ransformer equivalence circuit.pptx from K nsitu

1. Transformers equivalent circuit

2. Outline
• Exact equivalent circuit
• Approximate Equivalent Circuit

3. Equivalent Circuit of a transformer
• The equivalent circuit enables us to analyze and predict the
performance of the transformer by using electric circuit theory.
Exact Equivalent Circuit
• At no-load the current is used to create the magnetic flux and to supply
the core losses.
• The magnetizing current lags the voltage E1 by 90o while the core loss
current is in phase with E1
• Thus the phasor diagram of the magnetizing branch is shown in Figure

4. Equivalent Circuit of a transformer
•

5. Exact Equivalent Circuit
• At no-load the effect of magnetizing the core is represented by a
fictitious inductive reactance called magnetizing reactance (𝑋Φ )
across the emf E1
𝐸1 = 𝑋Φ𝐼Φ
• Core losses are represented by a fictitious resistance called the core loss
resistance (𝑅𝑐)

6. Exact Equivalent Circuit
• When the transformer is on load, secondary current flows causing the
primary current to increase over the no-load current.
• In the equivalent circuit the primary and secondary winding
resistances are represented by 𝑅𝑝 and 𝑅𝑠 respectively. The
corresponding copper losses are

7. Exact Equivalent Circuit
• The effects of flux leakages are represented by leakage reactances
𝑋1 and 𝑋2 in series with the primary and secondary winding resistances
respectively.
• The exact equivalent circuit of the double wound transformer is shown
in the figure below.

8. Exact Equivalent Circuit
• It is composed of the primary winding series impedance ,the
magnetizing impedance and the secondary winding series impedance

9. Exact Equivalent Circuit
•

10. Exact Equivalent Circuit

11. Exact Equivalent Circuit
•

12. Approximate Equivalent Circuit
• The circuit of Fig 9 can be simplified by connecting the magnetizing
branch across the supply to obtain the approximate equivalent circuit
shown in Fig 11.
• This increases the value of the no-load current and core losses slightly
while reducing the voltage drop across the series impedance ( ) and the
copper power loss in the primary winding slightly.
• Why?

13. Approximate Equivalent Circuit
•

14. Approximate Equivalent Circuit
•

15. Approximate Equivalent Circuit

16. Approximate Equivalent Circuit

17. Approximate Equivalent Circuit

18. Approximate Equivalent Circuit
•

19. Approximate Equivalent Circuit
•

20. • 4 kVA, 3200/320 V, 50 Hz step-down single phase transformer has
primary winding and secondary winding resistance of 1.25 Ω and 0.0125
Ω respectively. The primary winding and secondary winding reactances
are 3.75 Ω and 0.0375 Ω respectively. Determine:
• The equivalent resistance referred to the primary winding
• The equivalent resistance referred to the secondary winding
2.5
ep
R  
0.025
es
R  