Práctica basada en las relaciones al trabajar con niños y familias47

Another essential part of the electromechanical meter is a magnetic brake. Torque on the disk caused by interaction of fluxes tends to cause constant acceleration.

Without a brake, the speed of rotation would only be limited by the supply fre-quency, friction, and certain counter torques at higher speeds (discussed in later paragraphs concerning overload compensations). Therefore, some method of lim-iting the rotor speed and making it proportional to power is needed. A permanent magnet performs these functions. As the disk moves through the field of the per-manent magnet, eddy currents result in much the same manner as though the magnetic field were changing as previously described. These eddy currents remain fixed in space with respect to the magnet pole face as the rotor turns. Again, as in the case of eddy currents caused by fluxes from the voltage and current coils, the eddy currents are maximum when the rate of cutting flux lines is greatest. In this case the cutting of flux lines is caused by the motion of the disk, so the eddy cur-rents are proportional to the rotational speed of the disk. They react with the per-manent-magnet flux, causing a retarding torque which is also proportional to the speed of the disk. This balances the driving torque from the stator so that the speed of the disk is proportional to the driving torque, which in turn is propor-tional to the power flowing through the meter. The number of revolutions made by the disk in any given time is proportional to the total energy flowing through the meter during that time interval. The strength of the permanent magnet is chosen so that the retarding torque will balance the driving torque at a certain speed. In this way the number of watthours represented by each revolution of the disk is established. This is known as the watthour constant (K_h) of the meter.

ADJUSTMENTS

On modern electromechanical, single-stator watthour meters there are three adjustments available to make the speed of the rotor agree with the watthour constant of the meter. They are the “Full-Load” adjustment, the “Light-Load”

adjustment, and the “Power-Factor” adjustment.

Full-Load Adjustment

The eddy currents in the disk caused by the permanent magnets produce a retard-ing force on the disk. In order to adjust the rotor speed to the proper number of revolutions per minute at a given (or “rated”) voltage and current at unity power factor, the full-load adjustment is used.

Basically, there are two methods of making the full-load adjustment. One is to change the position of the permanent magnet. When the permanent magnet is moved, two effects result. As the magnet moves further away from the center of the disk, the “lever arm” becomes longer, which increases the retarding force. The rate at which the disk cuts the lines of flux from the permanent magnet increases and this also increases the retarding force.

The second method of making the full-load adjustment, by varying the amount of flux by means of a shunt, depends on the fact that flux tends to travel through the path of least reluctance. Reluctance in a magnetic circuit is resistance to magnetic lines of force, or flux. By changing the reluctance of the shunt, it is possible to vary the amount of flux that cuts the disk. One way of doing this is by means of a soft iron yoke used as a flux shunt, in which there is a movable iron screw. As the screw is moved into the yoke, the reluctance of this path decreases, more lines of flux from the permanent magnet flow through the yoke and less through the disk, so the disk is subject to less retarding force and turns faster.

In either case, the retarding force is varied by the full-load adjustment and, by means of this adjustment, the rotor speed is varied until it is correct. Normally the full-load adjustment is made at unity power factor, at the voltage and test current (TA) shown on the nameplate of the watthour meter, but the effect of adjustment is the same, in terms of percent, at all loads within the class range of the meter.

Light-Load Adjustment

With no current in the current coil, any lack of symmetry in the voltage coil flux could produce a torque that might be either forward or reverse. Because electrical steels are not perfect conductors of magnetic flux, the flux produced by the current coils is not exactly proportional to the current, so that when a meter is carrying a small portion of its rated load it tends to run slower. A certain amount of friction is caused by the bearings and the register, which also tends to make the disk rotate at a slower speed than it should with small load currents.

To compensate for these tendencies, a controlled driving torque, which is depend-ent upon the voltage, is added to the disk. This is done by means of a plate (or shading pole loop) mounted close to the voltage pole in the path of the voltage flux. As this plate is moved circumferentially with respect to the disk, the net driving torque is varied and the disk rotation speed changes accordingly. The plate is so designed that it can be adjusted to provide the necessary additional driving torque to make the disk revolve at the correct speed at 10% of the TA current marked on the nameplate of the meter. This torque is present under all conditions of loading. Since it is constant as long as applied voltage does not change, a change in the light-load adjustment at 10% of test amperes will also change full-load registration, but will change it only one-tenth as much as light-load registration is changed.

Inductive-Load or Power-Factor Adjustment

In 1890, Shallenberger presented the theory behind the inductive load adjust-ment. The theory is that in order to have correct registration with varying load power factor, the voltage-coil flux must lag the current-coil flux exactly 90° when the load on the meter is at unity power factor. This 90° relationship is essential to maintain a driving force on the disk proportional to the power at any load power-factor value. One way of doing this is to make the voltage-coil flux lag the current-coil flux by more than 90° by means of a phasing band, or coil, around the core of the center leg of the voltage coil. It is then necessary to shift the current-coil flux toward the voltage-coil flux until the angle is exactly 90°.

Figure 7-19 shows this in a phasor representation, E is the voltage and E is the flux caused by E. A voltage is induced by E in the phasing band which causes a current to flow, creating the flux shown as E_PB. This, added phasorially to E, gives E_T, which is the total resultant flux that acts on the disk and which lags E by more than 90°. Since this analysis is for unity-power-factor load, the current I and its flux I are in phase with E. But the flux I must be shifted toward E_Tuntil the angle is exactly 90°. A closed figure-8 circuit loop is inserted on the current magnet. The Flux, I, induces a voltage in this loop, which causes current to flow, creating the flux field I_PF. Varying the resistance of the power-factor loop can change this value. Adding I and I_PFgives I_Twhich is adjusted by varying I_PFuntil it is exactly 90° from E_T. Any reactance in the current coil which would cause I to be slightly out of phase with I is compensated for at the same time.

Figure 7-19. Phasor Diagram of Lag Adjustment.

As explained in the preceding discussion, the shift of resultant current-coil flux is done by means of a figure-8 conducting loop on the current electro-magnet. The coil usually consists of several turns of wire. The ends of this lag coil are twisted together and soldered at the point necessary to provide the 90°

angle. A change in the length of the wire varies the resistance of the coil and the amount of current flowing, which results in a variation in the amount of com-pensating flux.

The means of adjusting the flux angle may be located on the voltage-coil pole instead of the current poles, in which case it would vary E_PBinstead of

I_PF. The adjustment may be in the form of a lag plate or a coil with soldered ends, so that loop resistance may be varied. A lag plate would be movable under the voltage pole piece radially with respect to the disk. In this manner it would provide adjustable phase compensation with minimum effect on light-load characteristics.

Many modern meters use a fixed lag plate operating on voltage flux with the compensation permanently made by the manufacturer at the factory. Such plates may be located on the voltage coil pole or may form a single loop around both current poles.

For practical purposes, all modern meters leave the factories properly adjust-ed and, once calibratadjust-ed, this lag or power-factor adjustment seldom requires change regardless of the method used.

Once the proper phase relationship between the load-current flux and the voltage flux is attained, there will be no appreciable error at any power factor. If this adjustment is improperly made, an error will be present at all power factors other than unity and it will increase as the power factor decreases. This is calcu-lated as follows:

% error  100

(

)

Using the information supplied and the method explained in Chapter 3, this can be developed into a formula which may be resolved into the following:

% error  100

(

)

 100

(

)

where is the angle between the line current and voltage and  is the angle of error between the line-current flux and the voltage flux due to improper rela-tion within the meter. This error is computed without reference to errors of calibration at full load. Full-load errors are independent of those just calculated and add to or subtract from them dependent upon their relative signs. The errors indicated, while computed for lagging power factor, are also applicable for lead-ing power factor. The sign of the effect will change when golead-ing from a lagglead-ing power factor to a leading power factor. In other words, an improper lag adjust-ment, which causes the meter to run slow on lagging power factor, will cause it to run fast on leading power factor.

COMPENSATIONS

Although the three adjustments mentioned in previous paragraphs are the usual adjustments for a single-stator electromechanical watthour meter, several other factors must be compensated to make the meter accurate for the variety of field conditions in which it must operate. These compensations are built into the meter and provide corrections needed to make the meter register accurately under conditions of overload, temperature variation, frequency error, and volt-age fluctuation.

Overload Compensation

The meter may be adjusted to record correctly at its nominal load. However, the current sensing approach used in electromechanical meters is not perfect, and unless it is compensated, it will not record correctly as loads increase up to the maximum load of the meter (class current). Because electromagnetic steels are not perfect conductors of flux, the speed of rotation of the disk will tend to be pro-portionately less at higher loads. Also, as load currents increase, the damping caused by the interaction of the disk eddy currents with the fluxes that produce them also increases. This effect becomes more visible at the higher overload cur-rents of the meter. For example, the voltage coil produces eddy curcur-rents which interact with the current-coil flux to drive the disk, but the interaction of the volt-age-coil eddy currents with the voltvolt-age-coil flux retards the disk. The voltvolt-age-coil flux is practically constant regardless of load, so its retarding effect can be cali-brated out of the meter. The fluxes produced by the current coil will act with the current-coil eddy currents to retard rotation of the disk. At rated load these self-damping effects are in the order of only 0.5% of the total self-damping. However, the retarding action increases as the square of the current flux. This is true because the retarding force is a function of the eddy currents multiplied by the flux, and in this case the eddy currents increase as the flux increases, so the retarding force increases as the flux multiplied by itself.

Figure 7-20 shows the factors of accuracy for a meter with the typical load curve (6) of a model compensated meter. To negate the retarding or dropping accuracy shown as curve 4, which would result without overload compensation, a magnetic shunt is placed between but not touching the poles of the current

Figure 7-20. Factors of Accuracy.

electromagnet and is held in place by non-magnetic spacers. (See Figure 7-21.) This shunt has little effect below the point at which the accuracy curve of the meter would otherwise start to drop, but as the load increases the shunt approach-es saturation causing the current flux which cuts the disk to increase at a greater ratio than the current. This causes an added increase in torque, which counteracts the drop in the accuracy curve up to the point at which the shunt is saturated.

Beyond this point, which is usually beyond the maximum rated load of the meter, the accuracy curve drops very rapidly.

Figure 7-22 shows another diagram of the magnetic circuit for overload com-pensation on the current element. Other ways of minimizing the retarding effect are: (1) proper proportioning of the voltage and current fluxes, so that the effective voltage-coil flux (about 4% of the total damping flux) is proportionately higher than the effective current-coil flux; (2) by use of stronger permanent magnets and lower disk speed; and (3) a design which gives the greatest driving torque while getting the least damping effect from the electromagnets. Present-day meters will accurately register loads up to 667% of the meter’s nominal rating. Figure 7-23 shows comparisons of the accuracy of modern meters with that of those manu-factured in 1920, 1940, and 1955.

Figure 7-21. Simplified Diagram of Magnetic Circuit of Current Element for Overload Compensation.

Figure 7-22. Overload, Voltage, and Class 2 Temperature Compensations.

At the same time that these improvements were being made, similar improve-ments were effected in light-load performance as can be seen from the curves in Figure 7-24.

Figure 7-23. Heavy-Load Accuracy Curves.

Figure 7-24. Light-Load Performance Curves.

Voltage Compensation

Inaccuracies of registration in modern electromechanical meters over the usual range of voltage variations are very small. In a meter with no voltage compensa-tion, errors resulting from voltage change are caused by:

1. The damping effect of the voltage flux,

2. Changes in the electromagnet characteristics due to changes in voltage, 3. Changes in the effect of the light-load adjustment due to changes in voltage.

The damping effect of the voltage flux is similar to that of the current flux, with changes in effect being proportional to the square of the voltage.

The errors caused by the characteristics of electromagnets are due to the failure of the magnetic circuit to be linear under all conditions of flux density.

In an electromagnet the effective flux is not equal to the total flux. The ratio between the effective and the total flux determines many of the characteristics of the electromagnet. Improvements in the metals used have permitted a much closer approach to the desired straightline properties of the magnetic circuit.

Finally, by use of saturable magnetic shunts similar to those used in the current magnetic circuit, voltage flux is controlled and the errors due to normal voltage variations are reduced to a negligible amount.

Since the light-load compensation is dependent only on voltage, a voltage change varies the magnitude of this compensation and tends to cause error.

Increasing voltage increases light-load driving torque so that a meter tends to over-register at light-load current under over-voltage conditions. Good meter design, which maintains a high ratio of driving torque to light-load compensating torque, reduces these errors to very small values.

The reduction of voltage errors in some electromechanical meters of recent manufacture is to a degree that such a meter designed for use on 240 volts may (in most cases) be used on 120-volt services without appreciable error. Figure 7-25 shows a voltage characteristic curve for one of the modern meters.

Figure 7-25. Voltage Characteristic Curves.

Temperature Compensation

Watthour meters are subjected to wide variations in ambient temperature. Such temperature changes can cause large errors in metering accuracy unless the meter design provides the necessary compensation. Temperature changes can affect the strength of the retarding magnets, the resistance of the voltage and lag coils, the characteristics of the steels, the disk resistance, and other quantities that have a bearing on accuracy. Temperature errors are usually divided into two classes. Class 1 errors are those temperature errors which are independent of the load power factor, while Class 2 errors are those which are negligible at unity power factor, but have large values at other test points.

Class 1 temperature errors are caused by a number of factors which pro-duce a similar effect; namely, that the meter tends to run fast with increasing temperature. Since this is the effect caused by weakening the permanent magnet, the compensation for this class of error consists of placing a shunt between the poles of the permanent magnet to bypass part of the flux from the disk. This shunt is made of a magnetic alloy that exhibits increasing reluctance with in-creasing temperature. With proper design the shunt will bypass less flux from the disk with increasing temperature so that the braking flux increases in the proper amount to maintain high accuracy at unity power factor over the entire temperature range.

Class 2 temperature errors which increase rapidly with decreasing power factor, are due primarily to changes in the effective resistance of the voltage and lag circuits, which in turn, cause a shift in the phase position of the total voltage flux. Improved design has reduced these errors, and various forms of compen-sation have further minimized them. One compencompen-sation method consists of placing a small piece of material with a negative permeability temperature char-acteristic around one end of the lag plate (or a small amount of the alloy in the magnetic circuit of a lag coil) to vary the reactance of the lag circuit so that the lag compensation remains correct with temperature change. Another method

consists of over-lagging the voltage flux with a low-temperature-coefficient

consists of over-lagging the voltage flux with a low-temperature-coefficient