VECTOR DIAGRAM OF MOTOR 



129 



given exciting current, and I = armature current per phase. In 

 what follows, we shall consider the action of one phase only, so that 

 the reasoning will apply to both single- and polyphase motors. 



Let, in Fig. 97, 0V = V denote the p.d., and OE = E the e.m.f. 

 induced in the armature winding. The vector resultant of these two, 

 OR, gives the e.m.f. available for over- 

 coming the armature impedance. The 

 current is represented by 01 = I, 

 and lags behind OR by an angle 



armature reactance m 

 whose tangent = - .* 



armature resistance 



If 01' = projection of 01 on 0V, and 

 01" = projection of OI on OE produced 

 backwards, then V X OI'( = VI cos 

 /IOV) represents the total electrical 

 power supplied to the motor (excita- 

 tion not included, of course), while 

 E x OI"(= El cos 4>) represents that 

 portion of the total power which 

 undergoes conversion into mechanical 

 power. 



If the load is suddenly decreased, 

 acceleration begins to take place, and 

 the vector OE swings forward (i.e. in 

 a counter-clockwise direction), gradu- 

 ally gaining in phase on 0V. The effect of this is to reduce OR, 

 and so to reduce the current 01 in the same ratio. The driving 

 power f E X 01" is thereby decreased, and this decrease will go on 

 until the driving power becomes equal to that required to deal 

 with the decreased load. 



The opposite effect takes place with a sudden increase of load. 

 Thus the current taken by the motor automatically adjusts itself, 

 both as regards magnitude and phase, to the exact value required ; 

 hence the motor is running under stable conditions. 



* It is here assumed that the armature possesses a definite constant self-inductance 

 an assumption which is by no means legitimate. It roust, therefore, bo understood 

 clearly that the theory given is only approximate. 



t By the " driving power" is here meant the total power which actually undergoes 

 conversion into mechanical power ; the " driving power " therefore includes the power 

 lost in overcoming frictional resistances. 



FIG. 97. Vector Diagram of 

 Synchronous Motor. 



K 



