3 phase motor wiring

[ricicle]

Wow, Wikipedia is a wonderful thing...

Not wikipedia, just knowledge having worked in industrial control for many years

you have two identical motors under identical loads, one motor wired in star and the other in delta, which motor would have the lowest running current?

Delta

Star – high current, low speed
Delta – low current, high speed.

I think I know what you're trying to say but that statement is wrong on its own

In a star/delta starter, where do you put the overload?

Manufacturers vary, if it is put in the main power lines then it will set to FLC. If on the bottom of the main contactor then it will be set to FLC/1.73.
Anywhere else and there will be no O/L protection when in star and/or delta depending on location.

 

[JohnW2]

Ohm's and Kirchoff's Laws etc. were very much O-Level. Quite involved aspects of AC circuits, including complex algebra treatments thereof, and certainly including 3-phase concepts, were definitely part of A-Level Physics back then.

I did my Physics A-level between 1969 & 1971, and I don't recall doing that.

My A-Level Physics Course was 1964-1966 and, like Bernard, I definitely did - I might even be able to find some of my notes/books to prove it! I recall in particular that we were quite gratified to discover that what we were being taught about imaginery and complex numbers in A-Level Maths (which, in that context, appeared to be a rather weird abstraction, of no obvious practical use!) actually had some practical applications!

Because we are not allowed to have a system which recognises different levels of ability. Losing and failure cannot happen. Any suggestion that some people can go further than others is elitism. ...

That's all, IMO, very true, but it's a bit more complicated than that. If, like me, one lives in Bucks, one still enjoys selective secondary education - but that doesn't really help, because the curricula and exams have been dumbed down for everyone, included those selected to go to selective Grammar Schools. As you go on to say...

... Everybody must be told that they can do anything and everything. All you need to do to get to university is to want to go. Can't have A-levels because they are too hard for some. Can't tell people that they should acquire a foundation of knowledge of a subject because that is egocentric denigration.

Kind Regards, John

 

[matt1e]

Wow, Wikipedia is a wonderful thing... Shall we move away from motor theory gentlemen and go to the dark practical side of things?

Let’s talk about running current because that’s what you are really interested in. Assuming the motor can achieve design speed under the required load, would anybody care to answer this for me... you have two identical motors under identical loads, one motor wired in star and the other in delta, which motor would have the lowest running current? Come on, you know the answer....

Not enough info, what are the ratings of the windings and what is the supply voltage

Star – high current, low speed
Delta – low current, high speed.

You may have noticed that I have removed the reference to torque. To find any information on torque availability against voltage will be virtually impossible and I can’t think of a decent analogy to argue my case, so I have removed any reference to torque.

Nope A properly connected motor will always draw more current wired delta than wired star

The analogy is awful, it's as bad as
Star = high voltage = 400v
Delta = low voltage = 230v

Which may or may not be the case as it depends on the rating of the windings

Wow
Just to keep the cat amongst the pidgeons, I pose you a question; In a star/delta starter, where do you put the overload?

That's an unfair question as there are two positions either on the main contactor or the delta contactor and each has different advantages

Matt

 

[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.

 

[Spark123]

Wow
Just to keep the cat amongst the pidgeons, I pose you a question; In a star/delta starter, where do you put the overload?

On the contactor which pulls in to provide 400v to the system, not on the star or delta switching contactors.

 

[ricicle]

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.

I would have thought that wouldn't be the case if the output (work) and speed of the motors was exactly the same ?

 

[Spark123]

I think we're getting our wires crossed, I am talking about a single motor with a star delta starter.
The only time you are going to get the speed the same is where there are zero losses and no load.

 

[matt1e]

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

 

[bernardgreen]

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.

The speed of rotation is a factor in how much current the windings take.

The same winding can take LESS current at 400 volts applied with the motor turning than it would with 230 volts applied and motor not turning.

When the motor turns it acts like a generator producing a voltage in the windings that opposes the applied voltage thus reducing the current. In a perfect motor running without any load the current would be zero as the generated voltage would equal the applied voltage.

 

[Simon35]

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 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??

 

[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.

Kind Regards, John
Edit: crucial missing bit added!

 

[ricicle]

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 3-phase supply) - but that should be about 692.8V, not 680V.

Kind Regards, John

Surely the phase to phase voltage of a 400V three phase supply is 400V

 

[SaladFingers]

This thread is motoring on with lots of good information.

 

[JohnW2]

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.

Surely the phase to phase voltage of a 400V three phase supply is 400V

Whoops - typing fingers missed a crucial few characters - see above (and I'll edit the original)

Kind Regards, John

 

[Simon35]

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 3-phase supply) - but that should be about 692.8V, not 680V.

Kind Regards, John

Surely the phase to phase voltage of a 400V three phase supply is 400V

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...