Electrical Machine-I (EX-402) - Important Questions

What is voltage regulation of a transformer? Derive the conditions for zero, maximum and negative voltage regulation with phasor diagrams.

Derive the condition for maximum efficiency of a transformer and find the load current at which efficiency is maximum. Also discuss condition for maximum regulation.

A 20 kVA, 2000/200 V single-phase transformer has H.V. winding R1 = 3 ohm, X1 = 5.3 ohm and L.V. winding R2 = 0.05 ohm, X2 = 0.1 ohm. Find the voltage regulation at (i) 0.8 p.f. lagging, (ii) unity p.f., (iii) 0.707 p.f. leading at full load.

Explain the open-circuit test and short-circuit test on a single-phase transformer with circuit diagrams. How are iron loss, copper loss and equivalent circuit parameters determined?

Using the transformer EMF equation E = 4.44 f N Phi_m, determine the number of turns, induced EMF per turn, maximum flux and core cross-sectional area for a given rating. Give one numerical example.

Explain the Scott / T-T connection for 3-phase to 2-phase and 2-phase to 3-phase conversion with neat circuit and phasor diagrams, including teaser winding ratio and balanced/unbalanced operation.

Explain the function and construction of conservator and breather in transformers.

Explain the need, advantages and essential conditions for successful parallel operation of single-phase transformers. How is load sharing determined with unequal voltage ratios?

Explain the construction and working principle of a three-phase induction motor.

From fundamentals, derive the relationship between rotor input power, rotor copper loss and mechanical power developed (rotor output) in terms of slip s for a three-phase induction motor.

Explain the torque-speed (torque-slip) characteristics of a 3-phase induction motor, including starting, maximum and stable regions.

Explain the working of star-delta starter for starting a three-phase squirrel cage induction motor with neat sketch. Also list other starting methods for squirrel cage and slip-ring motors.

Explain the construction and torque-speed characteristics of a double-cage rotor induction motor and how it improves starting performance compared to a single squirrel-cage motor.

Give the construction and working principle of single-phase A.C. series motor with applications.

Write short notes on any two of the following: (i) Linear induction motor - construction, working principle and applications (ii) Why single-phase induction motor is not self-starting using double revolving field theory (iii) Servomotors - types, construction and working principle.