This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Operating Characteristics of DC Series Motor -1”.
1. DC series motors are used _______________
a) Where load is constant
b) Where load changes frequently
c) Where constant operating speed is needed
d) High starting speed
View Answer
Explanation: DC series motors are used in those applications where high starting speed is required at its initial phase. Since no load operation speed of DC series motor is very high, it is always operated on some non-zero load.
2. Where compensating winding is provided in case of conductively compensated DC series motor?
a) As separately wound unit
b) In series with armature winding
c) In parallel with armature winding
d) In parallel with field winding
View Answer
Explanation: Compensating winding is always excited with armature current in a given DC machine. So, compensating winding is provided in case of conductively compensated DC series motor in series with the armature winding.
3. No-load speed of which of the following DC motor will be highest?
a) Shunt motor
b) Series motor
c) Cumulative compound motor
d) Differentiate compound motor
View Answer
Explanation: At no load, armature current tends to zero, flux φ tends to zero, where speed is inversely proportional to the flux, speed will tend to infinity. Thus, no load speed of DC series motor is highest.
4. The direction of rotation of a DC series motor shaft can be changed by _____________
a) Interchanging supply terminals
b) Interchanging field terminals
c) Not possible
d) Cannot be determined
View Answer
Explanation: From speed current characteristics for a DC series motor, we get that speed is inversely proportional to flux. If flux direction is reversed, same flux will be available with negative sign in vectors, thus, speed will remain same but with the other direction.
5. Which of the following DC motor can be used at conveyors?
a) Series motor
b) Shunt motor
c) Differentially compound motor
d) Cumulative compound motor
View Answer
Explanation: A conveyor requires high torque with steady speed, which provided by DC series motor. From torque speed characteristic of a DC series motor, it is easily stated that speed of the series motor practically remains constant at high torques.
6.Which statement is correct about DC series motor?
a) Has its field winding consisting of thick wire and more turns
b) Has a poor torque
c) Can be started easily without load
d) Has its field winding consisting of thick wire and less turns
View Answer
Explanation: When field winding is connected in series it has to be of made by thick wire. When field winding is connected in parallel it has to be of made by thin wire. So, thick wire is used with less number of turns while thin one is used with more number of turns.
7. In which of the following applications DC series motor is invariably tried?
a) Starter for a car
b) Drive for a water pump
c) Fan motor
d) Home appliances
View Answer
Explanation: Starter of the car simply implies start of operation at maximum load. Thus, series motor can be used in this application. For higher torques, since speed remains constant car starter is one of the best application of DC series motor.
8. The speed of a DC series motor is ___________
a) Proportional to field current
b) Proportional to the square of the armature current
c) Proportional to the armature current
d) Inversely proportional to the armature current
View Answer
Explanation: Speed of DC series motor is inversely proportional to the armature current because, as armature current increases the flux produced also increases due to the series combination. Similarly if armature current is reduced flux is reduced which will increase speed.
9. In a DC series motor, if the armature current is halved, the torque of the motor will be equal to
a) 100% of the previous value
b) 50% of the previous value
c) 25% of the previous value
d) 10% of the previous value
View Answer
Explanation: Torque in the case of linear magnetization of DC series motor is directly proportional to square of the armature current. So, armature current is made 1/2th of the original value, then torque will be 1/4th of the original value.
10. Applications demanding large applications at starting and small torque while running use DC series motor.
a) True
b) False
View Answer
Explanation: Large starting torque, infinite speed at no-load, low running torque for steady speed operation are called as Series characteristics. This are shown by DC series motor, so applications like cranes, traction, etc. use DC series motor.
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