Current transformer

                                                              Current Transformer 

• When a large current carried by a conductor is to be measured, it is advisable to avoid the connection of ammeter in this high current circuit.
• For this purpose a current transformer is used. 
• A current transformer (C.T) is nothing but a coil wound on a toroidal core.
• The construction of a current transformer is shown in fig. and its equivalent circuit is shown in fig.
  

Construction of Current Transformer :

• Construction of C.T. is shown in fig.
• C.T. has a primary coil which has a primary coil of one or more turns. In Fig . the bar acts as the primary.
• The primary of C.T. carries the large current I(1) which is to be measured. so it is of large cross sectional area.
• The secondary of a C.T. made up pf large number of turns. It is wound on a core. The secondary winding is a low current winding hence its cross sectional are is small.
• An ammeter of small range (typically 0-5A) is connected across the secondary as shown in fig.

Operation :

• C.T. is basically a step up transformer. Hence the secondary is a high voltage low current winding. The current is stepped down.
• The current I(2) is measured by the ammeter. so if we know the turns ratio N1/N2 then it is possible to measured I(1) 
• The ammeter can be calibrated directly to measure the current I(1), if the turns ration is known. thus a C.T. can be used to measure a high current without actually connecting the ammeter directly in series with the high current.