Why DC voltage is not Applied in Transformer Input

What Happen if DC Voltage is Applied to Transformer Input?

Considering the importance we can say a transformer is the most efficient electrical machine ever invented on the earth.

The main function of the transformer is to step up or step down the voltage to the desired voltage level keeping the power level the same at both primary and secondary or in simple input power and output power are equal.

So, we can define a transformer as a static device that converts electrical power from one circuit to another without changing its frequency and it Steps up or Steps down the level of AC Voltage and Current.

Finally, we draw the working principle of a transformer that works on the principle of mutual induction of two coils or Faraday Law’s Of Electromagnetic induction.

When the current in the primary coil of the transformer has changed the flux linked to the transformer's secondary coil also changes; hence an EMF is induced in the secondary coil due to Faraday's law of electromagnetic induction.

Actually, the transformer is based on two principles:

i> an electric current can produce a magnetic field or electromagnetism;

ii> a changing magnetic field within a coil of wire induces a voltage across the ends of the coil.

Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

So far, we understand that the transformer works with an Alternating Current or AC voltage.

What is the Effect of Giving a Constant DC Voltage to a Transformer Input?

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Observing the following steps we can see the effects of applying a constant DC voltage input to a transformer:

1. As we know the properties of an inductor v=Ldi/dt, since v is constant here, i(current) will follow a ramp waveform and will keep on increasing till the core saturates;

2. This means, i will rise to a dangerously high value and then stop changing, if i stop changing then the induced voltage across the primary will be zero(as di/dt=0) thus short-circuiting the DC source;

3. Heavy short circuit current will flow, overheating the coils, resulting in i squared R losses. Temperature rise will exceed safe values and eventually, the apparatus will catch fire;

4. Short circuit currents will result in radial magnetic forces that will rip apart the winding and the wingdings become exposed to the atmosphere. Transformer oil may catch fire under short circuit conditions;

5. So we must never apply a DC voltage to a transformer during operating conditions.

Pulsating DC can be applied to transformers, such as the flux reset occurring in the negative pulse period. 

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.