xviii INTRODUCTION 



It was in 1891, at the Frankfort Exhibition, that synchronous poly- 

 phase motors, with flat ring or Gramme ring armatures, constructed 

 by the firms of Schuckert and of Lahmeyer, and of sizes as large as 

 50 H.P., were seen, for the first time, alongside the first non-synchronous 

 motors of Dolivo-Dobrowolski and of Brown. 



Since that time the principle of synchronous motor operation has 

 been extended to ordinary polyphase alternators, with any winding 

 whatever, stationary or movable, with poles alternating or not; and 

 the only improvements that have been made have been in the means 

 of their excitation or of starting them. 



In 1890-1 Swinburne had had the idea of producing, by means of 

 over-excited synchronous motors, the relatively considerable magnetiz- 

 ing current consumed by his " hedgehog " transformers. Under these 

 conditions the motor played the same role as a condenser. This very 

 interesting property was utilized industrially in 1893 in the Bulach- 

 Oerlikon power-transmission installation, at the suggestion of Dolivo- 

 Dobrowolski ; and it was also used in the distributing system at Bocken- 

 heim, by Lahmeyer, to compensate for the wattless current of the 

 non-synchronous motors, and even for raising the voltage of the gen- 

 erators. This method has come into extensive use at the present time, 

 especially in the United States. 



It constitutes an advantage in favor of synchronous motors, and 

 it has prevented them from disappearing from the commercial field, 

 after the development of motors of revolving field type, which are quite 

 superior to them from other standpoints, notably in regard to starting 

 power. It is desirable to utilize, wherever possible, both these types 

 of motors in distributions of mechanical power. The synchronous 

 polyphase motors are especially useful. They start readily without 

 load, are self-exciting, and have an efficiency equal to that of alternators. 



The principal objection to synchronous motors (which is, however, 

 in certain cases, an advantage) is the impossibility of modifying their 

 speed of rotation (without modifying that of the generators). Revolv- 

 ing field motors are superior to them in that respect, but this advantage 

 is obtained at the expense of efficiency. Commutator motors, of 

 which no mention will be made here, are alone capable of running 

 at all speeds, the same as direct current motors; and for this reason 

 they present a certain amount of interest. 



