o i 
  
o 
  
ALTERNATING MOTORS FOR RAILWAY WORK. 187 
rings. These rings permit the insertion of a three-part re- 
sistance in the secondary circuit for the purpose of speed 
regulation. The function and practical effect of such a 
rheostat has already been described. In this case it has 
been found to permit perfect control of the speed, as might 
be anticipated, but with poor efficiency at low speeds. 
The normal car speed is between nine and ten miles per 
hour. 'The starting torque of the motors has proved to be 
ample, quite sufficient to start a very heavily loaded car 
from rest on the steepest grade on the line, and the per- 
formance of the cars has been on the whole very good. 
The two trolleys perform well, and, what is rather extra- 
ordinary, the heavy alternating currents have not given 
  
FIG. 102, 
much trouble to the telephone system of the town. There 
must be a strong element of good luck in this immunity, 
for under ordinary circumstances induction would be at 
least quite perceptible, although the leakage difficulties, of 
course, are practically suppressed as are also all electrolytic 
troubles. 
In some recent two-motor car equipments made by 
Brown, Boveri & Company, a quasi-series connection has 
been employed for low speeds, the induced current from 
one motor serving as the inducing current in the other. 
Although such devices are, as indicated already, useful in 
giving a fair efficiency at low speeds, they can hardly be 
regarded as the full equivalent of the series parallel con- 
troller now so generally and successfully used with con- 
tinuous current motors. 
IV. Motors of the asynchronous type working on a 
monophase circuit are not as yet far enough developed to