DC Machines Multiple Choice Questions on “Operating Characteristics of DC Shunt Motor -1”.
1. For machine tools, which DC motor can be used?
a) DC Series motor
b) DC Shunt motor
c) DC cumulative compound motor
d) DC differential compound motor
Answer: b
Clarification: Shunt characteristics is the speciality of DC shunt motor. The speed of the motor almost remains constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.
2. In a DC shunt motor, speed is related to armature current as _____________
a) Directly proportional to the armature current
b) Proportional to the square of the current
c) Independent of armature current
d) Inversely proportional to the armature current
Answer: d
Clarification: When armature reaction is ignored in a DC shunt motor, flux almost remains constant but the speed of the motor decreases according to the increase armature current, increasing Ia*Ra drop. Hence, inverse proportionality.
3. In a DC shunt motor for zero armature current we get speed ______
a) Non-zero and minimum
b) Zero
c) Non-zero and maximum
d) Doesn’t depend on armature current
Answer: c
Clarification: For zero armature current we get some non-zero value, indicated by positive intercept on speed characteristics. As armature current is increased speed of DC shunt motor starts decreasing due to increase in voltage drop at armature resistance.
4. What will be the effect of opening of field of a DC shunt motor while motor is running?
a) The speed of motor will be reduced
b) The armature current will reduce
c) The motor will attain dangerously high speed
d) The motor will continue to constant speed
Answer: c
Clarification: In a DC shunt motor if supply for the field winding is cut down, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of DC shunt motor. So, if flux drops to zero theoretically speed will tend to infinity.
5. What will be the effect of reducing load on DC shunt motor?
a) Speed will increase abruptly
b) Speed will increase in proportion to reduction in load
c) Speed will remain almost constant
d) Speed will reduce
Answer: c
Clarification: For DC shunt motor, speed-armature current characteristics is called as a shunt characteristic as speed almost remains constant. Thus, by reducing load speed will increase negligibly, thus remaining almost constant.
6. How speed of the DC shunt motor can be increased?
a) Decreasing the field current
b) Increasing the field current
c) Decreasing the load current
d) Increasing the armature current
Answer: b
Clarification: For a constant load, load current will remain constant. Decreasing armature current will help in increasing speed for DC shunt motor. Since load current is addition of armature current and field current we’ll get less armature current for more field current.
7. Practical reason behind speed of DC shunt motor is proportional to back emf only is ____________
a) Back emf is equal to armature drop
b) Flux is proportional to field current
c) Flux is proportional to armature current
d) Flux is practically constant in DC shunt motors