Energy Meters

ENERGY METERS

The measurement of energy is the same process as measurement of power except that the instrument not merely indicates the power or rate of supply of energy but must take into account also the length of time for which the rate of energy is continued. There are basically three types of energy meters.

(a) Electrolytic meters

(b) Motor meters

(c) Clock meters.

Out of the above, motor meters are very widely used and among motor meters also induction type watthour meters are more commonly used and will be dealt with here.

Single Phase Induction Type Energy Meter

Single-phase induction watthour meters (or energy meters) are extensively used for the measurement of electrical energy in a.c. circuits. One can find such meters installed in homes.

An induction watthour meter is essentially an induction wattmeter with control spring and pointer removed but brake magnet and counting mechanism provided.

Construction.

Fig. 4.12 shows the various parts of a single-phase induction watthour meter.

(i) It consists of (a) two a.c. electromagnets; the series magnet and shunt magnet (b) an aluminium disc or rotor placed between the two electromagnets (c) brake magnet and (d) counting mechanism.

(ii) The shunt magnet is wound with a fine wire of many turns and is connected across the supply so that it carries current proportional to the supply voltage.

Since the coil of shunt magnet is highly *inductive, the current (and hence the -EB ni le flux) in it lags the supply voltage by 90°.

The series magnet is wound with a heavy wire of few turns and is connected in series with the load so that it carries the load current. The coil of this magnet is highly non-inductive so that angle of lag or lead is determined wholly by the load.

(iii) A thin aluminium disc mounted on the spindle is placed between the shunt and series magnets so that it cuts the fluxes of both the magnets.

(iv) The braking torque is obtained by placing a permanent magnet near the rotating disc so that the disc rotates in the field established by the permanent magnet. Eddy currents induced in disc produce a braking or retarding torque that is proportional to the disc speed.

(v) A short-circuited copper loop (also known as power factor compensator) is provided on the central limb of the shunt magnet. By adjusting the position of this loop, the shunt magnet flux can be made to lag behind the supply voltage exactly by 90°.

Frictional compensation is obtained by means of two adjustable short-circuited loops placed in the leakage gaps of the shunt magnet. Geared to the rotating element is counting mechanism which indicates the energy consumed directly in kilowatthours (kWh).

Theory

When induction watthour meter is connected in the circuit to measure energy, the shunt magnet carries current proportional to the supply voltage and the series magnet carries the load current. Therefore, expression for the driving torque is the same as for induction wattmeter.

Referring back to the phasor diagram in Fig. 4.13,

The braking torque is due to the eddy currents induced in the aluminium disc. Since the magnitude of eddy currents is proportional to the disc speed, the braking torque will also be proportional to the disc speed n i.e.,

Braking torque, T_B α n

For steady speed of rotation, T_d = T_B

.. Power α n

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

Power × t α n t

or Energy α N

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

The counting mechanism is so arranged that the meter indicates kilowatthours (kWh) directly and not the revolutions.

Meter constant:

We have seen above that:

N α Energy

or N = K × Energy

where K is a constant called meter constant.

Meter constant, K = N / Energy = No. of revolutions/kWh

Hence the number of revolutions made by the disc for 1 kWh of energy consumption is called meter constant.

The meter constant is always written on the name plates of the energy meters installed in homes, commercial and industrial establishments. If the meter constant of an energy meter is 1500 rev./kWh, it means that for consumption of 1 kWh, the disc will make 1500 revolutions.

Three-Phase Watthour Meter

In a 3-phase system, energy, like power, can be measured by means of two single- phase watthour meters. The total energy supplied will be equal to the algebraic sum of the two readings (a negative sign is used for the reading of the meter which runs backward). However, this is never done commercially as it would be more expensive and more troublesome than the use of a 3-phase meter.

A 3-phase meter is merely a combination of two single-phase meters (See Fig. 4.14), with their moving elements mounted on the same spindle. The total driving torque is equal to the sum of the torques exerted by both the moving elements. Thus only one counting mechanism is required which will directly indicate the energy being supplied to the 3- phase circuit. Fig. 4.14 shows how a 3-phase watthour meter is connected in a 3-phase circuit to measure energy. The current coils are connected in any two lines and each potential coil is joined to the third line.

In fact, the connections are similar to 2-wattmeter method used to measure power in a 3-phase circuit.

It is very important that the two elements are "balanced" i.e., the driving torque of the two elements be exactly equal for equal amounts of power flowing through each. If this is not done, the meter will not indicate correct reading on unbalanced load. The balancing adjustment is most conveniently made with the potential coils Connected in parallel and the current coils in series opposition.