Current transformers

Current transformers

For measurement currents in far more than concerning one hundred A a current electrical device is often used. With a d.c. moving-coil meter the present needed to convey full scale deflection is extremely small—typically many milliamperes. once larger currents square measure to be measured a shunt electrical device is value-added to the circuit .








 However, even with shunt resistors value-added it's uphill to live terribly giant currents. When a.c. is being measured a shunt can't be used since the proportion of the present that flows within the meter can rely upon its electrical phenomenon, that varies with frequency.

In current transformers the first sometimes consists of 1 or 2 turns while the secondary will have many hundred turns. A typical arrangement is shown in Figure twenty.19.

 If, as an example, the first has a pair of turns and therefore the secondary two hundred turns, then if the first current is five hundred A,

Current transformers isolate the meter from the most circuit and permit the utilization of a typical vary of ammeters giving complete deflections of one A, 2 A or 5 A.

For very large currents the transformer core can be mounted around the conductor or bus-bar. Thus the primary then has just one turn. It is very important to short-circuit the secondary winding before removing the ammeter. 

This is because if current is flowing in the primary, dangerously high voltages could be induced in the secondary should it be open circuited. Current transformer circuit diagram symbolsare shown in Figure 20.20.