POLYPHASE POWER 405 



power factor of the load is less than 0.5; when the power 

 factor is just 0.5 ($ = 60) one wattmeter gives zero indica- 

 tion. As the power factor decreases, this meter begins 

 to deflect backward. To make it give a readable deflection 

 its potential leads must be reversed; a reading is then taken 

 and recorded as negative and the true three-phase power 

 is the difference of the two wattmeter readings. 



A convenient method of determining whether or not 

 the reading of one meter should be recorded as negative 

 consists in increasing the power factor of the load slightly 



W 2 



FIG. 265. Connection of Wattmeters for Measuring the Power of a 

 Three-phase Line. 



and noting its effect 011 the readings of the two wattmeters. 

 This increase in the power factor may be brought about 

 by putting on an additional load which is non-inductive, 

 such as lamps. // the reading of one of the waitmeters 

 decreases when the power factor increases, its reading should 

 be recorded as negative. In measuring the input of an 

 induction motor running light, one of the wattmeters will 

 generally give a negative reading; as the load on the motor 

 is increased (thus increasing the power factor) it will be 

 noticed that the reading of one of the wattmeters decreases. 



