§ 7 Principles of adjustment and adjustment of the induction meter
For a more complete understanding of the operation of the induction meter, consider the methods and basic principles of its adjustment:
Counter and additional points.
The moment of rotation created by the resulting electromagnetic force will cause the disk to rotate.
The disk rotation speed will be determined by the network frequency and the number of pole pairs and will practically not depend on the load.
In order to turn the described induction system into a measuring device, it is necessary to create a counteracting torque, changing in proportion to the change in the measured quantity. Then, each value of the measured value will correspond to the opposing moment at which equilibrium occurs, that is, M BP = M against . Equilibrium can be static and dynamic. For all analogue analogue electrical measuring instruments, the equilibrium of moments is static, that is, when measuring, the arrow of the instrument deviates by a certain angle proportional to the measured value and remains stationary. The counteracting moment of such devices is usually carried out by twisting the coil spring.
In dynamic equilibrium, the movable element of the measuring system, for example, the disk of an induction counter, rotates at a uniform speed, and in this case the condition M bp = M against
The opposing moment for a rotating disk is due to the induction braking torque using a permanent magnet M (see Fig. 1), covering the disk with its poles. During rotation, the disk crosses the magnetic flux FT of a permanent magnet and induces an emf in it. e = c 2 * Ф t* n , creating a current i = e / r in the disk, where r is the resistance of the part of the disk in which the current closes, bn is the number of revolutions of the disk per unit time.
Since the flux FT and the current in the disk are spatially shifted by an angle of 90, a force of interaction between the flux and current, equal to FT i , is directed against the movement of the disk and creates a braking torque equal to:
M vs = s 1 * Ф т *i = c 2 * Ф т2* n = c 3 * n
Thus, the opposing moment created by the rotation of the disk with a permanent magnet is proportional to the frequency of rotation of the disk, and also depends on the radius of application of the braking force, i.e. from the position of the poles of the magnet from the center of rotation of the disk.
In addition to the main points - the moments of rotation and the opposing moment, the counter disk is affected by a number of additional moments, some of which are parasitic, such as the friction moment, induction braking moments from the intersection of the working flows by the disk, from the distortion of the electromagnet cores, and one is created artificially for friction compensation.
The moment of friction is created by the friction of the disk supports in bearings, the counting mechanism and the disk against the air. This moment consists of a constant part and a variable having a complex dependence on the speed of rotation of the disk. When designing, measures are taken to reduce the friction moment through the use of solid bearings and special materials, an increased class of processing gears, etc., as well as by creating a compensation moment.
The induction braking moment arising from the intersection of the working stream by the disk of the voltage circuit is almost constant (depends on the constancy of the applied voltage) and is added to the counteracting moment of the brake magnet. However, with increasing and decreasing voltage, this moment, which depends on Ф u2 = U 2 , introduces some additional error in the measurement. The induction braking torque of the series circuit is proportional to the square of the load current (since Ф2 ~ I2 ) and increases with the load, increasing the negative error of the counter. Moments from skewed cores are independent of disk speed and are not considered separately.
The compensation moment is usually created with the help of a steel screw located on the pole of the electromagnet of the voltage circuit parallel to the disk, as shown in Fig. 4.
Fig. 4 Schematic diagram of the regulation of the internal angle of the counter.
The currents induced in the rotating disk by an electromagnet of the voltage circuit interact with the magnetic flux of the steel screw branching off from the total flux and create a small torque, the magnitude of which can be controlled by screwing in and out of the screw. The direction of torque with the screw position shown in the figure is positive, i.e. from the pole Ф u to the extended end of the screw. If the screw is screwed so that its end protrudes more from the opposite side of the pole, then the direction of the moment will reverse. As you can see, the compensation moment will be proportional to the square of the voltage.
So, we draw conclusions from this paragraph:
1. To eliminate the so-called "self-propelled" counter, an adjustment screw is used, so sometimes, if it is possible to open the counter, you can screw the adjustment screw and the counter will slowly spin back when there is no load. But the method is too straightforward and easily detectable.
2. To adjust the internal angle of the meter, the adjusting resistance R is used , that is, this resistance is responsible for ensuring that the meter counts only active energy . If the adjustment is knocked down, then the meter, in addition to the active one, will also take into account reactive energy. This is an important note for ways to rewind a counter called "Reactive Energy Generator". These methods will only work if the counter adjustment is knocked down.