API ABDUL KALAM TECHNOLOGICAL UNIVERSITY
FIRST SEMESTER M.TECH DEGREE EXAMINATION, DECEMBER 2015

Electrical & Electronics Engineering
Streams: Electrical Machines, Power Systems, Power Control & Drives, Power system and

Control

OIEE6301: MODELLING OF ELECTRICAL MACHINES
Time: 3 hours Max marks: 60
Answer any two full questions from each part.

PART-A (Module | and ||)

1 a. Differentiate between transformer and speed voltages. (3) 0. A coil moves ina time
varying magnetic field whose spatial distribution along (6) the air gap periphery is
sinusoidal. Find an expression for voltage induced in the coil identifying the terms

therein.

2 ௨. Explain the reason for using transformation in electrical machines. (3)
0, Obtain identical transformations for currents and voltages from a rotating 116)
balanced 3 phase (a,b,c) winding to a rotating 2 phase (a, &) winding. Show that

power invariance is maintained in the transformation.

3 9. Explain the term power invariance applied to electrical machines (3) 0. Substantiate

the advantages of using per unit system with examples. (6)

Part B (Module ॥| and IV)

4 a.Derive the expression for motional inductance of a dc machine (3) b. A250 V dc series
motor running at 150 rad/sec takes 20 amperes from the (6) supply mains. The armature
and field resistances are ra + rt = 1.00. TotalJ=

5.4 kg.m ° 910 0; 0.02 Nm,sec/rad. Calculate the rotational mutual inductance
and the load torque. If the supply voltage is suddenly reduced to 220۷ withthe

load torque remaining constant, find the speed as a function of time.

5 a. Write the impedance matrix for a 3 phase salient pote synchronous machine. (6)
Hence obtain an expression for instantaneous electromagnetic torque and identify the

various terms in it.