Clamp Current Transformer

The clamp type current transformer is a transformer which has no primary conductor and primary insulation, and its magnetic circuit can be opened by hinge. The current-carrying conductor of clamp-type current transformer has less position correlation and stronger anti-interference ability. If it is connected to high-accuracy power meter, ammeter and phase meter, it can carry out more accurate on-site detection, which is both safe and efficient.

Introduction:

More and more attention has been paid to the high accuracy measurement of field current, phase, power and electric energy. In order to achieve this goal, it is often necessary to connect the current transformer of the high-precision measuring instrument into the circuit through repeated disassembly and wiring, which is not safe and time-consuming. Because of its low accuracy level, the existing clamp current transformer greatly limits the improvement of measurement accuracy. The key to solve this problem is to develop a high-precision clamp-type current transformer, hereinafter referred to as the current clamp.

Main technical performance and characteristics:

Rating ratio: 5A/5mA

Rated Voltammetric Number: 0100125VA

Working frequency: 50Hz (45Hz-65Hz)

Ratio difference and angle difference: better than 0.1 grade

Measurement range: (120%-1%) IN

Insulation Resistance (Primary Wire to Secondary Coil and Shell): > 1000M

Working power supply: 15V (battery power supply: +9V, capacity > 150mAh)

Power consumption: 180 mW

Self-heating time: 5 minutes

Working Environment Temperature: - 10 ~40

Dimension: 190mm x 50mm x 40mm

The small current characteristic of clamp-type current transformer is better. When the measured current changes in the range of IN (120%-10%), the ratio difference and nonlinearity are < 0.02%, and the angle difference is nonlinearity < 0.6'. The working power supply is designed for both AC and DC applications. It can be supplied by AC or rechargeable batteries. If an active load is connected to the output terminal, the precise AC current/voltage conversion can be realized, and the output voltage can be adjusted arbitrarily in the range of 0-5V. The current-carrying conductor of clamp-type current transformer has less position correlation and stronger anti-interference ability. If it is connected to high-accuracy power meter, ammeter and phase meter, it can carry out more accurate on-site detection, which is both safe and efficient.

Working principle and basic structure:

Electromagnetic current transformer has achieved high accuracy, but the magnetic conductivity of clamp-type current transformer will inevitably be greatly reduced due to its opening. Without compensation measures, its measurement accuracy is difficult to improve, and the position correlation is also very large.

In order to solve the above problems, three main measures are adopted in this design. First, the ferromagnetic material ferronickel alloy with high permeability is selected. Its initial relative permeability is 40,000, and the maximum is 100,000. Second, the transformer is compensated by self-balancing electronic compensation circuit. Third, special shielding measures are adopted. Reduce leakage interference to ensure the accuracy of residual flux detection in iron core.

The operational amplifier used requires that the offset voltage, current and temperature drift be as small as possible, the open-loop amplification multiple be large, and the frequency bandwidth be wide. In this sample, the operational amplifier offset voltage is 0.15 mV, the offset current is 1 mu A, and the AC zero voltage is less than 100 mu V. That is to say, the transformer will have errors at this time, which is commonly referred to as the zero flux principle.

It is impossible to achieve zero flux by itself, which can only be achieved by compensation of external conditions. The principle of self-balancing electronic compensation is adopted in this design. The magnetic flux produced by the output voltage of the operational amplifier and the current generated by the secondary winding in series is demagnetized to the core, so that the core can be balanced.

In ideal state, the amplification factor of operational amplifier is close to zero flux state. Because the secondary current of this kind of transformer is supplied almost entirely by external power supply through operational amplifier, and the current is not taken from the induced potential of secondary winding, the self-balancing electronic compensation current transformer can achieve high accuracy. In fact, the complete zero flux state can not be achieved, the core must have a little residual flux in order to make the amplifier output secondary current. The larger the amplifier is, the better the compensation effect will be, but too large will cause oscillation and make the system work unstable, which are mutually restrictive.

The open-loop amplification factor of operational amplifier is usually very large, so the input can be regarded as a virtual location, and the transformer can be regarded as no-load when it works in short-circuit state for the second time, so the influence of amplifier on clamp-type transformer is very small. It has been proved that the accuracy of the current/voltage converter can still reach 0.1 level by many practical measurements.

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