CONVERTER/CHOPPER FED DC DRIVE QUESTION BANK

1.     Derive the expression for critical speed which separates the continuous conduction mode from discontinuous conduction mode of operation of single phase fully controlled rectifier fed separately excited DC motor.                                                        8

2.     A 220 V, 1500 rpm, 10 A separately excited dc motor has an armature resistance of 1 Ω. It is fed from single phase supply fully controlled rectifier with a source voltage of 230 V, 50 Hz. Assuming continuous load current. Compute i)  motor speed at firing angle of  30 degree and torque of 5 Nm ii)  Developed torque at the  firing angle  of 45 degree and speed  of 1000 rpm.

3.     A 220V, 1500 rpm, 50 A separately excited motor with armature resistance of 0.5 ohm, is fed from a 3 phase fully controlled rectifier. Available ac source has a line voltage of 440 V, 50 HZ.A star delta connected transformer is used to feed the armature so that motor terminal voltage equals rated voltage when converter firing angle is zero.  i) Determine the value of firing angle when a) motor is running at 1200 rpm and rated torque.                                                                                                                           16

4.     Explain the chopper control of dc series motor drive                                                   8

5.     Explain in  detail the regenerative operation of 3 phase  fully controlled rectifier  control of separately excited DC motor

6.     Explain how regenerative braking is obtained in series motor with chopper control  8

7.     Explain in detail about time ratio control and current limit control

8.     Explain the operation of 3 phase fully controlled converter fed DC drive and obtain the expression of motor speed and torque for continuous conduction mode.

9.     A Chopper used to control the speed of a separately excited DC motor, has supply voltage of 230 V, Ton = 15ms Toff= 5ms. Assuming continuous conduction of motor current, calculate the average load current when the motor speed is 3000 rpm. assume voltage constant Kv =0.5 V/rad/sec and armature resistance is 4 Ω

10.  Explain the steady state analysis of the single phase controlled converter fed   separately excited dc motor for continuous current mode. Also explain its operation in motoring and regenerative braking mode.                                                                     16

11.  Explain the operation of four quadrant dc chopper drive.                                              10

12.  A 200 V, 875 rpm, 150A a separately excited dc motor has an armature resistance of 0.06 ohm. It is fed from a single phase fully controlled rectifier with an ac source of 220V, 50 Hz. Assuming continuous condition, calculate i) firing angle for rated motor torque and 750 rpm and 500 rpm. ii) Motor speed for α =160 degree at rated torque.                                                                                                                              16

13.  A 220V, 20A, 1000 rpm separately excited dc motor has an armature resistance of 2.5 Ω. The motor is controlled by a step down chopper with a frequency of 1 KHz. the input dc voltage to the chopper is 250v. What will be the duty cycle of the chopper for the motor to operate at a speed of 600 rpm delivering the rated torque?                                                  6

14.  A 230V, 960 rpm and 200A separately excited DC motor has an armature resistance of 0.02 ohm. The motor is fed from a chopper which provides both motoring and braking operations. The source has a voltage of 230 V.  Assuming continuous conduction.

i) Calculate duty ratio of chopper for braking operation at rated torque and 350 rpm.

ii) Calculate duty ratio of chopper for motoring operation at rated torque and 350 rpm.