DC MOTOR OPERATION DC MotorsDC MOTOR OPERATIONDC motors require special starting resistors for operation due to their uniquedesign. A knowledge of the operation of these starting resistors is necessary tounderstand DC motor operation.EO 1.11 EXPLAIN why starting resistors are necessary for largeDC motors.EO 1.12 LIST the four nameplate ratings for a DC motor.StartingofDCMotorsAt the moment a DC motor is started the armature is stationary and there is no counter EMFbeing generated. The only component to limit starting current is the armature resistance, which,in most DC motors is a very low value (approximately one ohm or less), as shown in Equation(6-9).(6-9)I_{a}E_{t}E_{CEMF}R_{a}In order to reduce this very high starting current, an external resistance must be placed in serieswith the armature during the starting period. To show why this is essential, let us consider a10-hp motor with an armature resistance of 0.4 ohms. If the motor were supplied by a 260 VDCsource, the resulting current would be as shown in Equation (6-9).I_{a}E_{t}E_{CEMF}R_{a}I_{a}260VDC00.4WI_{a}650 ampsThis large current is approximately twelve times greater than actual full-load current for thismotor. This high current would, in all probability, cause severe damage to the brushes,commutator, or windings. Starting resistors are usually incorporated into the motor design tolimit starting current to 125 to 200 percent of full load current.ES-06 Page 12 Rev. 0

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