Electrical Machines and Drives

Q.1. What are the losses that occur during electromechanical energy conversion? [2]

Q.2. What are the advantage and disadvantage of a separately excited generator over a self-excited generator? [2]

Q.3. How does sudden changes in load in a D.C. machine cause 'flash-over'? [2]

Q.4. How does higher permeability of magnetic material of a D.C. machine affect its performance? [2]

Q.5. Why do we use capacitor start-induction motor in applications requiring high starting torque in preference to repulsion induction motor? [2]

Q.6. What is torque speed characteristics of induction motor? [2]

Q.7. What are the reason for load equalization in an electric drives? [2]

Q.8. What are the various factors that influence the choice of electric drives? [2]

Q.9. What is the necessity of controlled rectifier for DC drives? [2]

Q.10. Why stator voltage control is suitable for speed control of induction motor in fan and pump drives? [2]

Q.1. Explain static rotor resistance control in closed loop speed control. [8]

Q.2. Explain using a power circuit, how the speed of an induction motor drive can be controlled by using current source inverter. [8]

Q.3. Describe the cyclo-converter fed induction motor drive. [8]

Q.4. Explain the basic construction and working of chopper controlled DC drives. [8]

Q.5. Describe the features of DC series generators and distinguish it from the other types of DC generators. Explain. [8]

Q.6. Derive the equation for torque developed under running conditions, by a 3-phase induction motor. Find the condition for maximum running torque. [8]

Q.7. Draw and explain the equivalent circuit of a single-phase induction motor, based upon double-field revolving theory. [8]

Q.1. Draw the torque-slip characteristic of a 3-phase induction motor and explain it. Find condition of maximum torque. Show the effect of rotor resistance on torque slip characteristic. [15]

Q.2. Establish an expression for the speed of dc motor with the help of neat connection diagram. Explain the method of controlling the speed of dc motor below and above the rated speed. Justify the statement that dc series motors are never started at no load. [15]

Q.3. Draw the characteristics of armature voltage controlled and field flux controlled D.C. drives. Explain the torque and power limitations in combined armature voltage and field control. [15]

Q.4. A 230V, 960-rpm, 12.8A separately excited dc motor has armature circuit resistance and inductance 2 ohm, 150 mH respectively. It is fed from a single-phase half controlled rectifier with an ac source of 230V, 50 Hz. Calculate - [15]

• (a) Torque for alpha = 60 deg and speed = 600 rpm.
• (b) Motor speed for alpha = 60 deg and T = 20 N-m.

Q.5. A 2200V, 2600 kW, 735 rpm, 50 Hz, 8 pole, 3-phase squirrel-cage induction motor has following parameters referred to the stator: Rs=0.075 ohm, Rr=0.1 ohm, Xs=0.45 ohm, Xr=0.55 ohm. Stator winding is delta connected and consists of two sections connected in parallel. [15]

• (a) Calculate starting and maximum torque as a ratio of rated torque, if the motor is started by star-delta switching. What is maximum value of line current during starting?
• (b) Calculate transformation ratio of an auto-transformer so as to limit the maximum starting current to twice the rated value. What is the value of starting torque?