Prob Unit 2_3 Q&A

Electric Drives and Control(EEE 402)

Introduction to Electric Drives

1. A 220 V, 500 A, 600 rpm separately excited motor has armature and field resistance of 0.02 and 10  respectively. The load torque is given the expression TL = 2000 – 2 N , where  N   is the speed in rpm. Speeds below the rated are obtained by armature voltage control and speeds above the rated are obtained by field control. i) Calculate motor terminal voltage and armature current when the speed is 450 rpm. ii) Calculate field winding voltage and armature current when the speed is 750 rpm. Assume the rated field voltage is the same as the rated armature voltage. V =220V , Ia1 = 500 A, N1 = 600 rpm, R a = 0.02 ohm, R f f =   10 ohm, TL = 2000 _ 2N Nm ( separately excited motor) motor) (i) V =? I a2 = ? (450 rpm) rpm) T2 = 2000_ 2 (450) = 1100 Nm E1 = V _ Ia1 R a = 220 _ 500 (0.02) = 210 V T1 = P / Wm = (Ea1 Ia1) / Wm = 210 x 500 / ( 600 x 2 / 60) = 1671 Nm T2 / T1 = Ia2 / Ia1 Ia2 = (T2 / T1) x Ia1 = (1100 / 1671 ) x 210 = 138.24 A E2= V _ Ia2 R a = 220 _ (138.24 x 0.02) = 217.2 V 600 rpm --------------E = 217.2 V 450 rpm --------------E = ? E= (450 / 600) x 217.2 = 163 V V = 163 + (138.24 x 0.02) = 165.8 V (ii) V f  =  = ?, Ia2= ?, N = 750 rpm T2 = 2000_ 2 (750) = 500 Nm T2 = T2 K eø Ia2 = 500 K eø =(500) / I a2 V = E2 + Ia2 R a 220 = K e ø N2 (2 /60) + 0.02Ia2 220 =(500 / Ia2 ) x 750 x (2 /60) + 0.02Ia2 0 = 0.02I2a2 _ 220Ia2 + 39270 Ia2 = 10818.5 A (or) 181. 5 A K e ø1 = E1 / Wm1 = 210 / (600 x 2 /60) = 3.342 K e ø2 = 500 / I a2 = 2.75 K e ø1 = 3.342-------------V 3.342-------------V f = 220 V K e ø2 =2.75--------- =2.75 ------------------- V f =? Dr Umashankar, Asso Prof, SELECT-VIT

Electric Drives and Control(EEE 402)

Introduction to Electric Drives

=(2.75 / 3.342) x 220 = 181.03 V Vf =181.03 V 2. A 220 V, 1500 rpm, 10 A separately excited D.C. motor has an armature resistance of 1 Ohm. It is fed from a single phase fully controlled bridge converter with an AC source voltage of 230 V, 50 Hz. Assuming continuous load current. Calculate (i) Motor speed at 0 the firing angle of 30  and torque of 5 N-m, (ii) Developed torque at the firing angle of 0 45  and speed of 1000rpm.

3. The chopper used for on-off control of a D.C. separately excited motor has supply voltage of 230 V DC, an on time of 10 ms and off time of 15 ms. Neglecting armature inductance and assuming continuous conduction of motor current. Calculate the average load current when the motor speed is 1500 rpm and has voltage constant of 0.4 V/rad/sec. The armature resistance is 2 ohms.

Dr Umashankar, Asso Prof, SELECT-VIT

Electric Drives and Control(EEE 402)

Introduction to Electric Drives

4. A three phase full wave bridge converter is connected to the armature of a dc motor. The motor runs at 1000 rpm at no load, when connected to a 220V dc source with R a=1 ohm. Find (i) Line to line voltage of the co nverter so that maximum output voltage of the co nverter is equal to the rated voltage of the motor. (ii) The speed of motor for α=45o and Ia =20A.

5. A 230V, 960rpm and 200A separately excited dc motor has an armature resistance of 0.02 ohms. The motor is fed from a chopper which provides both motoring and braking operations. Assume continuous conduction. i) Calculate duty ratio of chopper for motoring operation at rated torque and 350rpm ii) Calculate duty ratio of chopper for braking operation at rated torque and 350rpm iii) If maximum duty ratio of chopper is limited to 0.95 and maximum permissible motor current rating is twice the rated, calculate the maximum permissible motor speed obtainable without field weakening and power fed to the ac source 6. A 230 V, 1200 rpm, 15 A separately excited D.C. motor has an armature resistance of 1.2 ohms. Motor is operated under dynamic braking with chopper control. Braking resistance has a value of 20 ohms. (i) Calculate duty ratio of chopper for motor speed of 1000 rpm and braking torque equal to 1.5 times rated motor torque. (ii) What will be the motor speed for duty ratio o f 0.5 and motor torque equal to its rated torque?

Dr Umashankar, Asso Prof, SELECT-VIT

Electric Drives and Control(EEE 402)

Introduction to Electric Drives

7. A 220V, 970 rpm, 100A dc separately excited motor has armature resistance of 0.05 ohms. It is braked by plugging from an initial speed of 1000 rpm. Calculate i) Resistance to be placed in armature circuit to limit the braking current to twice the full load ii) Braking torque iii) Torque when speed has fallen zero

8. A 200 volts, 875 rpm, 150A separately excited dc motor has an armature resistance of 0.06 ohms. It is fed from single phase fully controlled converter with an source voltage of 220 volts, 50Hz. Assuming continuous conduction, Determine i) firing angle at rated motor torque and 750 rpm (3) Dr Umashankar, Asso Prof, SELECT-VIT

Electric Drives and Control(EEE 402)

Introduction to Electric Drives

ii) firing angle for rated motor torque and -500 rpm (3) iii) motor speed for firing angle is 160 degrees & rated torque (2+2)

Dr Umashankar, Asso Prof, SELECT-VIT