Ans. They are so mounted on the core that the currents in them produce a rotating or shifting field in the air gap, in somewhat the same manner that the currents in the primary windings of an induction motor produce a rotating field.
Fig. 2,544.—Fort Wayne single phase induction watthour meter with cover register and permanent magnets removed to show solid meter frame. A heavy steel back plate held in place by two screws inserted from the front of the central casting encloses the back part of the completely assembled meter. A felt gasket lying on a suitable ledge seals the joint against the entrance of dust or moisture when the back plate is drawn down firmly by the screws. The cover which encloses the back part of the meter is a non-magnetic metallic stamping. It is held in place by wing nuts on the two light brass studs extending forward from the meter frame. This joint between the main frame and the cover is also sealed against the entrance of dust and moisture by the use of a suitable felt gasket. Two glass windows are provided in this cover, one to permit the reading of the register dials, the other to permit observation of the disc's rotation. The cover is sealed in place in the usual way by passing a sealing wire through a hole drilled in the cover sealing stud and thence through a hole provided in the wing of the seal nut. The terminal chamber is an extension of the casting which supports all the inner parts of the meter. The heavy brass terminals used for connecting the meter in circuit are held permanently by a non-combustible insulating compound which is moulded in place around them. This construction gives excellent insulation and is a safeguard against accidental short circuits across terminals. A punched terminal cover which fits over the terminal chamber is hinged at the upper left hand corner so that it will of its own accord swing out of the way when the terminal cover sealing screw is removed. This hinged style of cover will be found convenient when installing and connecting the meter in circuit. When this cover is swung back into closed position it is fastened in place by passing a seal right through the seal screw and through a lug provided on the cover.
Ques. What is the strength of the rotating field with 90 degrees phase difference between the currents?
Ans. It is proportional to the product of the currents in the two coils and therefore proportional to the product of current and voltage in the metered circuit.
At any other power factor the field is proportional to this product multiplied by the sine of the angle of phase difference between the two meter currents. If the current in the voltage coil be in quadrature with the voltage of the metered circuit, at any power factor the sine of the angle of phase difference between the currents in the meter circuits will be equal to the cosine of the angular displacement between the current and voltage in the metered circuit. Under these conditions therefore the strength of the shifting field is proportional also to the power factor of the circuit. In other words, the strength of the rotating field is proportional to the product of the volts, amperes and power factor and is therefore a measure of the actual power.
Fig. 2,545.—Sangamo single phase induction watt hour meter; view with cover removed showing mechanism.
Ques. In what part of the meter is energy consumed?