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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.
c) 1500 r. p. m
a) 1 Hz square wave
b) 1 Hz sin e wave
c) d. c voltage
a) to change ac to dc
b) to change alternating voltage to direct voltage
c) for easy control of voltage
a) by interchanging the supply terminals
b) by interchanging the field terminals
c) either a) or b).
a) the armature resistance is high
b) the field current is not sufficiently high
c) there is no residual magnetism.
a) burn as the eddy currents in the field produce heat
b) run at its normal speed
c) run at a lower speed
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.
a) Induction motor
b) dc series motor
c) Synchronous motor
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.
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
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
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.
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.
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
16. The reversal of speed of motors used as electric braking is known as
i. regenerative braking
iii. dynamic braking.
Ans. . b
17. The method used for braking in rolling mills is
ii. regenerative braking
iii. dynamic braking