§ 6 How is the electric induction meter (for electricians) arranged?
For starters, I will quote an excerpt from the standard factory instruction for an induction meter. Immediately I will warn you that if you did not study at the electrical engineering faculty of the university, then the following text will be difficult for you. Even to me, as a person who carefully studied the theoretical foundations of electrical engineering, I had to reread this fragment 3-4 times to understand what the author meant. It seems that it is written in Russian, and from the electrotechnical point of view there are no mistakes, but it's hard to say that there simply is not any strength. Wrote profoundly abstruse theoretical professor or candidate. Not for people. Therefore, most not so sophisticated can immediately read the postscript, in which I tried to state all this in civil language.
To calculate the electrical energy consumed for a certain period of time, it is necessary to integrate instantaneous values of active power in time. For a sinusoidal signal, the power is the product of the voltage of the current in the network at a given time. Any electric energy meter works on this principle. The figure below shows the block diagram of the electromechanical counter.
So, the extract from the factory instruction:
The principle of the induction counter
The electricity meter is an electrical device 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 inducted in the moving part of the device. In the meter, one of the flows is created by an electromagnet, the winding of which is switched on to the mains voltage (in which the electricity is measured). This current crosses the movable aluminum disk and induces eddy currents in it, closing around the trace of the voltage electromagnet pole. The second flow 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, closing around the trace of the pole of its electromagnet. The interaction of the voltage electromagnet with induced currents in the disk by the flow of a current electromagnet with induced currents in the same disk by a voltage electromagnet flow, on the other hand, causes electromagnetic forces directed along the disk chord and creating a torque. Such counters are called two-threaded.
Modern meters are performed in a three-flow mode, in which the doubled torque is created by the fact that the magnetic flux of the current circuit crosses the aluminum disk twice.
Schematic arrangement of a single-phase induction three-current counter with a tangential magnetic system is shown in Fig.
Fig. 1 Schematic arrangement of an induction meter.
The magnetic system of the voltage circuit S u is Ш-shaped, located along the chord of the disk (hence the name, unlike the radial system, when the magnetic system of the U- shaped voltage circuit is located along the radius of the disk) and has the branches Ш - shunting magnetic flux and opposite Р , magnetically bound With lateral cores of the core.
Under the magnetic system of the voltage circuit is the U -shaped magnetic system of the current circuit S i .
In the gap between these systems is an aluminum mobile disk D. On the middle rod of the Ш-shaped core there is a multi-turn coil made of a thin wire, switched on to the voltage of the network U. The current Iu passing through this winding creates a common magnetic flux Ф of the common voltage circuit, a small part of which Φ u , called the working current, crosses the disk and through the opposite P closes to the lateral rods of the Ш-shaped core. A large part of the flow, 4 , does not intersect the disk, but is closed through the magnetic shunts , branching into two parts, This non-working flow Ф ш , as will be shown below, is necessary to create the necessary shift between the fluxes Ф u and Ф i (the internal corner of the counter).
On the lower magnetic system S i there is a malovit coil made 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 Ш of the upper core and partly through its lateral rods. A small non-working part of the stream Φ i closes without crossing the disk, through the opposite of Ρ . These components of the flow Ф i in the figure are not shown. A simplified vector diagram of the meter measuring element is shown in Fig. 2 for the general case, when the load current lags behind the voltage U by the angle j .
Fig. 2 Vector diagram of the induction counter.
The magnetic flux Φ i , passing along the magnetic circuit, creates in it the losses due to hysteresis and eddy currents, as a result of which the flux vector Φ i lags behind the current I creating it by an angle α 1 .
Usually this angle is small (about 10 ° ) and is used when adjusting the counter by the inner corner.
The voltage coil has a large inductive component, so that the current I u lags behind the voltage U applied to it by an angle of 70 ° . The current Φ generally lags behind the current Iu generated by it at an angle α2 due to the losses due to hysteresis and eddy currents in the core, and the component of this flux Φu crossing the disk lags behind by a larger angle due to additional losses due to eddy currents in the aluminum disk. The angle of phase shift Y between the fluxes Φ i and Φ u for the correct operation of the counter should be equal to 90 ° , as will be shown below.
In Fig. 3 shows an aluminum disk with traces of magnetic flux poles Ф u and fluxes + Ф i and -Φ i . Crosses are designated for the same time point flows directed from the observer, point - to the observer.
Fig. 3 The currents in the counter disk.
The flow of Ф u will be in the disk of the emf.
Eddy currents equivalent to the current I u ` ,
Which closes in the disk around the pole track, the stream Φ i , crossing the disk twice, will introduce equivalent currents - i i ` , closing around the tracks of " their " poles.
Since the emf induced in the disk Lag behind their magnetic fluxes by 90 ° , then, if we assume the resistance of the disk to be purely active, the currents in the disk caused by them will coincide in phase with the emf. And, consequently, lag behind the flow that generated them, too, by an angle of 90 ° . The direction of the induced currents is determined by the rule of the drill. The currents induced by the current Ф i , passing in the region of the trace of the pole Ф u in one direction, add up. The induced current I u ` passes in the region of the traces of the poles + Φ i and Φ i and also twice interacts with the flux Φ i , which leads to an increase in the electromagnetic interaction force, and this is the advantage of the three-stream magnetic systems before the two-stream ones.
PS . So, what does all this mean? Let's quote a quote from another source, it will accurately illustrate the first conclusion:
Induction electric meter, device for measuring electrical quantities in AC circuits. In contrast to the electrical measuring instruments of other systems, the ionosphere can be used in AC circuits of one particular frequency; Minor changes in it lead to large errors in the readings. In the USSR, induction ammeters, voltmeters of distribution have not received; Wattmeters from the early 50's. 20 cent. Also not available. Modern power plants 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 the action of an electric motor it 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, which induces a current in the moving part and causes its rotation. In terms of the number of variable magnetic fluxes that induce a current in the moving part of the instrument, one-stream and multi-
Structurally, the magnetic field 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 sequential inclusion in the load circuit; Movable part - a thin aluminum or brass disk placed in the field of the magnetic system; A permanent magnet creates a braking torque . They are insensitive to the influence of external magnetic fields and have a significant overload capacity.
REFERENCES Aluker Sh. M., Electrical measuring instruments, 2 ed., Moscow, 1966; Popov VS, Electrotechnical measurements and devices, 7 ed., M.-L., 1963.
1. That is, in fact an induction counter is a banal asynchronous engine and like any engine it can turn both in one direction and in another. To do this, it is sufficient to change the direction of the current in any of its windings.
2.I would also like to emphasize the point on one phrase from the factory instruction " The angle of phase shift Y between the fluxes of Φ i and Φ u for the correct operation of the counter should be equal to 90 ° ,
This means that in order for the counter to take into account only the 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 meters, which adjust this angle. About them will be told later. If the shunts are not tuned correctly, then the counter will in addition take active account of reactive energy, or simply inaccurately take energy into account.