DIRECT-CURRENT MOTORS

134. Direct-current motors. All dc motors must receive their excitation from some outside source of supply. Therefore they are always separately excited. The interconnection of the field and armature windings can be made, however, in one of the three different ways employed for self-excited dc generators. A dc motor may be designed, therefore, to operate with its field connected in parallel or in series with its armature, producing, respectively, a shunt or a series motor. If the machine is provided with two field windings, one connected shunt and the other series, it is a compound motor. The speed of a shunt motor connected directly to the line is nearly constant from no load to full load, while the speed of a series motor drops off rapidly as the load is increased. If a series motor were operated at no load with normal voltage, it would attain a dangerous speed, so high in most cases that it would throw itself apart by centrifugal force. A series motor should never be operated at no load unless there is sufficient external resistance connected in series with the motor to limit its speed to be a safe value. For this reason a belt drive should never be used with a series motor.

Standard compound motors are designed to operate with the shunt and series fields connected so as to aid each other (cumulative). Their operating characteristics therefore are a compromise between those of shunt and series motors. Their speed drops off...