3 ph question for Mr Sparks, please?

[ivan]

Like others a bit too long in the tooth, if you're not used to 3 ph distribution it can be a bit puzzling - not least because the supply people talk power all the time, and with voltage normalisation, "415 V" isn't really 415 V any more.

Living in indian country, I can see the 3 phases and neutral, strung along the poles outside in the road. Star generator configuration gives (a fairly wide tolerance) 230 V between any phase and neutral. Balancing single phase loads by normally connecting houses to each phase + neutral in turn, results in much lower current in the neutral line. Our house actually has a 4 wire feed. Occupied all day, and with offpeak elecricity cheaper than oil, (or even gas per kWHr) we have a lot of storage heaters. These are spread over 2 phases switched by a contactor, with the noramal 80A domestic supply on the other.

If I have a 3ph distribution board fitted I recon I'll have 230 x root3 = 398 V between each phase, which I presume is the modern equivalent of "415 V" suitable for a 3ph motors?

May moons ago in a holiday farm job, I had some experience logging, where timber was sawn for fencing etc on a large 35 Hp saw that had to be started in star and manually switched to delta when up to speed. Even then it markedly dimmed the lights for several seconds. I presume this was wound for 415 V (as it was then) in delta, so it would have been a " 718/415 V" motor?

If I have got it right so far, then why are modern 3ph motors wound as 400/230 V (star/delta connection available on the 6 terminals)? Where do you find 230 V 3ph?

thanks in anticipation,

"puzzled" of Devon!

 

[Nigel]

I am also long in the tooth and have spent my working life on electrical distribution,
The simplistic answer is on supply cables single phase =240v,
3phase =415v measured between phases,
A star /delta motor is run on 415v or in industrial use can be 3.3 kva
(never heard of 718v)
all these numbers are as you say pre harmonization,
If you are looking to run a three phase motor from 240volts you will need a converter and then not all 3 phase motors will suite,

Thats just lifting the lid of a very big can

Cheers Nigel

 

[Racers]

Hi,

I thought "normalization" just meant that they adjusted the tolerance so we could still have 240V instead of rewiring all the transformers on the national grid.
I am getting 242V at 20:28 in Nottingham.


Pete

 

[Nigel]

Hi Pete
when the power companys transmit the supplies to us they are allowed a certain tolerance it used to be IIRC +6% and -3% on 240volts

under harmonization these tolerances were expanded a bit but I don't have the exact figures to hand but it went up to something like 10% which gave them a bit more leeway although it makes no difference to the original question,

Cheers Nigel

The nominal European voltage is now 230V 50 Hz (formerly 240V in UK, 220V in the rest of Europe) but this does not mean there has been a real change in the supply.

Instead, the new "harmonised voltage limits" in Europe are now:
230V -10% +6% (i.e. 207.0 V-243.8 V)

in most of Europe (the former 220V nominal countries), and
230V -6% +10% (i.e. 216.2 V - 253.0 V)
in UK (former 240V nominal)