Thursday, September 10, 2020

Chapter 6 : DC Machines-Page 9

Chapter 6 : DC Machines

81. What happens when the supply terminals of a d.c. shunt motor are interchanged?
A. the motor will stop
B. the motor will run at its normal speed in the same direction as before
C. the direction of rotation will reverse
D. the motor will run much faster in the same direction

... Answer is (B)



82. Difference between the no load and full load speeds of a de shunt motor is of the order of

... Answer is (A)


83. In a dc motor, the effect of flux due to armature reaction on the main flux is to make
A. unequal flux density distribution across both sections
B. flux direction deviate from radial in the air gap
C. both (a) and (b) above
D. none of these

... Answer is (C)



84. Two dc series motors connected in series draw current 1 from supply and run at speed N. When the sae two motors are connected in parallel taking current I from the supply, the speed of each motor will be
B. 2N

  ... Answer is (C)


85. The torque-speed characteristic of a series motor is
A. parabola
B. rectangular hyperbola
C. linearly rising
D. linearly declining
Answer is (B)


86. The starter for a dc motor also provides protection to the motor against damage
A. due to short circuit in the equipment
B. from long term overload
C. from excessive starting currents
D. all of these

  ... Answer is (B)



87. Starting resistor connected in
A. series with the field winding
B. parallel with the field winding
C. parallel with the armature
D. series with the armature in a dc motor

  ... Answer is (D)


88. Direct online starters are not suitable for starting large dc motors because
A. the starting torque becomes very low
B. large voltage drop may occur in the supply mains
C. the moto may not start
D. the motor may run away

... Answer is (B)


89. In Ward Leonard method of speed control, the minimum number of machines needed is

A. one
B. two
C. three
D. four
  ... Answer is (C)


90. Ward Leonard method of speed control is basically useful where usually
A. a small range of very sensitive speed control is required
B. a wide range of less sensitive speed control is required
C. a wide range of sensitive speed control is required
D. a narrow range of less sensitive speed control is required

  ... Answer is (C)



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