Clamp ammeter structure and working principle
Magnetoelectric clamp ammeter structure
The magnetoelectric clamp ammeter is mainly composed of a special current transformer, a rectifying magnetoelectric system ammeter and internal circuits. The common models are: T301 and T302. T301 type clamp ammeter can only measure AC current, while T302 type 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 7-6. Its accuracy is 2.5, and the current range is: 10 A, 50 A, 250 A, 1000 A.
Structure of Digital Clamp Ammeter
The biggest difference in appearance between the digital clamp ammeter and the pointer clamp ammeter is that the display part of the digital clamp ammeter uses a liquid crystal display.
Working principle of clamp ammeter
The iron core of the transformer is made into a movable opening, which is clamp-shaped, and the movable part is connected with the iron core switch 6 . When the iron core switch is firmly grasped, the iron core of the current transformer is opened (as shown by the double-dot dash line in Figure 7-8), and the measured wire 4 can be placed in the jaws, and the current-carrying wire becomes the primary wire of the current transformer. winding. When the jaws are closed, an alternating magnetic flux passes through the iron core of the current transformer, and an induced current is generated in the secondary winding 5 of the transformer. ) The ammeter is connected to both ends of the secondary winding, and the current indicated by its pointer is proportional to the working current clamped into the current-carrying wire, and the measured current value can be read directly from the dial.
It is worth noting that: because the principle is to use the principle of the transformer, whether the iron 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 several times on the iron core to change the current ratio of the transformer to increase the current range. At this point, the measured current Ix should be:
Ix=Ia/N
Ia is the reading on the ammeter; N is the number of winding turns.
Matters needing attention in the use of clamp ammeter
(1) Since the clamp ammeter is in contact with the line under test, it is necessary to check whether the insulation performance of the meter is good before measurement. That is, the shell is not damaged, and the handle should be clean and dry.
(2) When measuring, you should wear insulating gloves or clean wire gloves.
(3) During measurement, attention should be paid to keeping a safe distance between each part of the body and the charged body (the safe distance for low-voltage systems is 0.1-0.3 m).
(4) Use the clamp ammeter to measure the current in the bare conductor, and the insulation measures must be done in advance before measurement.
(5) It is strictly forbidden to switch the gear position of the clamp ammeter during the measurement; if it is necessary to change gears, the measured wire should be withdrawn from the jaws before changing the gear position.
(6) Select the clamp ammeter strictly according to the voltage level: the clamp ammeter of the low voltage level can only measure the current in the low voltage system, and cannot measure the current in the high voltage system.
(7) When measuring in a small space (such as a distribution box, etc.), it is necessary to prevent short circuits between phases caused by the opening of the jaws.
(8) Only one phase conductor can be clamped for each measurement.
(9) The voltage of the circuit under test should be lower than the rated voltage of the clamp ammeter, otherwise, it is easy to cause accidents or electric shock accidents.
(10) After each measurement, the switch for adjusting the current range should be placed at the highest position, so as to avoid damage to the instrument due to measurement without selecting the range next time.
