DC MOTOR
59. To what polarity the
interpoles excited in dc motors?
For motor
operation the polarity of the interpoles must be that of the previous main pole
along the direction of rotation.
60. Name any four applications of
DC series motor. Electric traction
Mixies
Hoists
Drilling
machines
61.Why DC motors are not operated
to develop maximum power in practice?
The
current obtained will be much higher than the rated current.The effiency of
operation will be below 50%.
62.Name
the starters used for series motors.
Face
plate type.
Drum type
controller.
63. Name Different types of
starters.
1.
Three point starter
2.
Four point starter.
64.
Name the
Protective devices in a starter.
1, No volt release
2.
Overload Release.
65.
Draw
torque characteristics of shunt motor.
66.
What are
the modification in ward Leonard linger system?
1.
Smaller motor and generator set
2.
Addition of flywheel whose function is to reduce
fluctuations in the power demand from the supply circuit.
67.
What type
of DC motors are suitable for various torque operations?
1.
DC series motor
2.
DC cumulatively compound motor
. Define
speed regulation.
% Speed
regulation= NL speed- FL speed x 100 FL speed
68.
What are
the performance curves? Output Vs torque
Output Vs
current Output Vs speed Output Vs efficiency
69.
To what
polarity are the interpoles excited in dc generators?
The polarity
of the interpoles must be that of the next main pole along the direction of
rotation in the case of generator.
70. Why are carbon brushes
preferred for dc machines?
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.
71.
What are
the various types of commutation?
Linear commutation
Sinusoidal
commutation
72.
Name the
two methods of improving commutation.
(i) Emf
commutation. (ii)Resistance commutation
73. What is reactance emf in dc
machine?
The
self-induced emf in the coil undergoing commutation which opposes the reversal
of current is known as reactance emf.
74. Define the term commutation
in dc machines.
The
changes that take place in winding elements during the period of short circuit
by a brush is called commutation.
75. How and why the compensating
winding in dc machine excited?
As the
compensation required is proportional to the armature current the compensating
winding is excited by the armature current.
PROBLEMS
1.
A 10KW,240V dc shunt motor draws a line current of
5.2 amps while running at no load of 1200rpm from a 240V dc supply. It has an
armature resistance of 0.25 ohms and field resistance of 160 ohms . Estimate
the efficiency of motor when it delivers rated load.
2.
In a brake test the efficiency load on the branch
pulley was 40Kg, the effective diameter of the pulley 73.5 cm and speed 15 rps.
The motor takes 60A at 230V. Calculate the output power and efficiency at this
load.
Ans : η=
94.83%
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