Open Circuit Phase on STAR Transformer

Where are you referencing voltage?

to the neutral point of the STAR connection on the load side.

If you mean by "Star transformer" the secondary windings of a transformer to which load is connected, then the voltage does not depend on the load, as well as it does not depend on the load balance (so long as the power of the load does not ecxeed much the power of the transformer, if exceeds, then voltage will "drop").

What if one phase is not connected and left open circuited? Doesn't that mean the neutral point now has a voltage present, because the vector sum of voltages at this point is no longer zero? Between phases you would still see 415V, but if you were measuring from phase to neutral, then you would see <230V? (not sure what it would be)

No, you get 240V, even if you open two phases. So long as you don't exceed the individual phase rating of the transformer. If you have a 3KW transformer and draw 1KW on an individual phase, you'll still be getting near 240V line to neutral.

Hi

Imballance on Phases, I would be first having a look at the bonding of the main neutral and earth, if this bonding is not effective you will end up with a neutral floating above earth, CHECK THIS OUT FIRST!

Dont get to technical about it all, check the basic's first.

Best Regards

Joe

this is what I thought, I would like to mention that in this case, I'm assuming a floating neutral. If the neutral weren't floating, you would have higher voltages distributed across phases. How do you have 240V across each phase with an open circuited 3rd phase, if the net result of voltages is no longer 0 at the neutral point?

Your Transformer 2ndary is start connected, and Phase to neutral = 240V while Phase to phase = 415V.

Whether the Neutral is grounded or not, and whether one of the phases is not used on the load, the voltages remain the same: i.e. 240V between phase & Neutral and 415 v between phases. The Neutral is not Floating here. the terminology is not usefull in this context.

If your loading circuits are using equipments for 240V rating, and you connect them between a phase and the neutral at the beginning, and suddenly, you disconnect the Neutral from the transformer Neutral but kept the loading circuits as they are, THEN your Loadside Neutral becomes Floating since the equipment that are connected between P & N will become between P & P (P1 & P2 or P2&P3 etc), and since they are not of the same ratings, the artificial resulting neutral will be floating, such that the voltages present at the different equipments will no longer be 240V but somewhere between 0 (not exactly) and the 415 V (also not exactly). Meaning that on one line P to P, some will have less than 240V and some will have higher up to near 415. All depends on the impedence of each connected item.

A complete explanation will take more .... but I hope this will be enough for now.