Question

3. A 3-phase full-wave converter drive is feeding power to a 220 V, 20 A, 1500 rpm separately excited DC motor. The ac input to the converters are 415 V, 3-phase, 50 Hz, Y-connected supply system, connected through a step-down transformer so that the armature terminal voltage equals to rated voltage when converter firing angle is 0°. The resistance of the armature winding is 0.5 12. Determine the firing angle, i. when the motor is running at 1000 rpm and at rated torque and ii. at -800 rpm with twice rated torque. Assume continuous conduction

Answer 1

3 Vmi cora Vay : maximum line. of converter 3) output voltage of converter, Voang input voltage from transformer. firing angle Voerg: 220V . When a=0° 22.0 3 Vm! T Coso=1 => Vmir as 220 kn volt = 230.2667V 3 is Seperately excited DC motor mean flux is constant. at first, Back emf, El 220 - (0.5)(20) v as flax constant; ; Naspeed. = 210V 5 ExN Ebi 6 NI Ebz 7 210 1500 1000 Na => E62= 140v Ebz .. new terminal Voltage of motor; Voz E + IaRa Iaz : armature Current 2 in and Case : As 20 A, torque ue and flux constant, Iaz a Irated Vo2 = 140+ +(20) (0.5) - 150V

VO2 = 3 Vmi cosa 0.6818 3.14 x 150 3 x 230.2667 2. Cosa A Voz 3vmi TK = 44.0167 l'in) We know, Ebi Eba Eban N3 210 1500 => Eb2 = 112 volt 800 Eba As flua = constant ; Torque Ti T3 rated T & Ia : Ia: armature Current Ial 20A Ia, = larated Таз torque. I3 = 2T ; T Xi 20 Iaz .:. Iaz Voz .. new terminal voltage, Iaz - 40 A. Ebat Ra( faz) = 112 + (05X40) V = "12 + 2ov - 132 V Vos a 3ml Cosa . .. Cosa 0.6 a Voz 3 vmi 3.14 X 132 3 x 230.2667 Sunday 24 -I x = 53.131) Assuming continuous .1 conduction.