Measurement principle and usage of pointer clamp ammeter

1. Structure of magnetoelectric clamp ammeter

The magnetoelectric clamp ammeter is mainly composed of a special current, a rectifier magnetoelectric system ammeter and internal circuits. The common models are: T301 and T302. T301 clamp ammeter can only measure AC current, while T302 can measure both AC current and AC voltage. There are also AC and DC dual-purpose pocket clamp ammeters, such as: MG20, MG26, MG36 and other models.

The appearance of the T301 clamp meter is shown in Figure 1. Its accuracy is level 2.5, and the current range is: 10 A, 50 A, 250 A, 1000 A.

2. Working principle of clamp ammeter

The working principle of the clamp ammeter is: based on the working principle, when the clamp ammeter wrench is tightened, the iron core of the current transformer can be opened, and the wire of the measured current enters the jaws as the primary winding of the current transformer. When the wrench is loosened and the iron core is closed, an induced current is generated on its secondary winding according to the principle of the transformer, and the ammeter pointer deflects, thereby indicating the value of the measured current.

It is worth noting that: since the principle is based on the principle of mutual inductance, whether the core is tightly closed and whether there is a large amount of residual magnetism has a great influence on the measurement results. When measuring a small current, the measurement error will increase. At this time, the measured wire can be wound around the core a few more times to change the current ratio of the mutual inductance to increase the current range. At this time, the measured current ix should be:

In the formula, ia is the reading on the ammeter; n is the number of winding turns.

3. Steps for using a clamp ammeter

(1) Select the clamp ammeter correctly according to the type and voltage level of the current being measured. For AC lines below 500V, use the T301 type. When measuring the current of a high-voltage line, use a high-voltage clamp ammeter that matches its voltage level.

(2) Check the appearance of the clamp meter, the closing of the jaws and the meter head to see if they are normal. If the pointer is not at zero, perform mechanical zero adjustment.

(3) Select the appropriate range of the clamp ammeter according to the current being measured. The selected range should be slightly larger than the current being measured. If the current being measured is unknown, the maximum range should be used for estimation.

(4) Correct measurement. When measuring, press the wrench tightly to open the jaws. Place the wire to be measured in the center of the jaws, loosen the wrench and close the jaws tightly.

(5) After reading the value, open the jaws, remove the wire to be measured, and set the gear position to the highest current position or the off position.

Measurement example: Measuring the current of 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, correctly select the voltage level of the clamp-type ammeter, check whether its appearance insulation is good, whether there is any damage, whether the pointer swings flexibly, whether the jaws are corroded, etc. Estimate the rated current according to the motor power to select the range of the meter. When measuring, you can measure each phase once, or you can measure three phases 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 lines 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 determined whether the motor is overloaded (the measured current exceeds the rated current value), whether there is a problem inside the motor or the voltage, that is, whether the three-phase current imbalance exceeds the limit of 10%.

4. Matters needing attention when using a clamp ammeter

(1) Since the clamp ammeter needs to contact the circuit being measured, be sure to check whether the insulation performance of the meter is good before measurement. That is, the 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, be careful to keep all parts of your body at a safe distance from charged objects (the safe distance for low-voltage systems is 0.1 to 0.3 m).

(4) Clamp-on ammeters cannot measure current in bare conductors.

(5) It is strictly forbidden to switch the gear of the clamp ammeter during the measurement process; if it is necessary to change the gear, the measured wire should be removed from the jaws before changing the gear.

(6) Select the clamp ammeter strictly according to the voltage level: a low-voltage clamp ammeter can only measure the current in the low-voltage system, but cannot measure the current in the high-voltage system.