CHAPTER VII 

 SINGLE-PHASE POWER, WATTMETERS 



1. Required the power consumed in each of the following cases: 

 50 amperes at 1000 volts and the power factor 0.80; 



30 amperes at 400 volts with a 30-degree lag; 



1000 volts at 60 periods on a circuit of 10 ohms and 0.02 henry. 



(3 ram.) 



2. A single-phase motor takes 49 kilowatts at 350 volts with a 

 lag of 30 degrees; what will be the current and how much larger 

 must the conductors be than for a direct current of the same 

 voltage? (2 ram.) 



3. A 110-volt generator is rated at 120 kilo volt-amperes or 120 

 kilowatts on an inductionless load. What would be its rating on 

 the following loads: (a) A power factor of 0.9; (b) a power factor 

 of 0.7; (c) current lagging 30 degrees; (d) current leading 60 

 degrees. (1 ram.) 



4. The station voltmeter reads 1100, the ammeter reads 150 

 and the wattmeter reads 120 kilowatts; required the power 

 factor of the system and the lag; also if the generator is fully 

 loaded, what would be its kilowatt capacity at unity power factor. 

 (2 ram.) 



5. A motor is taking 100 amperes (maximum) at 250 volts and 

 with a lag of 30 degrees. Plot sine curves of e.m.f. and current 

 and by multiplication of the ordinates derive and plot a curve 

 of instantaneous power values. Integrate this curve (measure its 

 area) and thus check the formula for power in an inductive circuit. 

 (20 ram.) 



6. A motor circuit connected to a 100-volt alternating-current 

 system is taking 25 amperes; the wattmeter reads 1.8 kilowatts; 

 plot curves of e.m.f. (sine), current and, derived from these, watts, 

 and check by integrating (measuring area) the watt curve. Take 

 1 large division equal to 30 degrees. (20 ram.) 



7. An inductance coil on a 500- volt, maximum, circuit has r, 5, 

 and takes 10 amperes, co is 400; plot the e.m.f. current and hence 



70 



