Current Sensor and Current Transformer

The current sensor can detect both AC and DC. Measuring AC and DC flows out of AC and DC. External power supply is required.

The current transformer can only measure AC. when the output signal is AC 5A or ac1a, there is no need for external power supply.

Ordinary current transformer can only measure AC current. If the AC current transformer is open circuit, it will produce high voltage. In serious cases, it will break through the line insulation and produce electric shock, threatening personnel safety. Therefore, the secondary side cannot be open circuit during use, and the secondary side must be short circuited during disassembly.

Hall current sensor, DC and AC current can be measured. The secondary side does not have to be short circuited when the Hall current sensor is removed.

Ordinary current transformer can be measured directly without power supply. The Hall current sensor needs plus or minus 15V or plus or minus 12V working power supply.

Current transformers generally output AC 5 A or AC 1 A. The Hall current sensor generally outputs a small signal of 50mA or 100mA or 5V, and can also output 4-20mA, which is convenient for automatic control.

In terms of measurement accuracy, Hall current sensor is also better than current transformer. The measured current distortion, multiple harmonics and non sine wave have a great impact on the accuracy of current transformer measurement results. The influence on Hall current sensor is much smaller.

In the measurement frequency range, the Hall current sensor is much wider than the current transformer.

In addition, the response time of Hall current sensor is faster than that of current transformer.

The common current sensor / transformer converts a large current into a small current with the same frequency and phase for measurement or isolation. According to different transformation principles, there are current sensors / Transformers Based on electromagnetic induction principle, Hall effect and fluxgate.

Current transformer is similar to a transformer with few primary turns and more secondary turns. Ideally, the current ratio of the primary stage is inversely proportional to the turns ratio, and the current conversion ratio is marked with the rated current of the primary stage, such as "300A / 5A", indicating that the output current is 5A when the measured current is rated at 300A. Due to the leakage inductance and resistance of the primary coil, as well as the nonlinear excitation current and core magnetization curve, the ratio error and phase error of the transformer will be caused. The accuracy of transformer used for metering and billing is generally level 0.1 1. According to the principle of transformer, it can not measure DC current. It is usually designed for power frequency measurement. The accuracy is the parameter under power frequency. The bandwidth is narrow, so it is not suitable for harmonic analysis and non sinusoidal measurement. When using current transformer, be sure not to open the secondary circuit, otherwise high voltage will be generated, endangering personal and equipment safety.

Current transformer

The iron core in the current clamp is divided into two parts to avoid disconnecting the measured circuit. It is very easy to measure and widely used. There are two types based on the principle of electromagnetic induction and Hall effect.

The current clamp based on the principle of electromagnetic induction is the same as the transformer. The iron core is divided into two parts. When closed, the two parts of the iron core need to be closely combined. Some current clamps are connected to the secondary resistance, and the output is voltage signal, and the output without internal resistance is current signal. Affected by the closure degree of two iron cores, the accuracy of current clamp is usually worse than that of transformer. Similarly, the current clamp based on electromagnetic induction can only measure AC.

The current clamp based on Hall effect processes an air gap in the iron core to place the hall element. Hall element is used to measure the magnetic induction intensity in the air gap. According to different control modes, there are two types: open-loop and closed-loop. The open-loop hall type uses a Hall element with good linearity, and the output voltage of the hall element is directly proportional to the measured current. The closed-loop hall type uses zero flux technology, and there is a compensation coil on the iron core. When the primary measured current generates magnetic flux in the iron core, the hall element detects the magnetic induction intensity in the iron core and converts the error voltage into current to drive the compensation coil through negative feedback to offset the magnetic flux in the iron core. Finally, the measured current is consistent with the magnetic flux generated by the compensation coil in the opposite direction, By measuring the current of the compensation coil, the measured current can be converted according to the turns ratio.

Both open-loop and closed-loop Hall current clamp can measure DC and AC. The open-loop hall is affected by the nonlinearity of the iron core and the temperature characteristics of the hall element, the accuracy and linearity are poor, but the cost is low. The closed-loop hall has less dependence on the linearity of the hall element, and the iron core works under zero flux, so the accuracy is higher than that of the open-loop. However, the current clamp has the problem that the closing degree of the movable iron core is not ideal. There is almost no one equal to or better than 0.1%, and being able to achieve 1% is a very high index. The hall element needs to provide working voltage, so both current clamps need to supply power, and the closed-loop hall needs to drive the compensation coil, which consumes more power.

Current clamp

Generally, like Hall current clamp, there are two types of open-loop and closed-loop hall, and the output is current or voltage signal. Due to the closed form, the accuracy is higher than that of the same type of current clamp.

In addition, there are current sensors using fluxgate technology, whose accuracy is better than 0.05% or even 12ppm, but this type of sensor is very expensive and fragile. Once the sensor is not powered in use, the measured current will cause damage to the sensor.

Closed mouth current sensor

4 Connection between current sensor / transformer and power analyzer

The current measurement of pa5000 power analyzer has two groups of ports: direct input and sensor input. When the output of the used sensor / transformer is current signal, it is connected to the current direct input port of pa5000. Correctly set the "CT" proportion coefficient of PA according to the used sensor / transformer, such as 300A / 5A, then CT = 300A / 5A = 60.

When the output of the current sensor / current transformer used is a voltage signal, connect it to the external sensor port (BNC interface) of pa5000, and correctly set the "sensor ratio" of PA according to the sensor / transformer used, with the unit of MV / A. if the current clamp parameter is converted from 1000A to 1V, the sensor ratio is set to 1mV / A.