Step-up & Step-down Transformers

Updated: Nov 28, 2021

Step-up Transformer


Step-up transformer increases 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 increases the secondary voltage and in the same proportion of the increased voltage, it decreases 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 the number of turns in the secondary is more than the number of turns in the primary. Thus in a step-up transformer;


N2 > N1


Application of step-up transformer


A Step-up transformer is used for voltage boosting at the generation end for long-distance transmission purposes. The following advantages are achieved when the power is transferred at a high voltage through the 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 loss 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 is to be delivered at 33 kV, then, the 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 a voltage drop at higher voltage causes less difference between the sending end and receiving end voltage, thus the voltage regulation at higher voltage is better.


A Step-up transformer is also used in X-ray machines and electrostatic precipitators(ESP).


Step-down Transformer


A Step-up transformer decreases 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 decreases the secondary voltage and in the same proportion of the decreased voltage, it increases 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 the number of turns in the secondary is less than the number of turns in the primary. Thus in a step-down transformer;


N2< N1