Application of Current Transformer:

1. Measurement of AC Current

Whether the equipment runs at the rated current value or not, it is necessary to set up a current measuring device. The equipment with 40 KW or more must be monitored by ammeter.

The measurement of AC current includes direct measurement and enlarged measurement by current transformer. Direct measurement is to connect the appropriate ammeter in series with the current loop. The circuits shown in the figure below are all connected by current transformers.

(a) A current transformer is used to measure the current in a three-phase balanced circuit. Current transformer selection is based on the range of ammeter, while the range of ammeter is based on the actual value of load current, which should account for 2/3 of the total range of ammeter to the fullness value. For example, if the rated current of a device is 100A, an ammeter with a full degree of 150A is selected. At this time, the mutual inductance ratio of the current transformer should be 150/5A, and the ammeter with the corresponding ratio of 150/5A should be selected. When reading, read directly with the table indicating the value.

(b) Two current transformers are connected into incomplete stars to measure three-phase current in three-phase balanced or unbalanced lines.

(c) Three current transformers are used to measure the three-phase current in three-phase balanced or unbalanced lines by connecting them into full star connection.

2. Measurement of AC Electricity

In order to expand the measuring range of AC watt-hour meters, the most commonly used method in factories is to use current transformers to expand the measuring range.

3. Protection in Electric Drive Line

Overload protection in high-power motors is often unable to purchase corresponding thermal relays because of the high current. Under such circumstances, the method of installing current transformers is generally adopted to solve the problem. Its essence is that the thermal relay which converts large current into small current and uses less than 5A can satisfy the requirement of overload protection.