# Primary magnetizing impedance of non-ideal transformer

I am trying to determine the primary magnetizing inductance + core shunt resistance of a transformer. I attached the equivalent circuit of the transformer. I left the secondary open and have an AC voltage on the primary. This causes the coupling leakage inductance and coil resistance to appear as zero. How to I get the magnetizing inductance and resistance from measured V, I (sense resistor 10ohm )data.

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berkeman
Mentor
I am trying to determine the primary magnetizing inductance + core shunt resistance of a transformer. I attached the equivalent circuit of the transformer. I left the secondary open and have an AC voltage on the primary. This causes the coupling leakage inductance and coil resistance to appear as zero. How to I get the magnetizing inductance and resistance from measured V, I (sense resistor 10ohm )data.
Welcome to the PF.

That model circuit looks wrong to me. Can you link to where you saw it? If the secondary winding is open, you will mainly see the input leakage inductance Lk in series with the magnetizing inductance Lm, and the primary loss terms will interact as well...

berkeman
Mentor
magnetizing resistance
And I don't know what you mean by that term...

berkeman
Mentor
This would be more like the model I'' used to using in SPICE (although it may reduce to what you are asking about)... • Vikas Kasireddy
Yes, my question is about a real transformer. Equivalent circuit of a real transformer has the following components,
Rcore and Lprim for magnetizing impedance
R coil resistance and leakage inductance due to windings
Ideal transformer.
I know when the secondary is open, no current passes through the primary winding of the ideal transformer in the equivalence circuit diagram. Hence no current passes through Rs and Ls. The only current is through the primary magnetizing impedance Rcore and Lprim. I have data for V and I across primary winding. All I need to understand is how to calculate inductance and resistance from V I data

This would be more like the model I'' used to using in SPICE (although it may reduce to what you are asking about)... I was told that Rp and Xp can be shifted to after Rc and Xm terms since the current Io is very much less than Is. This is where the equivalent circuit I suggested comes from.

anorlunda
Staff Emeritus
since the current Io is very much less than Is
Not true when ##I_s=0##

• jim hardy and cnh1995
cnh1995
Homework Helper
Gold Member
I was told that Rp and Xp can be shifted to after Rc and Xm terms since the current Io is very much less than Is
As anorlunda said, this is not true when Is=0. You should keep the primary impedance "before" the magnetizing branch. But when the secondary is open, values of Xm and Rc are very much greater than R1 and X1, hence, the drop across primary impedance is neglected.
The only current we see is due to the magnetizing resistance and inductance.
What you are calling as megnatizing resistance actually represents core losses.
Connect a wattmeter in the primary and measure the no-load power W consumed at rated primary voltage. The power recorded W by the wattmeter is the core loss at rated voltage.
Zo=Vo/Io
cosΦo=W/VoIo.
Xm=ZosinΦo and
Ro=ZocosΦo.

Last edited:
• Vikas Kasireddy and jim hardy
Thanks guys! I did consider the primary windings inductance and resistance for open secondary and it makes sense.