Working Principle and Precautions of Current Transformer
The working principle of current transformer is similar to that of transformer. Its basic working principle is to work according to the electric magnetic electric conversion process. To a certain extent, it can be considered that current transformer is a special transformer. The function of making current transformer is mainly to measure the large current in the main circuit. During measurement, the large current in the main circuit is changed into small current according to the proportional relationship, and then sent to the meter to measure the current or power.Working process of current transformer
Current transformer is actually a special converter that changes a large current into a small current. This converter can only be used in AC circuit and can measure the large current in AC circuit. The following is mainly about its measurement process. Friends familiar with the transformer structure know how the transformer works, so the current transformer is also similar to the double winding transformer in structure. Different from the transformer, the number of turns of the primary winding of the current transformer around the iron core is very small, generally only one to two turns, and the enamelled wire is very thick (mainly because the primary winding needs to be connected in high current), it is connected in series in the measured circuit when connected, and the wiring mode is shown in the figure below
The number of turns of enamelled wire wound by the secondary of current transformer is many times more than that of the primary. Because the secondary is connected with the measuring instrument, the secondary is connected in series with the current coil of ammeter or watt hour meter to form a closed loop. The resistance value of these coils is very small, so the secondary can be similar to short circuit. We can deduce from the approximate short circuit of the secondary coil The voltage of the primary coil out of the transformer is almost zero. According to the electromagnetic principle, we can know that the primary input voltage determines the main magnetic flux. When the input side voltage is almost zero, the magnetic flux is zero and the excitation current is zero, so the total magnetic potential is zero. Finally, we will get such an expression: I1 = kxi2 (where I2 is the reading of the secondary connection ammeter of the current transformer).
We can know from the expression I1 = kxi2 that the current transformer is equivalent to a step-up transformer. When the primary current is large, the current induced by the secondary is the current reduced by a certain proportion. In the formula, K is the reduced proportion, which we call the transformation ratio. For example, when k is 30 / 5, it means that the current flowing through the primary is 30a, and the current induced by the secondary side is 5A, The transformation ratio is 6. In other words, we reduce the current of the main circuit by 6 times through the current transformer, which can not only expand the measuring range of the instrument, but also ensure the safety of personnel and equipment.
Precautions for use of current transformerThere are two points to pay attention to in the use of current transformer. One is that the shell and iron core of current transformer shall be reliably grounded for safety; the other is that its secondary cannot be disconnected when the current transformer is in use, otherwise it will induce high voltage and endanger personal safety.
