DC Machines Multiple Choice Questions on “Operating Characteristics of DC Shunt Motor
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
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
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
c) Non-zero and maximum
d) Doesn’t depend on armature current
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
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
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
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
Clarification: The field winding in DC shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage for motor is constant then flux also becomes constant. At the rated speed the back emf also becomes almost constant if the load is same.
8. In a DC shunt motor, what will be the armature current at maximum load?
a) Almost negligible
b) Rated full-load current
c) Less than full-load current
d) More than full-load current
Clarification: As the load will increase in rapid manner, speed change in DC shunt motor will be visible. As load increase, speed will decrease, though by some small value. From, current armature characteristic armature current will be more than the earlier case.
9. In which of the following motor, ratio of starting torque to full-load torque will be least?
a) DC series motors
b) DC shunt motors
c) DC compound motors
d) Synchronous motors
Clarification: From torque current characteristic, we get that torque at no load or starting torque is equal to zero. As load increases, speed decreases and armature current increases so as torque also increases linearly, if effect of armature reaction is neglected.