Earthed, of course.
Jordan
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Depends on if you are using a three phase generator or not. My 50kw three phase generator can do single phase by using two of the three legs but on heavy draw it creates an imbalance that shuts the generator off. Dixon
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That's interesting, but I think it's more likely I'll be in a situation where there's 3-phase line power (say, a factory) with no single phase outlet close by.
Jordan
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