3rd march 2009

3 transformer related questions today.....

Q1. Does transformer draw any current when secondary is open? why?

A1. Yes ,primary will draw the current from the main supply in order to magnetize the core and to supply for iron and copper losses on no load.There will not be any current in the secondary since secondary circuit is open.

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Q2. What is the difference between a power transformer and a distribution transformer?

A1.

Power transformers are used at the sending and receiving ends of a long ,high voltage power transmission line for stepping up and stepping down the voltage.They are manipulated to operate at or near their rated capacity(KVA rating).Therefore they are disconnected during light load periods.A power transformer does not come in direct contact with the consumer terminals.

The choice of a power transformer out of a large number of competing transformers should be based on full load efficiency.

Distribution transformers are those which change the voltage  suitable for utilisation purposes at the consumer's  premises. A distribution transformer must have its secondary directly connected with the consumer's terminals. The primary of distribution transformer are always energised.

The choice of a distribution transformer out of a number of competing transformers should be done using energy efficiency...and not full load eficiency.

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Q3. Why is transformer rated in KVA rather than in KW ?

A3. This is because rated transformer output is limited by heat and hence by lossesin the transformer.The two type of losses being core losses and ohmic (I^2 R)losses,

The core loss depend on transformer voltage and the ohmic losses depend on transformer current. As these losses depend on transformer's voltage and current and are almost independent of load power factor, the transformer rating is expressed in VA (VxI) or in KVA and not in KW.

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done for today .

bye for now.