3 phase Star and Delta Networks

Dubh Geannain

My old nemesis from my undergrad days has come back to haunt me. Three phase power. I've a question for anyone in the know.

Line to Line voltages for a Star network is sqrt(3)*Vphase so 1.73*230 = 400V
I've measured this in work and I get 410V, which is round about what I expected.

We don't have a Delta network so I can't verify my assumption but is Vline = Vphase i.e. is Vline = 230V for a Delta network?

Thanks.

Michael Collins

Yep, there generally is no neutral conductor in a delta configuration so there really are only line voltages. These are all equal to the voltage produced from a single phase alone, on the national grid in Ireland this is 230 V (rms).

Dubh Geannain

So if you measured the voltage between L1 and L3 you'd get 230V the same as if you measured between L1 and Earth for a Delta network.

Delta Network:
L1 - Earth = 230V
L1 - L3 = 230V

Star Network:
L1 - Earth or Neutral = 230V
L1 - L3 = 400V

Michael Collins

Dubh Geannain wrote: »

So if you measured the voltage between L1 and L3 you'd get 230V the same as if you measured between L1 and Earth for a Delta network.

Well really there is no earth for a delta network - or at least no natural, obvious choice, like there is for a star network.

In a star, the centre node is an obvious choice for the neutral, due to the symmetry involved.

In a delta network, there just is no "phase" voltage technically speaking. See the diagram below from All About Circuits



This is for a 120 V system like the USA, so just change those values to 230 V for Ireland or the UK. You can clearly see there's no obvious neutral. Between any two terminals, your meter would read 230 V.

Sometimes one coil may be centre-tap grounded, in which case you do have a neutral or earth terminal, but it's not analogous to a star system, due to a lack of symmetry. For example, if one coil was grounded at its centre-tap point, you'd have voltage between either end of that coil and ground equal to 115 V. You'd have a voltage across the complete coil of 230 V, and you'd have a voltage between the remaining node (called the High-Leg, or the Wild-Leg) and ground of 200 V. But this isn't very common, particularly in Ireland.

Dubh Geannain

Cheers for clarifying that MC.

FoxT

I am curious... Are there inherent advantages/disadvantages for star vs delta?

As a power generator - Star offers a 'neutral' point - which, when linked to earth, forms the basis of electrical safety... But what about, say, a car alternator?
All other things being equal, I would expect that a delta would be more appropriate ( Lower o/p impedance, more current) am I missing something?

Would appreciate any links/textbook reference/advice etc... part of a personal project I am doing.

Thanks.

Dubh Geannain

FoxT wrote: »

I am curious... Are there inherent advantages/disadvantages for star vs delta?

As a power generator - Star offers a 'neutral' point - which, when linked to earth, forms the basis of electrical safety... But what about, say, a car alternator?
All other things being equal, I would expect that a delta would be more appropriate ( Lower o/p impedance, more current) am I missing something?

Would appreciate any links/textbook reference/advice etc... part of a personal project I am doing.

Thanks.

Savings are made in terms of cabling requirements when transmitting power using Star Network as the current drawn is lower due to the higher voltage capacity of the configuration i.e. less copper is needed