Step down Transformer - Operation, Construction and Importance of them

Step down transformer is a device which changes voltage and current to lower level without changing frequency. A step down transformer consists of two or more coils of wire wrapped around a common ferromagnetic core. These coils are (usually) not directly connected. The only connection between the coils is the  common magnetic flux present within the core. 

There are basically two types of transformer by their function. One is Step Down Transformer and other one is step up transformer.


Transformer action in step down transformer​

Mutual induction between the windings is responsible for transmission action in a step down transformer. 

This Faradey’s law of Electromagnetic Induction explains this theory well. Faraday’s law of electromagnetic induction is whenever a conductor is placed in a varying magnetic field, EMF induces between conductor terminals  and this emf is called an induced emf and if the conductor is a closed circuit than the induced current flows through it


The magnitude of the induced EMF is equal to the rate of change of flux linkages. Following image represents the basic structure of a step down transformer.

Primary transformer winding is connected to a source of ac electric power, and the second (and perhaps third) transformer winding supplies electric power to loads. The transformer winding connected to the power source is called the primary winding or input winding, and the winding connected to the loads is called the secondary winding or output winding. If there is a third winding on the transformer, it is called the tertiary winding.


Basically there are three factors determining emf. Please read our article on this.

Number of turns in the coil – By increasing the amount of individual conductors cutting through the magnetic field, the amount of induced emf produced will be the sum of all the individual loops of the coil, so if there are 20 turns in the coil there will be 20 times more induced emf than in one piece of wire.

In transformer there are two windngs named as primary winding and secondary winding. Ration between number of turns in primary winding and secondary winding is know as Turns ratio. 

For an Step Down Transformer Number of turns in secondary winding is lesser than number of turns in primary winding. Hence, flux linkage of the secondary coil is less compared to flux linkage of the primary coil.

Therefore, induced emf will be less in secondary winding. Due to this, the voltage reduces at the secondary winding compared to primary winding. We can express the voltage ratio between coils with turns ratio by following equation. The Step Down Transformer equation will be,

step down Transformer Equation

Ns = number of turns in secondary

Np = number of turns in primary

Vs = Voltage in secondary

Vp = Voltage in primary

Sample calculation – Step down transformer example​

For example, consider the following situation. The number of turns in the primary winding of a transformer is 3000 and that in the secondary winding is 150. If the alternating voltage at the primary of the transformer is 240V, then the voltage at the secondary of the transformer can be calculated using the following equation.


Here, NP is primary winding turns = 30000

NS is secondary winding turns = 150

VP is voltage at the primary winding of the transformer = 240V

VS is the voltage at the secondary of the transformer =?

Using the above equation, VS = (VP * NS)/NP = 240*150/3000 = 12V

Hence, the voltage at the secondary winding of the transformer is 12V, which is less than that at the primary. Therefore, the transformer in this subject is a Step down Transformer. 

Importance of transformers in modern life​

The first power distribution system in the United States was a 120-V de system invented by Thomas A. Edison to supply power for incandescent light bulbs. Edison’s first central power station went into operation in New York City in September 1882. 

Unfortunately, his power system generated and transmitted power at such low voltages that very large currents were necessary to supply significant amounts of power. These high currents caused huge voltage drops and power losses in the transmission lines, severely restricting the service area of a generating station.

In the 1880s, central power stations were located every few city blocks to overcome this problem. The fact that power could not be transmitted far with low-voltage dc power systems meant that generating stations had to be small and localized and so were relatively inefficient.

First transformer rizqan engineering website

The first practical modern transformer, built by William Stanley in 1885. Note that the core is made up of individual sheets of metal (laminations). (Courtesy of General Electric Company.)

The invention of the transformer and the concurrent development of ac power sources eliminated forever these restrictions on the range and power level of power systems. A transformer ideally changes one ac voltage level to another voltage level without affecting the actual power supplied. 

If a transformer steps up the voltage level of a circuit, it must decrease the current to keep the power into the device equal to the power out of it. Therefore. ac electric power can be generated at one central location, its voltage stepped up for transmission over long distances
at very low losses, and its voltage stepped down again for final use. 

Since the transmission losses in the lines of a power system are proportional to the square of the current in the lines, raising the transmission voltage and reducing the resulting transmission currents by a factor of 10 with transformers reduces power transmission losses by a factor of 100. Without the transformer, it would simply not be possible to use electric power in many of the ways it is used today.

Transformer in transformer station rizqan engineering

Usage of transformers in a power generation, transmission and distribution systems. Voltages are step up and down at various stages in real systems. Transformers do the job.

In a modern power system, electric power is generated at voltages of 12 to 25 kV. Transformers step up the voltage to between 110 kV and nearly 1000 kV for transmission over long distances at very low losses. Transformers then step down the voltage to the 12- to 34.5 kV range for local distribution and finally pernit the power to be used safely in homes, offices, and factories at voltages as low as 120 V.

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