The structure and application of clamp ammeter

Motor and drive measurements:

Loading: The motor draw current measured as an average of the three phases should not exceed the motor's full load current rating (multiplied by the permissible overload factor). On the other hand, motors with load currents below 60% of the full load current (mostly) are becoming less powerful and their power factor is reduced.

Current Balance: Current imbalance may indicate a problem with the motor windings (e.g., different resistances on the field windings due to an internal short circuit). Generally, the imbalance should be less than 10%. (To calculate imbalance, first average the three-phase readings; then find the largest deviation from the average and divide by the average.) Extremely high current imbalance is single-phase imbalance when one of the three phases has no current. This is usually caused by a blown fuse.

Motors that are started directly at voltage (via a mechanical starter) have an inrush current. The inrush current can be around 500% on older motors and up to 1200% on energy-efficient motors. If the inrush current is too high, it often causes voltage dips and annoying tripping. Advanced clamp meters have a "surge" function that triggers on the inrush current and captures the original value.

Peak Load (Shock Load): Some motors are subject to shock loads, which can cause enough current surges to trip the overload circuit in the motor controller. Imagine encountering a hardened chainsaw at work. The Min/Max function can be used to record the worst-case current drawn by the shock load.

The use process of the clamp meter:

1.Accurately select the clamp ammeter according to the type and voltage level of the measured current. Generally, the T301 type is used for lines below 500V. When measuring the current of high-voltage lines, a high-voltage clamp ammeter that matches its voltage level should be used.
2.Accurately check the appearance of the clamp ammeter, whether the closing state of the jaws and the state of the meter head are normal. If the pointer is not at zero, mechanical zeroing should be performed.
3.Select the appropriate range of the clamp ammeter according to the size of the measured current. The selected range should be slightly larger than the value of the measured current. If the size of the measured current is unknown, the maximum range should be used for estimation first.
4.Measure accurately. When measuring, press the wrench to open the jaws. Put the measured wire into the base of the jaws, loosen the wrench and close the jaws tightly.
5.After reading, open the jaws, remove the measured wire, and set the gear to the highest current level or OFF gear.
Measurement example: measuring the operating current of a cage asynchronous motor in operation. According to the current size, it is possible to check whether the motor is working normally to ensure the safe operation of the motor and extend its service life. First, accurately select the voltage level of the clamp-type ammeter, check whether its external insulation is good, whether there is any damage, whether the pointer is swinging and flexible, whether the jaws are rusted, etc. Estimate the rated current according to the motor power to select the range of the meter. When measuring, each phase can be measured once, or three phases can be measured once. At this time, the number on the meter should be zero (because the sum of the three-phase current phase is zero). When there are two phase wires in the jaws, the value displayed on the meter is the current value of the third phase. By measuring the current of each phase, it can be distinguished whether the motor is overloaded (the measured current exceeds the rated current value), whether there is a problem with the internal or power supply voltage of the motor, that is, whether the imbalance of the three-phase current exceeds the limit of ten%.

Note when using the clamp meter:

1. Since the clamp ammeter has to touch the measured circuit, be sure to check whether the insulation function of the meter is good before measuring. That is, the outer shell should not be damaged and the handle should be clean and dry.
2. When measuring, you should wear insulating gloves or clean wire gloves.
3. When measuring, pay attention to keep a safe distance between each part of the body and the charged body (the safety distance of the low-voltage system is 0.1~0.3 m).
4. The clamp ammeter cannot measure the current of bare conductors.
5. Strictly select the clamp ammeter according to the voltage level: the clamp ammeter of low voltage level can only measure the current in the low-voltage system, and cannot measure the current in the high-voltage system.
6. Avoid switching the gear of the clamp ammeter during the measurement process; if you need to change the gear, you should first remove the measured wire from the jaws and then change the gear.

Key points for selecting clamp meters:

1. Inspection purpose:
select the model according to different inspection purposes, AC current, DC current, or leakage current;
2. The maximum conductor size that can be inspected is coordinated with the inspection site, with different sizes from 21mm diameter to 53mm diameter.
3. Whether the true effective value inspection is necessary, the clamp ammeter with the average value method cannot accurately inspect non-sinusoidal circuits such as motors and transformer circuits. To inspect such circuits, a clamp ammeter with the true effective value method should be used.
4. Other functions:
not only can the current be inspected, but there are also models that integrate the inspection function and the recording output.