The structure and working principle of a pointer type pliers pointer watch
Clamp Ammeter can generally be divided into magnetoelectric type and electromagnetic type. The method for measuring power frequency AC power is magnetoelectric, while the electromagnetic method is both AC and DC. This paper mainly introduces the measuring principle and using method of magnetoelectric clamp Ammeter.
1. Magnetoelectric clamp Ammeter structure
Magnetoelectric clamp Ammeter is mainly composed of a special current transformer, a rectifier magnetoelectric Ammeter and internal circuits. The commonly used models are T301 and T302. T301 clamp Ammeter can only measure AC current, while T302 can measure AC current and AC voltage. There are also AC and DC pocket clamp Ammeter, such as MG20, MG26, MG36, etc.
The shape of the clamp shaped table 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. Operating principle of clamp Ammeter
The working principle of clamp Ammeter is based on the working principle of current transformer. When the wrench of clamp Ammeter is tightly held, the iron core of current transformer can be opened, and the conductor of measured current enters the jaw as the primary winding of current transformer. When the iron core of the loosening wrench is closed, the induced current is generated on the secondary winding according to the principle of the transformer, and the pointer of the Ammeter 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
Where, Ia is the reading on the Ammeter; N is the number of turns wrapped.
3. Operation steps of clamp Ammeter
(1) Clamp Ammeter shall be correctly selected according to 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, the high-voltage clamp Ammeter corresponding to its voltage level shall be selected.
(2) Correctly check whether the appearance of clamp Ammeter, jaw closing and meter head are normal. If the pointer is not in the zero position, mechanical zero adjustment should be performed.
(3) Select the appropriate range of clamp type Ammeter according to 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. According to the current, it is possible to check and judge whether the motor works normally, so as to ensure the safe operation of the motor and extend its service life. First, correctly select the voltage level of clamp type Ammeter, and check whether its appearance insulation is good, whether there is damage, whether the pointer swings flexibly, and whether the jaw is rusted. Estimate the rated current based on the motor power to select the range of the meter. During measurement, it can be measured once for each phase or three phases. At this time, the number on the meter should be zero (because the sum of the three phase current is zero). When there are two phase lines in the jaw, the value displayed on the meter is the current value of the third phase. By measuring the current of each phase, it can be judged whether the motor has overload (the measured current exceeds the rated current value), Whether there is a problem with the power supply voltage inside the motor or (the device that converts other forms of energy into electrical energy is called the power supply), that is, whether the three-phase current imbalance exceeds the limit of 10%.
