Difference Between Current Sensor and Current Transformer
Current transformer is mainly used to measure the current of busbar and primary side with iron core coils in AC occasion. It can detect the DC current, can be single core or dual cores, generally, it's equipped with auxiliary DC winding. Such a structure is simple, reliable and slow. Technology and design of the current transformer are complex, its common precision is 5%, it's difficult to reach 0.5% because phase position and compensation are difficult. A current transformer is an electrical component that turns large current into small current, it doesn't change the current frequency but only the size. Its secondary side current is 5A, that means the output rated current is 5A. It's mainly used for electrical control, measurement, protection and signal.
Current sensor is a generalized concept, it generally refers to the secondary instrument now, particularly, in the field of semiconductors and microelectronics, such as hall sensor, fibre optic sensor. Semiconductor current sensor is a convenient commodity with fast response, easy compensation and correction, its accuracy can reach one thousandth. A current sensor is an electrical component that transforms AC into DC or DC into DC. Its output current is DC 4~20mA, which is an analog quantity.
Current transformer is based on the principle of electromagnetic induction. A current transformer is composed of closed cores and winding, it has few turns of primary winding and is connected in series to a circuit whose current needs to be measured. Therefore, full current of circuit often flows through it. It has more turns of secondary winding and is connected in series to measuring instrument and protection circuit. When the current transformer works, its secondary circuit is always closed, so the impedance of series coil of the measuring instrument and protection circuit are very small, and the working state of current transformer is close to short circuit.
When the primary side wire passes through the current sensor, primary side current IP would produce magnetic lines of flux. The primary side magnetic lines of flux concentrates at the magnetic core, the hall electrodes embedded in the air gap of the magnetic core can generate a voltage of only a few millivolts proportional to the primary side magnetic lines of flux. The electronic circuit can transform this tiny signal into a secondary current IS at the following relation: among, IS refers to the secondary current, IP refers to primary side current, NP refers to the turns of primary side winding, NS refers to the turns of secondary side winding, NP/NS refers to the ratio-turn, and generally the NP equals to 1. Output signal of the current sensor is the secondary side current IS, which is proportional to the input signal (primary current IP). IS is generally small, about 100~400mA. If the output current passes through the measured resistance RM, an output voltage signal of several volts proportional to the primary current can be obtained.
Secondary output of the current transformer is the secondary current which is directly proportional to the primary current, and the voltage is very low; However, secondary output of the voltage transformer is the secondary voltage which is directly proportional to the primary voltage, and the current is very low.
Current sensors can detect both AC and DC, external power supply is needed. Current transformers can only measure AC current and output signal is AC 5A or AC1A, external power supply isn't needed. Common current transformer can only measure AC current. If AC current transformer is open, it will produce high voltage to break down the insulation of the line and cause electric shock to threaten the safety of personnel, so the secondary side can't be open circuited when use. When removing, the secondary side must be shorted circuited.
Therefore, the current transformer is used to supply the current signal proportional to the primary scale to the protection and measurement, while the voltage transformer is used to supply the voltage signal proportional to the primary scale to the protection and measurement.
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