Introduction of ammeter

Overview

An ammeter is an instrument used to measure the current in AC and DC circuits. In the circuit diagram, the symbol of the ammeter is “circle A”. Current values are in “amps” or “A” as standard units. 

The ammeter is made according to the action of the current-carrying conductor in the magnetic field by the force of the magnetic field. There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles. There is a coil in the magnetic field. There is a hairspring spring at each end of the coil. Each spring is connected to a terminal of the ammeter. A rotating shaft is connected between the spring and the coil. On the front of the ammeter, there is a pointer. When there is a current passing through, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic field line, so the coil is deflected by the force of the magnetic field, which drives the rotating shaft and the pointer to deflect. Since the magnitude of the magnetic field force increases with the increase of the current, the magnitude of the current can be observed through the deflection of the pointer. This is called a magnetoelectric ammeter, which is the kind we usually use in the laboratory. In the junior high school period, the range of the ammeter used is generally 0~0.6A and 0~3A.

working principle

The ammeter is made according to the action of the current-carrying conductor in the magnetic field by the force of the magnetic field. There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles. There is a coil in the magnetic field. There is a hairspring spring at each end of the coil. Each spring is connected to a terminal of the ammeter. A rotating shaft is connected between the spring and the coil. On the front of the ammeter, there is a pointer. Pointer deflection. Since the magnitude of the magnetic field force increases with the increase of the current, the magnitude of the current can be observed through the deflection of the pointer. This is called a magnetoelectric ammeter, which is the kind we usually use in the laboratory.

Generally, currents of the order of microamps or milliamps can be directly measured. In order to measure larger currents, the ammeter should have a parallel resistor (also known as a shunt). The measurement mechanism of the magnetoelectric meter is mainly used. When the resistance value of the shunt is to make the full-scale current pass, the ammeter is fully deflected, that is, the indication of the ammeter reaches the maximum. For currents of a few amps, special shunts can be set in the ammeter. For currents above several amps, an external shunt is used. The resistance value of the high-current shunt is very small. In order to avoid errors caused by the addition of lead resistance and contact resistance to the shunt, the shunt should be made into a four-terminal form, that is, there are two current terminals and two voltage terminals. For example, when an external shunt and millivoltmeter are used to measure a large current of 200A, if the standardized range of the millivoltmeter used is 45mV (or 75mV), then the resistance value of the shunt is 0.045/200=0.000225Ω (or 0.075/200=0.000375Ω). If a ring (or step) shunt is used, a multi-range ammeter can be made.

Application

Ammeters are used to measure current values in AC and DC circuits.

1. Rotating coil type ammeter: equipped with a shunt to reduce sensitivity, it can only be used for DC, but a rectifier can also be used for AC.   

2. Rotating iron sheet ammeter: When the measured current flows through the fixed coil, a magnetic field is generated, and a soft iron sheet rotates in the generated magnetic field, which can be used to test AC or DC, which is more durable, but not as good as rotating coil ammeters Sensitive.   

3. Thermocouple ammeter: It can also be used for AC or DC, and there is a resistor in it. When the current flows, the heat of the resistor rises, the resistor is in contact with the thermocouple, and the thermocouple is connected with a meter, thus forming a thermocouple type Ammeter, this indirect meter is mainly used to measure high frequency alternating current.   

4. Hot wire ammeter: When in use, clamp both ends of the wire, the wire is heated, and its extension makes the pointer rotate on the scale.

Classification

According to the nature of the measured current: DC ammeter, AC ammeter, AC and DC dual-purpose meter;

According to the working principle: magnetoelectric ammeter, electromagnetic ammeter, electric ammeter;

According to the measurement range: milliampere, microampere, ammeter.

Selection guide

The measuring mechanism of ammeter and voltmeter is basically the same, but the connection in the measuring circuit is different. Therefore, the following points should be noted when selecting and using ammeters and voltmeters.

⒈ Type selection. When the measured is DC, the DC meter should be selected, that is, the meter of the magnetoelectric system measuring mechanism. When the measured AC, should pay attention to its waveform and frequency. If it is a sine wave, it can be converted to other values ​​(such as maximum value, average value, etc.) only by measuring the effective value, and any kind of AC meter can be used; if it is a non-sine wave, it should distinguish what needs to be measured For the rms value, the instrument of the magnetic system or the ferromagnetic electric system can be selected, and the average value of the instrument of the rectifier system can be selected. The instrument of the electric system measuring mechanism is often used for the precise measurement of alternating current and voltage.

⒉ The choice of accuracy. The higher the accuracy of the instrument, the more expensive the price and the more difficult the maintenance. Moreover, if the other conditions are not matched properly, the instrument with high accuracy level may not be able to obtain accurate measurement results. Therefore, in the case of selecting a low-accuracy instrument to meet the measurement requirements, do not choose a high-accuracy instrument. Usually 0.1 and 0.2 meters are used as standard meters; 0.5 and 1.0 meters are used for laboratory measurement; instruments below 1.5 are generally used for engineering measurement.

⒊ Range selection. In order to give full play to the role of the accuracy of the instrument, it is also necessary to reasonably select the limit of the instrument according to the size of the measured value. If the selection is improper, the measurement error will be very large. Generally, the indication of the instrument to be measured is greater than 1/2~2/3 of the maximum range of the instrument, but cannot exceed its maximum range.

⒋ The choice of internal resistance. When selecting a meter, the internal resistance of the meter should also be selected according to the size of the measured impedance, otherwise it will bring about a large measurement error. Because the size of the internal resistance reflects the power consumption of the meter itself, when measuring current, an ammeter with the smallest internal resistance should be used; when measuring voltage, a voltmeter with the largest internal resistance should be used.

Maintenance

1. Strictly follow the requirements of the manual, and store and use it within the allowable range of temperature, humidity, dust, vibration, electromagnetic field and other conditions.

2. The instrument that has been stored for a long time should be checked regularly and the moisture should be removed.

3. Instruments that have been used for a long time should be subject to necessary inspection and correction according to electrical measurement requirements.

4. Do not disassemble and debug the instrument at will, otherwise its sensitivity and accuracy will be affected.

5. For instruments with batteries installed in the meter, pay attention to check the discharge of the battery, and replace them in time to avoid the overflow of battery electrolyte and corrosion of the parts. For the meter that is not used for a long time, the battery in the meter should be removed.

Matters needing attention

1. Check the contents before the ammeter is put into operation  

a. Make sure the current signal is well connected and there is no open circuit phenomenon;

b. Make sure the phase sequence of the current signal is correct;

c. Make sure that the power supply meets the requirements and is connected correctly;

d. Make sure the communication line is connected correctly;   

2. Precautions for using ammeter  

a. Strictly follow the operating procedures and the requirements of this manual, and prohibit any operation on the signal line.

b. When setting (or modifying) the ammeter, make sure that the set data is correct, so as to avoid abnormal operation of the ammeter or wrong test data.   

c. When reading the data of the ammeter, it should be carried out in strict accordance with the operating procedures and this manual to avoid errors.   

3. Ammeter removal sequence  

a. Disconnect the power of the ammeter;

b. Short-circuit the current signal line first, and then remove it;   

c. Remove the power cord and communication line of the ammeter;

d. Remove the equipment and keep it properly.

Troubleshooting

1. Fault phenomenon

Phenomenon a: The circuit connection is accurate, close the electric key, move the sliding piece of the sliding rheostat from the maximum resistance value to the minimum resistance value, the current indication number does not change continuously, only zero (the needle does not move) or slightly moving the sliding piece to indicate Full offset value (the needle deflects to the head quickly).

Phenomenon b: The circuit connection is correct, close the electric key, the ammeter pointer swings greatly between zero and full offset value.

2. Analysis

The full bias current of the ammeter head belongs to the microampere level, and the range is expanded by connecting a shunt resistor in parallel. The minimum current in the general experimental circuit is milliampere, so if there is no such shunt resistance, the meter pointer will hit full bias.

The two ends of the shunt resistor are clamped together by the two solder lugs and the two ends of the meter head by the upper and lower fastening nuts on the terminal and the terminal post. The fastening nuts are easy to loosen, resulting in the separation of the shunt resistor and the meter head ( There is a failure phenomenon a) or poor contact (a failure phenomenon b).

The reason for the sudden change in the number of the meter head is that when the circuit is turned on, the sliding piece of the varistor is placed at the position with the largest resistance value, and the sliding piece is often moved to the insulating porcelain tube, causing the circuit to be broken, so the current indication number is: zero. Then move the sliding piece a little bit, and it comes into contact with the resistance wire, and the circuit is really turned on, causing the current indication number to suddenly change to full bias.

The method of elimination is to tighten the fastening nut or disassemble the back cover of the meter, weld the two ends of the shunt resistor together with the two ends of the meter head, and weld them to the two welding lugs.