The Disadvantage of Large Exciting Current in A Transformer

The magnetic flux in the core is created by the exciting current in the transformer. When the transformer is energized at no load, the transformer draws the primary current which has two components – loss current and exciting current component. More losses in the transformer are the disadvantage of a large exciting current and thus transformer efficiency deteriorates.

The exciting current in the transformer stays fairly constant whether the transformer is no load or a load. The main purpose of the exciting current is to release flux in the transformer.

 

The magnetic flux produced by the primary winding must perfectly link to the secondary and primary winding of the transformer. However, all the flux created by the primary does not connect with the winding. And some part of the flux is lost. The main reason for drawing a large exciting current is the loss of the flux in the core.

With the loss of flux, a leakage reactance is brought into the series of winding resistance. Consequently, the primary induced EMF reduces, and to maintain the same flux, the transformer draws more magnetizing current. Large rating transformers draws 0.5% of the full load current and small rating transformers( 1.5 kVA or less)  typically draw 5-10 % of full load current as an excitation current.

Reasons of large exciting current

There are the reasons for the large exciting current in the transformer as follows :

• The core of the transformer is made from a highly permeable material. The high permeable material causes magnetic flux lines to focus on the core material. If the quality of the core material is poor, the primary winding draws more current to create the required magnetic flux. 
• A smaller cross-section area leads to an increase of exciting current.
• If fewer primary turns are chosen to decrease the size of the transformer to reduce the cost, it will cause an increase in excitation current.

Disadvantages of large exciting current

• The main cause of the exciting current increase is the use of poor core material and the use of fewer turns in the primary winding. More hysteresis losses are caused by the large exciting current. The increased magnetic field with an increase of exciting current leads to increased core loss. 
• The core saturation may occur, which further causes distortion in the output voltage. The harmonic content in the output increases and power quality gets deteriorated. 
• The large exciting current causes higher system losses.
• The large exciting current causes insulation and heating of the transformer is apt to fail.