XFMR Primary current vs. secondary current

[Loffgren]

What is the formula for figuring out the primary current knowing the secondary current. Example 125 amps at 120/208 convert to primary current at 480V. I may be missing the obvious here, but thanks

 

[anbm]

What is the formula for figuring out the primary current knowing the secondary current. Example 125 amps at 120/208 convert to primary current at 480V. I may be missing the obvious here, but thanks

Assume this is 3 phase voltage system.

Total kVA = 125 x (208 x 1.732) = 45,032 VA

Primary current = 45,032 / (480 x 1.732) = 54.17A

 

[charlie b]

That is probably the most effective way to do it. I like converting things to VA, doing the math, then converting back to current. It is less likely to lead one to make an error.

But you can use the shortcut of applying the turns ratio. The ratio of the number of turns in the two windings is what makes the voltage drop from 480 (primary) to 208 (secondary). The same ratio makes the current rise from primary to secondary. The opposite ratio, therefore, would give you the current in the primary, if you know the secondary.

Primary current equals secondary current times (208/480). This gives you the same answer of 54.1 amps. You will note that it is the same set of numbers, except that you don't have to multiply, then divide, by the same factor of 1.732 (the square root of three).

 

[zog]

Ignoring any losses (Vp*Ip)=(Vs*Is)

 

[Loffgren]

re

re

Thanks for the help

 

[weressl]

What is the formula for figuring out the primary current knowing the secondary current. Example 125 amps at 120/208 convert to primary current at 480V. I may be missing the obvious here, but thanks

Example:

2*6=6*2.

What this illustrates that the transformer ratio of VOLTAGE is inverse of the CURRENT.

A transformer with 480/120V has a VOLTAGE ratio of 4:1, so the secondary CURRENT will be 1: 4 or 4 times of the primary current.

 

[Loffgren]

re

re

Perfect ! Thanks alot for the help

 

[LarryFine]

Don't forget to add a bit of extra headroom for losses and other stuff if applicable.