What is Step up Transformer? - Theory, Working Principle and Applications

Step up Transformer is a passive device used to increase the input A.C. voltage to higher level. It has less number of turns in the primary coil compared to the secondary coil. Primary coil is powered with alternating current-voltage and output voltage also A.C. voltage.

Electromagnetic induction is used to induce emf on secondary winding to induce a voltage on the secondary coil. There is no direct electrical connection between the primary coil and the secondary coil.

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What is a Transformer?

A transformer consists of two or more coils of wire wrapped around a common ferromagnetic core. The only connection between the coils is the  common magnetic flux present within the core. Primary coil is powered with A.C. voltage/current source and A.C. voltage will be induced in the secondary coil due to the common magnetic flux linking with the secondary coil. 

The turns ratio between primary coil and secondary coil will define a transformer to be step up transformer or step down transformer. If turns ratio between primary coil and secondary coil is greater than one (>1) then transformer will act as step down transformer and if turns ratio between primary coil and secondary coil is less than one(<1) then transformer will act as step up transformer.

step up transformer equation rizqan engineering blog
step up transformer equation rizqan engineering

As previously mentioned frequencies of both input and output voltage are the same, because the transformer is a very efficient piece of equipment – while the voltage and current values are usually different.

A transformer provides galvanic isolation in the electrical system. Due to these two main features, the transformer is the most important part of the electrical system and provides economical and reliable transmission and distribution of electrical energy.

Theoretically, transformers can transfer energy in both directions. But, practically it is not possible most of the time, because ratings and properties of the core, cross-section of copper wire and other features are specifically designed for a task. A specific requirement operating voltage and current ranges have a unique transformer design calculation. 

For the step up transformer, the primary coil contains less number of turns compared with the secondary coil. A secondary wire has a lesser cross-section than the primary copper wire, because of the higher current value on the primary side. Usually, we place the secondary coil close to the transformer core, and over them, we wound the primary coil. 

Equivalent circuit of a step up transformer

The equivalent circuit of a transformer is for both types of transformers, only the turns ration differs. In usual notation primary winding resistance, current, voltage reactance, etc… will be written in subscript 1 and secondary term in subscript 2 after notation.

diagram of a transformer rizqan engineering

Power in Step up Transformer

Ideally Input power is equal to the output power of the transformer. However electrical energy input through the primary coil is transformed in to magnetic energy and again magnetic energy is transformed in to electrical energy. In a step up transformer voltage is increased on secondary coil and current will be reduced. As input power is equal to output power the products of voltage and and current in primary side is equal to secondary side.

Input power on primary side

Step up transformer step down transformer power transfer equation rizqan

There are various types of losses in the transformer such as iron loss, copper loss, hysteresis loss, eddy current loss, stray loss, and dielectric loss. The hysteresis losses occur because of the variation of the magnetization in the core of the transformer and the copper loss occurs because of the transformer winding resistance. 

Therefore, in practical transformer output voltage and current vary from theoretical calculations.

How does Step up transformers work?

When the primary coil is supplied with A.C. voltage input, primary winding induces magnetic field with time-varying A.C. current.

This flux link with the secondary coil. Then linking magnetic flux induces a voltage on the secondary coil. As per the turns ratio, this output voltage will be step up or step down. 

Construction of Step up Transformers

Two major parts are there to build a step up transformer.
  1. Windings/Coils 

    Number of turns in the primary coil should be less compared to the secondary coil. As it is a step up transformer, the output voltage will be high, hence output current is low. Therefore, the secondary coil wire is thinner than the primary coil wire. 

    The material used in the winding is Copper and Aluminium. Copper being the expensive material increases the life of Step Up Transformer when compared to Aluminium which is less expensive.

    Lamination of Core reduces Eddy Currents. The most common type of Laminations are E-E Type and E-I Type to which Primary and Secondary winding is fixed and they are stacked to minimize the air gap.

  2. Transformer core
    • Less coercive materials are preferred such as Silicon Steel to build the Core. If the core is built with other Ferro-magnetic materials it might result in Hysteresis Loss and Eddy current Loss.
    • The characteristic of the Core restricts the magnetic field lines in the air which in turn increases the efficiency of the Transformer.
    • To build the core of the Transformer, a high penetrable material is used. To form the Core, thin Silicon Steel is assembled and tightly clamped which is laminated. The preamble material which is used in forming of the core is designed to let the magnetic flux to flow with less loss.

Applications of Step up transformers

  • Transmission lines – The power loss of the transmission line is directly proportional to the square of the current flows through it (Power = I2R). Therefore, low current – high voltage is used to transfer power within stations. Output of Step up transformer is high voltage and low current.
  • Generator step up transformer in engineering plants
  • The step up transformer is also used for starting the electrical motor
  • Inverters and Stabilizers in the transformers help in stabilizing the low voltage to the higher voltage.
  • Electrical and electronic devices where voltage boost is required
    Example: step up transformer 110v to 220v is used in electrical appliances made for both category of countries where 110v or 220v distribution lines used. 
  • On microwave ovens
  • X-rays machines
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