STARTING TORQUE 217 



127. Heating Test 



In addition to the special tests described, it is necessary, in an 

 exhaustive test of an induction motor, to carry out the insulation 

 ( 89) and heating tests, which apply to all classes of electrical 

 machinery. The heating test may be conveniently carried out by 

 clamping and short-circuiting the rotor, and applying a p.d. to the 

 stator windings sufficient to give rise to the full-load currents. The 

 rise of temperature should be determined by resistance measure- 

 ments ( 93). 



128. Induction Motors with Large Starting 



Torque 



In an induction motor having permanently short-circuited 

 windings, high efficiency (which involves low rotor resistance) is 

 incompatible with large starting torque. Such motors are, therefore, 

 incapable of starting against a heavy load, and it is usual to provide 

 a fast and loose pulley, the motor being first allowed to run up to 

 speed on the loose pulley, and the driving belt being then shifted to 

 the fast pulley. Instead of this arrangement, various forms of 

 friction clutch may be used. In a device recently patented by 

 De Lignieres,* the stator as well as the rotor is capable of rotating, 

 and the stator is provided with a suitable brake, by means of which 

 it may be gradually reduced to rest. In starting, the stator is 

 allowed to run up to full speed, and the brake is then gradually 

 applied to it, the torque increasing as the speed decreases, until the 

 rotor is started ; the stator being then gradually reduced to rest, and 

 the rotor gaining speed. 



M. D6ri has devised an extremely ingenious method of securing 

 large starting torque with high efficiency in motors having per- 

 manently short-circuited rotors. | This method is used by the Helios 

 Co. in their induction motors. It consists essentially in halving 

 the number of poles at starting, and in using a special form of short- 

 circuited rotor winding, such that while under normal running 

 conditions, with the full number of poles in use, the resistance of the 

 rotor winding is low, its resistance when only half the number of 

 poles is used is considerable, thereby enabling the rotor to develop a 

 powerful starting torque. The change in the number of stator poles 

 is effected by means of a switch. The arrangement of the rotor 



* Wettem Electrician, vol. xxxiii. p. 228 (1903). 



t Zeittchrift fur Elelctrotechnik, vol. xvi. p. 285 (1898); also Elektrotechnische 

 liundschau, vol. xxi. p. 24 (1903). 



