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§ 6 How an electric induction meter works (for electricians).


To begin, I will give an excerpt from the standard factory instructions for the device induction counter. Immediately I will warn you that if you have not studied at the Faculty of Electrical Engineering of the University, the following text will be hard for you. Even I, as a person who carefully studied the theoretical foundations of electrical engineering, had to re-read this fragment 3-4 times to understand what the author meant. It seems that it is written in Russian, and from an electrical point of view, there seems to be no errors, but it is tricky in such a way that there is simply no strength. He wrote a deeply abstruse theoretical professor or candidate. Not for people. Therefore, the majority of those who are not so sophisticated can immediately read a postscript in which I have tried to state all this in a civil language.

To calculate the electric energy consumed over a certain period of time, it is necessary to integrate the instantaneous values ​​of active power over time. For a sinusoidal signal, the power is equal to the product of the voltage on the current in the network at a given time. On this principle, any meter of electrical energy. The figure below shows a block diagram of an electromechanical meter.

Induction Counter Block Diagram

So, an excerpt from the factory instructions:

The principle of operation of the induction counter

The electricity meter is an electrical measuring instrument for measuring the amount of electricity.
The principle of operation of induction devices is based on the mechanical interaction of variable magnetic fluxes with currents induced in the moving part of the device. In the meter, one of the currents is created by an electromagnet, the winding of which is turned on to the mains voltage (in which electricity is measured). This flow crosses the movable aluminum disk and induces eddy currents in it, which are closed around the voltage trace of the electromagnet pole. The second stream is created by an electromagnet, the winding of which is connected in series to the current circuit. This flow induces in the disk also eddy currents, closed around the pole trace of its electromagnet. The interaction of the electromagnet flux voltage with induced currents in the disk current of the current electromagnet with induced currents in the same disk voltage electromagnet voltage, on the other hand, cause electromagnetic forces directed along the chord of the disk and create torque. Such counters are called double-flow.
Modern counters are three-threaded, in which double the torque is created due to the fact that the magnetic flux of the current circuit twice crosses the aluminum disk.

A schematic device of a single-phase three-flow induction counter with a tangential magnetic system is shown in Fig.1.

Schematic device of induction counter

Fig. 1 Schematic device induction counter.

The magnetic system of the voltage circuit S u is W-shaped located along the chord of the disk (hence the name, unlike the radial system, when the magnetic system of the voltage circuit of the U- shaped form is located along the radius of the disk) and has branches C - shunting magnetic flux and a counter pole P 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 there is an aluminum movable disk D. On the middle core of the W - shaped core there is a multi - turn coil of thin wire, which is switched on to the mains voltage U. The current I u passing through this winding creates a common magnetic flux F of the common voltage circuit, a small part of which F u , called the working flow, crosses the disk and through the counter pole P closes on the side rods of the U-shaped core. Most of the flow F total , without crossing the disc, closes through the magnetic shunts W , splitting into two parts ½ F w . This non-working stream Ф ш , as will be shown below, is necessary to create the necessary shift between the flows Ф u and Ф i (the internal angle of the counter).
On the lower magnetic system S i there is a low-turn coil of thick wire, connected in series to the load current circuit I. The magnetic flux Φ i crosses the aluminum disk twice and closes along the magnetic shunt of the upper core Ø and partially through its side rods. The insignificant non-working part of the flow F i closes, without crossing the disc, through the antipole P. These components of the flow f i are not shown in the figure. A simplified vector diagram of the measuring element of the meter is shown in Fig.2 for the general case when the load current lags the voltage U by angle j .

Vector diagram

Fig. 2 Vector diagram of induction counter.

The magnetic flux Φ i , passing through the magnetic conductor, creates in it losses for hysteresis and eddy currents, as a result of which the flux vector Φ i lags behind the current I that created it by an angle α 1 . Typically, this angle is small (about 10 ° ) and is used when adjusting the counter on the inside corner.
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 ° . Flux F generally lags behind the current I u that generated it by an angle α 2due to hysteresis losses and eddy currents in the core, and the flux component of this flux crossing the disk lags a greater angle due to additional eddy current losses in the aluminum disk. The phase shift angle Y between the flows Φ i and Φ u for the meter to work correctly must 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 F u and the fluxes + F i and -F i . Crosses indicate flows for one and the same moment of time, directed from the observer, point to the observer.

Fig. 3 Currents in the disk counter.

The flow f u induces in the disk zs eddy currents equivalent to the current I u ` , closing in the disk around the pole trace, the flux Φ i , crossing the disk twice, induces equivalent currents - I i ` , closed around the traces of " their " poles.
As induced in the drive emf. lagging behind their magnetic flux by 90 ° , then, if we assume that the resistance of the disk is purely active, the currents caused by them in the disk will coincide in phase with the emf. and, therefore, to lag behind the flow that generated them also at an angle of 90 ° . The direction of the induced currents is determined by the rule of the gimlet. The currents induced by the flow Φ i , passing in the wake field of the pole Φ u in one direction, are added. The induced current I u ` passes in the region of the traces of the poles + F i and -F i and also interacts twice with the flux F i , which leads to an increase in the electromagnetic interaction force, and this is the advantage of three-stream magnetic systems over double-flow ones.

Ps . So what does all of the above mean? We will quote from another source; it will definitely illustrate the first conclusion:

Induction about p electric measuring instrument, a device for measuring electrical quantities in AC circuits. Unlike electrical devices of other systems, I. p. Can be used in alternating current circuits of one specific frequency; minor changes in it lead to large errors of indication. In the USSR, induction ammeters, voltmeters, did not spread; wattmeters since the early 50s. 20 in. also not available. Modern operating systems are manufactured only as counters of electrical energy for single-phase and three-phase AC circuits of industrial frequency (50 Hz ). According to the principle of action of an IC, it is similar to an asynchronous electric motor: the load current, passing through 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. According to the number of variable magnetic flux inducing current in the moving part of the device, single-flow and multi-stream flows are distinguished.

Constructively, an I. n. Consists of a magnetic system, a moving part, and a permanent magnet. The magnetic system contains 2 electromagnets with cores of complex shape, on which windings are placed with parallel and series connection to the load circuit; the moving part is a thin aluminum or brass disk placed in the field of the magnetic system; permanent magnet creates a braking moment . I. n. Insensitive to the influence of external magnetic fields and have significant overload capacity.

Lit .: Aluker Sh. M., Electrical measuring devices, 2 ed., M., 1966; Popov, V.S., Electrotechnical Measurements and Instruments, 7th ed., Moscow-L., 1963.

1. That is, inherently, an induction counter is a banal asynchronous motor and, like any other motor, it can spin either way or the other. It is enough to change the direction of the current in any of its windings.

2. And I would also like to emphasize the moment on one phrase from the factory instructions " The phase shift angle Y between flows Φ i and Φ u for the meter to work properly must be equal to 90 ° ,"
This means that in order for the meter to take into account only active energy, the magnetic fluxes generated by the voltage coil and the current coil must be shifted in phase by 90 degrees. For this purpose, special shunts are used in the counters that regulate this angle. About them will be discussed later. If the shunts are configured incorrectly, then the meter will take into account, in addition to the active, reactive energy, or simply not accurately account for energy.