3 phase motor wiring

[ricicle]

400V RMS but peak to peak will be 400 x √2 = 565V peak to peak I think, not √3....but its a long old time since I actually learnt any of that 3 phase theory so may be talking complete rubbish...

The motor ratings will always be RMS values.

 

[Simon35]

400V RMS but peak to peak will be 400 x √2 = 565V peak to peak I think, not √3....but its a long old time since I actually learnt any of that 3 phase theory so may be talking complete rubbish...

The motor ratings will always be RMS values.

Very true...only relevance of peak to peak is for the motor and cable insulation. I dont come across many star delta started motors any more, used to work a lot on process automation software, lots of pumps on star delta, and packaging machine software, lots of obscure motor windings, two speed windings, two-phasing motors at start up...all interesting stuff though

 

[matt1e]

I think what he meant was that when connected in star, across each winding will be L-N voltage, 230V. When in delta, L-L voltage, which is 400.

Where does 680V come from??

The 680 is supposed to be 690v Simon, John is correct in his presumption of the working out
And it is the volage rating of the windings in a typical large motor when it is in star formation

A motor as such would be started by either the use of a star/delta starter or soft started in delta via a VSD

Matt

 

[JohnW2]

400V RMS but peak to peak will be 400 x √2 = 565V peak to peak I think,not √3....

Not quite. 400V RMS has a peak of (400 x √ 2), as you say about 566V, but the peak-to-peak would then be 1132V. However, we're not talking about either peak or peak-to-peak - all the figures being discussed are RMS.

...but its a long old time since I actually learnt any of that 3 phase theory so may be talking complete rubbish...

As above, almost correct (once you've doubled your answer to get peak-to-peak) - so not 'complete rubbish', although 'completely irrelevant'

What I'm talking about is the relationship between the RMS (or peak if you wanted) phase-neutral voltage and phase-phase voltage in a 3-phase supply. The latter is √ 3 times the former - hence,for example, the standard UK supply has a nominal voltage of 230V between each phase and neutral, but just under 400V (230 x √ 3) between any two phases. As I was saying, if there were 400V between each phase and neutral, the voltage between any two phases would then be (400 x √ 3), which is about 692.8V.

Kind Regards, John

 

[ban-all-sheds]

If, like me, one lives in Bucks, one still enjoys selective secondary education

That's where I lived when I enjoyed mine.

 

[Simon35]

400V RMS but peak to peak will be 400 x √2 = 565V peak to peak I think,not √3....

Not quite. 400V RMS has a peak of (400 x √ 2), as you say about 566V, but the peak-to-peak would then be 1132V. However, we're not talking about either peak or peak-to-peak - all the figures being discussed are RMS.

...but its a long old time since I actually learnt any of that 3 phase theory so may be talking complete rubbish...

As above, almost correct (once you've doubled your answer to get peak-to-peak) - so not 'complete rubbish', although 'completely irrelevant'

What I'm talking about is the relationship between the RMS (or peak if you wanted) phase-neutral voltage and phase-phase voltage in a 3-phase supply. The latter is √ 3 times the former - hence,for example, the standard UK supply has a nominal voltage of 230V between each phase and neutral, but just under 400V (230 x √ 3) between any two phases. As I was saying, if there were 400V between each phase and neutral, the voltage between any two phases would then be (400 x √ 3), which is about 692.8V.

Kind Regards, John

I have no problem being completely irrelevant

 

[JohnW2]

If, like me, one lives in Bucks, one still enjoys selective secondary education

That's where I lived when I enjoyed mine.

Not a bad place! However, in the 60s, you could presumably have enjoyed it anywhere in the country. My Grammar School was in Middx, and therefore didn't survive as such for more than a year or four (can't remember exactly) after I left in '67.

More recently Bucks education has remained good. Quite apart from having retained selective schools, even the non-Grammar Schhols are very good. The academic highlight of our family is my younger daughter, who got 5 A-grade A-Levels, a First Class Hons degree and various higher degrees and qualifications - but she is the one who (to everyone's amazement, and despite appeals) failed her '12-plus' (not '11-plus' any more) and went to a non-Grammar School- but that didn't seem to do her any great harm.

Kind Regards, John

 

[JohnW2]

I have no problem being completely irrelevant

It's certainly peferable to 'talking complete rubbish' - which you didn't do!

Kindest Regards, John

 

[JohnW2]

Very true...only relevance of peak to peak is for the motor and cable insulation.

... and quite large would the figures be with the numbers we've been talking about. As I've explained, 400V (RMS) phase-neutral would correspond to 692.8V (RMS) between phases, which in turn would translate to about 1959V peak-to-peak between phases (400 x √ 3 x √ 2 x 2).

Kind Regards, John

 

[SaladFingers]

If, like me, one lives in Bucks, one still enjoys selective secondary education

Kind Regards, John

And some very fine breweries (depending which side of the chilterns you are)

 

[echoes]

If, like me, one lives in Bucks, one still enjoys selective secondary education

That's where I lived when I enjoyed mine.

Not a bad place! However, in the 60s, you could presumably have enjoyed it anywhere in the country. My Grammar School was in Middx, and therefore didn't survive as such for more than a year or four (can't remember exactly) after I left in '67.

More recently Bucks education has remained good. Quite apart from having retained selective schools, even the non-Grammar Schhols are very good. The academic highlight of our family is my younger daughter, who got 5 A-grade A-Levels, a First Class Hons degree and various higher degrees and qualifications - but she is the one who (to everyone's amazement, and despite appeals) failed her '12-plus' (not '11-plus' any more) and went to a non-Grammar School- but that didn't seem to do her any great harm.

Kind Regards, John

I enjoyed mine in Trafford, Manchester, where they still select. I was the first year to do GCSE rather than O-Level, and remember the teachers saying that about 25% of the old O-Level syllabus content had gone.

 

[JohnW2]

I enjoyed mine in Trafford, Manchester, where they still select. I was the first year to do GCSE rather than O-Level, and remember the teachers saying that about 25% of the old O-Level syllabus content had gone.

I can believe that, and I suspect that a fair bit more has gone subsequently. When my daughters were doing GCSE's, 15 or so years ago, I was absolutely amazed (since my mind still 'thought O-Level') at how high a proportion of the exam questions were almost ludicrously basic/trivial - I suppose an inevitable consequence of trying to design a 'one size fits all' exam. One consequent problem was that the brighter students realised that they could get high marks just by getting all of those very simple questions right, which gave them less incentive to apply themselves particularly diligently to the more challenging (I would say 'proper'!) material!

Kind Regards, John

 

[Spark123]

Star delta starters connect the windings at 230v then after a timed period switch them to 400v each.
The same winding with 400v across it is going to take more current than with 230v across it.

No they don't
Star delta starters connect the windings in star
Then switch over to a delta configuration
The voltage which will be 400 will be in essence connected to a winding rated at 680v not 230
When it switches over to delta it will be connected to a winding rated at 400 v
It will draw less current in the first instance

Matt

I'm not talking about ratings, I'm talking about what happens with a star delta starter when it starts up. Initially the motor is connected star which has 230v applied across each coil, phase to the star point (400 / root 3).
It then switches to delta and each coil is connected directly to two phases each which is 400v per coil.
All figures I quoted are RMS.

 

[Spark123]

Where does 680V come from??

I presume (but obviously may be wrong) that it's meant to be (400 x √ 3) (i.e. the between-phase voltage of a 400V phase-N 3-phase supply) - but that should be about 692.8V, not 680V.

Kind Regards, John
Edit: crucial missing bit added!

I think that is meant to be divided by, not multiplied by. For this very same reason your normal street 3 phase transformer gives out 400v between phases and 230v between each phase and neutral.

 

[JohnW2]

Where does 680V come from??

I presume (but obviously may be wrong) that it's meant to be (400 x √ 3) (i.e. the between-phase voltage of a 400V phase-N 3-phase supply) - but that should be about 692.8V, not 680V.

I think that is meant to be divided by, not multiplied by. For this very same reason your normal street 3 phase transformer gives out 400v between phases and 230v between each phase and neutral.

All I did was to offer a probable answer (which I've subsequently been told was correct) as to "where the '680V' came from". As I indicated, it came from multiplying 400V by √ 3 (with a bit of an arithmetical/typographical error).

What I wrote above is correct - that a 3-phase supply with 400V between each phase and neutral would have about 693V between any two phases (just as, as you say, a supply with 230V between each phase and neutral has about 400V between any two phases). Whether any of that makes any sense in the context of the discussion is a totally different matter. Although 230V/400V 3-phase supplies are obviously incredibly familiar to us, I've personally never seen or heard of a 400V/693V one, in industrial settings or anywhere else - but that may just be because my knowledge about, let alone experience of, such settings is extremely limited. If one were talking about inverter-derived power, I suppose ones can be found that produce a whole range of voltages.

If one simply has a standard 230V/400V 3-phase supply, then, as has been said (amidst all the confusion!), the motor connection situation is incredibly simple. One starts with star connection, which gives 230V across each winding, and then switches to delta, which gives 400V across each winding. Full stop. All references to 680V/690V (or 400/690V 3-phase supplies) are then a completely irrelevant distraction, corresponding to nothing in the real world of a motor fed from a standard 230V/400V 3-phase supply.

Kind Regards, John