A 7.5-hp 120-V series dc motor has an armature resistance of 0.1 ohm and a series field resistance of 0.08 ohm. At full load, the current input is 56 A, and the rated speed is 1050 r/min. Its magnetization curve is shown below. The core losses are 220 W, and the mechanical losses are 230 W at full load. Assume that the mechanical losses vary as the cube of the speed of the motor and that the core losses are constant. 160 150 140 Speed = 1200 r/min 130 120 110 * 100 90 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 Series field current, A NOTE: Magnetization curve is given for 1200 rpm. (a) What is the efficiency of the motor at full load? (b) What is the speed of the motor if it is operating at an armature current of 40 A? (c) What is the efficiency of the motor if it is operating at an armature current of 40 A? Internal generated voltage EA, V

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Speed control by DC motor-speed controller is required to realize variable speed drive. Whenever a motor drives a particular load, it is called drive, and variable speed drive refers to a load requiring variable speed. All this is obtained by a DC motor speed controller.