T1IK INDUCTION .VoynA' _>}:; 



larger sizes of motor, open slots are often used for the stator, 

 as shown in Kig. 231, because of the expense and difficulty of 

 placing the winding in semi-closed slots, and also because the 

 -ity for semi-closed slots is usually less in the larger motors. 

 That is, in the large, higher-speed motors the pole-pitch is large 

 and therefore the ampere-conductors per pole is large. Conse- 

 quently, the desired flux density in the gap may be readily ob- 

 tained without an excessive magnetizing current, even if open 

 Eire used. 



In practically all motors of the squirrel-cage type, the slots 

 of the rotor are semi-closed, as there is little difficulty encountered 

 in placing the rotor bars in this type of slot. 



The advantage of the semi-closed slot is that the effective 

 sectional area of the air-gap is increased and the magnetizing 

 current is therefore reduced. Semi-closed slots reduce the pulsa- 

 tions of flux in the individual teeth, and therefore reduce the 

 tooth losses, which otherwise might be serious. On the other 

 hand, the semi-closed slot gives a much higher slot inductance 

 than the open slot and this inductance in the stator and in the 

 rotor lowers the power-factor and decreases the starting and the 

 break-down torques of the motor. 



105. Operating Characteristics of the Squirrel-cage Motor. 

 Th<- squirrel-cage motor, like the direct-current shunt motor, 

 it substantially constant speed. As the rotor cannot 

 reach the speed of the rotating magnetic field, it must at all 

 time- operate with a certain amount of slip. At no load the slip 

 is very small. As load is applied to the rotor, more rotor current 

 is required to develop the necessary torque in order to carry the 

 increased load. Consequently, tin- rotating magnetic field 

 must cut the rotor conductors at an increased rate, in order to 

 produce the necessary increase of current. The slip of the rotor 

 must accordingly increase, so that the rotor speed drops. 



f tin- slip to the total j)ower delivered to the rotor /x proportional 

 to the II: lots in Uif rotor. As the resistance of tin 4 squirrel 

 is very low, the /''/.' loss i- 1<>\\ and, therefore, t he >lip for 

 ordinary loads is small. In large motors, ."><) hp. or greater, the 

 slip is of the order of 1 to 2 per cent, at full load. In the 

 small' the >\\\) may be a.- high M > s 1" l (l p'i 



at full load. 



