What Is a Current Transformer?
A current transformer (CT) is a device that transforms a magnitude of current, often abbreviated as CT.
The magnitude of the current input to the current transformer is transformed according to the ratio of the number of Coils or turns at the input and output. It is used to convert the current of a large-current circuit or a high-voltage circuit into a small current. Also, it is used for current measurement and relays.
Uses of Current Transformers
Current transformers are widely used in industry and infrastructure equipment.
The following are examples of current transformer applications:
- Current monitoring of pumps for pumping water
- Monitoring of falling water in wastewater and sewage treatment pumps
- Monitoring of current at the destination of power transmission in substations
- Monitoring of the amount of electricity generated at power plants
Current transformers are used for circuit insulation and current conversion. Circuit isolation is used to isolate the main circuit from the control circuit where large current flows, and Current Transformers are also used to monitor the current of small pumps (5 A or so) for the purpose of circuit isolation.
Current transformers are used to output large currents as small current circuits. Since it is uneconomical to introduce several thousand A of current into the control circuit due to the large size of the control wiring, Current Transformers are used to transform the current to a maximum of about 5 A for control and monitoring.
Current Transformer Principle
Current transformers for AC consist of an iron core, a primary coil, and a secondary coil. Each of these is wound around an iron core.
When current flows through the primary coil, a magnetic flux is generated in the iron core to excite the secondary coil. The secondary coil generates a current on the secondary side according to the magnitude of the magnetic flux excited.
The current value generated in the secondary coil is determined by the current in the primary coil and the turn ratio: the more turns in the primary coil, the higher the current value, and the less turns in the secondary coil, the lower the current value. Generally, the current value of the secondary coil is adjusted to 1 A or 5 A for the maximum allowable current in the primary coil.
Types of Current Transformers
Current transformers are classified into wire-wound current transformers and feed-through current transformers. The following are examples of Current Transformer types:
1. Current Transformer
Current transformers consist of an annular iron core and input/output coils.
It works as described in the principle section. It is mainly used for small current transformers of several A to several tens of A, because the larger the current in the measurement circuit, the larger the allowable wiring current on the primary side. It is characterized by its ability to measure small currents with high accuracy.
2. Through-Hole Type Current Transformer
Current transformers are transformers that do away with the primary-side coil. The circuit wiring to be measured is inserted between the iron core and the primary coil.
Current transformers are generally selected for large current circuits ranging from several tens of A to several hundred A or more, and can be installed at low-cost since there is no primary-side coil.
3. DC Current Transformer
Current transformers for AC circuits are described in the principle section. It cannot be used for DC circuits because no magnetic flux is generated in the iron core. A DC current transformer using a Hall element is used to measure DC current.
When a magnetic field is applied to a circuit in which Current flow, a voltage corresponding to the current value is generated. This is called the Hall effect, and a Hall element is an element that outputs a voltage based on this principle. Most DC Current Transformers are current transformers using Hall elements.
However, in the case of direct current, there are high current power supplies, such as for trams. Their current values may be tens of thousands of A, which is difficult to measure with a Hall element Current Transformer.
4. Shunt Resistor Type Current Divider
A shunt resistor is a low-resistance resistor for measuring current. Since the resistance value is predetermined, it can be converted to a current value by measuring the voltage at both ends.
Shunt resistors are used to measure large DC currents of several tens of thousands of A. Since shunt resistors in high current circuits generate a large amount of heat, they are often water-cooled.
How to Select a Current Transformer
Current transformers are basically selected according to the value of the current flowing on the primary side, and a current transformer with a current specification larger than the maximum current flowing in the primary side should be selected.
Current transformers are connected to an overcurrent relay or current indicator on the secondary side. Depending on the intended application, it is recommended to select a current transformer with a secondary side current value of 0-1A or 0-5A,
Since the overcurrent relay is often placed close to the current transformer, a product with a secondary side current specification of 0-5 A is often selected for the current transformer. This is because 0-5A makes it possible to measure current values more precisely.
On the other hand, a current indicator may be separated from a current transformer, in which case the secondary side current specification is often set to 0-1A. This is because the 0-1A specification makes it possible to reduce the power supply capacity required for the current transformer.