Line Current in a 2 phase transformer

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Discussion Overview

The discussion revolves around calculating the primary line current in a two-phase transformer setup, specifically a 30kVA transformer with a 415:650 voltage rating, utilizing two phases from a three-phase source. The conversation includes theoretical considerations and calculations related to transformer operation and phase relationships.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant seeks to calculate the primary current for a two-phase transformer using two phases from a three-phase source, questioning how to derive the current for a 30kVA load.
  • Another participant requests clarification on the transformer's configuration, specifically whether it is an open delta or another arrangement, and suggests providing a diagram for better understanding.
  • A participant clarifies that the calculation for line current can be approached as a single-phase problem when considering the voltage between the two lines, indicating that the line current can be calculated using the formula KVA/V.
  • There is mention of safety concerns regarding working with transformers, emphasizing the need for qualified personnel, although the original poster clarifies that their inquiry is theoretical.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and assumptions about the transformer's configuration and the calculation method. There is no consensus on the exact approach to calculating the line current, and some aspects of the discussion remain unresolved.

Contextual Notes

The discussion includes assumptions about the transformer's phase relationships and the implications of using two phases from a three-phase source. The lack of clarity regarding the transformer's configuration and the specific voltage relationships may affect the calculations presented.

anoor
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I am a graduate engineer and have been set a small task to work out the current (primary side) in a 2 phase 30kVA 415:650 transformer. The 2 phases have been utilized from a 3 phase source and are hence 120 degrees out of phase.

I understand had it been a 3 phase 30kVA transformer I could calculate my current by 30kVA / 415 / route*3 = 42A.

If it was a single phase then simply 30kVA / 415 = 73A.

However if I am only using 2 phases from a 3 phase source yet still need to produce 30kVA of power, how do I calculate the line current?

Thanks in advance.
 
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Hello Anoor and Welcom :

I think we will need a little more info -- you are describing a 30KVA trans but the Voltage is 415:650? And by 2 phase - is the secondary like an Open delta - or some other arrangement? Best bet is to post a diagram of the nameplate info.

Lastly -It is not clear if you are actually working on this physically? In the USA you would not be a " qualified person" - working on this is extremely dangerous, does your University have a qualified electrician that you can work with?.
 
I am not working on it physically its just in theory.

Maybe its my poor explanation ill try again!

If I have a standard 230/400V switchboard and want to connect a transformer it would normally be a 3 phase (400V) or single phase-neutral (230V). However if I wanted to connect a transformer to phases L1 & L2, instead of the neutral (giving me 400V). What would my line current be on the primary and how would I calculate it? I apologise if my ignorance doesn't allow me to adequately suscribe the situation! I have attached a pdf showing an extract of the schematic, I hope this helps!

Thanks again.
 

Attachments

OK the diagram help a little. If you think of the "phase" as between the lines, as in L2-L3 - this is really a single phase question - IN the USA we refer to this as Line to Line. In this case there is no phase angle - so the Line current is KVA/V - also the transformer input or out put is only a 2 wire device - right?

The lines L2 and L3 are only out of phase 120Deg relative to the neutral
 

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