ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 5 
\ a 
out by applying solid poles or heavy dampers, which will 
act in a similar manner to that of the rotor of an induction 
motor when the stator circuit at the suppiy mains is closed. 
The starting torque obtained in such a self starting device of 
a polyphase synchronous motor, is less than 25% of the 
normal, thus producing a heavy rush of current. 
Another matter to be considered is, that through a trans- 
former action, excessive voltage will be induced in the 
field coils, especially if they are wound for high excitation 
voltage with a large number of turns. The high voltage 
induced in the field coils is likely to cause breakdowns and 
accidents, but this rise may be diminished by short 
circuiting the field coils, or by grouping them in parallel 
- during the operation of starting. 
429. Data Required by the Designer.—A motor is 
intended to develop a certain mechanical energy ex- 
pressed in horse power, when running at a certain speed 
of WV revs. per minute and fed by a current of N cycles 
under a pressure of # volis. Further, there are specificd 
or assumed, as the case may be, the values of the efficiency 
and overload capacity. 
The design of a synchronous motor and its calculation is 
identical with that of a generator; by converting the 
mechanical output in H.P. into the electrical input 
expressed in KVA., one can follow up the method de- 
veloped in the previous examples. | 
430. Example 27.—Method of converting the mechanical out- 
put into electrical energy and calculation of the input: 
A three phase synchronous motor has to be capable of 
developing a normal power of 450 H,P. when running at 
a speed of 500 revs. per minute, the machine being fed 
by a current at 50 cycles under a terminal pressure of 
3500 volts. Further, it is specified that for the normal 
working, the armature should carry a leading current 
with a power factor, cos g =0.9, whereby the efficiency: 
shall not be less than 94%. 
