Q for helpful 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 (now 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,

[old]

I don't understand the question. a 3 phase supply and a neutral four wires.phase to phase 415v .any phase so three times 240v to neutral. So its like three 240v supply's at 120 degree phase spacing of each one so connect to a motor and you have a rotating magnetic field.

[ivan]

Sorry, Old;

A) I was seeking confirmation that my somewhat unusual domestic supply carries the same 3 phases as an idustrial supply. I am a bit confused as the interphase voltage is just under 400 V, not the 415 usually talked about.

B) Big 3ph motors have the 3 windings brought out to 6 teminals to allow star-delta starting to reduce start up load on the distribution system. It seems to me that attempting this on a 400/220V motor (which also has the 6 teminals nowadays) would burn out the windings when switched to "run" (ie delta) - so why are motors made as 400/220V? Where is there a 220V 3ph supply? This is an area I'm puzzled about, and am hoping for clarification!

[Scrit]

Jun 10, 2008 19:01:25 GMT ivan said:

Where is there a 220V 3ph supply? This is an area I'm puzzled about, and am hoping for clarification!

The United States, Canada or anywhere else (mainly the Americas) which runs the American 60Hz generation system.

The European standard 3-phase (phase-to-phase) voltage is 380 volts (which corresponds to 220 volt in single phase - ours is technically 230 volts these days.....)

Scrit