3 phase open delta?

[whipit]

When measuring line voltage at the secondary taps of a 3 phase open delta 4 wire(2 pot transformer configuration) wired for 120v/240v. Why wouldn't you read 480v instead of 240v when voltage is measured across the two outside taps? When measuring the voltage from the high leg to the grounded tap you read a higher voltage because your measuring all of one winding and half of the other. Wouldn't you be reading the voltage of two windings or is it because the two phases are out of sync and they are canceling out each other at point? Thanks for any clarification.

 

[jbartos]

Suggestion: Visit

http://www.federalpacific.com/university/transbasics/chapter3.html

etc. for more info on open delta transformer connection

 

[jbartos]

Suggestion: The voltages are added vectorially on the open delta transformer connection. To obtain 480V the transformer windings would have to be connected in parallel on the primary and in series on the secondary. The series connection would have to be additive such that 480V would materialize.

 

[rbulsara]

Because it is the vector summation. The voltage from the midpoint to one end is 120 degree apart from the voltage from the midpoint to the other end.

V/_0 + V /_120 = V /_ -120.

Please refer to my response to thread238-67436 started by ossie for 'unbalanced three phase loads'. Although it is for current, the math is still applicable for the voltage summation.

Removing one of the tranformers of a delta system (creating an open delta) does not change the voltage and current relationhips, only capcity of the system is affected.

 

[whipit]

Thanks rbulsara, I have another question, is this really 3 phase?

 

[busbar]

Visualize the situation as a right triangle with 60° between adjacent {1 per-unit length or ½ coil} and hypotenuse {2 per-unit length or full coil.} The opposite length is 1.732 per unit. This holds for unloaded open-delta versus delta transformer-secondary configurations. Reference last two illustrations at

http://home.att.net/~benmiller/elecsys.htm

 

[rbulsara]

Whipit:

Yes. I presume you are talking about a two transformer VT (PT) connected to a 3 phase supply on the primary side, and 120V secondary(or 240V). It is not equivalent to a trasformer with a mid-tap.


If an open delta were not a 3 phase system, it would not be used as emergency (at reduced capacity) two trasnformer system to feed 3 phase loads when one of the 3 phase bank unit fails.

 

[jbartos]

Suggestion: The open delta is a three phase source; however, the power rating is given by two transformer power ratings that are added. Therefore, it is not the same as close delta formed by three single phase transformers. Also, the open delta voltage source triangle is not very symmetrical, when under the load.

 

[rbulsara]

Just for infromation:
Power rating of a Open delta system will be 57.7% of the completed delta system with 3 transformers of same rating.