 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 Dynamics of DC machines

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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

Describe by sketches.

Simple lap & wave windings, their characteristics, winding calculations

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: 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.

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

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 of shunt and compound wound generators

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