I have a 3 phase tranformer that I trying to figure the secondary current on. It has a 480V primary and a 120/208 Secondary. I am trying to figure the secondary current. i come up with 625A when using 120V and 360A when using 208v. Which of the two do I use?
75KW Secondary current
- Thread starter Benton
- Start date
- Status
- Location
- Massachusetts
75*1000/208/1.73=secondary current
- Location
- Massachusetts
zog
Senior Member
- Location
- Charlotte, NC
What current? Full load? Available fault?
iwire gave you the correct formula.
It comes from rearranging the general formula for kVA:
kVA = (sqrt(3)*VL*IL)/1000
Note that transformer is rated in kVA, not kW. It's an important distinction.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
110528-1515 EDT
A simple intuitive way to approach the question is:
(1) The KVA rating for the whole transformer system is 75 KVA. This needs to be equally proportioned to the 3 phases. Thus 25 KVA per phase. It does not matter whether it is a true 3 phase transformer or 3 single phase transformers.
(2) If you view the secondary as a Y, whether it is a delta or Y, then a resistive load from a line to the neutral (in the case of a delta an imaginery neutral), then use the line to neutral voltage and the KVA for one phase to calculate the current. In this case 25,000 / 120 = 208.3 A. You know for a resistive load that the line current equals the line to neutral current, and the resistor's current is in phase with the line to neutral voltage.
If you were only given the delta voltage, then you would need to divide that voltage by 1.732 to get the imaginery line to neutral voltage. But usually from memory you know this value without doing the division.
Whether the load is a delta or Y the line current will be the same for a given KVA rating.
.
A simple intuitive way to approach the question is:
(1) The KVA rating for the whole transformer system is 75 KVA. This needs to be equally proportioned to the 3 phases. Thus 25 KVA per phase. It does not matter whether it is a true 3 phase transformer or 3 single phase transformers.
(2) If you view the secondary as a Y, whether it is a delta or Y, then a resistive load from a line to the neutral (in the case of a delta an imaginery neutral), then use the line to neutral voltage and the KVA for one phase to calculate the current. In this case 25,000 / 120 = 208.3 A. You know for a resistive load that the line current equals the line to neutral current, and the resistor's current is in phase with the line to neutral voltage.
If you were only given the delta voltage, then you would need to divide that voltage by 1.732 to get the imaginery line to neutral voltage. But usually from memory you know this value without doing the division.
Whether the load is a delta or Y the line current will be the same for a given KVA rating.
.
- Status