Precautions after using the clamp ammeter
After the measurement is completed, if the clamp ammeter of the clamp ammeter is measured by a common magnetoelectric clamp meter, the indicated value will be very different from the actual value to be measured, or even no indication. The reason is that the magnetoelectric system The head of the clamp meter is connected to the secondary coil of the transformer, and the voltage of the meter head is obtained from the secondary coil. According to the principle of electromagnetic induction, the mutual induction electromotive force is E2 = 4.44fWФm. It is not difficult to see from the publicity that the magnitude of the mutual induction electromotive force is proportional to the frequency. When this kind of clamp meter is used to measure the rotor current, due to the low frequency on the rotor, the voltage obtained on the meter head will be much smaller than the voltage when measuring the same power frequency current (because this kind of meter head is based on AC 50Hz power frequency design). Sometimes the current is so small that the rectifying element in the meter head cannot even be turned on, so the clamp meter has no indication, or the indication value is very different from the actual value.
If the clamp meter of the electromagnetic system is selected, since the measuring mechanism has no secondary coil and rectifying element, the magnetic flux generated by the measured current passes through the meter head, magnetizes the static and moving iron pieces of the meter head, and deflects the pointer of the meter head, which is consistent with the measured meter. The frequency of the current does not matter, so the value of the rotor current can be correctly indicated.
When using a clamp ammeter to measure a three-phase balanced load, the current indication value when two-phase wires are placed in the jaws is the same as the current indication value when one phase is placed. When using a clamp ammeter to measure a three-phase balanced load, there will be a strange phenomenon, that is, the indicated value when two-phase wires are placed in the jaws is the same as that when one-phase wire is put in, this is because the three-phase balanced load In the circuit, the current value of each phase is equal, and the following expression is used to express Iu=Iv=Iw. If one phase wire is put in the jaw, the clamp meter indicates the current value of the phase, when two phase wires are put in the jaw, the value indicated by the meter is actually the sum of the phasors of the two phase currents, According to the principle of phasor addition, I1+I3=-I2, so the indicated value is the same as when one phase is put in.
If the three phases are put into the clamp at the same time, when the three-phase load is balanced, I1+I2+I3=0, that is, the reading of the clamp ammeter is zero.
