This set of DC Machines Quiz focuses on “Operating Characteristics of DC Shunt Motor – 2”.
1. As the load is increased the speed of DC shunt motor will ___________
a) Reduce slightly
b) Increase slightly
c) Increase proportionately
d) Reduce rapidly
Explanation: As the load is increased, speed of the DC motor will reduce slightly, the change is so negligible that in many cases it is assumed that speed of the DC motor remains constant. Hence, characteristic is also called as shunt characteristic.
2. The armature torque of the DC shunt motor is proportional to
a) Field flux only
b) Armature current only
c) Field flux and armature current
d) Field current
Explanation: Torque of the DC shunt motor is directly proportional to the armature current. It’s almost a straight line if effect of armature reaction is neglected. If armature reaction is taken in consideration then torque will increase but non-linearly with armature current.
3. If a DC shunt motor is working at full load and if shunt field circuit suddenly opens _____________________
a) Will make armature to take heavy current, possibly burning it
b) Will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
c) Nothing will happen to motor
d) Motor will come to stop
Explanation: At no load or lower loads, there is possibility of excess speed in such cases but here, when full load is given, armature circuit will draw very high current in order to maintain back emf. So, if fuses or circuit breakers are not used then, armature circuit may burn.
4. Speed of DC shunt motor is directly proportional to___________
b) Back emf
c) Terminal voltage
d) Armature resistance drop
Explanation: The armature circuit equation for DC shunt motor is given by Ea = Vt -IaRa. But back emf is also equal to kφn. Thus, kφn= Vt -IaRa, which further gives n = (Vt -IaRa)/ kφ. Hence, speed is directly proportional to back emf.
5. Speed torque characteristic of DC shunt motor is _______________
a) Starting from origin
b) Starting from speed axis and increasing
c) Starting from speed axis and decreasing
d) Starting from speed axis and constant
Explanation: Speed-torque characteristic falls as torque increases, because armature reaction causes decrease in flux whose square is in inverse proportion with speed, causing decrease in speed rapidly compare to increase in torque.
Explanation: Speed-torque characteristic equation is obtained by n = (Vt -IaRa)/ kφ and Torque = k1φIa. Thus, by combining these equations and by assuming all constants equal to k and kl we get the final equation shown above.
7. For some percentage increase in the torque, which DC motor will have the least percentage increase of input current?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Separately exited motor
Explanation: The mechanical torque T is directly proportional to the product of flux per pole φ and armature current. In case of DC series motor, up to saturation point flux is proportional to field current because Ia = If. Now if small percentage increase in armature current (before saturation) will occur the same percentage of torque will increase. Whereas in DC series motor the torque is proportional to square of the armature current (before saturation).
8. DC Shunt Motor has a disadvantage __________
a) Constant speed
b) Less expensive
c) Not suitable for rapidly changing loads
d) Cannot be determined
Explanation: In DC shunt motor, due to shunt characteristics speed variation depends not only upon the controlling resistance but on the load current also. This double dependence makes it impossible to keep the speed sensibly constant on rapidly changing load, which is in contrast with characteristic.
Sanfoundry Global Education & Learning Series – DC Machines.
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