5-A] 



STUDY AND OPERATION. 



H7 



A wattmeter* may be connected in two ways, as follows. 

 38. In the first and usual method, Fig. 6, the potential coil is 

 connected between the line wires on the supply side. In this method, 





FIG. 6. First method. FIG. 7. Second method. 



Methods for connecting a wattmeter. 



the wattmeter reading is too large, including not only the true watts 

 of the load but also the RI 2 loss in the current coil of the wattmeter. 

 This error, which may amount to several watts, can frequently be 

 neglected; correction for it is not easily made. In measuring small 

 amounts of power, in order that the error may be neglected, the 

 current should not exceed one half the rating of the current coil of 

 the wattmeter. With current greater than this, the loss in the 

 series coil, which increases with the square of the current, may be 

 too large to neglect. 



39. In the second method, shown in Fig. 7, the potential coil is 

 connected across the terminals of the load. In this method the watt- 

 meter reading is also too large, since it includes the RI 2 loss in the 

 potential coil of the wattmeter. This error is larger than the error 

 in the first method and should be corrected for by subtracting E 2 -f- R w 

 from the wattmeter reading, E being the line voltage and R^ the 



* (37a). Lag Error. A wattmeter measures El X power factor. For 

 accuracy, a wattmeter must have the resistance of the potential circuit so 

 large, compared with its reactance, that the circuit is practically non- 

 inductive. The current in the potential circuit is then practically in phase 

 with the electromotive force; in reality it lags by a small angle, 

 = tarT* (La -- R) . The error due to this lag angle is different for dif- 

 ferent power factors, cos 0, of the load. The true watts are equal to the 

 wattmeter reading multiplied by 



cos cos (0 0) 



In commercial wattmeters, at commercial frequencies, this correction can 

 be neglected. It becomes appreciable on higher frequencies, particularly 

 on loads of low power factor and at low voltages i. e., when the resistance 

 of the potential circuit is small. 



