(1) Measuring the rotor current of the wound asynchronous motor: When measuring the rotor current of the wound asynchronous motor with a clamp ammeter, a clamp ammeter with an electromagnetic head must be selected. If a common magnetoelectric clamp ammeter is used During the measurement, the indicated value and the actual value to be measured will be very different, or even not indicated. The reason is that the meter head of the magnetoelectric clamp meter is connected to the secondary coil of the transformer, and the voltage of the meter head is determined by the secondary coil. owned. According to the principle of electromagnetic induction, the mutual inductance electromotive force is E2=4.44fwPm. It is not difficult to see from the public announcement that the magnitude of the mutual inductance electromotive force is proportional to the frequency. When using this clamp meter 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 type of meter head is 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 indicated value is very different from the actual value.
If the clamp meter of the electromagnetic system is used, since the measuring mechanism has no secondary coil and rectifier element, the magnetic flux generated by the current to be measured passes through the meter head, magnetizing the static and moving iron pieces of the meter head, so that the pointer of the meter head is deflected, and the measured The frequency of the current does not matter, so the value of the rotor current can be correctly indicated.
(2) When measuring a three-phase balanced load with a clamp-type ammeter, the indicated value of the current when the two-phase wire is placed in the jaw is the same as the indicated value of the current when the --phase is placed. When a clamp-type ammeter is used to measure a three-phase balanced load, a strange phenomenon occurs, that is, the indicated value when the two-phase wire is placed in the jaw is the same as the indicated value when the --phase wire is placed. This is because in the three-phase In a balanced load circuit, the current value of each phase is equal, and the following formula is used to express Iu=Iv=Iw. If one-phase wire is put into the jaw, the clamp meter indicates the current value of that phase. When two-phase wires are put into 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 the -phase is placed.
If three phases are put into the jaws 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.
