§ 6 How is an electric induction meter arranged (for electricians).
To get started, I will cite an excerpt from the standard factory instructions for the construction of an induction meter. I’ll immediately warn you that if you did not study at the Faculty of Electrical Engineering of the University, then the following text will be difficult for you. Even I, as a person who studied the theoretical foundations of electrical engineering very carefully, had to re-read this fragment 3-4 times to understand what the author meant. It seems that it was written in Russian, and from an electrotechnical point of view there are no errors, but it is so surprising that there simply are no forces. He wrote a deeply abstruse professor-theorist or candidate. Not for people. Therefore, the majority of those not so sophisticated can immediately read the postscript, in which I tried to put it all in civilian language.
To calculate the electric energy consumed for a certain period of time, it is necessary to integrate the instantaneous values of active power in time. For a sinusoidal signal, the power is equal to the product of the voltage and current in the network at a given time. Any electric energy meter works on this principle. The figure below shows a block diagram of an electromechanical meter.
So, an excerpt from the factory instructions:
The principle of operation of the induction meter
An electricity meter is an electrical meter for measuring the amount of electricity.
The principle of operation of induction devices is based on the mechanical interaction of alternating magnetic fluxes with currents induced in the moving part of the device. In the counter, one of the flows is created by an electromagnet, the winding of which is connected to the mains voltage (in which electricity is measured). This flow crosses the movable aluminum disk and induces eddy currents in it, closing around the trace of the pole of the voltage electromagnet. The second stream is created by an electromagnet, the winding of which is connected in series to the current circuit. This flow also induces eddy currents in the disk, which are closed around the trace of the pole of their electromagnet. The interaction of the flow of the voltage electromagnet with the induced currents in the disk by the flow of the current electromagnet with the induced currents in the same disk by the flow of the voltage electromagnet, on the other hand, causes electromagnetic forces directed along the chord of the disk and generate torque. Such counters are called dual-threaded.
Modern counters are three-threaded, in which a double torque is created due to the fact that the magnetic flux of the current circuit crosses the aluminum disk twice.
A schematic diagram of a single-phase induction three-flow meter with a tangential magnetic system is shown in Fig. 1.
Fig. 1 Schematic device of an induction counter.
The magnetic system of the voltage circuit S u of the U-shape is located along the chord of the disk (hence the name in contrast to the radial system, when the magnetic system of the voltage circuit of the U shape is located along the radius of the disk) and has branches W - shunting the magnetic flux and the pole P , magnetically connected with side core rods.
Under the magnetic system of the voltage circuit is a U - shaped magnetic system of the current circuit S i .
In the gap between these systems is an aluminum movable disk D. A multi-turn coil of thin wire, connected to the mains voltage U, is located on the middle rod of the Ш-shaped core . The current I u passing through this winding creates a common magnetic flux Ф of the general voltage circuit, a small part of which Ф u , called the working flux, crosses the disk and through the opposite pole P closes to the side rods of the U-shaped core. Most of the flux Ф total , without crossing the disk, is closed through magnetic shunts Ш , branching into two parts ½ Ф ш . This non-working flow Ф ш , as will be shown below, is necessary to create the necessary shift between the flows Ф u and Ф i (internal angle of the counter).
On the lower magnetic system S i there is a small-coil coil of thick wire connected in series in the load current circuit I. The magnetic flux Ф i twice crosses the aluminum disk and closes along the magnetic shunt Ш of the upper core and partially through its side rods. A negligible non-working part of the flow Ф i closes, without crossing the disk, through the opposite pole P. These components of the flow Ф i are not shown in the figure. A simplified vector diagram of the meter measuring element is shown in Fig. 2 for the general case when the load current lags the voltage U by an angle j .
Fig. 2 Vector diagram of the induction counter.
The magnetic flux Ф i passing through the magnetic circuit creates losses in it for hysteresis and eddy currents, as a result of which the flux vector Ф i lags behind the current I created by an angle α 1 .
Usually this angle is small (about 10 ° ) and is used when adjusting the counter according to the internal angle.
The voltage coil has a large inductive component, as a result of which the current I u lags behind the voltage U applied to it by an angle of 70 ° . The flux Φ generally lags behind the current I u that generated it by an angle α 2due to losses in hysteresis and eddy currents in the core, and the component of this flux Φ u crossing the disk lags by a larger angle due to additional losses in eddy currents in the aluminum disk. The phase angle Y between the flows Ф i and Ф u for the correct operation of the meter should be equal to 90 ° , as will be shown below.
In Fig. 3 shows an aluminum disk with traces of the poles of the magnetic flux Ф u and fluxes + Ф i and- Ф i . Crosses denote for the same moment in time the flows directed from the observer, with a dot to the observer.
Fig. 3 Currents in the counter disk.
The stream f u will induce z.s.
eddy currents equivalent to current I u ` ,
closing in the disk around the trace of the pole, the stream Ф i , crossing the disk twice, will induce equivalent currents - I i ` , closing around the traces of " their " poles.
Since the emf induced in the disk 90 ° behind their magnetic fluxes, if we assume that the disk resistance is purely active, the currents caused by them in the disk will coincide in phase with the emf. and, therefore, lag behind the flow that generated them also by an angle of 90 ° . The direction of the induced currents is determined by the rule of the gimlet. The currents induced by the flux Ф i passing in the region of the trace of the pole Ф u in one direction are added. The induced current I u ` passes in the region of the trace of poles + Ф i and- Ф i and also interacts twice with the flux Ф i , which leads to an increase in the electromagnetic interaction force, and this is the advantage of three-flux magnetic systems over double-flux ones.
PS . So what does all of the above mean? We quote from another source, it will definitely illustrate the first conclusion:
Inductive electrical instrument, a device for measuring electrical quantities in AC circuits. Unlike electrical measuring instruments of other systems, I. p. Can be used in alternating current circuits of one specific frequency; its minor changes lead to large errors of evidence. In the USSR, induction ammeters, voltmeters did not receive distribution; wattmeters since the beginning of the 50s 20 century also not available. Modern industrial electronics are manufactured only as electric energy meters for single-phase and three-phase alternating current circuits of industrial frequency (50 Hz ). According to the principle of operation, an inductor is similar to an asynchronous electric motor: the load current, passing along the working circuit of the device, creates a traveling or rotating magnetic field that induces a current in the moving part and causes its rotation. By the number of variable magnetic fluxes inducing current in the moving part of the device, single-threaded and multi-threaded I. p.
Structurally, a magnetic field consists of a magnetic system, a moving part, and a permanent magnet. The magnetic system contains 2 electromagnets with complex cores, on which windings with parallel and series connection to the load circuit are placed; moving part - a thin aluminum or brass disk placed in the field of the magnetic system; a permanent magnet creates a braking torque . And. Items are insensitive to the influence of external magnetic fields and have significant overload capacity.
Lit .: Aluker Sh. M., Electrical meters, 2nd ed., M., 1966; Popov V.S., Electrical Measurements and Instruments, 7th ed., M. —L., 1963.
1. That is, in essence, an induction meter is a trivial asynchronous motor, and like any motor, it can spin in one direction or the other. To do this, it is enough to change the direction of the current in any of its windings.
2. And I would also like to accentuate the moment on one phrase from the factory instruction " The phase angle Y between the flows Ф i and Ф u for the meter to work correctly must be 90 ° ,"
This means that in order for the meter to take into account only active energy, the magnetic flux generated by the voltage coil and current coil must be 90 degrees out of phase. To do this, special shunts are used in the counters, which adjust this angle. About them will be discussed later. If the shunts are not configured correctly, then the counter will take into account reactive energy in addition to the active one, or it is simply inaccurate to take into account energy.