312 



ALTERNATING CURRENTS 



motor the component of terminal voltage which is necessary to 

 balance the back emf . is found by subtracting the IR drop from 

 the terminal voltage. 



To subtract IZ from V, IZ is added to V. It will be noted 

 that the emf. E is numerically greater than the terminal 

 voltage V. That is, by taking a leading current, the synchronous 

 motor is able to operate with an induced emf. greater numerically 

 than the terminal voltage. This is analogous to the alternator 

 delivering leading current with its induced emf. less than its 

 terminal voltage. In each case the flow of power is towards the 

 higher voltage. 



127. Effect of Decreasing the Field Excitation. When the 

 field of a direct-current shunt motor is weakened, the motor 

 speeds up until its back emf. reaches a value which gives the 

 proper armature current for the particular load condition. 



N 



Terminal 

 Voltage a 



Maximum 



j Current a 

 1 Maximum 



FIG. 289. Magnetizing effect of lagging current on the poles of a synchronous 



motor. 



When the field of a synchronous motor is weakened, it cannot 

 speed up permanently for it must run at a constant average 

 speed. However, it takes a lagging current. This current has 

 two effects. 



Figure 289 shows a coil, dotted, whose axis is in position Y. 

 In this position the coil sides are opposite the centers of the pole 

 faces and the back emf. is therefore a maximum. The terminal 

 voltage, which is nearly 180 from the back emf., has its maximum 

 value also for this position of the coil, its direction being indi- 

 cated in the dotted coil. If the current is lagging the terminal 

 voltage by 90, it will not reach its maximum value until the 

 coil axis reaches position X. The current under these conditions 

 is in such a direction as to strengthen the S-pole. Therefore, 

 in a synchronous motor a lagging current strengthens the 

 field through the effect of armature reaction. When the fie ' 



