ELECTRICAL ENGINEERING

B.E./B.Tech. DEGREE EXAMINATION, MAY / JUNE 2006

THIRD SEMESTER

ELECTRONICS AND COMMUNICATION ENGINEERING

EE 253 – ELECTRICAL ENGINEERING

TIME: THREE HOURS                                             MAXIMUM: 100 MARKS

ANSWER ALL QUESTIONS

PART A – [10 X 2 = 20 MARKS]

1) How the instantaneous power in a balanced three phase circuit is a constant?

2) What is transformer all day efficiency?

3) Define the term back emf in DC motor.

4) Draw the torque – speed characteristics of DC motor.

5) What is importance of slip in electrical motors?

6) What are the basic requirements for starting of induction motor?

7) For a 50 Hz generator list four possible combinations of the number of poles and the speed?

8) Define voltage regulation of a synchronous generator at a given load.

9) What is the function of circuit breaker in power system?

10) Name any three ways of electric power generation.

PART B – [5 X 16 = 80 MARKS]

11) (i) A three phase balanced load has 10Ω resistance in each of its phases. The load is supplied by a 220V 3phase source. Calculate the power absorbed by the load if it is connected in wye; calculate the same if it is connected in delta connection. [8 MARKS]

(ii) a 220V/110V,10kVA transformer has a primary widning resistance of 0.25Ω and a secondary winding resistance of 0.06Ω. determine the primary and secondary currents at rated load, the total winding resistance referred to the primary, and the total winding resistance referred to the secondary. [8 MARKS]


12) (a) (i) Derive the torque equation of DC motor from first principle. [8 MRAKS]

(ii) Classify DC machine according to forms of excitation and draw its circuit diagram. [8 MARKS]

OR

(b) (i) A 230V, 10HP shunt motor takes a full load line current of 40A. The armature and field resistances are 0.25Ω and 230Ω respectively. The total brush contact drop is 2V, and the core and friction losses are 380W. Calculate the efficiency of the motor. Assume that the stray load loss is 1% of the rated output. [11 MARKS]

(ii) Write short notes on starting of DC motor. [5 MARKS]


13) (a)(i) A six pole three phase 60Hz induction motor runs at 4% slip at a certain load. Determine the synchronous speed, rotor speed, frequency of the rotor currents, speed of the rotor rotating field with respect tot the stator, and speed of the rotor rotating field with respect to the stator magnetic field. [12 MARKS]

(ii) List out the similarities between a transformer and an induction motor in developing an equivalent circuit. [4 MARKS]

OR

(b)(i) An induction motor is designed to run at 5% slip on full load. If the motor draws 6 times the full load current at starting at the rated voltage, estimate the ratio of the starting torque to the full load torque. [8 MARKS]

(ii) Explain various methods of starting of three phase induction motor. [8 MARKS]


14) (a)(i) Calculate the percentage voltage regulation for a three phase wye connected 2500kVA, 6600V alternator operating at full load and 0.8 power factor lagging. The per phase synchronous reactance is 10.4Ω. Neglect armature resistance. [8 MARKS]

(ii) Explain the constructional details of synchronous machine. [8 MARKS]

OR

(b) Explain the constructional details, principle of operation and applications of the following special machines:

                                  (i) Stepper motor [8 MARKS]

                                  (ii) Reluctance motor [8 MARKS]


15) (a)(i) Briefly explain the principle divisions in power system. [8 MARKS]

(ii) List out the important substation equipment and their function. [8 MARKS]

OR

(b) Write short notes on:

                         (i) Underground cable distribution. [8 MARKS]

                         (ii) EHVDC transmission [8 MARKS]