Delta and Star Connections

If you searched this forum first you would get a thousand replies instead of one. I get the impression that a whole classroom of people are coming on here to get the easy answer.

Basically in three phase power the 3 phases can be utilised or supplied by a Delta device which operates on a higher voltage and lower current - good for transmitting over distances.

Or if you wish to utilise lower voltages then the equipment is Star wound with the common mid point connected to earth - this gives a neutral point. The higher voltage is still available between any two or all three of the phases and the lower voltage is available between any phase and the neutral. Operating loads on lower voltage require higher current - so bigger cable sizes. Not good for transmitting.

There is another benefit on motor starting where starting in the a Star connection winding formation puts the impedance of two windings in series and reduces the inrush starting current, then switching back to one winding impedance in Delta for the full running load.

I was of the impression that greater resistance gave greater voltage over ampage.

given that the windings and wire size/area are equal for both star and delta connection, how is it that star windings has lower ampage/less resistance?

perhaps it is in the construction of delta phase components? but that doesn't make koolaid in my book!

"given that the windings and wire size/area are equal for both star and delta connection, how is it that star windings has lower ampage/less resistance?"

that should read

given that the windings and wire size/area are equal for both star and delta connection, how is it that Star windings has higher ampage and Delta has lower ampage/less resistance?

people come on here for advice,

better to say nothing than give bad advice. also, if you take this advice without checking it, you're an idiot.

I went to pull out my old electrical engineering pocket manual, but I couldn't find it.

anyway, if I remember rightly, its because the circular current of the delta configuration has an accumulative resistance configuration.

still, I didn't check, and I've only used delta configurations.

hi,

good reply from friend.i want to add one thing more if have selected higher rating motor rather than desired capacity (especially in fan drive) we should go for star connection inspite of delta connection .one can get power saving by this practice.

byeeeeeeee

Please explain the logic and mathematics of the power saving of the delta over the WYE connection,

With 40 years in motor design, I was not aware of this.

Thanks

Snakers

I think some people are getting confused about 3 phase.

I'm looking at thisissue sideways, upside down, backwards, and every which way to a month of sundays, and I still can't see how savings can be made!!

I could add more components to produce savings, but as a stand alone unit, it makes no sense

Thats an interesting idea.

savings?

LOL, sometimes this forum amuses me to no end... At the risk of doing someone else's homework assignment, I feel compelled to offer a voice of sanity.

The only time this is worthy of consideration as a stand-alone issue is if you are designing a motor from the outset, AND it will be a single voltage design, AND it will be a single speed design AND you are not interested in Star-Delta starting. Only in that case does there present a pause to consider how to configure the windings. Only in that one case is there a slight advantage of star over delta because there is slightly lower losses in star due to the fact that current in each winding, and therefore IR losses in the wire itself, will be slightly lower. But why someone would design such a limited application motor in the first place is where I would start. My opinion is that if you were designing a motor for sale, design it for the MAXIMUM flexibility in it's application so that you can sell it to the most number of customers, weighed of course with economic factors so as to not make it unaffordable.

So that said, the difference is in the application. If you want a dual voltage motor where the high voltage rating is 1.732 times the low voltage rating, one way to do that is to set up the windings so that you connect in star for high voltage, delta for low voltage. For example, a lot of IEC motor designs are such that if you want to connect to a 415V supply, you use the star connections, but if you need to connect to a 240V system, you use the delta connections. kW will be the same, but in the star connection, the current will be lower by the voltage ratio (1.732).

If on the other hand, you want a single voltage motor which can be started with lower starting current (and torque), you can take 6 motor leads out to a star-delta starter and connect it in star to start with, then switch it into delta to run. By applying the same voltage to both connections, the star connection will yield less torque (33%) and draw less starting current (33% or nominal starting current).

There are also methods of changing the number of poles via star and delta connections in order to make 2 or 3 speed motors.

So the bottom line is, unless you frame your question in the context of how you intend to use your motor, there is no right or wrong answer.

Other than that rubbish of saving energy! That made me laugh out loud! If you connect motors in star that were designed for delta, you will in fact draw less current, BUT you will produce 1/3 of the torque. If your load does not change, your motor will burn up! If it doesn't, your motor was 3 times the size it needed to be in the first place! Soooo funny...

I just gotta say it.

HA!!!

OK nuff said

No...

you haven't done anyones homework for them. It's great advice for advancing ones understanding tho.

and you reminded me that I need to square the current in order to gain the ratio differencial. thanks. otherwise, I would have only consider the union value, such that 3/1 : 3/2 which is a consideration of the resistence

Excellent Presentation. Good engineering report.

Snakers ....