8.5. Induction Type Watt-hour Meter. This is the most commonly used meter on A.C. circuits for measurement of energy.
(i) Simple in operation
(ii) High torque/weight ratio
(iii) Cheap in cost
(iv) Correct registration even at very low power factor
(v) Unaffected by temperature variations
(vi) More accurate than commutator type energy meter on light loads (owing to absence of a commutator with its accompanying friction).
Induction Type Single Phase Energy Meters. These are, by far, the most common form of A.C. meters met with in every-day domestic and industrial installations. These meters measure electric energy in kWh.
The principle of these meters is practically the same as that of induction watt meters. Instead of the control spring and the pointer of the watt-meter, the watt-hour meter, (energy meter) employs a brake magnet and a counter attached to the spindle. Just like other watt-hour meters, the eddy currents induced in the aluminium disc by the brake magnet due to the revolution of the disc, arc utilised to control the continuously rotating disc.
Construction. The construction of a typical meter of this type is shown in Fig. 30. The brake-magnet and recording wheel-train being omitted for clearances. It consists of the following:
(i) Series magnet M1
(ii) Shunt magnet M2
(iii) Brake magnet
(iv) A rotating disc.
M1 = Series or current magnet
M2 = Shunt or voltage magnet
D = Disc
C = Copper shading bands
CC = Current coil
PC = Pressure coil
Fig. 30. Induction type single phase energy meter.
current coil and is connected in one of the lines and in series with the load to be metered. The series electromagnet is energized and sets up a magnetic field cutting through the rotating disc, when load current flows through the current coil C.C. The rotating disc is an aluminum disc mounted on a vertical spindle and supported on a sapphire cup contained in a bottom screw. The bottom pivot, which is usually removable, is of hardened steel, and the end, which is hemispherical in shape, rests in the sapphire cup. The top pivot (not shown) merely serves to maintain the spindle in a vertical position under working condition and does not support any weight or exert any appreciable thrust in any direction.
The shunt magnet M2 consists of a number of M shaped iron laminations assembled together to form a core. A core having large number of turns of fine wire is fitted on the middle limb of the shunt magnet, this coil is known as pressure coil P.C. and is connected across the supply mains.
The brake magnet consists of C shaped piece of alloy steel bent round to form a complete magnetic circuit, with the exception of a narrow gap between the poles. This magnet is mounted so that the disc revolves in the air gap between the polar extremities. The movement of the rotating disc through the magnetic field crossing the air gap sets up eddy currents in the direction react with the field and exerts a braking effect. The speed of the rotating disc may be adjusted by changing the position of the brake magnet or by diverting some of the flux there from.
Working. The shunt electromagnet produces a magnetic field which is of pulsating
character; it cuts through the rotating disc and induces eddy currents there in, but normally does not in itself produce any driving force. Similarly series electromagnet induces eddy currents in the rotating disc, but does not in itself produce any driving force. In order to obtain driving force in this type of meter, phase displacement of 90° between the magnetic field set up by shunt electromagnet and applied voltage V is achieved by adjustment of copper shading band C (also known as power factor compensator or compensating loop). The reaction between these magnetic fields and eddy currents set up a driving torque in the disc.
Sources of Errors. The various sources of errors in an induction-type energy meter are given below:
(i) Incorrect magnitude of the fluxes. These may arise from abnormal voltages and load currents.
(ii) Incorrect phase relation of fluxes. These may arise from defective lagging, abnormal frequencies, changes in the iron losses etc.
(iii) Unsymmetrical magnetic structure. The disc may go on rotating while no current is being drawn but pressure coils alone are excited.
(iv) Changes in the resistance of the disc. It may occur due to changes in temperature.
(v) Changes in the strength of the drag magnets. It may be due to temperature or ageing.
(vi) Phase-angle errors due to lowering of power factor.
(vii) Abnormal friction of moving parts.
(viii) Badly distorted waveform.
(ix) Changes in the retarding torque due to the disc moving through the field of the current coils.
Example 7. A 5 A, 230 V meter on full load unity power factor test makes 60 revolutions in 360 seconds. If the normal disc speed is 520 revolutions per kWh, what is the percentage error?
Example 8. The constant of a 230 V, 50 Hz, single phase energy meter is 185 revolutions per kWh. The meter takes 190 seconds for 10 revolutions while supplying a non-inductive load of 4.5 A at normal voltage. What is the percentage error of the instrument?
Example 9. The name plate of a meter reads “1 kWh= 15000 revoltions”. In a check up, the meter completed 150 revolutions during 45 seconds. Calculate the power in the circuit.
Solution. Power metered in 150 revolutions
= 1 × 150/15000 = 0.01 kWh
Example 10. A 230 V ampere-hour type meter is connected to a 230 V D. C. supply. If the meter completes 225 revolutions in 10 minutes when carrying 14A, calculate:
(i) The kWh registered by the meter, and
(ii) The percentage error of the meter above or below the original calibration.
The timing constant of the meter is 40 A-s/revolution.