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
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?
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.
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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