Tutorial 5 Answer PDF

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ANSWER TUTORIAL 5 An 8 pole induction motor, the supply frequency was 50 Hz and the shaft speed was 735 rpm. What where the magnitudes of the following : (i) synchronous speed; (ii) speed of slip; (iii)per unit slip and; (iv)Percentage slip. Solution (i). Ns = 120f / p = (120 x 50) / 8 = 750 rpm (ii...

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ANSWER TUTORIAL 5

1. An 8 pole induction motor, the supply frequency was 50 Hz and the shaft speed was 735 rpm. What where the magnitudes of the following : (i) synchronous speed; (ii) speed of slip; (iii) per unit slip and; (iv) Percentage slip. Solution (i). Ns = 120f / p = (120 x 50) / 8 = 750 rpm (ii) Speed of slip = 750 -735 = 15 rpm (iii) Per unit slip = 15/750 = 0.02 (iv) Percentage slip = 0.02 x 100% = 2%

2. A 14 pole, 50 Hz induction motor runs at 415 rpm. Deduce the frequency of the current in the rotor winding and the slip. Solution Ns = (120x 50)/14 = 428.57 rpm s = (428.57-415)/428.57 = 3.17% fr = 0.0317 x 50 = 1.585 Hz

3. An induction motor has four poles and is energized from a 50 Hz supply. If the machine runs on full load at 2 percent slip, determine the running speed and the frequency of the rotor currents. Solution Ns = (120 x 50)/4 = 1500rpm Nr = (1-0.02) x1500 = 1470 rpm fr = 0.02 x 50 = 1Hz

4. A 400 V, 40 Hz, three phase slip ring type induction motor has rotor resistance of 0.1Ω and standstill reactance of 0.5 Ω per phase with the rotor circuit star connected. The full load slip is 4 percent. Calculate the extra resistance to be connected in the rotor circuit to achieve maximum torque at 50 percent slip. Solution T = (k.s1.Rr)/(Rr2+s12.Xro2) = (k x 0.04 x 0.1)/{0.12 + (0.042 x 0.52)} = 0.3846 k Tmax = (k x 0.02)/Rr’ = 0.3846 k Rr’ = 0.02 / 0.3846 = 0.052 Ohm Rex = Rr + Rr’ = 0.1 + 0.052 = 0.15 Ohm...