An ammeter is a device that supplements a voltmeter. It is used where it is not possible to install a full-fledged wattmeter or use a multimeter. Its purpose is to facilitate the maintenance and repair of electrical installations under constant load, to identify impending breakdowns in time and to take prompt measures to eliminate them. For example, an ammeter allows you to assess the condition of the battery in a car and predict when a worn-out battery will need to be replaced with a new one.
For dial ammeters, the basis of the device is a simple electromagnetic (or other type) galvanometer or an electrical measuring head.
The galvanometer itself works like a cross between a millivoltmeter and a microammeter. It is impossible to include it in the circuit without load and ballast resistances - the winding of the coils is not designed for a significant current strength, which is needed by power electrical installations and consumers connected to them: with a high degree of probability, its winding will burn out. An analog galvanometer is constructed as follows. A coil rotates in the field of permanent magnets, through which a current begins to flow at the moment the device is connected. By generating its own magnetic field, the coil turns through a certain angle - in proportion to the current passed through it. And it is forced to turn by the torque generated by the interaction of the field of a permanent magnet and the field of the coil.
Together with the coil, the arrow, rigidly fixed to it, also rotates. This whole structure is fixed on a fixed axis located in the center of the magnetic gap. A flat spring, attached at one end to the rigid base (frame) of the device, and the other to the axis with the arrow, when the galvanometer is turned off from the electrical circuit, makes the arrow return to its original position.
In addition to the return spring, on the counterweight of the arrow there is a balance bar - a metal thread made of a soft and sufficiently elastic metal (for example, platinum), which balances the arrow and prevents its end from touching the scale - an aluminum plate with graduated divisions, fixed as a flat frame on the front of the insides galvanometer. In some cases, in order not to waste expensive platinum, a drop of some low-melting alloy (exactly in milligrams or in hundreds of micrograms) is soldered onto the counterweight of the arrow . If the balancer breaks, the measurement results will be inaccurate and intermittent, or none at all. The rules for handling the galvanometric ammeter strictly prohibit throwing it, subjecting it to severe vibration and strong shocks - the measuring heads break very easily.
Finally, the dial ammeter has a flat screw that slightly displaces the arrow spring in both directions . By turning this screw, the arrow may not be strictly at the zero of the scale, but at the near-zero point. By how many divisions the zero will be violated - by the same amount the ammeter will "lie" by taking the reading of the measured current. The manufacturer, after testing the galvanometer, independently calibrates zero - on a scale. The user himself will adjust the zero accuracy by eye using a flat screwdriver - when he finds that the pointer zero is lost and does not coincide with the scale one.
Supports made of wire with rubber gaskets, located at the ends of the scale , hold the arrow at the near-zero and maximum position when the galvanometer "rolls over" or is connected "backwards". They prevent the spring from twisting, and the end of the arrow from hitting the edges of the visual measuring field, if the measured parameter, due to the user's inattention, turns out to be several times larger than the real one that the device itself is capable of displaying.
Holds constant current of small values. The measuring head is a magnetoelectric system with a scale containing a certain graduation.
Designed for fast-alternating current circuits with a frequency of hundreds and thousands of hertz. The basis of such an ammeter is a magnetoelectric head. It consists of a piece of conductor to which a thermocouple is connected. The current that heats the conductor generates heat, which is captured by the thermocouple. Heat radiation deflects the frame with an arrow by a calculated angle that is linearly dependent on the value of the throughput current.
Its purpose is to measure current in fast-changing electrical installations operating at increased (up to 200 Hz) frequency. For electrodynamic ammeters, a room or compartment is suitable, where unnecessary electromagnetic fields are completely absent. However, they are highly accurate and are used for regular verification of all other types of meters.
The frame of the ferrodynamic meter is made of a ferromagnet, an iron core and a static coil. It has an inherent accuracy class for an electrodynamic ammeter, but is insensitive to electromagnetic interference (parasitic fields).
A digital ammeter (ideally a multimeter) does not have a galvanometric measuring head. Instead of a galvanometer, a whole system is used: sensors for the current supplied to the measurement, an ADC, a processor with ROM and random access memory, a display with a controller for outputting the values of the readings. To record readings on an external medium, a microUSB port or a Wi-Fi / Bluetooth radio module can be attached - this allows you to connect an ammeter or multimeter to a PC, smartphone or tablet, and use a special application to work with it.
Schematic diagram of an ammeter
In the simplest ammeter, designed for one measurement range - for example, 0 ... 10 A - a shunt is connected parallel to the galvanometer. This is a low-resistance resistor with a resistance of 0.01 ... 1 Ohm. A rough calculation is made according to Ohm's law - the current is equal to the quotient of the division of the EMF (voltage) of the power supply network (circuit) by the resistance value.
Having an idea of the resistance of the ammeter shunt, you already know how to connect it correctly.
Inclusion in the chain
The device is always connected in series and not in parallel with the load. If you dare to connect the device in parallel, the fuse will blow and the device will turn off. At a current of several amperes, the galvanometer coil and shunt burn out. A burnt-out arrowhead cannot be restored. Disconnect the line first. With a low EMF - up to 12 V - the power supply can be dispensed with without de-energizing. Connect the ammeter to open circuit. Make sure that the current carrying capacity of the ammeter (for example, the device is rated for 10 A), the total current load does not exceed the measurement limit for which the ammeter is designed. If the device is not "two-sided" (for example, -10 and +10 A with a zero in the middle), observe the polarity. After turning it on, it will show how many (miles) amperes per hour your electrical appliance or electrical circuit is consuming.
Nuances when connecting an ammeter to a car
For cars, a "two-sided" ammeter is used, in which zero is not at the beginning, but in the middle of the scale. "Minus" current (negative reading of the device) is the current consumed by the car electronics. "Positive" - when the current flows in the opposite direction - the charging current coming from the oscillator. In the same way, ammeters are connected and work in special equipment (truck crane, tractor, excavator, etc.).
The factory equipment for most foreign cars already provides for a shunt and ammeter calibrated by the same car factory, connected in series with the battery through the positive wire. If the arrow after a successful start of the car goes off scale and does not return to the zero position, the battery is damaged and must be replaced with a new one.
The shunt resistance is equal to the external resistance of the connected load (for example, a powerful lamp or headlamp), multiplied by the ratio of the current passing through the ammeter itself to the difference between the total current in the circuit and the current of the ammeter itself. The current flowing through the shunt resistor is many times the current flowing through the galvanometer winding. The opposite is true for the resistance of the galvanometer and shunt.
In the simplest case, a shunt is a short coil or strip of thick copper, steel, or aluminum wire. A galvanometer is connected to its conclusions. It is like a "lightning rod" for high currents, allowing to keep the coil of the device intact and safe - perhaps ten-thousandths of the current that the shunt will pass through it will pass through it. In practice, the galvanometer will turn into a millivoltmeter - it senses only that small voltage drop across the strip or shunt resistor. The shunting value is a multiple of 10 units.
A single shunt can be used to measure dc currents or low ac voltages. If we are talking about measuring alternating current with high potential, then, in addition to a rectifier diode bridge, the device requires current measuring transformers. Knowing what voltage is in the electrical circuit (for example, 1 kV), you can use a voltage boosting transformer. Its primary winding, which has a low resistance, wound with a thick wire, is connected in series with the power supply line (in its break). The secondary, which generates a high EMF, is connected to an ammeter. Due to the low resistance of the device itself, the transformer goes into short circuit mode, that is, it is maximally loaded.
If you have correctly selected the ratio of the turns of both windings, you will have the opportunity, with a small current passing through the device itself, to measure the current of large values in the external circuit. To obtain the value of the current flowing through the primary winding, multiply the measured value by a transformation factor. In ammeters, where the current transformer is permanently built-in and is not disconnected after the end of measurements, but remains in the device further, the scale is already optimally calibrated. So that none of the personnel is accidentally killed by a high voltage current, one of the terminals from the secondary winding and the magnetic circuit (plates) of the transformer are grounded.
The secondary winding and the magnetic circuit are insulated separately. They are placed inside the bushing, through the channel of which the shunt with the current measured in the circuit passes. Such current transformers are called bushing.
You cannot open the secondary winding of the current transformer by disconnecting the ammeter from it. If this does happen, an abrupt increase in the magnetic flux in the magnetic circuit automatically becomes a source of a very dangerous for the life of the "gauge" voltage of hundreds or even thousands of volts. Connecting a low ohmic resistance in parallel with an ammeter (or shunt) will reduce this voltage, making it possible to measure its remainder with an ammeter - it is estimated as the current passing through the device circuit.
Measuring current transformers have their own error - in terms of phase angle of currents and transformation ratio. In the first case, a phase shift (rotation) from a position of 180 degrees is considered, which causes a significant error in the readings of a wattmeter included in the same circuit. Accuracy class by transforming factor is estimated by losses from nominal - 0.02 ... 1 and more.
Current Clamp Application
The 50 amp shunts do not protrude outside the instrument case. But if we are talking about a current greater than 50 A, the so-called current-carrying crocodiles or current clamps are used. In the second variant, not the galvanometer itself is calibrated, but the shunt. In parallel, a millivoltmeter with a voltage of 45-150 millivolts is turned on. The goal is to ensure the deviation of the instrument arrow at a distance not more than the entire scale.
Determination of DC and AC
For direct current, no special circuits are required - there is a milliammeter, a powerful shunt with resistance in hundredths and thousandths of an Ohm. They are connected in parallel with each other - and the entire installation is placed in an open circuit. For alternating current, a method with a current transformer connected according to the above scheme is required. So that the arrow does not fluctuate around the zero of the scale with a frequency of 50 or more hertz, a diode rectifier is used. This is one diode or diode bridge. The voltage rating of the diode must be high enough. Thus, you will avoid electrical breakdown and subsequent failure of the device.
The graduation of the galvanometer scale (not a ready-made ammeter) is conditional - it depends on the following parameters:
- the weight of the arrow and the coil with the enamel wire, which carries it along;
- the strength of the magnet (or the magnetizing stator coil, if a permanent ferrite magnet was not installed);
- the stiffness of the return spring;
- the clarity of the arrow balancer.
Depending on which device is assembled on the basis of the galvanometer - ammeter, voltmeter or ohmmeter - the calibration is performed according to the shunts and the schematic diagram of the device.
For example, to calibrate the device at 15 volts (car generator voltage) to 15 amperes, the shunt must have a resistance of 1 ohm. If the charging current is large - by 75 A, then a powerful shunt element of 0.2 Ohm is installed. The correction for the resistance of the galvanometer winding in this case will be very small - it itself is at least hundreds of times higher than that of the shunt connection, and the error of such an ammeter will be 0.2% or less. An accurate calculation can be carried out according to the above formula, taking into account the resistance of the galvanometer winding. If we are talking about high currents, it is no less logical to include a fuse or automatic fuse in the circuit break in series with the ammeter - in case the device goes off scale.
For information on how to properly connect an ammeter, see the next video.