3 6 DIRECT CURRENT MOTORS. [Ex p. 



Under load the compound winding, by strengthening the field, 

 causes the motor to slow down. For certain kinds of service 

 as in operating rolling mills, cranes, elevators, etc. this is desir- 

 able in that the motor can work at great overload without the 

 excessive demand for power which would be made by a constant 

 speed motor. As compared with a shunt motor, it works under 

 load at greater torque ^nd less speed, and can stand a greater 

 overload. In this respect it is similar to the series motor (see 

 18). It differs from the series motor in that at light load 

 there is still a certain strength of field due to the shunt winding, 

 and the speed, therefore, cannot exceed a certain value, whereas 

 a series motor will attain a dangerous speed if the load is thrown 

 off. Under some operating conditions the compound motor can 

 accordingly be used where neither the shunt nor the series motor 

 would be suitable. 



If slowing down with load is not wanted and a constant speed 

 is desired at all loads, together with a large torque at starting, 

 the series winding is used during starting only and is then cut 

 out or short-circuited. 



14. Differential Motor. Since a differential winding weak- 

 ens the field as the load increases, such a winding makes possible 

 a speed which increases with load. This is practically not desir- 

 able. In some cases, however, it is desirable to have the same 

 speed at full load as at no load and to use a series winding just 

 sufficient to overcome the tendency which a shunt motor has to 

 slow down with load. If the series turns are too many for this, 

 their effect can be cut down by a shunt of proper resistance con- 

 nected in parallel with the series winding. 



The starting torque of a differential motor is poor, particularly 

 under load, inasmuch as the large starting current in the differ- 

 ential winding greatly weakens the field. For this reason, when 

 a differential winding is used, it is usually cut out of circuit or 

 short-circuited during starting. 



