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DC Machines

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Lec 01: Introduction of DC machines

Description

• Basics of electromechanical energy conversions, principles of Generators and Motors. Works on the “faradays laws”. are 2 types i.e. DC Generators & DC motors.

• E.M.F equation of Generator, types of Generators and their current equations, armature reaction and commutation & the characteristics of the generators are studied.

• Various losses of electrical machines, efficiencies and different types of speed control methods are also studied in this course.

Prerequisite

Better to have knowledge on electro mechanical energy conversion, faraday’s law & lenz’s law.

For whom:

• For Electrical and Electronics students to help them understand the basic to advanced concepts

• Students who are preparing them for Engineering syllabus

REFERENCE BOOKS:

1. Electrical Machines, P.S. Bimbra, Khanna Publishers.

2. Principles of Electrical Machines, V. K. Mehta, Rohit Mehta, S. Chand publishing.

3. Electric Machines, Mulukutla S. Sarma, Mukesh K. Pathak, Cengage Learning.

4. Electric Machines by I.J. Nagrath & D.P. Kothari, Tata Mc Graw – Hill Publishers.

5. Fundamentals of Electric Machines, B. R. Gupta, Vandana Singhal, New Age International Publishers.

6. Electrical Machines, M. V. Deshpande, PHI Learning Private Limited.

7. Electrical Machines, R. K. Srivastava, Cengage Learning.

Q & A
Exam Questions

Q1 . a) Explain the effect of armature reaction.
b) A 4 - pole generator supplies a current of 143A. It has 492 armature conductors (a) wave wound (b) lap - wound. When delivering full load, the brushes are given an actual leadoff 100. Calculate the d
Q2 . a) Explain the methods of Improving Commutation.
b) A 500 - kW, 500V, 10 pole DC generator has a lap-wound armature with 800 conductors. Calculate the number of pole - face conductors in each pole of a compensating winding if the pole face covers
Q3 . Draw and explain the external characteristics of
(a) shunt generator
(b) Series generator
(c) Compound generator
Q4 . The following information is given for a 300 - kW, 600 - V, long shunt compound generator: shunt field resistance = 75Ω, armature resistance including brush resistance = 0.03Ω, communicating field winding resistance = 0.011Ω, series field resista
Q5 . Explain speed current and speed torque characteristics of
(i) DC shunt
(ii) DC series
(iii) DC compound motors
Q6 . A 2 - pole DC shunt motor operating from a 200V supply take a full load current of 35A, the no-load current being 2 A. The field resistance is 500Ω and the armature has a resistance of 0.6Ω. Calculate the efficiency of the motor on full load. Tak
Q7 . A 230V series motor is taking 50A. Resistance of armature and series field windings is 0.2Ω and 0.1Ω respectively. Calculate
(a) Brush voltage
(b) Back e.m.f
(c) Power wasted in armature
(d) Mechanical power developed
Q8 . a) Enumerate different losses occur in DC machine and explain.
b) Draw the power stages of DC motor and dc generator.

1) Electric traction
2) Mixies
3) Hoists
4) Drilling machines

The current obtained will be much higher than the rated current.The effiency of operation will be below 50%.

% Speed regulation= (NL speed- FL speed) / FL speed x 100

The high contact resistance carbon brushes help the current in the coil undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.

The self-induced emf in the coil undergoing commutation which opposes the reversal of current is known as reactance e.m.f..

The changes that take place in winding elements during the period of short circuit by a brush is called commutation.

As the compensation required is proportional to the armature current the compensating winding is excited by the armature current.

Self-excited generator are those whose field magnets are energized by the current produced by the generator themselves. • Separately excited generator are those whose field magnets are energized from an independent external source of dc current.

E = (ФNZ/60)(P/A)V.
P ---> No. of Poles
Z ---> Total no. of conductor
Ф ---> Flux per pole
N ---> Speed in rpm.

In lap winding, due to the difference in the induced emf in various parallel paths, there may be circulating currents in brushes and winding. The connections that are made to equalize the difference in induced emf and to avoid circulating currents through brushes are called equalizer connections.

It is the distance measured in terms of armature conductors away from commutator.

The magnetic flux retained in the poles of the machine even without field supplies called the residual flux.

The speed for which the given field winding resistance act as critical resistance is called as critical speed.

a) Residual magnetism may not be there in the poles
b) Direction of rotation may be wrong
c) The field resistance may be more than critical resistance
d) There may be disconnection in the field winding
e) Brush contact may be poor
f) The field coils may be connected with the armature to oppose the emf due to residual magnetism

1. Provide Extra De- magnetizing AT on field windings
2. Using compensating winding

These are small poles fixed to yoke in between main poles & its winding is connected in series with armature winding.

Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemical etc. to protect the motors against these elements the motor frames are used partially closed or totally closed or flame proof.

T = PIZ Ф / 2πA
Number of Poles (P)
Number of parallel paths (A)
Flux (Ф)
Number of conductors (Z)
Armature current (Ia)

DC series motor is used for traction purpose because its starting torque is high.

1. Field control method
2. Armature control method
3. Ward Leonard method

The absence of back emf at the time at starting causes the armature current to shoot up to about 20 times the normal current, if no limiting resistance is included.

1. Brake test
2. Swinburne’s test
3. Field test

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Course Feature

Category Electrical
Language English
Price 89.00
Validity 1 Year
Q & A 22
Exam Questions 8
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Electrical

DC Machines

Lec 01: Introduction of DC machines

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1. Introduction & Working

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DC Machines

Lec 01: Introduction of DC machines

1. Introduction & Working

2. Construction of DC Machines

3. Types of DC Machines

4. Armature Reaction

5. Commutation

6. Types of Speed Drives

Q1 . Name any four applications of DC series motor.

Q2 . Why DC motors are not operated to develop maximum power in practice?

Q3 . Define speed regulation.

Q4 . Why are carbon brushes preferred for DC machines?

Q5 . What is reactance e.m.f in DC machine?

Q6 . Define the term commutation in DC machines.

Q7 . How and why the compensating winding in DC machine excited?

Q8 . What is meant by self excited and separately excited DC generator?

Q9 . Write down the emf equation for DC generator?

Q10 . What is meant by equalizer connections?

Q11 . What is Back Pitch?

Q12 . What do you mean by residual flux in DC generator?

Q13 . What is critical speed of a DC shunt generator?

Q14 . A DC generator fails to self excite. List the cause for the failure.

Q15 . What are the methods to eliminate armature reaction effect?

Q16 . What are Interpoles?

Q17 . How does DC motor differ from DC generator in construction?

Q18 . What are the factors affecting torque?

Q19 . Which type of motor is used for traction? why?

Q20 . What are the various methods of controlling speed?

Q21 . Why is the starting current high in a DC motor?

Q22 . What are the different types of tests done on DC motor?

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