Transformer currents in primary and secondary question

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  • #1
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Main Question or Discussion Point

I'm reviewing transformers and I have some confusion over the current

s is for secondary and p is for primary

I know that
Vs = Vp(Ns/Np)
and
Ip = Is(Ns/Np)

So when you have a load across the secondary terminal you get your current through Vs/Rload.
But does this mean for current in the primary coil, it has nothing to do with the inductance value or resistance (if I added a resistor before the inductor in the primary section)? V/Z = I does not work here?

If this is true what would the current be if I left the secondary terminal open so no current will flow. What do i use to calculate current in the primary coil? Is it affected by the inductance value then?

thanks in advance
 

Answers and Replies

  • #2
vk6kro
Science Advisor
4,081
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I'm reviewing transformers and I have some confusion over the current

s is for secondary and p is for primary

I know that
Vs = Vp(Ns/Np)
and
Ip = Is(Ns/Np)

So when you have a load across the secondary terminal you get your current through Vs/Rload.
But does this mean for current in the primary coil, it has nothing to do with the inductance value or resistance (if I added a resistor before the inductor in the primary section)? V/Z = I does not work here?

If this is true what would the current be if I left the secondary terminal open so no current will flow. What do i use to calculate current in the primary coil? Is it affected by the inductance value then?

thanks in advance
Those formulas are for ideal transformers. There will be a current in the primary due to the load in the secondary, but there will also be a current due to the inductance of the primary winding.
It is just a coil and it has to have some value of inductance and so it has a reactance which will allow a certain amount of current to flow from the supply.

Cheap transformers are sometimes wound on inadequate iron formers and so don't have enough inductance or reactance to stop a relatively large current flowing even if there is no load.
 

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