Download MEEPP 202 Electric Drives

Reg. No: …..………………….
Name: …….………………...
M.TECH. DEGREE EXAMINATION
Model Question Paper - II
Second Semester
Branch: Electrical and Electronics Engineering
Specialization: Power Electronics and Power Systems
MEEPP 202 ELECTRIC DRIVES
[Regular/Supplementary - 2011 Admission onwards ]
Time: Three Hours
Maximum: 100 Marks
Answer all questions
All questions carry equal marks
1. (a) Explain the steady state stability of a Motor-Load System with necessary equations and
graphs.
(15 marks)
(b) Half hour rating of a motor is 100kW.Heating time constant is 80min and the maximum
efficiency occurs at 70% of full load. Determine the continuous rating of the motor.
(10 marks)
OR
2. (a) A drive has following parameters :J=10kg-m2,T=(100-0.1N)Nm Passive load torque,
Tl=(0.05N)Nm , where N is the speed in rpm. Initially the drive is operating in steady
state . When it is reversed, motor characteristic is changed to T=(-100-0.1N) Nm.
Calculate the time of reversal.
(10 marks)
(b) Derive the Moment of inertia of a flywheel to prove the load equalization.
(15 marks)
3. (a) Explain with neat circuit diagram and waveforms, a three phase fully controlled dc motor
drive. Get the expression for the average output voltage and speed.
(10 marks)
(b) The magnetization characteristics of a dc series motor when running at 500 rpm is given
by
Current(A)
Emf(V)
20
215
30
310
40
381
50
437
60
485
70
519
80
550
Total resistance of the armature and field windings is 0.5 ohm. When connected for
dynamic braking against an overhauling load of 500 Nm ,motor speed is to be
maintained at 600 rpm. What resistance must be connected across the motor terminals?
(15 marks)
OR
4. (a) A 220 V, 960 rpm, 12.8A separately excited dc motor has armature circuit resistance and
inductance of 2Ω and 150mH respectively. It is fed from a single phase half controlled
rectifier with an ac source voltage of 230V,50Hz.Calculate i) Motor torque for α=60˚ and
speed=600rpm.ii) Motor speed for α=60˚ and T=20 Nm.
(15 marks)
(b) Explain with neat circuit diagram and waveforms, a single phase half controlled dc motor
drive. Get the expression for the average output voltage and speed. Draw the speedtorque characteristics.
(10 marks)
5. (a) Analyze and compare the operation of an induction motor with balanced and unbalanced
source voltages.
(10 marks)
(b) A 440V,50Hz,6 pole,950 rpm,Y-connected induction motor has the following parameters
referred to the stator : Rs = 0.5Ω, Rr’ =0.4Ω, Xs=Xr’=1.2Ω ,Xm= 50Ω .Motor is driving a
fan load, the torque of which is given by TL=0.0123ωm2 .Now one phase of the motor
fails. Calculate motor speed and current. Will it be safe to allow the motor to run for a
long period?
(10 marks)
(c) Derive the torque equation of a salient pole synchronous machine and draw the speed –
torque characteristics.
(5 marks)
OR
6. (a) Explain the slip speed control of Induction motor with relevant equations.
(10
marks)
(b) The wound rotor motor is rated at 30kW,1170r/min,460V,60Hz.the open circuit voltage is
400V,the load resistance is 0.5 Ω. If the chopper frequency is 200Hz, calculate the time
Ton so that the motor develops a torque of 200Nm at 900r/min.
(10 marks)
(c)
With neat waveforms and circuit diagrams, explain electronic commutation of
synchronous motor drive.
(5 marks)
7. (a)
Explain the closed loop speed control of load commutated synchronous motor drive.
(10 marks)
(b) Derive the no load speed equation of static Kramer drive.
(15 marks)
OR
8. (a) A 3-Ø SCIM drives a fan type load. No load rotational losses are negligible. Show that
rotor current is maximum when motor runs at a slip s=1/3.Find also an expression for
maximum current.
(10 marks)
(b) If 3-Ø SCIM runs at speed of (i)1450 rpm and (ii)1300 rpm, determine the maximum
current in terms of rated current at these speeds. The IM drives a fan and no-load
rotational losses are neglected.
(15 marks)
***