Choose the correct answer of the following: In a three-phase AC system, the three voltages have a phase difference of 0°
90°

(iii) 120°
240° Under maximum power transfer condition, the efficiency of a circuit is 50%
zero

(iii) 100%

(iv) 200% In Rolling mills, which of the following motors is most suitable?
DC series motor

(ii) DC shunt motor

(iii) AC motor

(iv) DC compound motor
For an ideal transformer, regulation will be Zero

(ii) 50%

(iii) 100%

(iv) None of the above voltage
In a 3-phase Induction motor, Rotor copper losses are same as Rotor input power

(ii) s (Rotor input power)

(iii) s (Rotor output power)

(iv) None of the above
The form factor for sinusoidal emf is 4.44

(ii) 3.33

(iii) 2·22

(iv) 1.11 Which of the following motors can operate as leading power factor load 3-phase induction motor

(ii) Overexcited synchronous motor

(iii) Underexcited synchronous motor

(iv) DC motor

1.(B) State True or False for the following statements: Resonance in an RLC series circuit is voltage resonance. In AC circuits, for maximum power transfer,ZL Zs. Three-point starter is used for DC series motor. Potential transformer is used to measure large currents. In a three-phase induction motor, the magnetic flux produced by stator winding current rotates at synchronous speed. Salient pole rotor type alternators operate with steam turbines only. A synchronous motor has zero starting torque.

2. Find the current I using Node Voltage Analysis.

<br><br> <img src='./qimages/8520-2a.jpg'> State and prove the maximum power transfer theorem in DC circuits.

3. Explain the following in brief:
Speed control of DC shunt motor
Working of DC generator
Characteristics of DC series motor
Torque equation of DC motor

4. Explain open circuit and short circuit tests conducted on transformers.
A single-phase transformer has a core, whose cross-sectional area is 150 cm^2• it operates at a maximum flux density of 1 Tesla at 50 Hz. The secondary winding has 66 turns. Determine the output in kVA when connected to a load of 5 ohm impedance.

5.(a) Explain the concept of rotating magnetic field and working of a 3-phase induction motor. Draw and explain the torque-slip characteristic of a 3-phase induction motor. Discuss the effect of rotor resistance on torque-slip curves.

6. A 4-pole 50 Hz star-connected alternator has a flux per pole of 0·12 Wb. It has 4 slots per pole per phase and conductors per slot being 5. Winding coil span is 150°. Calculate the emf induced between two phases.

(b) Explain the working principle of a synchronous motor.

7. Write short notes on any two of the following: Parallel Operation of Alternators Synchronous Condenser Speed Control of 3-phase Induction Motor Autotransformer Starting of 3-phase Induction Motor