Electrical Engineering ⇒ Topic : Mercury Motor Watthour Meter

Mercury Motor Watthour Meter

The weight of the moving system of a commutator is quite large and the torque-weight ratio is low. Also the friction is large because of the brush pressure required for good commutation. Both these disadvantages are largely overcome in the mercury motor meter. This type of instrument is used for direct currents only.

Principle. The working principle of this meter is the same as that of commutator motor meter. Here the rotor is a copper disc (instead of armature) floating in a mercury chamber through which current is passed. The mercury serves the same purpose as the brushes and commutator in the commutator meter.

Construction. Fig. (a) shows the essential parts of a mercury motor watt-hour meter. It consists of a thin copper disc mounted on the spindle and floating in a mercury chamber. Below this disc is placed an electromagnet whose coils are in series with a high resistance and connected across the supply. Therefore, current in the coils of this magnet is proportional to the supply voltage. The circuit current is passed through the disc at its circumference through mercury. This current leaves the disc at a point on the circumference diametrically opposite to the entrance point. In order to ensure radial flow of current through the disc, radial slots are cut in the disc. As the disc is slotted,it cannot be used for braking purposes. Another aluminium disc is used to provide the necessary braking torque. This disc is mounted on the same spindle and rotates between the poles of two permanent magnets.

Theory. When the meter is connected in the circuit to measure energy, the coils of the electromagnet carry current proportional to the supply voltage. The flux produced by the electromagnet is, therefore, proportional to the supply voltage. The reaction between the current in the disc and magnectic field due to electromagnet produces the necessary driving torque.

                                                           figure (a)

Driving torque, T_d ∝ Flux of electromagnet x Load current

                                ∝ VI

                                ∝ Power

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  ∝ n

For steady speed of rotation, T_d = T_B.

                                           Power ∝ n

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

                                      Power x t ∝  nt

                                     or Energy ∝   N

where N (= n t) 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.

Note. The meter can be used as an ampere-hour meter with slight modifications. The construction of mercury ampere-hour meter is the same except that a permanent magnet is used with disc in place of the electromagnet to provide the driving torque.