# Delta-Star transformer current flow problem.

#### dreglands

Joined Feb 13, 2007
2
I have a problem I hope someone may help me out with. Ive got a 100KVA 480V L-L primary and a 415V L-L/240V L-N, Delta-Star transformer. The transformer is to be assumed to have not losses. The secondary winding will supply the full 100KVA only on one phase 240Vac L-N the other two phases will not be connected.

What will be the transformers primary side Phase and Line currents (Primary side being delta)? Assume the P.F of the single phase load is 0.8.

Any help will be much appreciated.

#### dreglands

Joined Feb 13, 2007
2

#### JoeJester

Joined Apr 26, 2005
4,390
What have you done to solve the problem?

Can you show your work to date?

#### mohammed123

Joined Dec 12, 2013
1
if u connect fullload to only single phase your transformer coil will be burnt of that phase.it should be balanced..in delta line current=phase current i think?

#### jegues

Joined Sep 13, 2010
733
The transformer is to be assumed to have not losses. The secondary winding will supply the full 100KVA only on one phase 240Vac L-N the other two phases will not be connected.
Can you give us the exact wording the question uses, because something seems goofy here...

Typically a three phase transformer will be composed of three identical single phase transformers, where the KVA rating for the three phase transformer is three times that of the single phase transformer.

$$S_{B 3 \phi} = 3S_{B 1 \phi}$$

So to think that you can transfer the 3 phase rated power through one single phase transformer is silly.

if u connect fullload to only single phase your transformer coil will be burnt of that phase.it should be balanced..in delta line current=phase current i think?
In a delta configuration, the line current and phase current are not equal.

#### t_n_k

Joined Mar 6, 2009
5,455
Notwithstanding the improbable nature of the question and the glaring error concerning the specified rating and proposed loading, it is a simple matter to state what the primary line currents would be.

For an ideal transformer the ampere-turns balance between mutual phase primary and secondary windings must be maintained. There is clearly a 2:1 turns ratio between a primary delta winding and its corresponding secondary star mutual pairing. The sole winding carrying the full load current on the secondary must induce its primary mutual delta winding to carry exactly half of the secondary load current. The two primary source line conductors terminating on that same primary delta winding will carry that same primary delta winding current. The third source line conductor will have no current. The primary delta windings with mutual connection to the unloaded secondary star windings will carry no current by virtue of their being no current in those unloaded secondaries.