The structure and working principle of a pointer type clamp watch
Clamp type ammeters can generally be divided into two types: magnetoelectric and electromagnetic. The method for measuring power frequency AC power is magnetoelectric, while the electromagnetic method is both AC and DC. This article mainly introduces the measurement principle and usage method of magnetoelectric clamp ammeter.
1. Structure of magnetoelectric clamp ammeter
The magnetoelectric clamp ammeter mainly consists of a special current transformer, a rectified magnetoelectric ammeter, and internal circuits. The commonly used models are T301 and T302. The T301 clamp type ammeter can only measure AC current, while the 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 shaped watch 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 a clamp type ammeter is based on the working principle of a current transformer. When holding the clamp type ammeter wrench tightly, the iron core of the current transformer can be opened, and the measured current wire enters the clamp as the primary winding of the current transformer. After the iron core of the loosening wrench is closed, according to the principle of the transformer, an induced current is generated on its secondary winding, and the ammeter pointer deflects to indicate the value of the measured current.
It is worth noting that since its principle is based on the principle of a transformer, whether the iron core is tightly closed and whether there is a large amount of residual magnetism has a significant impact on the measurement results. When measuring a small current, it will increase the measurement error. At this point, the measured wire can be wound several more turns on the iron core to change the current ratio of the transformer and increase the current range. At this point, the measured current Ix should be:
Ix=Ia/N
In the formula, Ia is the reading on the ammeter; N is the number of turns wrapped.
3. Usage steps of clamp ammeter
(1) Choose a clamp ammeter correctly based on the type and voltage level of the measured current. T301 type is selected for general AC lines below 500 V. When measuring the current of high-voltage lines, a high-voltage clamp ammeter that matches its voltage level should be selected.
(2) Correctly check the appearance, jaw closure, and head condition of the clamp ammeter for normal operation. If the pointer is not in the zero position, mechanical zero adjustment should be performed.
(3) Select the appropriate range of the clamp type ammeter based on the measured current. The selected range should be slightly larger than the measured current value. If the magnitude of the measured current is not known, the maximum range estimation should be selected first.
(4) Correct measurement. When measuring, the wrench should be tightened to open the jaws. Place the tested wire in the center of the clamp, loosen the wrench, and close the clamp tightly.
(5) After reading, open the jaws and exit the measured wire, placing the gear in the highest or OFF position of the current.
Measurement example: Measure the working current of a cage asynchronous motor during operation. Based on the current level, it is possible to check and determine whether the motor is working properly to ensure safe operation and extend its service life. Firstly, select the voltage level of the clamp type ammeter correctly, check its appearance for good insulation, damage, flexible pointer movement, and rust on the jaw. Estimate the rated current based on the motor power to select the range of the meter. When measuring, it is possible to measure once in each phase or once in three phases. At this point, the number on the meter should be zero (due to the sum of the three-phase current phasors being zero). When there are two phase lines in the clamp, 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), Is there a problem with the power supply voltage inside the motor or the device that converts other forms of energy into electrical energy, that is, whether the three-phase current imbalance exceeds the 10% limit.
