Primary magnetizing impedance of non-ideal transformer

Click For Summary

Discussion Overview

The discussion revolves around determining the primary magnetizing inductance and core shunt resistance of a non-ideal transformer using measured voltage and current data. Participants explore the implications of leaving the secondary open and the resulting effects on the equivalent circuit components.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that with the secondary open, the input leakage inductance appears in series with the magnetizing inductance, while others question the validity of the equivalent circuit model presented.
  • There is a discussion about whether current flows through the primary winding of the ideal transformer when the secondary is open, with some asserting that only magnetizing resistance and inductance contribute to the current.
  • One participant shares a link to a model circuit used in SPICE, suggesting it may align with the original inquiry.
  • Another participant emphasizes that the primary impedance should be considered before the magnetizing branch, especially when the secondary is open, and discusses the relationship between core losses and magnetizing resistance.
  • Mathematical relationships involving power measurements and impedance calculations are introduced, including the use of a wattmeter to measure core loss at rated voltage.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the transformer under open secondary conditions, particularly regarding the flow of current and the significance of various components in the equivalent circuit. The discussion remains unresolved with multiple competing interpretations of the transformer model.

Contextual Notes

Participants highlight the importance of distinguishing between ideal and real transformers, and there are mentions of specific assumptions regarding current flow and component interactions that are not fully resolved.

Who May Find This Useful

Individuals interested in transformer modeling, electrical engineering, and those working with equivalent circuits in practical applications may find this discussion relevant.

Vikas Kasireddy
Messages
8
Reaction score
0
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.
 

Attachments

  • Screen Shot 2017-02-26 at 3.30.10 PM.png
    Screen Shot 2017-02-26 at 3.30.10 PM.png
    5.8 KB · Views: 1,299
Engineering news on Phys.org
Vikas Kasireddy said:
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...
 
Vikas Kasireddy said:
http://nptel.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/chapter_1/1_18.html

Correct me if I am wrong, but I thought when the secondary is open, no current passes through the primary of the ideal transformer part. The only current we see is due to the magnetizing resistance and inductance.
Sorry, are you asking about an ideal transformer or a real transformer? I thought you were asking about a real transformer.
 
Vikas Kasireddy said:
magnetizing resistance
And I don't know what you mean by that term...
 
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
 
berkeman said:
This would be more like the model I'' used to using in SPICE (although it may reduce to what you are asking about)...

https://upload.wikimedia.org/wikipedia/commons/3/39/TREQCCT.jpg
TREQCCT.jpg

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.
 
Vikas Kasireddy said:
since the current Io is very much less than Is

Not true when ##I_s=0##
 
  • Like
Likes   Reactions: jim hardy and cnh1995
  • #10
Vikas Kasireddy said:
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.
Vikas Kasireddy said:
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:
  • Like
Likes   Reactions: Vikas Kasireddy and jim hardy
  • #11
Thanks guys! I did consider the primary windings inductance and resistance for open secondary and it makes sense.
 

Similar threads

Replies
10
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 8 ·
Replies
8
Views
3K
Replies
8
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 81 ·
3
Replies
81
Views
8K
Replies
64
Views
8K
Replies
2
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 36 ·
2
Replies
36
Views
12K