Thread: 3 Phase Star and Delta

Can someone explain in simple terms what the difference is, and the result of using a 3 phase motor in both configurations with a VFD.

Cheers Shane.

I'll try again, I missed half the question the first time, I swear I'm going blind.

The difference is simply in the way the windings are connected.
In a "normal" 3 phase motor when the same V is applied, Star has (about) the same rpm and (about*) half the power of Delta. This can be used for two things, generally on small motors it is used to allow dual voltage. i.e. 415V Star and 240V Delta. In larger motors it can be used as a "soft start" Star star, Delta run. The motor is wound to run on 415V Delta, when started in Star 415V this reduces the current required and the torque though the drive train. Once at speed the windings are switched to Delta for full power.
Then there are multi pole motors for 2 speed, but I dont think you want to go there just yet

BobL's tests cover the VSD use of star and delta. The short version is if you use a 240V VSD on a 415V motor you'll get about half the hp. I wouldnt use a 415V VSD on a 240V wired motor.



Stuart

*BobL tests this(I'd start about page 10) VSD power tests
p.s. I assume you have seen a diagram
https://www.elprocus.com/wp-content/...figuration.jpg

If "Star has (about) the same rpm and (about*) half the power of Delta" then why use it? Using my Waldown drill motor as an example. I think it is 1Hp. It was wired for Star and I pulled out the Star point and connected it for Delta. What is the advantage of connecting it for Star in the first place?

Dean

Hi Dean
I'm not sure I understand your question.

Originally Posted by Oldneweng

If "Star has (about) the same rpm and (about*) half the power of Delta" then why use it?

For the same applied V

Originally Posted by Oldneweng

Using my Waldown drill motor as an example. I think it is 1Hp. It was wired for Star and I pulled out the Star point and connected it for Delta. What is the advantage of connecting it for Star in the first place?

Your VSD is 240V and the motor was made to run on 415V.
Do you mean "why to the manufacturers make 415V star motors instead of 415V delta motors?"
I really dont know, I've read its so the same motor can be use here and in the US, not so sure I believe that is 100% of the story. I wonder if its easier or uses less copper or less turns of thicker copper or, well who knows , a handy sideeffect its that we can rewire a 415V motor for 240V 3 phase.

Stuart

Originally Posted by Oldneweng

If "Star has (about) the same rpm and (about*) half the power of Delta" then why use it? Using my Waldown drill motor as an example. I think it is 1Hp. It was wired for Star and I pulled out the Star point and connected it for Delta. What is the advantage of connecting it for Star in the first place?

Dean

Stu is on track - it's basically economics.
The Y connection uses lower currents which means reduced heat which means longer life. So for a given amount of copper windings, where motors run continuously, need to last for a long time, and don't start under much of a load then Y is to go.

Where the motor use is intermittent, and are under a high load start then for the same amount of copper ∆ is usually preferred.

Originally Posted by BobL

Stu is on track - it's basically economics.
The Y connection uses lower currents which means reduced heat which means longer life. So for a given amount of copper windings, where motors run continuously, need to last for a long time, and don't start under much of a load then Y is to go.

Where the motor use is intermittent, and are under a high load start then for the same amount of copper ∆ is usually preferred.

Thanks Bob. That makes it clear.

Dean

Originally Posted by BobL

The Y connection uses lower currents which means reduced heat which means longer life. So for a given amount of copper windings, where motors run continuously, need to last for a long time, and don't start under much of a load then Y is to go.

Where the motor use is intermittent, and are under a high load start then for the same amount of copper ∆ is usually preferred.

That doesn't sound right(For what thats worth lol).
We are talking about two motors, one 415V Delta and the other 415V Star? and not about star/delta start. Now I maybe looking at this to simplistically, but.
Wont the average phase to phase current be the same? Wont that mean each Star winding needs half the number of turns with the same wire size as a Delta winding. If thats the case won't there be double the copper in a Delta wound motor than a Star wound motor?

I cant help but feel I'm well out of my depth.

Stuart

Originally Posted by Stustoys

That doesn't sound right(For what thats worth lol).
We are talking about two motors, one 415V Delta and the other 415V Star? and not about star/delta start. Now I maybe looking at this to simplistically, but.
Wont the average phase to phase current be the same? Wont that mean each Star winding needs half the number of turns with the same wire size as a Delta winding. If thats the case won't there be double the copper in a Delta wound motor than a Star wound motor?

I cant help but feel I'm well out of my depth.

Stuart

Well thanks for nothing Stuart. Just when I thought I had some idea, you go and throw another spanner in the works!

It took it to mean that when a motor (like my example above) has both windings possible, the use of the Star windings will cause less stress and therefore the motor will likely last longer.

There is a slight possibility that I am wrong tho.

Dean

Originally Posted by Oldneweng

Well thanks for nothing Stuart. Just when I thought I had some idea, you go and throw another spanner in the works!

I wouldnt get to worried Dean, there is a better than even bet I've got it all backwards

Originally Posted by Oldneweng

It took it to mean that when a motor (like my example above) has both windings possible, the use of the Star windings will cause less stress and therefore the motor will likely last longer.

Well if you're talking about the same V in Star or Delta on your motor then yes it would possibly last longer in Star as it would only make half the power.

If a motor is star/delta it is normally that way for because it needs a "soft start" capability (start in star, switch to delta when running) or it is meant as a dual voltage motor.

It is unusual to see a star/delta motor run continuously in star config using it's delta voltage i.e. you select a motor based on HP/kW you require and voltage you intend to run it on and that is how you run it.

Try this - I don't necessarily agree with the way some of the things are expressed but it shows the differences in Phase voltage and currents.
Comparison between Star and Delta Connections

So if my maths is correct.
415V 10A
Star connection
I_L = I_{PH so 10A=10A}
V_L = √3 V_{PHso 415V= 1.73 x 240V}


Delta
V_L = V_{PH so 415V=415V}
I_L = √3 I_{PH​ so 10A=1.73 x 5.7A = 10A}

10Ax240V=5.7AX415V

Star connection increases current

What he seems to have left out it that while each Star winding requires less turns it needs thicker wire. I think......

Where did I go wrong?

Stuart

p.s. granted its not exactly what I said in my earlier post

Originally Posted by Stustoys

Star connection increases current

What he seems to have left out it that while each Star winding requires less turns it needs thicker wire. I think......

Or better cooling?

This is why the used Toshiba motor I picked for my compressor has me stumped.
It's a 5HP Y motor with no fan!!!

If I may, Shane asked for simple terms.
3-phase power is delivered as 3 lots of powere (in three cables) with 415V (in Oz) between any two 'phase' wires. The arrangment of that power gives us 240V between any one phase wire and Neutral. So the house power is connected between one of the phases and Neutral.
Smaller 3-phase motors are wound with 3 lots of windings for 240V each (for economic and simple space reasons) and connected in Star form, with the star point (centre of the 3-pointed star) not connected to anything. So effectively there are 2 coils between each of the 3 phase wires and 415V.

When you only have single phase in your house and you use a VFD to change that back into three phases, you end up with only 240V between the 3 'phase wires'. So that the motor can develop its design output, we have to rearrange the coils inside the motor to feed 240V to EACH coil. The way to do that is to arrange them in delta or triangle. For motors that don't have that facility built in (many motors do), we have to find the star point and separate it, bringing out all three wires which were joined there and make the wiring change externally.
That way a motor designed to run on 415V in star form can run exactly at the same speed and power output on 240V 3-phase power.

threephase.jpg

The issue some of the above responses referred to have to do with BIGGER motors, that use the same logic for soft staring - and this has nothing to do with VFDs at all.

One last comment: multi speed 3-phase motors are wound completely differently and cannot be easily rewired to run on 240V 3-phase. The only three options for connecting them to a single phase supply are to run them on reduced power, get a VFD that increases the output voltage to 415V or get them rewound as a single speed motor suitable for 240V 3-phase supply from a VFD.

Hope that helps a little.

Well, that is a good explanation of what's going on, thanks.

Shane, do you have a specific application in mind? Using 240v or 415v? Last year I converted my 1.5hp single-phase Taiwan VO-AIS mill to use a Huanyang VFD ($139 bargain) and a new 1hp 3-phase motor. I was trying to understand this star-delta thing too.

The VFD will accept single-phase or 3-phase power. I am using single-phase 240v going in and 240v 3-phase goes out. The motor has terminals for connecting to 415v (star) or 240v (delta), both 3-phase. I set up an external control box with stop/start/rev switches and a potentiometer for the frequency change.

I still really don't understand the electrical side of things, but it is great not having to change the belts any more, well hardly ever, as these motors seem ok running from under 15Hz to 100Hz or more. Just turn the knob to change speed...