|> |> Having been in a situation where we had tens of thousands of dollars of
|> |> damage and even more loss due to down time, all because of 240 volt
|> |> being run at 208 volts, and burning up, I can definitely say there are
|> |> some issues with the way they have done three phase.
|> | Motors rated at 208 (OK, namplated 200) are there for a reason. Draw
|> | power at all, and 480 becomes a option for motor loads.
|> The motor was in an air conditioner that was probably being overworked due
|> to the added equipment. It was replaced per manufacturer spec, twice and
|> still kept burning out. Finally someone had the idea to find a matching
|> motor of another manufacture rated for 208, after he realized the motors
|> that were being put in were for 240. That finally did the trick. But it
|> was with 4 times total shutdown of equipment when the A/C was down, while
|> they went up to the roof to repair it.
|> So basically, as I understand it, motors design for 240, especially if not
|> well designed for heavy duty, will fail at 208 if worked hard enough. But
|> with more 208 around these days, it would see that's what things should be
|> designed directly for, instead of retrofitting.
| Many motors specify a service factor for 240V and no service factor for
| 208V. Refrigeration systems are a common problem since the designer of the
| system knows exactly what the load will be and they don't put a service
| factor on their motor load. That's why AC's and fridge's are so dang
| sensitive to low voltage. Because of this problem, 208V AC units are widely
| available. If it was done 20 years ago I understand the mistake, but if it
| was recent, the AC contractor was negligent.
The very same AC system worked for every other tenant in the building.
Each tenant had the same system per unit (though a couple tenants used
2 systems because they rented 2 units together). The AC system was
there before the owners rented the place, which was before I came to
work there. It was failing at a point we had about 50 computers in
one room. This had been an issue brought up with the landlord who
didn't want anything added on by us. He said if the AC would not do
the job then he'd upgrade the AC at no cost to us. They did have a
rather substantial about of BTUs and rough calculations suggested they
would work fine as there was plenty of capacity. I think the problem
was that the voltage got changed after the AC was put in. Each unit
has its own 208Y/120 transformer fed by 600 volts. And that was a
change from something they had before that. But the tenant in the
middle building (of 3) who had the whole building, needed 600 volts
for some reason. So they changed things around to give them 600.
So I suspect things were 240 prior to that, and the AC system had
originally been selected for that. It all did have the look that the
electrical had been majorly redone at one point. The mater bank was
all new. The 600 volt main in each unit was new. The transformer was
new. But the build was older as was the AC. So I suspect the power
company foisted a change from 240 to 208 on the landlord as part of
the agreement to get 600 volts in (unusual in the USA). Or it could
have been the electrical contractor. But I do know as soon as they
had a different model of motor (it was the blower motor) put in, the
AC system worked and worked well (it definitely had the capacity but
it would run constantly in the peak of Texas summer). The building
looked to be 30 to 50 years old (all 3 of them).
| Was this in Canada? The standard industrial voltage is 600Y/346. That
| votlage is also common in many mills since their EEs figured out that they
| could get more power through the same breaker.
No. It was in Texas. I don't know what necessitated the 600 volts,
but I know the tenant in the middle building had 800 amps of it going
in, whereas the small tenants in the other 2 buildings had only 80 amps
per unit based on the transformer's disconnect. I don't specifically
recall the transformer rating, but thinking back on it I'd have to say
it must have been 75 kVA. The padmount transformer that was feeding
the 3 buildings looked like a well oversized unit, so I might guess it
was 1500 to 2500 kVA.
|> If we could just standardize on 480Y/277 for everything three phase and do
|> lower voltage only in single phase 120/240, and say bye-bye to 208, that
|> would seem to make life easier.
| Somewhat. They would have to worry about phase ballance more. Plus they
| can share a neutral with a three wire multi-wire branch circuit on 208Y/120
| instead of two wire ones with 120/240. Doing as you propose would eliminate
| the problem with neutral current.
Or you could have 3 single phases, which would be a "6" phase if you put
them all together. That would be my preference. Instead of a 150 kVA
208Y/120 you'd have 3 separate 50 kVAs for 120/240.
|> As long as the USA is a country where it
|> is standard to supply single phase to homes (exceptions exist, but this is
|> the standard), this is what I think the need is. While I do occaisionally
|> see some heavier duty single phase appliances that are designed to work
|> fine on 208 volts (like a stove with special elements), most only have
|> 240 volt versions. And some that do have 208 specify a 60 amp circuit
|> instead of a 50 amp circuit for 240.
| The place where I absolutly agree is with network service to homes &
| apartments. That would just suck to have only 208 available as the higher
| power level, other than the fact that with a transformer you could make the
| third phase and have three phase to use in a shop.
Those situations tend to be big apartment buildings, so a shop is unlikely.
But I do know how to wire up a transformer to re-derive the third phase from
just the 2 provided.
What I would do if designing a big building intended for multi-residential
is have a vertical stack of electrical closets and run 480Y/277 vertically.
Then on each floor put in a single phase 480 -> 120/240 transformer. The
phases would be diversified by floor. If it was larger than one transformer
could handle, then maybe 2 or 3 per floor. But I would definitely want to
provide genuine 120/240 for residential and even for office tenants. Then
if there was any specific need for three phase somewhere (e.g. elevator in
the commons, central A/C), run that from the 480. Exterior security lights
could be run from 480 or 277.
|> Their big loads are 400 volts. Three phase in Europe is 400Y/230 and it
|> more commonly available to homes in continental Europe.
| In homes? Just how often are residential loads 400V? I don't see much
| difference in saftey between 120V or 240V. But I see a big one with 400V.
| 400V is cabable of systaining arcing faults.
Three phase to home is common in places like Germany. And you know they
have 230 volts L-N. You can do the math.
BTW, I have been looking for some information on the physics of arcs and
how and when arcs can or cannot be sustained. What little I have found
suggests that what type of metal (e.g. copper, aluminum) is involved is
part of the determination of what voltage would sustain an arc. That
gives me the idea that it has a specific voltage drop at the arc interface
and once exceeded by 2 times (once for each wire) the arc can sustain.
Most of that is guessing, and I can't find much about it, otherwise. What
I'd like to know is if what I've been guessing is true, and if so, what
the particulars of different metals are with regard to sustaining an arc.
That and how many amps would I need to sustain one.
I've seen where people can get a nice arc from a Tesla coil or a pole pig
in reverse, but I'm sure that's way more voltage than they need just to
sustain the arc.
| I could argue that their industrial systems are inefficient since theirs are
| only 400V and ours are 480V.
They do have a next step up at 690Y/400. But either way it's 50 Hz.
I do find it interesting that the ratio between their voltage in the same
class, and ours, is also the same as the ratio between their frequency and
ours. A transformer could be designed for 400Y/230 50 Hz or 480Y/277 60 Hz
and have the same saturation point. The same thing also exists between
North American (120 60 Hz) and the eastern half of Japan (100 50 Hz).
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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