A current transformer (CT) is nothing but a coil wound on a toroidal core. When a large current carried by a conductor is to be measured, it is advisable to avoid the direct connection of ammeter in this high current circuit. For this purpose a current transformer is used.
Fig. 1 : Connection of Current Transformer (CT) to measure current.
We have to take an important precaution while operating with a CT. The precaution is that the CT should never be operated with its secondary open circuited. The secondary should be either shorted or a small when it is not connected to the meter resistance should be connected across it. If the secondary is open circuited, then the secondary ampere turns will become zero. These secondary ampere turns generally oppose the primary turns. So as the secondary ampere turns become zero, this opposition is reduced to zero and the primary ampere turns (mmf) will produce a large flux in the core. The excessive flux has two effects on the operation. First effect is increase in core loss resulting in core heating beyond safe limits. The second effect is that a large emf is induced on the primary and secondary sides, which can damage the insulation of windings. The excessive secondary induced voltage is dangerous for the user as well. Hence operation of a CT with open circuited secondary should always be avoided. The secondary of CT is generally connected to ground to protect the user against the possible shocks.
Connection Diagram of Current Transformer (CT)
The construction of a current transformer is shown in Fig. 1 (a) and its equivalent circuit is shown in Fig. 1 (b). CT has a primary coil which is of one or more turns. In Fig. 1 (a), the bar acts as the primary. The primary of CT carries the large current I1 which is to be measured, so the bar is of large cross sectional area. The secondary of a CT is made up of large number of turns. It is wound on a core. The secondary winding is a low current winding hence its cross sectional area is small. An ammeter of small range (typically 0-5 A) is connected across the secondary shown as Fig. 1 (b).
Working Principle of Current Transformer (CT)
CT is basically a step up transformer. Hence the secondary is a high voltage low current winding. The current is stepped down. The secondary current is given by,
\[{{I}_{2}}={{I}_{1}}\times \frac{{{N}_{1}}}{{{N}_{2}}}\]
Where,
N2 >> N1
The current I2 is measured by the ammeter. So if we know the turns ratio N1 / N2 then it is possible to measure I1 (i.e. the large current). The ammeter can be calibrated directly to measure the current I1, if the turns ratio is known. Thus a CT can be used to measure a high current without actually connecting the ammeter directly in series with the high current.
\[{{I}_{1}}=\frac{{{N}_{1}}}{{{N}_{2}}}\times {{I}_{2}}\]
Advantages of Current Transformer (CT)
Some of the advantages of a CT are as follows :
- It is possible to use the ammeters of normal current range to measure very high currents.
- We can calibrate the same ammeter to measure different currents by adjusting the turns ratio of the CT
- The user is safe due to the isolation provided by the current transformer between high current primary and low current secondary.
- The CT can be used to operate indicating and protecting devices such as relays.
- The same CT can be used to provide the measured current to several instruments.
Disadvantages of Current Transformer (CT)
- It can be used to measure only the ac current. The DC current cannot be measured.
- CT cannot be operated with open circuited secondary