1 8 ELEMENTS OF ELECTRICAL ENGINEERING. 



much as the greatest disadvantages of the alternating-current 

 system relate to motor operation, and especially to the problem 

 of starting and the problem of speed control 



(a) The synchronous motor requires an auxiliary source of 

 mechanical power to start it, or indeed it may be started and 

 brought up to speed by inherent but very weak induction motor 

 action. The mechanical load of a synchronous motor must 

 usually be connected to the motor after the motor is in operation 

 at full speed. 



Once in operation, however, the synchronous motor runs at 

 absolutely constant speed, frequency of alternating-current supply 

 being constant, its efficiency is quite high, and its power-factor 

 may be adjusted at will by varying its field excitation. The syn- 

 chronous motor is often used on account of its adjustable power- 

 factor in order to compensate for lagging current taken by 

 induction motors. 



The synchronous motor is not suitable for variable speed 

 service, and it is used only where frequent starting is not required. 



(&) The polyphase induction motor starts readily under load and 

 it runs at nearly constant speed, very much like a direct-current 

 shunt motor which is driven from a constant voltage supply. 

 The speed of an induction motor cannot be varied except by 

 completely altering the arrangement of its stator winding, or by 

 inserting resistance in circuit with its rotor windings. The first 

 method requires a specially designed motor with an elaborate 

 arrangement of switches for quickly altering the winding con- 

 nections, and, under practical conditions, it gives but two speeds. 

 The second method is unsatisfactory, because it means a con- 

 siderable loss of power, because it gives a speed which varies 

 greatly with the motor load, and because it necessitates the use 

 of collector rings on the rotor. The induction motor is not suit- 

 able, therefore, for variable speed service. 



(c) The single-phase induction motor to be self-starting must be 

 arranged to start either as a repulsion motor or as a polyphase 

 induction motor. In the former case the motor may develop 



