College : Vardhaman College Of Engineering
Degree : B.Tech
Department :Electrical & Electronics Engineering
Semester : III
Subject :DC Machines
Document type : Question Paper
Website : vardhaman.org
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Vardhaman Dc Machines Question Paper
B. Tech III Semester Regular Examinations, December – 2013
(Electrical and Electronics Engineering)
Date : 4 December, 2013
Time : 3 Hours
Max. Marks : 75
Related : Vardhaman College Of Engineering Elements Of Electrical Engineering B.Tech Question Paper : www.pdfquestion.in/6308.html
Answer ONE question from each Unit
All Questions Carry Equal Marks
All parts of the question must be answered in one place only
Unit – 1
1. a) By means of block diagram represent the flow of energy during the conversion of electrical energy into mechanical energy. Also explain what is meant by coupling field.7M
b) The relationship between mmf ? , flux ? and the position coordinate x is given by ? ??2×2 . Find the mechanical force f acting in the x direction using
i. Energy of the system
ii. Co-energy concept 8M
2. a) Give the examples of single excited and doubly excited electromechanical energy conversion devices. 5M
b) Two coils have self and mutual inductances of If the current I1 is maintained constant at 5A and I2 at -2A, find the mechanical work done when x increases from 0 to 0.5m. What is the direction of force developed? 10M
Unit – 2
3. a) Define Armature Reactions. The effect of armature reaction is to decrease the flux under one pole tip and to increase it under the other pole tip. Explain this with respect to a dc generator by using the suitable diagram. 7M
b) Develop from first principle, EMF equations for a dc. machine. 4M
c) Four terminals of a dc shunt machine are available, but these are unmarked. How would you identify the field and armature terminals? 4M
4. a) The terminal voltage of a 8 pole dc. shunt generator with 780 wave connected armature conductors and running at 500 rpm is 240 V. The armature resistance is 0.24O and the field resistance is 240 O. Find the armature current, the induced emf and the flux per pole if load resistance is 12 O. 7M
b) What is commutation? Explain the process of commutation in a dc. machine. 8M
Unit – 3
5. a) Explain the external (V/I) characteristics for a shunt, series generator and compare them. 8M
b) A 6-polewave connected dc machine has 300 armature conductors and each conductoris capable of carrying 80A without excessive temperature rise. The flux per pole is 0.015 Wb and the machine is driven at 1800 rpm. Calculate the total current, emf, power developed in the armature. 7M
6. a) A 4-pole compound generator has armature, series field and shunt field resistances of 1O, 0.5O and 100O respectively. This generator delivers 4kW at a terminal voltage of 200 V. Allowing 1V per brush for contact drop, calculate for both short-shunt and long-shunt connections
i. The generated emf and
ii. The flux per pole if the armature has 200 lap-connected conductors and is driven at 750rpm. 8M
b) Explain the voltage build up process in dc shunt generators. It is found that the voltage of a dc shunt generator does not build up. Explain the various possible causes of this failure. 7M
Unit – 4
7.a) The armature of a 6 pole lap wound dc shunt motor takes 400 A at a speed of 350 rpm. The flux per pole is 80 m Weber’s, the number of turns is 600 and 3% of torque is lost in friction and iron losses. Calculate the brake horse power, back emf and supply voltage if armature resistance is 0.1 O. 8M
b) Draw a neat labeled diagram of a three point starter used for starting of shunt motors. 7M
8. a) Explain the significance of back emf in a dc machine. 5M
b) A 4 pole dc motor runs at 600 rpm on full load taking 25 A at 450 V. The armature is lap wound with 500 conductors and flux per pole is expressed by relation F = 1.7 x 10-2 x I0.5 Weber’s where I is the motor current. If supply voltage and torque are both halved, calculate the speed at which the motor will run. Ignore stray losses. 10M
Unit – 5
9. a) Derive the efficiency of dc machine as motor and generator using Swinburne’s test. 8M
b) A full load brake test on a small dc shunt motor, gave the following data: Spring balance readings 25kg and 9 kg Outside pulley diameter 19.5 cm Motor speed 1500 rpm Applied voltage 230V Line current 12.5A Calculate the shaft torque, shaft power and the motor efficiency at rated load. 7M
10. a) Explain the various losses in dc machine and draw the power flow diagrams for long shunt
i. dc compound generator
ii. dc compound motor 10M
b) A 200 V dc shunt motor delivers an output of 17 kW with an input of 20 kW. The field winding and armature circuit resistances are 50O and 0.04O respectively. Calculate the value of total armature ohmic losses at maximum efficiency. 5M