11-06-2013, 02:09 PM
SEMESTER EXAMINATIONS FOR DYNAMICS OF ELECTRICAL MACHINES
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Answer any five questions
All questions carry equal marks
1.a) Explain the principle of operation of Inductor motor using an electromechanical analogy.
b) Draw the equivalent circuit of an induction motor with reference to its stator.
2. Discuss in detail about each of the following: a) Lagrange’s equation for two mutually coupled coils, b) Induction machine dynamics during normal operation.
3. Obtain a) Transient model of a separately excited DC generator. b) Steady state analysis of a separately excited DC motor
4.a) What are the state variables and input variables in the linear dynamic representation of a separately excited DC motor?
b) A 240-V, 12-hp, separately excited dc motor operating on a load of 15 N-m in the linear region of its magnetization characteristic has the following parameters.
Ra =0.28Ω, La = 2.81 mH, Rf = 320 Ω, Lf = 2 H, J = 0.087 kgm2, D = 0.02 N-ms, and KT = 1.03. Determine the variation of the motor speed, armature cur-rent, and field current as a function of time when the field voltage is suddenly reduced from 240 V to 192 V a t = 0.
5.a) Explain Ward-Leonard system of speed control.
b) A separately excited DC generator running at 4500/π rpm has the following parameters:
Rf =80, lf=40H, Ra=0.1, la= 0.3mH, motional inductance Md=0.8H, Kg=120V/field ampere
i. The field is unexcited and armature is open. Find the armature voltage as a function of time and sketch it, if a constant voltage of 160V is suddenly impressed across the field terminals?
ii. Sketch the rise of armature current in part (i) if armature terminals are initially short circuited?
6.a) Derive the power expression of separately excited DC motors.
b) What is the procedure to be adopted for interconnection of DC machines? Explain any.
7.a) Derive the maximum torque expression during dynamic braking of Induction Machine.
b) Write the equations for dynamical response of Induction Motor and explain.
8.a) Explain what is meant by small oscillations and how they are encountered.
b) When we have to compute swing curves and why?