A permanent-magnet dc motor has RA=7 Ω, VT=240 V and operates under no-load conditions at a speed of 1500 rpm with IA=1 A. A load is connected and the speed drops to 1300 rpm. Determine the efficiency of the motor under loaded conditions. Assume that the losses consist solely of heating of RA and frictional loss torque that is independent of speed.

(c) A dc series motor, connected to a 440 V supply, runs at 500 rpm when taking a current of 40 A. Calculate the value of resistance which, when inserted in series with the motor, will reduce the speed to 300 rpm, the gross torque being then half its previous value. The resistance of the motor is 0.2 ohm. Assume flux to be proportional to field current.

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A 230 V dc series motor has an armature resistance of 0.2 ohm and field resistance of 0.1 ohm at rated voltage. The motor draws a line current of 40 amperes and runs at a speed of 1000 rpm. Find speed of the motor for line current of 20 A at 230 V. Assume that flux at 20 A line current is 60% of flux at 40 A line current.

pole. 50-Hz. 3-ϕ induction motor has a full-load speed of 950 rpm. At half load its speed would be ..... rpm a) 475 b) 500 c) 975 d)1000

19 A permanent magnet motor equipped with cobalt-samarium magnets loses 3% of its magnetism per 100°C increase in tempera- ture. The motor runs at a no-load speed of 2500 r/min when connected to a 150 V source in an ambient temperature of 22°C. Estimate the speed if the motor is placed in a room where the ambient temperature is 40°C.

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