Question

Assignment 02 SET YOUR CALCULATOR TO FIX 3 DIRECT CURRENT MACHINES 0 QUESTION1 An 800-rpm, 440 V, 120 A (armature current), shunt motor has an armature resistance of 0,2 ? The delivered torque is reduced to 60% of the full load value and 15-? resistor is inserted in the armature circuit. Determine the new speed if the supply remains unchanged. QUESTION 2 An 800 rpm, 500 V. 120 A (armature current), series motor has a field resistance of 0.04 ? 191 and an armature resistance of 0.11 ?. Determine the speed of the motor 2.1 For a supply voltage of 550 V and a current of 60 A. 2.2 For a torque of 50% of full-load, a supply voltage of 500 V and a diverter across the field of 0.08 ?. in series with the armature, the torque being 75% With a resistance of 1 of full load and a supply voltage of 500 V 2.3 (10) [30] TRANSFORMERS QUESTION 3

Answer 1

Equivalent circuits of DC short Motor v_t = 440 v I_a_1 = 120 A R_a = 0.2 Ohm Now, v_t - I_a r_a - E_b = 0 Rightarrow E_b = v_t - I_a r_a or, E_b_1 = 440 - (120 times 0.2) = 416 v Now, 1.5 Ohm resistor is inserted in the circuit. New back emf wiil be E_b_2 = v_f - I_a (r_a + r) or, E_b_2 = 440 - 120 (0.2 + 1.5) = 236 v But, T alpha E_b/N (for I_a = consult) or, N alpha E_b/T or, N_2/N_2 = E_b_2/T_2 times T_1/E_b_1 or, N_2/800 = 236/416 times T_1/60/100 times T_1 Rightarrow N_2 = 756.41 rmp new speed.