How to choose the best sensor?

Current sensors and transformers convert large currents into small currents of the same frequency and in phase to facilitate measurement or isolation. According to different transformation principles, there are generally current sensors/transformers based on electromagnetic induction principle, Hall effect, and fluxgate. Here we will introduce how to use the right sensor in the field to ensure the best test results.

Due to the sensor's measurement "dead zone" problem, large-scale sensors cannot test small currents, so on-site testing often requires selection of multiple sensors to match different test scenarios. How to choose in the end?

A current transformer

The current transformer is similar to a transformer with few primary turns and many secondary turns. Under ideal conditions, the ratio of the primary and secondary currents is inversely proportional to the ratio of the turns, and the current conversion ratio is marked with the primary and secondary rated current. For example, "300A/5A" indicates that the measured current is 5A when the rated current is 300A. Because the primary and secondary coils all have leakage inductance and resistance, and the excitation current and the core magnetization curve are nonlinear, it will cause the ratio error and phase error of the transformer. The accuracy of transformers used for metering and billing is generally 0.1~1. According to the principle of transformer, it can not measure DC current. It is usually designed as a power frequency measurement. The accuracy is the parameter under the power frequency, and the bandwidth is narrow. It is not suitable for harmonic analysis and non-sinusoidal measurement. If the measured model With a lot of harmonics, the result is very small. The use of current transformers must be careful not to open the secondary circuit, otherwise high pressure will endanger personal and equipment safety.

Second, current clamp

Current clamps can be said to be the most used sensors, they are compact, flexible, and can be adapted to almost all test situations. From the principle, it can be divided into two types: electromagnetic induction principle and Hall effect.

Based on the principle of electromagnetic induction current clamp and transformer, the core is divided into two parts, closed when the two parts of the core need to be tightly combined, some current clamp secondary connected to the resistance output voltage signal, no internal resistance output current signal This is why some manufacturers have the same type of current clamp current and voltage output of two types, the most typical is the French CA's C117 and C116. Affected by the degree of closing of the two cores, the current clamp accuracy is usually worse than that of the transformer. Similarly, current clamps based on electromagnetic induction can only measure AC.

The Hall effect-based current clamp is to process an air gap in the core to place the Hall element, and use the Hall element to measure the magnetic induction strength in the air gap. According to different control methods, there are two types of 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 proportional to the measured current. The closed loop Hall type uses zero flux technology and the core has a compensation coil. When the primary has a measured current in the iron core, the Hall element senses the magnetic induction in the iron core. This error voltage is converted into a current-driven compensation coil by negative feedback to cancel the magnetic flux in the iron core. The measured current and the magnetic flux generated by the compensating coil are in the same direction. By measuring the current of the compensating coil, the measured current can be converted according to the turns ratio.

Both open-loop and closed-loop Hall-type current clamps can measure DC and AC. Open-loop Hall is affected by core nonlinearity and Hall element temperature characteristics. Both accuracy and linearity are poor, but the cost is low. The closed-loop Hall is less dependent on the linearity of the Hall element and the core operates at zero flux, so the accuracy is higher than that of the open-loop. However, the current clamp has an unfavorable degree of closing of the active iron core. Almost no equal to better than 0.1%, and 1% is already a very high standard. Hall elements need to provide operating voltage, so both current clamps need to be powered, and the closed-loop Hall needs to drive the compensation coil to consume more power.

Third, the closed current sensor

In common with Hall-type current clamps, there are also two types, open-loop and closed-loop Hall, and the output is a current or voltage signal. Due to the closed form, the accuracy of the current clamp is higher than that of the same type, and most of our most common LEM LF series sensors are this principle.

There are also current sensors that use fluxgate technology, with accuracy better than 0.05% and even 12ppm, but this type of sensor is very expensive and fragile. If the sensor is not supplied with power, the measured current will cause damage to the sensor. The most common type of power supply is the LEM IT series of sensors.

Fourth, precision resistance

Precision resistors can be said to be often overlooked, and perhaps people do not know when to use it. Its essence is actually a resistor that can convert the acquired current signal into a voltage signal. When do you use it? An example is as follows.

When using the power meter PA333H and LEM LF 205-S sensor to test, the sensor's transformation ratio is 1:2000, power meter PA333H can accurately test the minimum current of 10mA, converted back, through the sensor we can measure the smallest current size 20A! This combination is obviously unreasonable, but there are still many occasions that need to be used. What if I need to test the current below 20A at this time? In addition to sensors with smaller change ratios and power conversion analyzers (smaller currents can be measured), tight resistance can also be used. Such as Zhi Yuan Electronics's PATV-33, its resistance is about 3.3 ohms. After adding it, the ratio of this combination becomes 1.65mV/A, and the minimum can be measured to about 0.3A. This greatly improves the sensor's ability to measure small currents.

The testing program of the test instrument combined with the Hall sensor has been applied in various fields such as industrial production, automotive, and new energy, and the overall measurement accuracy is determined not only by the test instrument, but also the sensor plays an indispensable role. In the testing process, it is very necessary to choose the right sensor and use the right measurement method.