How to use clamp meter and three-phase four-wire meter?
1. Power and energy measurement methods
1. Power measurement method
1) Direct method: Measuring power can directly use electric power meter, digital wattmeter or three-phase wattmeter. Measuring three-phase power can also use single-phase wattmeter to connect to two-meter method or three-meter method, although there is a summation process , but it is generally still classified as a direct method.
2) Indirect method: DC power can be obtained indirectly by measuring voltage and current. AC needs to obtain power through voltage, current and power factor.
2. Electric energy measurement method
1) Direct method: direct measurement of electric energy, electric energy meter can be used for DC, induction system or electronic energy meter can be used for AC.
2) Indirect method: The indirect method is generally not used for electric energy measurement, and only when the power is stable and constant, the power meter and timing clock are used for measurement.
Second, the application of clamp ammeter
According to different structural principles, clamp ammeters are divided into two types: magnetoelectric type and electromagnetic type. The magnetoelectric type can measure AC current and AC voltage;
Vector calculation graph
1. Measuring the rotor current of the wound asynchronous motor: When measuring the rotor current of the wound asynchronous motor with a clamp ammeter, the clamp ammeter with the head of the electromagnetic system must be selected. , the indication value and the measured actual value will be very different, or even no indication, 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 obtained by the secondary coil of.
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 the AC frequency of 50Hz of 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. The vector calculation diagram of the current is shown in Figure 2.1.
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.
2. When using a clamp ammeter to measure a three-phase balanced load, the current indication value when two phase wires are placed in the jaw 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, expressed by the following public expression:
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.
