This set of DC Machines Questions and Answers for Aptitude test focuses on “Speed Control Using Armature Control in DC Motor”.
1. For which speed control method we get minimum efficiency?
a) Voltage control method
b) Field control method
c) Armature control method
d) Cannot be determined
Explanation: In armature control method, we vary armature voltage by adjusting variable resistance value. So, when we define armature efficiency, it is equal to η = [ (Vt-IaRe) Ia] / VtIa= 1-IaRe / Vt. Thus, efficiency is reduced drastically for large speed reductions.
2. The speed of a DC motor can be varied by changing ______________
a) Field current
b) Applied voltage
c) Resistance in series with armature
d) Field current, applied voltage or resistance in series with armature any method will work
Explanation: In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.
3. For constant torque drive which of the following speed control method is preferred?
a) Field control
b) Armature voltage control
c) Shunt armature control
d) Voltage control
Explanation: Armature control method provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value full motor torque can be obtained at full-load armature current at all speeds.
4. When the armature of a DC motor rotates, emf induced is in machine is called as ___________
a) Self-induced emf
b) Mutually induced emf
c) Back emf
d) Cannot be determined
Explanation: According to terminology used in DC machines, emf induced in a machine when armature rotates is called as back emf. This back emf value is directly proportional to the speed of the motor.
5. Which of the following method is used for DC motor with 12+ HP requiring frequent start, stop, speed reversal?
a) Drum type controller is used
b) Three-point starter is used
c) Four-point starter is used
d) Cannot be determined
Explanation: Drum controllers are used when an operator is controlling the motor directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane motors, elevators, machine tools, and other heavy applications.
6. Flux density distribution is distorted by armature control method.
Explanation: Unlike field control method, the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited. Unlike field control scheme, speed reversal can be easily implemented here.
7. In rheostatic series control method of armature we add ____________
a) Variable resistor in parallel with armature
b) Variable resistor in series with armature
c) Fixed resistor in parallel with armature
d) Fixed resistor in series with armature
Explanation: In rheostatic series control method, we add variable resistance in series with the armature so that armature voltage can be varied, by varying the voltage drop at series resistor. By increasing value of series resistor speed can be decreased.
8. By series armature resistance method, we can get _________________________
a) Speed above rated speed
b) Speed equal to rated speed
c) Speed below rated speed
d) All speeds are possible
Explanation: The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.
9. Consider the armature control method, where 10 Ω resistance is connected in series with armature in first case. For second case resistance value is changed 100 Ω. Then ____________
a) Speed-torque characteristic of both will start from same point
b) For first case speed-torque characteristic will start below to the case 2
c) For first case speed-torque characteristic will start above to the case 2
d) Can’t say because other parameters are unavailable
Explanation: Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance value will lie below the other.
10. Speed regulation of armature series control method is __________
a) Very good
d) Cannot comment
Explanation: The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors (fractional kW) or for short-time, intermittent show-downs for medium-sized motors.
11. In shunted rheostatic armature control method ______________
a) Variable resistor is added in parallel with armature
b) Variable resistor is added in series with armature
c) Variable resistor is added in parallel with armature and another variable resistor is added in series
d) Variable resistor is not added in whole circuit
Explanation: In shunted armature speed control method we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.
12. Which of the following is correct statement?
a) Speed regulation of rheostatic armature control method is better than that of shunted armature control
b) Speed regulation of rheostatic armature control method is worse than that of shunted armature control
c) Speed regulation of rheostatic armature control method is almost equal to that of shunted armature control
d) Speed regulation of rheostatic armature control method and of shunted armature control are equally worst.
Explanation: Speed regulation of shunted armature control method is better than that of rheostatic armature control. External power loss in shunted armature control method is very high compare to that of rheostatic armature control method.
13. By series parallel method of armature control how many different speeds are possible?
Explanation: Here two identical motors are coupled together mechanically to a common load. Two speeds at constant torque are possible in this method—one by connecting the motors armatures in series and the other by connecting them in parallel.
14. Which method is best for series traction motors?
a) Rheostatic armature control
b) Shunted armature control
c) Series parallel control method
d) Any method can be employed
Explanation: Series-parallel control method is superior to the rheostatic control insofar as efficiency is concerned. However, it is limited to two speed steps. The method is commonly employed for speed control of series traction motors.
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