Step-up & Step-down Transformers

Step-up Transformer


Step-up transformer increase the voltage in its secondary when voltage is fed to its primary without changing the frequency and power. The power at primary and secondary remain the same. The step-up transformer increase the secondary voltage and in the same proportion of the increased voltage it decrease the secondary current. Thus, the power remains the same.

The voltage induced in the primary and secondary depends on the number of turns used in the primary and secondary. The voltage induced is directly proportional to the number of turns.





In a step-up transformer, the secondary voltage is more than the primary voltage.(E2>E1). It is possible when number of turns in the secondary is more than number of turns in the primary. Thus in a step-up transformer;


N2 > N1


Application of step-up transformer


Step-up transformer is used to for voltage boosting at the generation end for long distance transmission purpose. The following advantages are achieved when the power is transferred at high voltage through transmission line.


a. Reduced line losses


Current gets reduced in the same proportion of the increased voltage. For example to deliver the same power if the voltage is 3 times increased then the current will reduce by 1/3 rd times. The line losses is proportional to the square of the current. In this case the line loss will be 1/9 th of the line losses with lower voltage transmission. If generation power 5MW to be delivered at 11 KV voltage at unity power factor, then the current is equal to;






If the same power to be deliverd at 33 KV, then current is equal to







Thus, the current at 33 KV is 1/3 rd of current at 11 KV.


The lines losses is equal to;




Thus by increasing transmission voltage 3 times, the line losses become 1/9th.


b. Reduced Voltage drop


The voltage drop in transmission line at 11 KV ,262 ampere is;



The voltage drop in transmission line at 33 KV ,87 ampere is;




Reduction in Voltage drop



b. Improved Voltage Regulation


The reduction in voltage drop at higher voltage cause less difference between the sending end and receiving end voltage, thus the voltage regulation at higher voltage is better.


Step-up transformer is also used in X-ray machine and electrostatic percipitator(ESP).


Step-down Transformer


Step-up transformer decrease the voltage in its secondary when voltage is fed to its primary without changing the frequency and power. The power at primary and secondary remain the same. The step-down transformer decrease the secondary voltage and in the same proportion of the decreased voltage it increase the secondary current. Thus, the power remains the same.

In a step-down transformer, the secondary voltage is less than the primary voltage.(E2<E1). It is possible when number of turns in the secondary is less than number of turns in the primary. Thus in a step-down transformer;


N2< N1


Step-down transformer is used for utilizing the power at equipment rated voltage level It is used for distribution and isolation purpose.


Related posts:

  1. Working principle of transformer, turn ratio & transformation ratio

  2. Percentage impedance of transformer and its calculation

  3. Leakage Reactance of Transformer

  4. Transformer winding Resistance

  5. Difference between power and distribution transformer

  6. Cooling classes of transformer

  7. Difference between an ideal and practical transformer

779 views0 comments

Recent Posts

See All