Ammeter

A current measuring device, also referred to as ammeter, colloquially shortened as ammeter and in the English language as Ammeter, used to measure the electrical current. It belongs to the field of electrical measuring techniques.

In the measurement, the measured variable is transformed into a display of their multiples of the unit ampere. For laboratory, service and field applications, there are switchable multiple instruments with multiple ranges, called multimeter. For industrial applications, there are display type measuring devices.

• 3.1 Devices with moving coil movement
• 3.2 Device with moving iron
• 3.3 Digital ammeters

Use

To measure the current to be measured current must flow through the meter. It is therefore connected to the load in series. There must always be installed. To install or remove the ammeter, the circuit must be switched off and disconnected: Measurement requires an intervention in the circuit to be tested and at this time an interruption of the current flow. At best, a " bypass " will be placed around the forward current to the point to be separated.

There are several ways to prevent this separation of the electric circuit:

Embodiments

Digital devices

Digital ammeters are the usual in practical operation instruments and are among the field of digital measurement technology. They represent, in principle, strain gauges which measure the voltage drop across an internal or external sense resistor, which is referred to as a shunt. Digital ammeters are often combined with other methods such as voltmeter and ohmmeter in a digital multimeter. The specific function is selected, normally by a rotary switch mounted on the device by the user.

There are also models for use in control centers and equipment rooms, which mounted firmly in control panels ( panels ) are introduced and are used to display instantaneous values ​​for the control and operating personnel. These so-called built-in power meters are configured once with a matching task for measuring shunt during assembly.

Analog devices

Moving-coil

Because of the low self- consumption, because of reliability and low noise sensitivity, the moving coil instrument is still widely used for current measurement. It measures the ( positive or negative ) DC and at higher currents must be complemented by parallel connection of a shunt.

Alternating current can be measured by a moving coil movement using a rectifier, the 1.11 times the direct guidance value is displayed; However, this is only for sinusoidal form of the alternating current and at the same time of rms. This method requires a larger voltage drop (captive ) than in DC due to the forward voltage of the diodes of the rectifier measurement. Moreover the non-linear voltage drop must be countered by a non-linear at the bottom of the scale division. Analog multimeter with built-in amplifier around this problem.

Moving iron

To measure the RMS value, the moving iron is. The devices are mainly designed for 50 Hertz, but are also suitable for harmonic rich power and for direct current. Moving iron works can be made ​​for very large currents without a shunt must be used.

Hitzdrahtmesswerk

A Hitzdrahtmesswerk uses a through which the power wire that heats up due to the Joule heat and according to its thermal expansion coefficient increases its length and of a lever mechanism drives a pointer. Hitzdrahtmesswerke measure the rms value and are suitable for DC and AC power, even for high frequency. These devices are no longer manufactured for decades.

Transmitters

In industrial measurement technology and automation technology is also used no -indicating instruments, but transmitters that provide a standardized electrical signal to the central processing. This may be analogous - technically a unit signal, eg 4 ... 20 mA. It can also be a digital output signal for transmission over a data bus, the bus is referred to in this context, the fieldbus. These gauges with digital measuring signal at the output then are called measurement converter.

Current measurement using the magnetic field

A current probe evaluates to measure the current from its occurring magnetic field around the conductor.

Folding a ferrite or laminated iron core strap forming the transformer core, the closed pliers included current-carrying conductor, the primary winding and which is arranged around the core, the secondary winding. The manager has guided through the winding nprim = 1 The current of the secondary winding, which is inversely proportional to the ratio of the number of turns is displayed on a moving iron or moving coil movement with rectifier or a digital electronics. This device is only suitable for the measurement of alternating current.

Equally suitable for DC and AC current measurement devices also have a folding nucleus, but are fitted with current sensors. They work on the principle of compensation (compensation current transformer) or the Hall effect ( Hall sensors) or with magnetic field dependent resistors together with an electronic transmitter.

Clip-on current knives are made to measure in the power grid as indicating devices; they are also available as an accessory for multimeter for range extension, or as an accessory for oscilloscopes for potential free measurement of rapidly changing current flows. Then you are also called current clamp or current probe.

Sometimes current probe are additionally provided with a voltage input and can also measure the electrical power and power factor in addition to the voltage.

Current measurement with current transformer

Alternating currents can be measured via current transformers ( special measuring transformers); is measured by the ammeter which ( according to the nominal ratio smaller ) secondary current. Alternatively, a resistor is connected thereto, and the measured voltage to the secondary winding. Except for the measurement of large currents (from about 10 A and up) to use converter for potential-free measurement on conductors at high voltage.

Current transformers can dine ( the burden ) only a maximum external resistance, otherwise the measurement result is wrong, or they are overloaded. Is not connected to the secondary winding, they generate dangerous stresses due to the power source character. Unused current transformers must therefore be secondary side always shorted.

Current measurement with shunt

For large direct currents or streams of the same content, a measuring resistor is often used, to which a voltage measuring device is connected. The shunts are often performed with four-wire connection, in order to avoid the measurement error due to the contact resistance of the connectors. Are usual types that produce the specified maximum current a voltage drop of 60 mV. In a measuring resistance from 1 milliohms an associated voltage meter is practically not labeled with a scale value 60 mV but with 60 A.

Measuring range adjustment, errors due to self-consumption

Devices with moving coil movement

The measuring unit has a measuring range final value (mostly full scale on the associated scale ) at their design maximum permissible current Imax. At the same time it has an internal resistance Ri. This means that if the maximum current is flowing, falls a maximum voltage Umax from which to him according to Ohm's law

Calculated. Exceeding the maximum amperage the measuring system can be overloaded. For encoders with a class mark a permissible overload is defined by standardization.

To adapt to the desired measuring range, a shunt resistor Rn is connected to the measuring unit in parallel which receives that portion of the current should not flow through the meter. You calculate it so that the desired scale value IMBE the proportion Imax by the measuring system and the rest of In = IMBE - Imax flows through the shunt.

Ri can be formed not only by the coil resistance RSp because the copper used for this purpose its resistance with temperature by 3.9 ... 4.5 % / 10 K changes, resulting in temperature-dependent changes the display. To minimize this error of measurement an independent on the temperature resistance Rv still has to be in series to the work: For Rv = 2 ⋅ RSp reduces the temperature influence of Ri to a third, or about 1.5 % / 10 K; this value represents the case of instruments of accuracy class 1.5 represents the upper limit permitted

For switchable measuring ranges on multiple ammeters see analog multimeter.

The fact that the current measuring device establishes the voltage required for the measurement, causes each measurement, the original conditions at the measurement object distorted, since the voltage required for measuring missing elsewhere. Therefore, these should be kept as small as possible, ie the resistance of the current meter should be as small as possible. Examples of this type of measurement errors are described in feedback deviation.

Devices with moving iron

The coil of a moving iron work has a considerable inductance component of the internal resistance, so that the use of a shunt with AC current measurements is not possible. Also with direct current, the use of a shunt measurement problems, since the copper windings of the measuring equipment has a different temperature coefficient than the shunt.

Moving iron works are therefore made ​​for different measuring ranges up to about 100 A by different numbers of turns and wire cross sections are used for the coil for the measurement ranges.

For measuring range matching at alternating but also current transformers can be used. Provided for this purpose moving iron works usually have a full scale of 5 A, corresponding to the rated output current of the current transformer. The scale is labeled according to the ratio of the connected current transformer, has thus, for example, the final value of 300 A at a current transformer 300 A: 5 A ( turns ratio 1: 60 ).

Digital ammeters

The measuring resistor in digital multimeters and panel meters is usually designed so that the voltage drop at the end of each current measurement range corresponds to the smallest voltage range. This is 200 mV at most instruments. Changes the current range of series- shunt is along multiple, uninterrupted switched to one or more measuring resistors.

Digital instruments have often a smaller power consumption than moving-coil instruments, and thus the retroactivity deviation ( circuit influence error ) is lower. However, this is not true for DC measurement, since in moving-coil devices Umax ( required for full scale voltage ) is almost always less than 200 mV.

Web Links / References / Footnotes

• Elektrotechnisches meter