PHASE METERS 



535 



When the power-factor angle of the load is the reading will 

 be given by 



= sin cos A + tan sin ft sin A " cot ftt 

 The change of deflection is D D , 



cot (D -- Do) = cot0 



1 cos cos A + I (cos2A cos cos Al + (cos/3cos A -f sin2/3^-- 1) (tan 6 cos sin A)l 



COS p J 



sin /3 sin A 



Inspection shows that if j8 = A, this equation reduces to 



or 



cot (D - Do) = cot B 



D- 



i _ 



B 



sin a /3 



= cot 6 



Consequently if the crossed coils be adjusted once for all so 

 that the angle between their planes is equal to the electrical angle 

 between their currents, the deflection from the initial position 

 will be equal to the power-factor angle of the load. 



An explanation of the action of the Tuma phase meter may 

 also be based on the fact that the crossed coils set up a rotating 

 field (see page 444), for in the original design of the instrument 

 these coils are 90 apart in space and are traversed by currents 

 differing 90 C in time phase. 



FIG. 317. Showing effect of frequency on a single-phase power-factor meter. 



Single-phase Power-factor Meters. In the application of the 

 principle of the Tuma phase meter to the construction of power- 

 factor meters for use on single-phase circuits a difficulty is en- 

 countered. For though the windings and the angle between 

 the crossed coils may be adjusted so that the instrument reads 



