478 ELECTRICAL MEASUREMENTS 



are used for measuring energy supplied for industrial purposes. 

 Induction motors which are commonly used may be only par- 

 tially loaded and therefore operating at low power factors. 

 This is the condition at which it is most necessary to keep the 

 potential and current fluxes of the meter in the proper phase 

 relation. 



Light-load Adjustment. The principle underlying the devices 

 used for the light-load adjustment is that of the shaded pole 

 motor. In this type of motor the flux from the stator is split 

 into two portions which are displaced in time phase. Con- 

 sequently, the forces acting upon the movable element are un- 

 balanced and a tendency towards rotation results. Referring 

 to Fig. 272, which shows the electric and magnetic circuits of 

 one type of watt-hour meter made by the General Electric Co., 

 PL is the potential lug. Immediately below it is a stamping LLA 

 which forms a short-circuited coil of a single turn; it is made of 

 sheet metal of the appropriate resistivity, and so mounted that 

 it can be displaced in its own plane either to the right or to the 

 left by moving the lever L. 



Suppose that the potential coil is energized, that there is no 

 load on the meter and that LLA is placed symmetrically with 

 respect to the potential pole. Currents will be induced in LLA 

 which will cause a back magnetomotive force, but as LLA is 

 symmetrically placed with respect to the pole tip, the flux cut- 

 ting the disc will be symmetrical with respect to the pole and all 

 in the same time phase. Consequently there will be no tendency 

 for the disc to turn. Now suppose the loop to be displaced to- 

 ward the left the part of the pole covered by it will be " shaded," 

 that is, owing to the induced currents in the loop, the flux from 

 that portion of the pole will be decreased and displaced in phase 

 when compared with that from the unshaded portion at the 

 right . of the loop. Thus the disc is acted on by two sets of 

 fluxes which differ in time phase and there is a travelling field 

 and a tendency to rotation. By giving the loop the proper 

 displacement the friction may be compensated so that the disc 

 will begin to move as soon as a very small load is put on the 

 circuit. 



Sources of Error in Induction Watt-hour Meters. The read- 

 ings of the induction watt-hour meter are subject to a number of 



