106 ELEMENTS OF ELECTRICAL ENGINEERING. 



This method of speed control is limited as follows : (a) It is 

 not feasible to increase 4> much beyond a certain value on ac- 

 count of what is called the magnetic saturation of the iron 

 through which this flux <I> passes. To increase <> to this satu- 

 ration value requires an excessive value of field current and may 

 involve excessive heating of the field magnet coils, (ft) When <f> 

 is decreased more and more, the operation of the motor becomes 

 unsatisfactory ; the torque that can be developed by the machine 

 [see equation (25)] becomes very small when <> is small, and 

 the magnetizing action of the current in the armature begins to 

 preponderate over the weakened magnetizing action of the field 

 windings, and serious sparking at the brushes results. Experience 

 shows that the maximum practicable range of speed control of 

 the shunt motor by means of the field rheostat is about one hun- 

 dred per cent, that is, a shunt motor which runs at speed n when 

 its field magnet is near saturation (largest feasible value of <>) 

 will operate more or less satisfactorily down to a value of <3> equal 

 to about half the saturation value, and the corresponding speed 

 is of course 2n. 



(3) Method by altering the reluctance of the magnetic circuit. 

 If the reluctance of the magnetic circuit of a shunt motor is in- 

 creased, the flux < is decreased, the field excitation being con- 

 stant. The speed of the motor will, therefore, be increased as 

 explained above. If the increase of reluctance is produced by 

 widening the gap space, then the sparking difficulties which 

 usually accompany a decrease of <I> are largely obviated by the 

 increased width of the gap space as explained in Arts. 77 and 78. 



This method of speed control is exemplified by the " multi- 

 speed" motors of the Stow Manufacturing Company. The 

 details of one of these motors are shown in Fig. 8ia and a gen- 

 eral view of the same motor is shown in Fig. 8 1 b. Each of the 

 four field magnet cores in Fig. 8 1 a is hollow and contains a solid 

 cylindrical plunger of iron A. In motors of large size the ends 

 of these plungers are the central parts of the pole faces, whereas 

 in the smaller motors of the Stow Company a thin shell of iron 



