The resistance of the armature and field windings is 02 Ω and 150 Ω

The resistance of the armature and field windings is

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The resistance of the armature and field windings is 0.2 and 150 respectively. (a) If the terminal voltage and armature current remain unchanged, what will be the speed of the machine if it is run as a motor with the flux per pole reduced by 20%? (b) Calculate the percent change in flux per pole if the machine, has to run as a motor, at the speed of the generator. [(a) 953 rpm (b) 4.6 % reduction] Solution Data: Power output P = 10 kW; V L = 300 V; N = 800 rpm; r a = 0.2 ; R sh = 150 ; rpm 953 ; 934 . 292 066 . 307 N 0.8 1000 Hence, 8 . 0 934 . 292 motor a as operating is machine hen the w 1000 066 . 307 generator a as operating is machine hen the w ; Since (a) V 934 . 292 2 . 0 33 . 35 300 motor, a as operating is machine When the V 066 . 307 2 . 0 33 . 35 300 emf Generated 33 . 35 0 . 2 33 . 33 A 0 . 2 150 300 A; 33 . 33 300 10000 current Load b sh N N E E N E r I V E r I V E A I I V P I b a a a L a a L a a L L Nagsarkar & Sukhija Basic Electrical Engineering,2/E Copyright © Oxford University Press , 2011
4.6 flux in change Percentage 954 . 0 or ; 934 . 292 066 . 307 800 800 . rpm 800 at running is motor when the , is gap air in the flux that the Assume (b) x x x 9.19(b) A separately excited dc motor has a full load rated output of 300 HP (metric) at 500 rpm at a rated voltage of 500V. The armature has a lap connected winding with 1000 conductors. If the I 2 R loss in the armature winding is 10 kW, calculate the flux per pole. Assume that the motor has an efficiency of 90%. [0.0576 Wb] Solution Data: Rated output = 300 metric hp; N = 500 rpm; V L = 500 V; Z = 1000; Armature I 2 R loss = 10 kW, motor efficiency η = 90% A HP 33 . 490 500 5 . 735 33 . 333 V Input current Armature 33 . 333 9 . 0 300 Output Input 042 . 0 33 . 490 10000 , 1000 10 2 2 a a r r I E b = 500 490.33 0.042 = 479.41. Assume that the machine has P poles and the flux per pole is . Since the machine is lap wound, a = P Wb 0.0575 or 41 . 479 P 60 1000 500 P 9.20 A separately excited dc motor delivers 50 hp (metric) at full load and 1000 rpm. Its no load speed is 1100 and the armature voltage is 250 V. If the field current is held constant calculate (a) power at full load, (b) full load current. What is the resistance of the armature? If the motor terminals are now connected to a supply source of magnitude 500 Nagsarkar & Sukhija Basic Electrical Engineering,2/E Copyright © Oxford University Press , 2011