DESIGN OF ALTERNATING 
GENERATORS AND SYNCHRONOUS 
MOTORS. 
DESIGN OF SYNCHRONOUS MOTORS. 
419. The Principle of the Synchronous Motor.— 
A synchronous motor is simply a reversed alternator and 
most manufacturing firms use their generator designs 
without any alteration, for synchronous motors. If a 
machine, instead of being driven by a prime-mover and 
generating a current at a certain voltage, is fed by a 
current at the same terminal pressure, it will be capable 
of developing mechanical energy approximately equal to 
KW 0.746 horsepower, when running at a synchronous 
speed which may be determined by Equation ('7). 
420. Two Principal Features will be noted in the working 
of synchronous motors : The synchronous motor does not 
start up by itself when the armature is switched on to 
the supply circuit, the field at the same time being fully 
excited, but the armature must be run up to the synchron- 
ous speed, (see Par. 42'7) before it can exert its power. 
The characteristic working condition of this type of motor 
is, that the speed is strictly constant according to the fre- 
quency and number of poles. 
421. Starting of Synchronous Motors.—The method 
of starting and switching a synchronous motor into 
circuit isas follows: The motor being at rest, the main 
switch S,, Fig. 93, as well as that of the excitation cir- 
cuit S, is kept open and all the resistance of the 
rheostat FR, is inserted. The armature is then run up 
