I'm not sure whether this was satisfactorily cleared up. Anyway, the message
was still on my list of Classiccmp mails to be answered when I finally caught up
with reading them this pm.
>> > Realy 234V (strange) and 180 degree ? Where do you know ? By definition ?
>> > Just remember, if you tap between 2 phases of a 3~ signal you still get
>> > a perfect sinus and you can't decide if it is a single phase or part of
>> > a 3 phase signal. Ok, I'm always learning new things on this earth :9
>> > The numbers just saemed to fit well.
>> I'm sure the power company has a legal obligation to keep the voltage
>> within a certain range of values. Although, I don't know exactly what that
>> range is. I think 117 is probably the optimal value for each of the legs
>> giving 234 for the total. I've heard it referred to as 110, 115, 117, and
>> 120. Since the actual voltage varies any of these could be correct.
>
> Anybody out there knowing the US regulations ? AFAIR somewhen in the
> late 70s the acording definitions have been worldewide standardized
> on 115V/200V and 230/400V - the intention was to allow manufacturers
> (especialy the ones for power transmission systems) to develop more
> efiecently and to lower building costs, since within the lower power
> range (up to some MW) designs could be standardized on a simple level.
>> Since it is a simple transformer, the phases are 180 degrees out of phase
>> (by definition). That's assuming a balanced resistive load. Of course, that
>> could change under a heavy inductive or capacitive load but, that's an
>> extreme and won't normally present a problem.
>
> Still not an argument against 2 phases from an 3~ signal.
1. Voltage
The voltage will be declared by the supply company to be some value +/- some
tolerance. This will usually be according to a national standard.
The transformer will usually have a higher voltage on its rating plate - if the
nominal voltage is 115 or 117 volts, the transformer may be rated at 120V.
People near the transformer will get 120V, people at the far end of the cable
will get 110V. My house is very near the transformer so I get 248V on a nominal
230V supply (NB when it was nominally 240V I got 248V. Nobody has adjusted the
transformer...)
2. Phase.
As I understand it the usual practice in the US is take _either_ a phase and
earth (neutral) _or_ two phases of the HV supply and feed a single phase
transformer from this. The LV side of the transformer is (say) 240V with a
centre tap. The centre tap is earthed and provides the neutral connection, the
outer two taps are then both hot at 120V, 180 degrees apart. This is true _even
if_ the primary (HV winding) is connected between two phases of the HV 3-phase
system.
In the UK the usual practice is to connect all 3 phases of the HV (almost
invariably 11kV phase to phase) to a delta-star 3-phase transformer. The
secondary, the star-connected winding provides 3 phases and neutral. Neutral is
earthed at the transformer. Protective earth can be provided by _either_ a
separate wire from the main earth at the transformer _or_ an earthing electrode
at each customer's site _or_ the neutral is earthed at a large number of points
on the system and the customer's protective earth is bonded to neutral at the
point of entry of the supply. In all cases the customer sees 240V line to
neutral; if he gets more than one line (hot) connection, they will be 120
degrees apart.
In remote locations, only 2 phases of the 11kV system are taken to the site.
Small loads get a single phase 11kV/250V transformer with one end of the LV
earthed; larger loads get a single phase 11kV/500V transformer with a centre tap
earthed. Some farm equipment (I am told - I have never seen this) is rated for
480V single phase.
It is very unusual for a domestic installation to get more than one phase and
neutral. But 3-phase can be done - the supply company don't mind putting it in
if they think you'll buy a lot of electricity!
Philip.
Received on Mon Apr 26 1999 - 12:17:13 BST
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