CONDITIONS OF STABILITY OF SYNCHRONOUS MOTORS. 141 



.Z? 2 



and - A , and the counter E.M.F. lies between the values and 

 4r 



E sec 9, the plant will be in a condition of stable working. 



EFFICIENCY. 



93. The electrical efficiency of the motor is 



w 



w + i*r 



where w is the output of the motor, r the resistance of its armature, 

 and i the current. To make the efficiency high, the resistance of 

 the armature should be small, and the plant should work in the 

 neighbourhood of minimum current in the line for the given 

 output. 



It appears from practical results that 3 horse-power machines 

 can be made to have an electrical efficiency of over 85 per cent., 

 and 6 horse-power machines of over 90 per cent. ; also machines 

 have been made to develop their full-rated powers at frequencies 

 of 60 to 80 complete periods per second. 



It has been shown that when the motor is excited to a slightly 

 greater extent than that which corresponds to minimum current, 

 the fields of both generator and motor are strengthened by arma- 

 ture reaction, so that it appears advisable for the armatures of 

 both machines to possess a fair amount of self-induction, not only 

 on account of the strengthening of the fields, but also because, in 

 the case of a breakdown, the counter E.M.F. of self-induction 

 would then prevent too large a current flowing, and would diminish 

 the risk of burning up the armatures. 



PROBLEMS ON CHAPTER XIII. 



1. A synchronous motor whose armature has a resistance of 0*5 ohm and 

 self-induction OO25 henry runs with a P.D. of 100 volts between its terminals. 

 What is its maximum output (irrespective of heating limit) ? and what is the 

 corresponding current and counter E.M.F., the frequency being 50 periods per 

 second? 



Answer. Maximum output = 11, 250 watts; current = 100 amperes; counter 

 E.M.F. = 1180 volts. 



