Friday, September 25, 2015

DC Machine Multiple Choice Questions with Solutions: Part-12

DC Motor DC Generator Multiple Choice Questions with Solutions:

[1] Which of the following statements is false?
(a) A DC motor converts electrical energy to mechanical energy
(b) The efficiency of a DC motor is the ratio input power to output power
(c) A DC generator converts mechanical power to electrical power
(d) The efficiency of a DC generator is the ratio output power to input power
Options:
1. C only
2. C and D only
3. B only
4. None of the above



[2] If the speed of a DC machine is doubled and the flux remains constant, the generated e.m.f.
(a) remains the same
(b) is doubled
(c) is halved
(d) None of the above



[3] If the flux per pole of a shunt-wound DC generator is increased, and all other variables are kept the same, the speed
(a) decreases
(b) stays the same
(c) increases
(d) None of the above



[4] Which of the following statements is false?
(a) A commutator is necessary as part of a DC motor to keep the armature rotating in the same direction
(b) A commutator is necessary as part of a DC generator to produce unidirectional voltage at the terminals of the generator
(c) The field winding of a DC machine is housed in slots on the armature
(d) The brushes of a DC machine are usually made of carbon and do not rotate with the armature
Options:
1. D only
2. C and D only
3. None of the above
4. C only



[5] If the flux per pole of a shunt-wound DC generator is halved, the generated e.m.f. at constant speed
(a) is doubled
(b) is halved
(c) remains the same
(d) None of the above



[6] In a series-wound generator running at constant speed, as the load current increases, the terminal voltage
(a) increases
(b) decreases
(c) stays the same
(d) None of the above



[7] Which of the following statements is false for a series-wound DC motor?
(a) The speed decreases with increase of resistance in the armature circuit
(b) The speed increases as the flux decreases
(c) The speed can be controlled by a diverter
(d) The speed can be controlled by a shunt field regulator



[8] The armature resistance of a DC motor is 0.5 , the supply voltage is 200V and the back e.m.f. is 196V at full speed. The armature current is:
(a) 4A
(b) 8A
(c) 400A
(d) 392A



[9] In DC generators iron losses are made up of:
(a) hysteresis and friction losses
(b) hysteresis, eddy current and brush contact losses
(c) hysteresis and eddy current losses
(d) hysteresis, eddy current and copper losses



[10] The effect of inserting a resistance in series with the field winding of a shunt motor is to:
(a) increase the magnetic field
(b) increase the speed of the motor
(c) decrease the armature current
(d) reduce the speed of the motor



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1:07 AM