The Locomotive . he current —within certain limits. When starting or when the motor is slowed downdue to load, less counter-voltage — or counter-electromotive force* as itis usually called — is generated and hence more current is drawn from the line. The torquetherefore increases withthe load. On the otherhand, as the load fallsoff, the motor speeds updue to the decrease inthe field current, and ifthe speed is not checkedit will race to destruc-tion. Series motorsought, on this account,to be hand controlled. Fig. I is an illustra-tion of what may hap-pen to an automaticallycontrolled series wo
The Locomotive . he current —within certain limits. When starting or when the motor is slowed downdue to load, less counter-voltage — or counter-electromotive force* as itis usually called — is generated and hence more current is drawn from the line. The torquetherefore increases withthe load. On the otherhand, as the load fallsoff, the motor speeds updue to the decrease inthe field current, and ifthe speed is not checkedit will race to destruc-tion. Series motorsought, on this account,to be hand controlled. Fig. I is an illustra-tion of what may hap-pen to an automaticallycontrolled series woundmotor when the controlfails to operate. Thismotor was used on a 10ton hoist and over-speeded, with the resultthat the commutatorflew to pieces, the arma-ture wires were forcedout of the slots and thefield windings werecompletely destroyed. Infact the motor was so badly wrecked that it was not considered worthrepairing and was replaced by a compound wound motor, which is notliable to this type of Fig. I. *It is, of course, generally understood that a direct current generator and motorare practically the same. With current in the field windings, if the machine is drivenby mechanical power—say by a belt — it will be a generator and generate a voltagein the armature circuit. Conversely, it current from some outside source Is causedto flow in the armature circuit, the machine will be a motor and drive the machinebelted to it. In fact even when operating as a motor it is also creating a counter-voltage. This is called counter-voltage because is opposes the line voltage, and themotor speed varies so that the load plus the counter-voltage always equals the impressed(line) voltage. 102 THE LOCOMOTIVE. [October, The shunt wound direct current motor is characterized as a constantspeed machine. The field winding is connected directly across the line,or in parallel with the armature, and as the line maintains an approxi-mately constant voltage, the field current remai
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