A clamp meter measures the actual current of a single phase line. Since a clamp meter is essentially a CT with an open primary winding, the current detected in the primary winding will be reflected and displayed in the secondary winding. This measurement is independent of the connection method of the three-phase motor .
The first step is to measure each phase separately. You can directly measure the phase voltage on the secondary side of the voltage transformer (method as above). You can also directly measure the phase voltage (using the input terminal of a clamp meter). However, you need to insert the test leads and select the appropriate range. Its "jaw" is only for measuring AC current; it's a dedicated clamp meter. Position the pointer to the right. The usage is the same as a regular multimeter. When the voltage level is unknown, start with a high range and gradually decrease to a low range.
The second step is to measure the voltage by connecting the probes in parallel. The clamp meter measures the voltage by using the multimeter attached to it. The clamp meter cannot directly "clamp" the voltage. The selector switch is adjusted to an appropriate range.
I. How to use a digital clamp meter
1. Mechanical zeroing is required before measurement.
2. Select the appropriate range. Start with a larger range and then select a smaller range, or estimate the range based on the nameplate value.
3. When the reading is not obvious when using the minimum range measurement, the wire to be measured can be wound around the wire a few times. The number of turns should be based on the number of turns in the center of the clamp jaws. Then the reading = indicated value × range / full deflection × number of turns.
4. After measurement, set the selector switch to the maximum range.
5. During measurement, the conductor being measured should be centered in the jaws and the jaws should be tightly closed to reduce errors.
II. Precautions for Digital Clamp Tables
1. The voltage of the circuit being tested must be lower than the rated voltage of the clamp meter.
2. When measuring the current of a high-voltage line, wear insulating gloves, insulating shoes, and stand on an insulating mat.
3. The jaws must be tightly closed and the range must not be changed while the circuit is energized.
Measuring the current of a single winding of a motor with a clamp meter results in phase current, but this can be problematic for practical measurements. You either need to connect all three pairs of windings externally, or the current on the power line is always line current. Calculating the power of a motor is difficult and impractical because the power factor is an uncertain factor unless you also connect a power factor meter. However, you should know that the power factor changes due to the load.
