CHAPTER III. 

 POWER AND POWER-FACTOR. 



Power in an Alternating Circuit. The nature of power is 

 that of a force exerted at a certain speed : for instance, the 

 resistance to turning of a shaft overcome at the rate of some 

 number of revolutions per minute. 



In a direct-current electric motor the force exerted by 

 the armature is proportional to the current in the armature, 

 since the turning effort is produced by the magnetic action 

 of the armature current upon the field. The speed of the 

 motor is independent of current, but is proportional to the 

 voltage applied to the motor. Hence the power, which is 

 proportional to the product of turning effort and speed, is 

 proportional to the product of current and volts supplied to 

 the motor armature. The actual power supplied to the 

 motor is measured in watts, which are numerically equal to 

 the product of current and voltage. 



It would, however, be possible to apply to the armature 

 current and voltage alternately, instead of simultaneously. 



Thus, if the armature were held stationary a large current 

 could be sent through it at a very low voltage. If the motor, 

 on the other hand, is allowed to run without load, a high 

 voltage may be applied, causing the motor to run very 

 rapidly, while taking only a very small current. In either of 

 these cases the power supplied by the motor would be very small . 



A somewhat analogous case to the one just suggested 

 may arise in an alternating circuit, and with corresponding 

 results. The voltage and current may be considerable, but 

 unless they have a high value simultaneously the power which 

 they represent may be small. 



Power of an Alternating Current. The power of an 

 alternating current at any instant in any part of a circuit is 

 numerically equal to the product of the current in the circuit 

 multiplied by the voltage measured at the terminals of the 

 portion of the circuit under consideration. Since both current 

 and voltage change their direction twice in every cycle, 

 this product may be either positive or negative, and will 

 usually be positive during part of each period, and negative 

 during the remainder. 



