281.  The series parallel system of speed control of D.C. series motors widely used in traction work gives a speed range of about
A. 1 :10
B. 1: 8
C. 1 :6
D. 1: 4

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282.  If conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
A. less than 50
B. 60 to 70%
C. 80 to 90%
D.100%

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283.  In which of the following tests only one motor is required?
A. Brake test
B. Hopkinson's test
C. Field's test
D. Swinburne's test

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284.  In Field's test for series motors one motor drives the other machine as
A. cumulative compound generator
B. differential compound generator
C. separately excited generator
D. series generator

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285.  A motor for punching machines is usually subjected to
A. no load
B. continuous part load
C. continuous full load
D. intermittent load

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286.  Which of the following is not necessarily the advantage of D.C. motors over A.C. motors?
A. Better speed control
B. Low cost
C. High starting torque
D. Wide speed range

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287.  The armature shaft of a D.C. motor must be able to withstand
A. any unbalanced magnetic pull on the armature core
B. twisting strains due to transmission of torque
C. bending moment due to the weight of the armature and commutator
D. all of the above

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