178 ELECTRICAL MACHINERY 



is not enough to be objectionable when the main field is 

 operated somewhere near saturation, but when this field 

 is weakened, the effect of the armature reaction is much 

 exaggerated and the brushes will spark badly unless properly 

 shifted with every change in load. 



Commutating Poles used with Field-weakening Control 

 Scheme. The type of motor best adapted for speed varia- 

 tion by the field control method is the commutating pole 

 motor and practically all motors intended for service where 

 a variable speed is desired are equipped with commutating 

 poles. In such motors the commutating pole provides 

 the flux required for sparkless commutation, irrespective 

 of the strength of the main field. Such motors are designed 

 for a speed range as great as 1:6; ordinarily, however, 

 the speed variation obtainable is not greater than 1:3 

 or 1 : 4. 



Variable Speed Motor Compared to a Constant Speed 

 Motor. The torque obtainable from a variable speed motor 

 is greatest when the main field has its greatest strength, 

 i.e., at minimum speeds. As the speed is increased by 

 field weakening, the torque decreases in about the same 

 ratio as the speed increases. This is due to the fact that the 

 torque of a motor is proportional to the product of the 

 armature current and the field density; the safe armature 

 current is nearly the same at high speeds as at low speeds 

 so that the torque goes down as the speed goes up. The 

 product of torque and speed is practically the same what- 

 ever the speed and this means that the motor has the same 

 capacity in horsepower over its whole speed range. 



Now a motor to give a certain output must be of larger 

 size the slower the speed; the size of all variable-speed 

 motors is, therefore, larger than that of standard constant- 

 speed motors of the same horsepower capacity. A field 

 frame designed for a standard constant-speed 5-h.p. motor 

 would be used in the construction of a 3-h.p. variable-speed 

 motor: other sizes would be in about the same ratio. 



