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D.C.MACHINES

 Introduction

 Constructional features & types of DC machines

 Windings

 Emf, torque and speed equations

 Commutation & armature reaction

Characteristics of series, shunt, and compound wound generators & motors

 Starting & speed control

 Parallel operation

 Testing & efficiency

 

Dynamics of DC machines

 

 

 Objective questions

 

HOME takes you to the start page after you have read these Topics. Start page has links to other topics.

 

Introduction

 

Electricity, as is well known, is the most flexible form of energy that can be transported with ease from one place to another for ultimate consumption by its user. In this scenario electrical machines & transformers play a vital role whose importance cannot be overemphasized. Electrical machines are energy converting links between electrical & mechanical networks. Therefore working principles & characteristics of basic machines must be well understood by electrical engineers. In case of DC machines, the following topics must be covered:

Provision of self and separate e excitation

Ability of the same machine to work as a generator and a motor.

Brief reference to the field of application of DC machines

 

Constructional features & types of DC machines

 

Describe by sketches.

Windings

 

Simple lap & wave windings, their characteristics, winding calculations

 

Emf and torque equations

 

Emf equation, open circuit characteristics, voltage build -up process for generator, failure of shunt generators to build up, critical resistance.

Torque equation; torque speed characteristics of shunt, series & compound wound motors

 

Commutation & armature reaction

 

Commutation: retarded, accelerated and linear commutation. Methods of improving commutation

Armature reaction: shifting of magnetic neutral plane, effect of brush shift, calculation of demagnetizing & cross magnetizing ampere- turns, Compensating winding, Effect of armature reaction on external characteristics.

 

Characteristics of series, shunt, and compound wound generators & motors

 

Load characteristics of shunt generators (external & internal), their pre-determination from given open-circuit characteristics and armature resistance drops

Starting & speed control

 

Starting of DC motors, starters

Basic principles of sped control of a DC machine, flux and armature voltage control, Speed control of shunt, series & separately excited motors

 

Parallel operation

 

Parallel operation of shunt and compound wound generators

 

Testing & efficiency

 

Losses in DC machines, efficiency calculation from measurement of losses (Swineburne's method, Hopkinson Test, Field's Test

 

 

Dynamics of DC machines(moduleh.xls

 

This Excel spreadsheet module uses a separately excited DC motor. The armature inductance is included in the equivalent circuit. The student can study the response of the motor armature current & speed with respect to time for a sudden change in load or applied voltage. The student can vary the parameters like damping (D), armature inductance (L), moment of inertia (J), armature resistance Ra, applied voltage (V), and the load torque (TL). The student observes the effect of parameter variations on the steady -state & transient response of the system. The response can be obtained either by the Laplace Transform method or by the Runge-Kutta routine.

 

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Objective questions

 

  1. The speed of a d. c series motor at no load is

 

a)       Zero

b)       Infinity

c)       1500 r. p. m

 

Ans. b

 

  1. A sinusoidal voltage of frequency 1 Hz is applied to the field of a d. c generator. The armature voltage will be

 

a)       1 Hz square wave

b)        1 Hz sin e wave

c)        d. c voltage

 

Ans. b

 

  1. The function of the commutator in a d. c machine is

 

a)       to change ac to dc

b)        to change alternating voltage to direct voltage

c)        for easy control of voltage

 

Ans. b

 

  1. The direction of rotation of  a dc series motor  can be reversed

 

a)       by  interchanging  the supply terminals

b)       by  interchanging  the field terminals

c)       either a) or b).

 

Ans. b

 

  1. A dc shunt generator  driven at normal speed  in the normal direction fails to build up armature voltage because

 

a)       the armature resistance is high

b)       the field current is not sufficiently high

c)       there is no residual magnetism.

 

Ans. c

 

  1. If  a dc motor is connected across an ac supply , the dc motor will

 

a)       burn as the eddy currents in the field produce heat

b)       run at its  normal speed

c)       run at a lower speed

 

Ans. a

 

  1. What would happen if the  field of  dc shunt motors is opened?

 

a)       the speed of the motor will be reduced

b)       it will run at its normal speed

c)       the speed will be enormously high , and the motor will destroy itself.

 

Ans. c

 

  1. Which one of the following motors is used in a  locomotive?

 

a)       Induction motor

b)       dc series motor

c)       Synchronous motor

 

Ans.   b

 

9.  The efficiency of   a dc shunt generator has  a maximum  value when

 

a)       magnetic losses are equal to mechanical losses

b)        armature copper loss is equal to the constant loss

c)        field copper loss is equal to the constant loss.

 

Ans. b

 

10.     The dummy coil in the dc machine  is used to

 

d)       eliminate armature reaction

e)       bring about mechanical balance of  armature

f)        eliminate reactance voltage

 

Ans. b

 

                     11. The sparking at the brushes in  a   dc generator is attributed to

 

                      a) quick reversal of current  in the coil under commutation

                      b) armature reaction

                      c) reactance voltage

 

Ans. . c

 

                 12.  Interpoles   in the armature of   a dc generator are used to

 

a)       neutralize  the reactance voltage

b)        neutralize the reactance voltage  and the cross-magnetizing effect of the armature reaction

c)       neutralize the cross-magnetizing effect of the armature reaction

 

Ans. b

 

13.     The mechanical power  develop by the dc motor  is maximum when

 

a)       back emf = applied voltage

b)       back emf = 0

c)       back emf = 0.5  times the applied voltage.

 

Ans. c

 

14.     In series –parallel control method when two dc series motors  are connected in  series , the speed of the set is

 

(a)     half of the speed  of he motors when connected in parallel

(b)      ¼ th of the speed of the motors  when connected in parallel

(c)     same as  in parallel.

 

Ans. b

 

15.     The armature torque  of the  dc shunt motor  is proportional to

 

                                                                           i.      armature current only

                                                                          ii.      field flux only

                                                                        iii.      armature current and flux  both

 

Ans. c

 

16.     The reversal of speed of motors  used as electric braking  is known as

 

                                                                           i.      regenerative braking

                                                                          ii.      plugging

                                                                        iii.      dynamic braking.

 

Ans. . b

 

17.     The method used  for braking in rolling mills is

 

                                                                           i.      plugging

                                                                          ii.      regenerative braking

                                                                        iii.      dynamic braking

 

 Ans. a