ALTERNA TING-CURRENT INSTRUMENTS 



67 



meter, cuts a field of constant strength produced by permanent 

 magnets. This causes a retarding torque which is proportional 

 to the speed of the disc. Therefore, both the driving torque 

 (motor action) and the retarding torque (generator action) are 

 produced on the same disc. 



Calibration and Adjustment of the Induction Watthour Meter. 

 The induction watthour meter is calibrated in much the same 

 manner as the direct-current watthour meter. A standard indi- 

 cating wattmeter is used to measure the average power over a 

 stated interval and the revolutions of the disc of the watthour 

 meter are counted with the aid of a stop watch. The average 

 meter watts are calculated by means of the equation 

 w = K X N X 3,600 

 t 



(28) 



Line 



4=51 



Load 



64. Connections for testing alternating-current watthour meter. 



when- A* is the meter constant, N the revolutions of the disc and 

 / the time in seconds. 



As a rule, an ammeter and a voltmeter are used in connect inn 

 with such a test, as shown in Fin-, ill. in order to determine the 

 power-factor. Instrument losses should be carefully investigated 

 and correct inns made if necessary. 



After the meter is adjusted at full load and unity power-factor 

 liy mean- of the retarding mannet-. it is adjusted at light load 

 by means of the litfht-load adjustment. The power-factor is 

 then lowered. Any error occurring nw must l>e due to improper 

 ktion i- then made correct ly adju>lini: 

 the resistance /.'. Fin. ill. which i- in Mliefl with the l.-muin. 

 If the meter r6gl0ten low with 1 urrent. the resi-' 



//should l>e decreased ; if r ' lie resistance A' should 



l>e increased. With leading these operations should 



ed, 



