Transformer on No-load

 

The answer to the question lies in one of the assumptions made for an ideal transformer. The core of an ideal transformer is supposed to have infinite permeability. So, no MMF is needed to produce working flux. But, in a practical transformer, the permeability is not infinite so we need a certain current for a certain amount of MMF which will produce the required working flux.

No load current has two components (a)Iron loss component and (b) Magnetizing Component.

Iron loss component: It is the component of no load current responsible for resistive loss in the core…

Magnetizing Component: It is the component of no load current responsible for hysteresis loss in the core.

Hench, the primary current I₀ is vector summation of I_µ & I_w, So, we can write that I₀ = ( I_µ ² +  I_w²) and is not a 90⁰ behind V₁, but lags it by an angle φ < 90⁰ Which is shown in the figure. And no-load input power, W₀ = V₁ I₀Cose φ0. 

The magnitude of the no-load primary current is very small as compared to the full-load primary current. 

It is 1% of the full-load current. As I₀ is very small, the no-load primary Cu loss is negligible which means that no-load primary input is practically equal to the iron loss in the transformer.

The no-load loss of a transformer arises at its core, a part that experiences lower heating than the transformer windings depending on the quality of lamination and the thickness and resistance of the core.

In the case of no load, the second terminal of the transformer is open means the circuit is not complete on the secondary side. This situation clearly indicates that no path is available for the current to flow on the secondary side. And if there is no current flowing in the secondary side, there is no de-magnetizing flux generated which means there is no need to draw more current from the source. So primary current would contain only the exciting current (i.e. no-load current)

  

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.