THE INDUCTION MOTOR 341 



and hence the rotor can speed up no more. It follows, 

 therefore, that the limiting speed for the rotor is the same 

 as the speed of rotation of the magnetic field. 



Actual Rotor Speed always Less than that of the Field. 

 Actually, there is always some friction for the rotor to 

 overcome, and hence the rotor can never run at exactly 

 synchronous speed (i.e., the speed of the field), but it 

 approximates it very closely. In a certain motor having 

 a synchronous speed of 900 r.p.m. the rotor turned 898 

 r.p.m. at no load, or, we might say, the rotor had a slip 

 of 2 r.p.m. or about 0.2%. 



95. Starting Characteristics. The three important char- 

 acteristics of an induction motor when starting are torque, 

 current, and power factor. When at rest an induction 

 motor with a short-circuited rotor is essentially a short- 

 circuited transformer, the rotor corresponding to the short- 

 circuited secondary and the stator to the primary winding. 

 We know that a short-circuited transformer draws from the 

 circuit, to which it is connected, a very heavy current, 

 and this is also true of a squirrel-cage induction motor, 

 when at rest. 



Large Starting Current. The first bad feature of a low- 

 resistance squirrel-cage rotor then is the large starting current; 

 a certain 5 h.p. motor having a full-load current of 27.5 

 amperes, takes 130 amperes when first connected to the 

 supply line of normal voltage and larger squirrel-cage motors 

 draw a proportionately greater overload current when 

 starting. 



The Power Factor is Low. The power factor of this 

 starting current is very low and this, combined with the 

 fact that the current is large, produces bad fluctuations 

 in the line voltage when such a motor is switched to the 

 supply line. The 5-h.p. motor mentioned above caused 

 the line voltage to drop from 110 volts to 102 volts when 

 starting, although when taking its normal full-load current 

 and running at its normal speed, the line voltage held up 



