Step Down Transformer reduces voltage and hence used in almost all household electrical appliances. Our present day electronics is heavily dependent on it. In this post, we will try to understand what is it, its working principle, equation, types, advantages and disadvantages.

What is Step Down Transformer

A Step Down Transformer is a device which converts high primary voltage to a low secondary voltage. In a Step Down Transformer, the primary winding of a coil has more turns than the secondary winding. Figure 1 below shows the winding representation of a typical Step-Down Transformer.

Representation of Windings in Step Down Transformer

Fig 1: Representation of Windings in Step Down Transformer

Working Principle of Step Down Transformer

Transformer work on the principle of “Faraday’s law of electromagnetic induction”. Mutual induction between the windings is responsible for transmission action in a transformer.

Faraday’s law states that “when the magnetic flux linking a circuit changes, an electromotive force is induced in the circuit proportional to the rate of change of the flux linkage”.

The emf (Electro Motive Force) induced between the two windings is determined by the number of turns in primary and secondary winding respectively. This ratio is called as Turns Ratio.

The voltage reduction capability of step down transformers depends on the turn ratio of the primary and secondary coil. As the number of windings in secondary coil is less as compared to the number of windings in primary coil, so the amount of flux linkage to the secondary coil of the transformer will also be less compared to the primary coil.

Accordingly, the emf induced will be less in the secondary coil. Due to this, the voltage reduces at the secondary winding compared to primary winding

Step Down Transformer Equation

The formula used to design a Step Down Transformer is

Transformer EquationWhere,

  • Ns = number of turns in secondary
  • Np = number of turns in primary
  • Vs = Voltage in secondary
  • Vp = Voltage in primary

The number of turns in secondary winding should always be less than the number of turns in the primary winding of the transformer i.e Np>Ns to work transformer as “Step-Down Transformer”.

As the number of turns will be less in secondary winding, so will be total induced emf and hence the output voltage in the secondary will also be less than the primary input voltage.

Let’s understand by considering the situation of Step-Down Transformer in which the secondary turns [Ns] are 250, primary turns [Np] are 5000 and the input voltage [Vp] is 240. Then the voltage at the secondary [Vs] can be calculated by using the formula:

Transformer Equation

Buy rearranging the equation we get:

Secondary winding voltage calculation in Step Down Transformer

Hence, the voltage at the secondary winding of the transformer is 12V, which is less than that at the primary winding. Therefore, the transformer in said to be a Step-Down Transformer.

Types of Step Down Transformer

Step Down Transformers can be classified into three categories based on tappings in secondary coil. They are:

  • Single Phase Step Down Transformer
  • Center Tapped Step Down Transformer
  • Multi Tapped Step Down Transformer

Single Phase Step Down Transformer

Used to step down the current ratings and input voltage, and it gives low voltage and current output.

Ex: 12V AC.

Symbol and Physical View of Single Phase Step Down Transformer

Fig. 2 – Symbol and Physical View of Single Phase Step Down Transformer

Center Tapped Step Down Transformer

This type of Step Down Transformers will be having one primary winding and a center split in the secondary winding, by which it gives the output voltage with the center pint.

Ex: 12v-0-12v.

Symbol and Physical View of CenterTapped Step Down Transformer

Fig. 3 – Symbol and Physical View of Center Tapped Step Down Transformer

Multi Tapped Step Down transformer

This type of Step Down Transformers have multiple tapping in Secondary coil. Multiple Tapping are used to get the desired varied output with secondary coils.

Ex: 0-12v, 0-18v.

Symbol and Physical View of MultiTapped Step Down Transformer

Fig. 4 – Symbol and Physical View of Multi Tapped Step Down Transformer

Applications of Step Down Transformer

The various applications of Step Down Transformers includes:

  • In main adapters and chargers for cell phones, stereos and CD players
  • To step down the voltage level in transmission line
  • In welding machines by reducing voltage and increasing current.
  • In televisions, voltage stabilizers, inverters, etc.

Advantages of Step Down Transformer

The advantages of Step Down Transformers are as follows:

  • Useful in stepping down the voltage, thereby making transmission power easier and cheaper
  • More than 99% of efficiency
  • Provides varied voltage requirements
  • Low Cost
  • High Reliability
  • High Durability

Disadvantages of Step Down Transformer

The disadvantages of Step Down Transformers are as follows:

  • Requires a lot of maintenance failing which can damage the transformer
  • Volatility in feedstock costs
  • Fault rectification takes more time

Role of Step Down Transformer in Voltage Transmission

Voltage Ditribution Chain

Fig. 5 – Voltage Distribution Chain using Transformer

In power stations, an AC electricity is generated at a nearly low peak voltage around 440v. A general end user uses a voltage of 220V to 240V for household and business. The generated voltage output of power station is partaken to a step-up transformer, which increases its peak voltage from few hundred volts to few kilovolts.

The step-up transformer output is given to a high tension transmission line which transports the power/electricity over the miles. This is done to reduce voltage drop. Once this power reaches its consumption point/ end sub station, then with the help of step-down transformer, it is reduced to the desired value i.e. 220V-240V.

Author: Megha S Badad, M.TECH  (Electronics)

 

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