§ 7 Principles of adjustment and adjustment of the induction counter
For a more complete understanding of the operation of the induction counter, we consider the methods and basic principles of its adjustment:
Counteracting and extra points.
The torque generated by the resulting electromagnetic force will cause the disk to rotate.
The frequency of rotation of the disk will be determined by the frequency of the network and the number of pairs of poles and practically will not depend on the load.
In order to convert the described induction system into a measuring device, it is necessary to create an opposing torque, changing in proportion to the change in the measured value. Then each value of the measured value will correspond to the opposing moment at which equilibrium occurs, that is, M bp = M vs. The balance can be static and dynamic. For all indicating analog electrical measuring instruments, the moment equilibrium is static, that is, when measured, the instrument arrow deflects by an angle proportional to the measured value and remains fixed. The opposing 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, an induction counter disk, rotates at a uniform rotation frequency, in which case the condition M bp = M against
The opposing moment for a rotating disk is carried out at the expense of the induction braking moment with the help of a permanent magnet M (see Figure 1), covering the disk with its poles. During rotation, the disk intersects the magnetic flux Ft of the permanent magnet and induces an emf in it. e = c 2 * Ф т* n , creating a current in the disk i = e / r , where r is the resistance of the part of the disk in which the current closes, bn is the number of disk revolutions 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 the current appears, equal to Ft i , directed against the movement of the disk and creates a braking torque equal to:
M vs = s 1 * F t *i = c 2 * F t2* n = c 3 * n
Thus, the opposing moment created when a disk is rotated 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 moments - the torque and the opposing moment, the counter disk is affected by a number of additional moments, of which part is parasitic, such as friction torque, induction braking moments from the intersection of the working flows by the disk, from the skew of the electromagnets cores, friction compensation.
The moment of friction is created by the friction of the disk bearings in the bearings, the counting mechanism and the disk against the air. This moment consists of a constant part and a variable that has a complex dependence on the speed of rotation of the disk. When designing, measures are taken to reduce the frictional moment by using solid supports and special materials, increased processing class of gear wheels, etc., as well as by creating a compensation moment.
The induction braking moment arising from the intersection of the working circuit of the voltage circuit by the disk is almost constant (depends on the constancy of the applied voltage) and is added to the opposing torque of the braking magnet. However, with increasing and decreasing voltage, this moment, depending on Ф u2 = U 2 , introduces some additional error in the measurement. The induction braking moment of the series circuit is proportional to the square of the load current (since F2 ~ I2 ) and increases with the load, increasing the negative error of the counter. The moments from skewed cores do not depend on the speed of the disk and are not considered separately.
The compensation moment is usually created using a steel screw located at the pole of an electromagnet of the voltage circuit parallel to the disk, as shown in Fig. four.
Fig. 4 Schematic diagram of the regulation of the internal angle of the counter.
The currents induced in the rotating disk by the electromagnet of the voltage circuit, interact with the magnetic flux of the steel screw branching off from the total flow, and create a small torque, the value of which can be regulated by screwing in and unscrewing the screw. The direction of the moment at the position of the screw indicated in the figure is positive, i.e. from pole F u to the extended end of the screw. If the screw is screwed in so that its end protrudes more from the opposite side of the pole, the direction of the moment will be reversed. It is easy to see that 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 adjusting screw is used, therefore sometimes, if it is possible to open the counter, the adjusting screw can be screwed in and the counter, when there is no load, will slowly turn back. But the way is too straightforward and easily detectable.
2. To adjust the internal angle of the meter, an adjusting resistance R is applied, that is, this resistance is responsible for ensuring that the meter read only active energy . If the adjustment is knocked down, the meter will also take into account reactive energy. This is an important note for methods of winding the counter called "Reactive Energy Generator". These methods will work only when the counter adjustment is knocked down.