What Is a Current Meter?
Ammeter is a meter used to measure current in AC and DC circuits. In the circuit diagram, the symbol of the ammeter is "Circle A". The current value is in standard units of "A" or "A".
- Chinese name
- Ammeter
- Foreign name
- Ammeter
- nickname
- Ammeter
- Presenter
- ampere
- Applied discipline
- physics
- Scope of application
- Electricity
- Scope of application
- Electromagnetism
- Ammeter is a meter used to measure current in AC and DC circuits. In the circuit diagram, the symbol of the ammeter is "Circle A". The current value is in standard units of "A" or "A".
- The ammeter is made according to the effect of the magnetic field force on the conducting conductor in 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. On the front of the ammeter, there is a pointer. When a current passes, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic induction line, so the force of the magnetic field causes the coil to deflect and drives the rotating shaft and 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 by the degree of deflection of the pointer. This is called a magnetoelectric ammeter, which is what 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
Ammeter principle
- The ammeter is made according to the effect of the magnetic field force on the conducting conductor in the magnetic field. There is a permanent magnet inside the ammeter,
- SJKJ ammeter and voltmeter (10 photos)
- Generally, it can directly measure currents on the order of microamperes or milliamps. In order to measure larger currents, the ammeter should have a parallel resistor (also called a shunt). The measuring 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 ammeter indicates the maximum. For currents of several amps, a dedicated shunt 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 four terminals, that is, two current terminals and two voltage terminals. For example, when using an external shunt and a millivolt meter to measure a large current of 200A, if the standardized range of the millivolt meter used is 45mV (or 75mV), then the resistance value of the shunt is 0.045 / 200 = 0.000225 (or 0.075 / 200 = 0.000375). If a ring (or step) shunt is used, a multi-range ammeter can be made.
Ammeter development process
- William Edward Weber's contribution to electromagnetics is manifold. He for
- Werber
Ammeter classification
Ammeter DC Ammeter
- Figure Expansion of the range of magnetoelectric and electric systems
- (a) shunt; (b) two static coils in series; (c) two static coils in parallel
- DC ammeters mainly use magnetoelectric system or electric system measurement mechanism (see mechanical indication meter measurement mechanism). The basic quantity measured by these measurement mechanisms is current, which can be used to directly measure small currents. For a large amount of DC current, the magnetoelectric measuring mechanism uses a shunt, that is, a parallel resistance. Its role is to shunt most of the measured current. For currents below 10A, an internal shunt is often used; for larger current values, a dedicated shunt is used. It adopts a four-terminal structure (Figure a), and has two current terminals and two potential terminals. The selection conditions of its resistance value are: when the nominal current passes through the shunt, the voltage between its potential terminals is 45mV or 75mV; this voltage value is measured with a magnetoelectric millivolt meter with a range of 45mV or 75mV, and the dial It is scaled by the current value. For the measurement system of the electric system, the method of expanding the measuring current range is: thicken the wire of the static coil, while reducing the number of turns to keep the ampere-turn value unchanged; change the two static coils from series to parallel [Fig. (c)], can double the range. A DC digital ammeter can be formed by using a shunt and a digital voltmeter. [1]
Ammeter AC ammeter
- The AC ammeter can adopt an electromagnetic or electric measuring mechanism. In order to make the magnetoelectric system measurement mechanism also be used to measure AC current, you can use rectifiers or thermocouples to convert AC to DC first; the meters made from them are called rectifier ammeters (see rectifier ammeters) Thermoelectric ammeter. In order to expand the range to measure large currents, the rectifier type ammeter also uses a shunt; the method of the electric series ammeter is the same as before; the electromagnetic series is to thicken the coil wire and reduce the number of turns. For larger measured current value, it should be used with current transformer. AC shunts and AC digital voltmeters are usually used to form AC digital ammeters. [1]
- See the table for the limits of various ammeters, the frequency range of use, and the highest accuracy level possible.
- The non-sinusoidal waveform has a small impact on the electromagnetic, electric, and thermoelectric ammeters. Rectified ammeters are limited to sinusoidal waveforms, and digital ammeters have similar restrictions. For measuring non-sinusoidal current in the power system, a converter-type ammeter can be used. When measuring high current, it must be used with a special shunt. When measuring high current, it must be used with current transformer. [1]
- The AC current meter mainly adopts the measurement mechanism of the electromagnetic system electric meter, the electric system electric meter and the rectifier type electric meter. The lowest range of the electromagnetic measuring mechanism is about tens of milliamps. In order to increase the range, it is necessary to reduce the number of coil turns and thicken the wire. When an electric current measuring mechanism is used to form an ammeter, the moving coil and the static coil are connected in parallel, and the lowest range is about several tens of milliamps. In order to increase the range, if you want to reduce the number of turns of the static coil and thicken the wires, or change the two static coils from series to parallel, the range of the ammeter will be doubled. When using a rectifier meter to measure AC current, the ammeter reading is correct only when the AC has a sinusoidal waveform. A shunt can also be used to increase the range. In addition, high-frequency currents can also be measured with thermoelectric meter measurement mechanisms. The large-range AC ammeters used in power systems are mostly 5A or 1A electromagnetic system ammeters, and are equipped with current transformers with appropriate current conversion ratios.
Digital ammeter
- The display ammeter is divided into single-phase digital ammeter and three-phase digital ammeter. The meter has functions such as transmission, LED (or LCD) display and digital interface. It displays the measurement results in digital form by AC sampling of various parameters in the power grid. . Data processing by CPU. The three-phase (or single-phase) current, voltage, power, power factor, frequency and other electrical parameters are directly displayed by the LED (or liquid crystal), and the corresponding analog power of 0 ~ 5V, 0-20mA or 4-20mA is output at the same time. RTU is connected to the moving device; and it has RS-232 or 485 interface.
Ammeter selection
- 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.
- Selection of type. When the measurement is DC, a DC meter should be selected, that is, an instrument of a magnetoelectric measurement mechanism. When the measurement is AC, pay attention to its waveform and frequency. If it is a sine wave, it can be converted into other values (such as maximum value, average value, etc.) by measuring the effective value, and any type of AC meter can be used. Value, the effective value can choose the instrument of the magnetic system or the ferromagnetic electric system, and the average value is the instrument of the rectifier system. The instruments of the electric measuring mechanism are often used for the precise measurement of AC current and voltage.
- Choice of accuracy. The higher the accuracy of the meter, the more expensive it is and the more difficult it is to repair. In addition, if other conditions are not matched properly, even a high accuracy level instrument may not be able to obtain accurate measurement results. Therefore, in the case of using a lower accuracy instrument to meet the measurement requirements, do not choose a high accuracy instrument. In general, instruments of class 0.1 and 0.2 are used as standard meters; instruments of class 0.5 and 1.0 are used for laboratory measurement; instruments below class 1.5 are generally selected for engineering measurement.
- Selection of range. In order to give full play to the accuracy of the instrument, it is also necessary to reasonably select the instrument measurement limit according to the size being measured. If it is not selected properly, the measurement error will be very large. Generally, the indication of the meter to be measured is greater than 1/2 to 2/3 of the maximum range of the meter, but cannot exceed its maximum range.
- Selection of internal resistance. When selecting the meter, the internal resistance of the meter should also be selected according to the size of the measured impedance, otherwise it will cause a large measurement error. Because the size of the internal resistance reflects the power consumption of the meter itself, when measuring the current, a current meter with the smallest internal resistance should be used; when measuring the voltage, a voltmeter with the largest internal resistance should be used.
Notes for ammeters
- Correct wiring . When measuring current, the ammeter should be connected in series with the circuit under test; when measuring voltage, the voltmeter should be connected in parallel with the circuit under test. When measuring DC current and voltage, you must pay attention to the polarity of the instrument, and make the polarity of the instrument consistent with the polarity being measured.
- High voltage and high current measurement . When measuring high voltage or high current, a voltage transformer or current transformer must be used. The range of the voltmeter and ammeter should be consistent with the secondary rating of the transformer. The general voltage is 100V and the current is 5A.
- Expansion of range . When the circuit being measured exceeds the range of the instrument, an external shunt or voltage divider can be used, but it should be noted that its accuracy level should be consistent with the accuracy level of the instrument.
- In addition, you should also pay attention to the environment in which the instrument is used, and keep it away from external magnetic fields .
Ammeter DC current meter structure
- Ammeter
- Three terminals [There are two types of terminals "+", "-", such as (+,-0.6A, -3A) or (-, 0.6A, 3A)], pointer, scale, etc. Binding post)
Rules for using ammeter
- Ammeter
- The ammeter should be connected in series with the consumer (cannot be connected at both ends of the battery, otherwise the ammeter will be burned.);
- The current should enter from the "+" terminal post and exit from the "-" terminal post (otherwise the pointer is reversed and it is easy to bend the needle.);
- The measured current should not exceed the range of the ammeter (you can use the test method to see if it exceeds the range.);
- Never allow the ammeter to be connected to the two poles of the power supply without passing through the electrical appliances. Ammeter, power supply, wires.).
- Note: burn the meter (ammeter) first, then destroy the source (power supply)
Steps for using ammeter
- Zero calibration, adjust the zero calibration button with a flat-blade screwdriver.
- Selected range (estimated by experience or touch method)
- Ammeter
- One look: range. Measuring range of ammeter.
- Second look: dividing value. How much is a small cell on the dial?
- Third look: pointer position. How many division values are included in the position of the pointer.
Ammeter reading
- See clearly
- Ammeter
- Look at the division value (in general, the range 0 ~ 3A division value is 0.1A, 0 ~ 0.6A is 0.02A)
- Look at the hand stop position (must be viewed from the front)
- Selecting the range {estimated by experience or using the touch method}
Ammeter modification
- Conversion of a sensitive galvanometer into
- Ammeter (2 photos)
- Pointer ammeters are modified from sensitive ammeters. Even if the sensitivity of the sensitive galvanometer is even higher, the current passed by it will not exceed 30 microamperes at most, and the current intensity measured by the students with the ammeter is 0.6A, or 3A, far exceeding the maximum value. The ammeter should not only allow the entire current on the circuit to pass, but also not allow the current through the coil to exceed the safety limit. The ammeter is connected in series with the appliance under test, so shunting is required when retrofitting. Connect the sensitive ammeter in parallel with a resistor with a small resistance value, so that most of the current will pass through the resistor, and a small part will pass through the meter head. At this time, you can mark the meter head with a new scale.
- There is a formula for the resistance value required for the modification: R1 = R / [(I1 / I) -1], where R1 is the resistance required during the modification, R is the coil resistance of the sensitive ammeter, and I1 is the modification The maximum range of the rear ammeter, I is the maximum range of the sensitive ammeter.
- The current meter used by students has two ranges, that is, two resistors; the range selection on the pointer multimeter is actually a potentiometer.