DC Motors DC MOTOR OPERATIONThe amount of starting resistance necessary to limit starting current to a more desirable value iscalculated using Equation (6-10.R_{s} = (6-10)E_{t}I_{s}R_{a}whereR_{s} = starting resistanceE_{t} = terminal voltageI_{s}= desired armature starting currentR_{a} = armature resistanceExample: If the full load current of the motor mentioned previously is 50 amps, and it isdesired to limit starting current to 125% of this value, find the required resistancethat must be added in series with the armature.R_{s}E_{t}I_{s}R_{a}R_{s}260VDC125%(50 amps)0.4WR_{s}3.76WThe starting resistors are used in a DC motor by placing them in the starting circuit of the motorcontroller that is used to start the DC motor. Starting resistors are normally of variableresistances, with the value of resistance in the circuit at any time being either manually orautomatically controlled. The maximum amount of resistance will always be inserted when themotor is first started. As the speed of the motor increases, counter EMF will begin to increase,decreasing armature current. The starting resistors may then be cut out, in successive steps, untilthe motor reaches full running speed.DCMotorRatingsThe nameplate ratings of a DC motor refer to the conditions of voltage, current, speed, and powerat which the motor is normally operated. The principal rating is known as the continuous rating,which is the rating described on the nameplate of a motor. The continuous power rating is athermal rating. At this power, the motor can be operated for long periods of time without a largerise in temperature and beyond the limits of the conductor insulating material, bearings and othercomponents, which are greatly affected by temperature.The speed rating of a DC motor is often given on the nameplate. This speed is the upper limitat which a motor can be operated without mechanical damage occurring.Rev. 0 Page 13 ES-06

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