A Current transformer operates on the same principle as a power transformer.The cross-sectional area of the core of a C.T. is made large so that the magnetic flux density is low.The core is in the form of a toroid and is made of high-permeability steel.This reduces magnetising current and thus the radio of primary and secondary currents is constant under all working conditions.
The primary of a C.T. is generally a single-turn winding B(called a bar primary)and carries full-load current.The secondary winding S has a large number of turns.Current transformer ratios are generally specified in terms of full-load primary and secondary currents.Usually,the secondary winding are designed for rated values of 5 A,although 1A and 2A ranges are also used.For example,1000/5 A current transformer may be used with 5 A ammeter to measure currents upto 1000 A.fig.shows the connections of a current transformer.
There is one important difference between a current transformer and a power transformer and a power transformer.The power transformer is supplied at constant voltage,and the primary current automatically adjusts itself so that the primary ampere-turns balance the secondary ampere-turns.In the current transformer,the primary current is determined by the external circuit and,therefore,the variation of the load on the secondary side does not affect the primary current.The secondary current adjusts itself to a value so that the primary ampere-turns balance the secondary ampere turns.
The primary of a C.T. is generally a single-turn winding B(called a bar primary)and carries full-load current.The secondary winding S has a large number of turns.Current transformer ratios are generally specified in terms of full-load primary and secondary currents.Usually,the secondary winding are designed for rated values of 5 A,although 1A and 2A ranges are also used.For example,1000/5 A current transformer may be used with 5 A ammeter to measure currents upto 1000 A.fig.shows the connections of a current transformer.
There is one important difference between a current transformer and a power transformer and a power transformer.The power transformer is supplied at constant voltage,and the primary current automatically adjusts itself so that the primary ampere-turns balance the secondary ampere-turns.In the current transformer,the primary current is determined by the external circuit and,therefore,the variation of the load on the secondary side does not affect the primary current.The secondary current adjusts itself to a value so that the primary ampere-turns balance the secondary ampere turns.
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