Electrical Engineering ⇒ Topic : Energy Meter

 Energy Meter

The energy absorbed by a load or energy consumed by a consumer can be determined directly by connecting an energy meter in the circuit. The connection of an energy meter is identical to that of a wattmeter and therefore the circuit  can be used for measurement of energy also. The energy consumed over a given period of time can be obtained by measuring the number of revolutions made by the aluminium disc of the energy meter. The number of revolutions (N) made by the aluminium disc in a given time can be counted with the help of a stop-clock or can be registered in a register or a counter attached to the spindle of the arrangement. If K is the constant of the energy meter expressed as the number of revolutions per kilowatt hour, then the energy is kilowatt hours (kWh) per revolution will be equal to 1/K. Then the total energy consumed in N revolutions will be

The value of K is generally given in the name plate details of the energy meter.

ENERGY METER

Energy meters are of three types: (i) Electrolytic meter, (ii) Motor meter, and (iii) Clock.

The measurement of electrical energy is, from the viewpoint of economics, the most important of all  electrical measurements. An instrument which measures electrical energy is called an energy meter or a watthour meter. Since electrical energy consumed by a load adds up as the time goes on (watt-hours = watts x hours), it is evident that watthour meter is an integrating instrument. The following three types of energy meters are most commonly used :

(1) Commutator motor meter          .............            for d.c. and a.c. circuits.

(2) Mercury motor meter                 .............            for d.c. circuits.

(3) Induction motor meter                ............            for a.c. circuits only.

In general, energy meters designed for d.c. circuits can be used on a.c. circuits but the reverse is not true. All energy meters are essentially * wattmeters with control spring and pointer removed  but braking torque and counting mechanism provided. Since there is **no controlling torque, the moving system will rotate continuously like an electric motor and hence the name motor meters. The driving torque on the rotor will obviously be proportional to the power being supplied. A braking or retarding torque proportional to the disc speed is provided by a brake magnet. Consequently, the disc speed will be proportional to the power and the number of revolutions made by the disc in a given time which is a measure of the electrical energy taken by the load in that time. The counting mechanism is so designed that it counts the revolutions of the disc in terms of kilowatthours (kWh).

General Theory. The general operating principle of all watthour meters is the same. Fig. a (i) shows the general construction of a watthour meter. As indicated, the operating mechanism has a shunt circuit and a series circuit connected in the same manner as in a wattmeter.The shunt circuit carries current proportional to the supply voltage and the series circuit carries the load current. Consequently, the driving torque on the ***rotor is proportional to the power being supplied i.e.,

                                                                                figure (a)

Driving torque, T_d ∝ Power

Since there is no controlling torque, the disc will rotate continuously like an electric motor.If the speed of rotation of the rotor is made proportional to the driving torque (i.e., power), then its rate of rotation can be used as a measure of power and the total number of revolutions in a given time is a measure of the electrical energy taken by the load in that time. This is achieved by providing a braking torque proportional to the rotor speed. Such a braking torque is produced by rotating an aluminium disc mounted on the same spindle between the poles of a permanent magnet (called brake magnet).

Braking torque. Fig. a (ii) shows how braking torque is provided in a watthour meter. An aluminium disc is attached to the spindle and intercepts the flux of a permanent magnet (i.e., brake magnet). As the disc rotates, eddy currents are induced in the disc. These eddy currents react with the flux of the permanent magnet to produce a braking (or retarding) torque proportional to the disc speed.

Multiplying both sides by t, the time for which the power is supplied, we have,

                                           Power x t ∝ n t

                                       or Energy ∝ N

where N (= n t) is the total number of revolutions in time t.

The number of revolutions of the rotor or disc are recorded on dials which are geared to the shaft [See Fig. a (i)]. The counting mechanism is so arranged that it indicates kilowatthours (kWh) directly and not revolutions.

Note. Eq. (i) above reveals that braking torque will be proportional to disc speed only if the values of Φ and R are constant. This implies that strength of the brake magnet should remain constant throughout the life of the meter. Also the resistance of the disc should be substantially constant.