# Dumb question regarding short circuit test equivalent resistance

Hi,
Silly little question about the transformer short circuit test.
When I perform it with the secondary shorted and I get Req = Power / current2

is this Req = Rpri + Rsec?
If I measured Rpri and Rsec with a multimeter?

Thanks

## Answers and Replies

cnh1995
Homework Helper
Gold Member
is this Req = Rpri + Rsec?
It is Rpri+Rsec*(Npri/Nsec)2.
The second term on the RHS is the reflected secondary resistance or the secondary resistance "seen" by primary.

It is Rpri+Rsec*(Npri/Nsec)2.
The second term on the RHS is the reflected secondary resistance or the secondary resistance "seen" by primary.
Can we move this a = (Npri/Nsec)2 term to get the primary and secondary resistances as seen by themselves?
So if I measure Rpri and Rsec with a multi meter then will the value of Rpri I get on the meter be
Rpri = Req - Rsec*(Npri/Nsec)2

Where Rsec is the value of the secondary from the meter?

cnh1995
Homework Helper
Gold Member
Can we move this a = (Npri/Nsec)2 term to get the primary and secondary resistances as seen by themselves?
So if I measure Rpri and Rsec with a multi meter then will the value of Rpri I get on the meter be
Rpri = Req - Rsec*(Npri/Nsec)2

Where Rsec is the value of the secondary from the meter?
Yes (provided the test is performed with secondary shorted).

Yes (provided the test is performed with secondary shorted).
Just to be 100% clear: When you say the test I presume you mean the Short Circuit test in finding Req, and not measuring the coils resistance with the multimeter? (because I can't see how it would make any difference to that)
So bottom line is that other than the values I can derive from Short Circuit and Open Circuit tests, I still need either the Primary coil resistance, or the Secondary coil resistance (via multimeter) to accurately and specifically know every individual parameter of the transformer? (i.e. to separate the primary and secondary resistance values out from Req)
Thanks again

cnh1995
Homework Helper
Gold Member
Just to be 100% clear: When you say the test I presume you mean the Short Circuit test in finding Req, and not measuring the coils resistance with the multimeter? (because I can't see how it would make any difference to that)
So bottom line is that other than the values I can derive from Short Circuit and Open Circuit tests, I still need either the Primary coil resistance, or the Secondary coil resistance (via multimeter) to accurately and specifically know every individual parameter of the transformer? (i.e. to separate the primary and secondary resistance values out from Req)
Thanks again
Yes to all.

tim9000
Yes to all.
Thank you