EE 2251 ELECTRICAL MACHINES — I ANNA UNIVERSITY PREVIOUS YEAR QUESTION PAPER, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS FOR EEE DEPARTMENT

ANNA UNIVERSITY PREVIOUS YEAR QUESTION PAPER EE2251 ELECTRICAL MACHINES — I, IMPORTANT QUESTIONS, 2 MARKS AND 16 MARKS QUESTIONS FOR EEE DEPARTMENT

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2011

Fourth Semester

Electrical and Electronics Engineering

EE 2251 — ELECTRICAL MACHINES — I

(Regulation 2008)

Time : Three hours Maximum : 100 marks

Answer ALL questions

PART A — (10 × 2 = 20 marks)

1. What are the three types of basic rotating electric machines?2. A conductor 80 cm long moves at right angle to its length at a constant speed of

30 m/s in a uniform magnetic field of flux density 1.2 T. Find the emf induced

when the conductor motion is normal to the field flux.

3. Which equivalent circuit parameters can be determined from the open-circuit

test on a transformer?

4. The emf per turn for a single-phase 2200/220 V, 50 Hz transformer is 11 V.

Calculate the number of primary and secondary turns.

5. Based on the principle of conservation of energy, write an energy balance

equation for a motor.

6. What are the three basic principles for the electromechanical energy

conversion?

7. What is magnetic leakage flux?

8. Why is the efficiency of a three-phase induction motor less than that of a threephase

transformer?

9. Draw the circuit model of dc shunt motor.

10. What is the function of no-volt release in a three-point starter?

PART B — (5 × 16 = 80 marks)

11. (a) (i) Discuss in detail the magnetic circuits and the electrical analog of

magnetic circuits. (10)

(ii) What is eddy-current? Explain in detail the eddy-current loss. (6)

Or

(b) (i) Explain the power losses that occur in a magnetic material when it

undergoes cyclic magnetization. (10)

(ii) The total core loss of a specimen of silicon steel is found to be

1500 W at 50 Hz. Keeping the flux density constant the loss

becomes 3000 W when the frequency is raised to 75 Hz. Calculate

separately the hysteresis and eddy current loss at each of those

frequencies. (6)

12. (a) The following data were obtained on a 20 kVA, 50 Hz, 2000/200 V

distribution transformer :

Voltage (V) Current (A) Power (W)

OC test with HV open-circuited 200 4 120

SC test with LV short-circuited 60 10 300

Draw the approximate equivalent circuit of the transformer referred to

the HV and LV sides respectively. (16)

Or

(b) (i) A 3-phase transformer bank consisting of three 1-phase

transformers is used to step-down the voltage of a 3-phase, 6600 V

transmission line. If the primary line current is 10 A, calculate the

secondary line voltage, line current and output kVA for the

following connections :

(1) / Y and

(2) Y / . The turn’s ratio is 12. Neglect losses. (8)

(ii) A 20 kVA, 2500/500 V, single-phase transformer has the following

parameters :

HV winding : r1 = 8 and x1 = 17

LV winding : r2 = 0.3 and x2= 0.7

Find the voltage regulation and the secondary terminal voltage at

full load for a pf of 0.8 lagging and 0.8 leading. The primary voltage

is held constant at 2500 V. (8)

13. (a) (i) Describe the flow of energy in electromechanical devices. (6)

(ii) Discuss about the ‘field energy’ and ‘coenergy’ in magnetic system.

(4)

(iii) The magnetic flux density on the surface of an iron face is 1.6 T

which is a typical saturation level value for ferromagnetic material.

Find the force density on the iron face. (6)

Or

(b) A doubly-excited magnetic field system has coil self- and mutualinductances

of

L11 = L22 = 2H and L12 = L21 = θ cos

Where θ is the angle between the axes of the coils.

(i) The coils are connected in parallel to a voltage source t V v

m

ω sin = .

Derive an expression for the instantaneous torque as a function of

the angular position θ. Find there from the time-average torque.

Evaluate for θ = 30°, t v 314 sin 100 = . (8)

(ii) If coil 2 is shorted while coil 1 carries a current of t I i

m

ω sin 1

= ,

derive expressions for the instantaneous and time-average torques.

Compute the value of the time-average torque when θ = 45° and

t i 314 sin 2 1

= . (8)

14. (a) (i) A 3-phase, 50 Hz. star-connected alternator with 2-layer winding is

running at 600 rpm. It has 12 turns/coil, 4 slots/pole/phase and a

coil-pitch of 10 slots. If the flux/pole is 0.035 Wb sinusoidally

distributed, find the phase and line emf’s induced. Assume that the

total turns/phase are series connected. (8)

(ii) A 4-pole, lap-wound dc machine has 728 armature conductors. Its

field winding is excited from a dc source to create an air-gap flux of

32 mWb/pole. The machine (generator) is run from a prime mover

(diesel engine) at 1600 rpm. It supplies a current of 100 A to an

electric load.

(1) Calculate the electromagnetic power developed. (4)

(2) What is the mechanical power that is fed from the primemover

to the generator? (2)

(3) What is the torque provided by the prime mover? (2)

Or

(b) (i) A 3-phase, 50 kW, 4-pole, 50 Hz induction motor has a winding (ac)

designed for delta connection. The winding has 24 conductors per

slot arranged in 60 slots. The rms value of the line current is 48 A.

Find the fundamental of the mmf wave of phase-A when the current

is passing through its maximum value. What is the speed and peak

value of the resultant mmf/pole? (12)

(ii) A 4-pole synchronous generator driven at 1500 rpm feeds a 6-pole

induction motor which is loaded to run at a slip of 5%. What is the

motor speed? (4)

15. (a) (i) A 220 V dc generator supplies 4 kW at a terminal voltage of 220 V.

the armature resistance being 0.4 . If the machine is now

operated as a motor at the same terminal voltage with the same

armature current, calculate the ratio of generator speed to motor

speed. Assume that the flux/pole is made to increase by 10% as the

operation is changed over from generator to motor. (6)

(ii) A 220 V, 7.5 kW series motor is mechanically coupled to a fan.

When running at 400 rpm the motor draws 30 A from the mains

(220 V). The torque required by the fan is proportional to the square

of speed. Ra = 0.6 , Rse = 0.4 . Neglect armature reaction and

rotational loss. Also assume the magnetization characteristic of the

motor to be linear.

(1) Determine the power delivered to the fan and torque

developed by the motor. (5)

(2) Calculate the external resistance to be added in series to the

armature circuit to reduce the fan speed to 200 rpm. (5)

Or

(b) A 250-V dc shunt motor has Rf = 150 and Ra = 0.6 . The motor

operates on no-load with a full field flux at its base speed of 1000 rpm

with Ia = 5 A. If the machine drives a load requiring a torque of 100 Nm,

calculate armature current and speed of motor. If the motor is required to

develop 10 kW at 1200 rpm what is the required value of the external

series resistance in the field circuit? Assume linear magnetization.

Neglect saturation and armature reaction. (16)

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