68 SYNCHRONOUS MOTORS 



seek to encourage as much as possible the use of such apparatus by 

 their patrons. 



When no such apparatus exists on the system, it may be desirable 

 to place synchronous motors running without load at the distributing 

 centers for the special purpose of furnishing reactive current. Let 

 us see what the nominal power of such a motor should be. 



The reactive current i being then only the current necessary to 

 keep the apparatus in motion, if we designate by p the sum of the losses 

 without load (i.e., friction, hysteresis, eddy currents and power 

 necessary for excitation), we will have, approximately, 



._p+rl 2 

 i f, 

 EI 



If the apparatus is efficient i 2 will be very small with respect to 

 j 2 and consequently we can write, as an approximation, 



[In a good motor the active current with no load, i, does not exceed 

 o.io or 0.20 of the full-load current. The lag of the current of an over- 

 excited motor running without load can therefore approach very near 



-, without nevertheless approaching as closely thereto as would the 

 2 



current of a condenser.] 



Since is very small, under these conditions, E 2 becomes approx- 

 imately 



The power for which the motor is rated by the manufacturer is 

 equal to the product of the ordinary E.M.F. (usually with a margin of 

 15 to 25 per cent increase), by the normal current. For example, under 

 this hypothesis, it would be possible to use a motor having a catalogue 



E / F \ 



voltage of - ( more generally, - - ) and having the normal current /. 



I - 2 5\ ~ 



Its rated power will be 



in which / has the value already given. This value of / shows that, in 

 an installation comprising a certain number of machines in permanent 

 operation, the capacity of synchronous motor necessary for compensa- 

 tion may be taken lower in proportion as the load-factor will be higher, 



