Fig. 2,578.—Westinghouse rotating field type power factor meter. The rotating field is produced by the currents of the metered circuits in angularly placed coils, one for each phase of the system, in the case of polyphase meters. In the three phase meter the rotating field is produced by three coils spaced 60° apart; in the two phase meter by two coils spaced 90°; in the single phase meter the positions of voltage and current coils are interchanged and the rotating field is produced by means of a split phase winding, connected to the voltage circuit. There are no movable coils or flexible connections. Single phase meters indicate the power factor of a single phase circuit, or of one branch of any polyphase circuit. Special calibration is necessary in order to use a single phase instrument on a three phase circuit unless the voltage coil be connected from one line to the neutral. Polyphase meters indicate the average angle between the currents and voltages and are superior for polyphase service to meters having only one current coil.
In the wattmeter type, the phase relation between the pressure and the current fluxes is such that on a non-inductive load the torque is zero.
For instance, in a dynamometer wattmeter, the pressure circuit is made highly inductive and the instrument then indicates volts × amperes × sin φ instead of volts × amperes × cos φ, that is to say, it will indicate the wattless component of the power. A dynamometer of this type is sometimes called an idle current wattmeter.
Fig. 2,579.—Single phase power factor meter of the rotating field or disc type.
Ques. Describe a single phase power factor meter of the disc or rotating field type.
Ans. It consists of two pressure coils, as shown in fig. 2,579, placed at right angles to each other, one being connected through a resistance, and the other through an inductance so as to "split" the phase and get the equivalent of a rotating magnetic field.
The coils are placed about a common axis, along which is pivoted an iron disc or vane. The magnetizing coils FG are in series with the load. If the load be very inductive, the coil M experiences very little torque and the system will set itself as shown in the figure. As the load becomes less inductive, the torque on S decreases and on M increases so that the system takes up a particular position for every angle of lag or lead.
Ground Detectors.—Instruments of this name are used for detecting (and sometimes measuring) the leakage to earth or the insulation of a line or network and are sometimes called ground or earth indicators, or leakage detectors.