100 ELEMENTS OF ELECTRICAL ENGINEERING. 



motor is that which is expended in heating the armature windings 

 by the armature current ; that is to say, field loss and friction 

 losses are ignored. The total power delivered to a motor, 

 neglecting field loss, is equal to E x l a , where E x is the impressed 

 voltage, and / a is the current- flowing through the armature. 

 Part of this power is consumed in heating the armature windings, 

 and part is used in forcing the current through the armature in 

 opposition to the electromotive force, E n which is induced in the 

 armature windings by the motion of the armature. This latter 

 part is equal to EJ n and it is, according to Lenz's law, all con- 

 verted into the mechanical power which turns the armature. 



Therefore, ignoring the power used for field excitation, the in- 

 take of electrical power by the motor is E x l a ; and, ignoring the 

 losses of mechanical power in the motor, by bearing friction, air 

 friction, and magnetic friction,* the output of mechanical power is 

 E a l at so that the approximate efficiency is : E a l a -4- E x l a or 

 EJE x . This expression for efficiency, known as Siemens' law, 

 always gives an overestimate of the efficiency of a motor; and 

 when a motor is running under a small load the ratio EJ E x may 

 be many times as great as the actual efficiency. 



55. Constant voltage and constant current motor driving. The 

 discussion of the electromagnetic behavior of a generator, that is, 

 the discussion of the variation of its voltage with its current out- 

 put, is impossible unless the mechanical conditions of driving are 

 specified. In fact, constant speed driving is nearly always taken 

 as the basis for the discussion of generator characteristics. 



Similarly, the discussion of the mechanical behavior of a 

 motor, that is, the discussion of the variation of its speed with its 

 power output or with its torque, is impossible unless the elec- 

 trical conditions of driving are specified. Usually the discussion 

 of the mechanical characteristics of a motor is based upon the 

 assumption, either of a constant voltage between the motor ter- 



* The effect of eddy currents and hysteresis in a dynamo armature is to create a 

 drag which opposes the motion of the armature, very much like ordinary mechanical 

 friction. 



