THE SYNCHRONOUS MOTOR 311 



" incoming " alternator.* The starting device used is 

 generally a small induction mo tor f (of 5 to 10% the 

 rating of the synchronous motor) mounted on the same 

 shaft with the armature of the synchronous motor. 



Induction Motor as Starter. The induction motor must 

 have at least one pair of poles less than the synchronous 

 motor or else the synchronous motor could not be brought 

 up to synchronous speed. This is because of the fact 

 that the speed of an induction motor is from 5 to 10% 

 less than synchronous speed. If a 10-pole synchronous 

 motor is to be started by an induction motor this will have 

 only 8 poles. Suppose the power supply is 60 cycles, then 

 the synchronous speed is 720 r.p.rn. for a 10-pole machine 

 and 900 r.p.m. for an 8-pole machine. The induction motor 

 would therefore be designed to run at a speed 20% (180 

 r.p.m.) less than synchronous when it is supplying a load 

 just equal to the stray power losses of 'the synchronous 

 motor. It is always wound for the same voltage and num- 

 ber of phases as the synchronous motor so that the same 

 bus-bars may be used to feed both motors. 



Induction Motor Method of Starting. Another method 

 of starting is called the induction motor method. In this 

 method no extra starting motor is necessary as the syn- 

 chronous motor itself is made to act as an induction motor. 

 We shall not analyse the theory of the method now as it 

 will be taken up in the chapter on induction motors but it 

 is sufficient to say that an armature wound with a polyphase 

 winding supplied with polyphase currents generates a rotat- 

 ing magnetic field. This rotating magnetic field produces 

 eddy currents in the pole faces and damping grids (see 

 next paragraph) and these eddy currents react on the 

 armature to make it revolve. By this action the armature 

 is accelerated until synchronous speed is reached. During 



* For a description of the operations necessary to put an incoming 

 alternator on the line see page 262. 



f Sec Chapter X for explanation of induction motor. 



