Electrical Engineering ⇒ Topic : Commutator Motor Meter

Commutator Motor Meter

The first electrical energy meter was the commutator meter developed by Elihu Thomson in 1889. This meter can be used to measure d.c. as well as a.c. energy, although it is rarely used to measure a.c. energy. It is because induction watthour meter (See Art.) is far superior to the commutator meter for the measurement of a. c. energy.

Construction. Fig. (a) shows the various parts of a commutator motor meter. It is essentially a small motor (no iron is used in the fields or armature) with a magnetic brake. The field coils consist of a few turns of heavy copper wire and are connected in series with the load so that they carry the load current. In the field of these coils rotates an armature pivoted in jewelled bearings. The armature has a number of coils connected to the segments of a small commutator. The commutator is made of silver and the brushes are silver tipped in order to reduce friction. The armature A and a high resistance R (multiplier) are in series and connected across the supply. Therefore, armature carries current proportional to the supply voltage. An aluminium disc mounted on the spindle (or shaft) rotates between the poles of two permanent magnets and provides the necessary braking torque. The braking torque prevents the armature from rotating too fast and from continuing to rotate after the load is disconnected

                                                                    figure (a)

Theory. When the meter is connected in the circuit to measure energy, the field coils carry the load current and the armature (or rotor) carries current proportional to the supply voltage. In this respect, it resembles dynamometer wattmeter so that driving torque is proportional to the power being supplied i.e.,

                                             Driving torque, T_d ∝ Power

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

The counting mechanism is so arranged that the meter indicates kilowatthours (kWh) directly and not the revolutions as shown in Fig.(b). The armature is connected through a set of gears to a series of dials that indicate the electrical energy (in kWh) consumed. The right-hand dial indicates kWh in units of 1, the next in units of 10, the next in units of 100 and the left-hand dial in units of 1000.

Friction Compensation. There are a number of sources of mechanical friction in the Thomson watt-hour meter viz (i) bearing (ii) gear train and meter register (iii) brush pressure on the commutator and (iv) windage. The frictional effects may cause considerable error unless compensation for friction is provided. This is accomplished by means of a compensating coil See Fig. (a) placed co-axially with the two current coils and connected in series with the armature. Its flux adds to that of the field coils which carry the load cun-ent. Since the compensating coil carries current proportional to the supply voltage, its reaction with the armature current (constant at constant voltage) will contribute a substantially constant driving torque on the motor. Its torque contribution may be adjusted by changing its position relative to the armature to alter the portion of its flux which the armature intercepts.

                                                figure (b)