1. Attempt the following objective type questions.
A fully controlled converter has only diodes

(ii) only thyristors

(iii) both diodes and thyristors

(iv) only resistors

(b) SCR can be turned off if a reverse biased voltage comes across it.
if the current is below holding current.

(iii) Both and Either or
Which motor is having almost flat speed-torque characteristics? DC series motor

(ii) Induction motor

(iii) DC shunt motor

(iv) DC compound motor The average value of output voltage for half wave fully controlled rectifier with R-Load is Vm(1 cos a)/2n
2Vm cos a/n

(iii) 2Vm/n

(iv) Vm/2n (cos In dc chopper, if Ton is the on period and f is the chopping frequency, the output voltage in terms of input voltage Vs is given by Vs.Ton/f

(ii) Vs.f/Ton

(iii) Vs/f. Ton

(iv) Vs.f.Ton
For a three-phase bridge inverter having 1800 connection, which statement is true? Each thyristor conducts for 180°
Thyristors conduct for 120°

(iii) Two thyristors conduct at any instant.
Conduction period cannot be controlled. In a CSI, the is constant.

2.(a) Describe the working of a single-phase half-wave converter-fed dc drive with proper voltage and current waveforms.

(b) A single-phase full converter. connected to 230 50 Hz ac supply is feeding a dc motor having armature resistance of 0·5 .0. Calculate the firing angle, if back emf E is 100 V

(ii) -100 V.

3. What are the methods of speed control of a dc separately excited motor? Explain the concept of constant power and constant torque drives for field flux control and armature voltage control.

4.(a) Explain the concept of electric drives. What is the function of feedback in electric drives? Explain each block with example.

(b) Draw the waveforms of a semi-converter f or R and R-L Load.

5.(a) What are the distinct features of two quadrant and four quadrant drives Explain with quadrant diagram. Explain the methods to control the speed of a dc series motor using dc chopper.

6.(a) A dc chopper has input voltage of 200 V and output voltage of 150 V. Load resistance is 10ohm. Find duty cycle,

(ii) average and rms load currents.


(b) Why is forced commutation necessary for choppers?

7.(a) What are the merits and demerits of ac drives over dc drives . Explain the vlf method of speed control of an induction motor.

8. Write short notes on any four of the following:

(a) Step-up Chopper Speed -Torque Characteristics of Induction Motors Comparison of VSI and CSI PWM Control Bridge Type Cycloconverter Thyristor Characteristics