D.C. Motor Objective Questions With Explanation Part-1

1. No-load speed of which of the following motor will be highest ?
A. Shunt motor 
B. Series motor
C. Cumulative compound motor
D. Differentiate compound motor

Answer: B. Series motor

Explanation: 
  • At no load DC series motor rotates at dangerously high speed. Hence dc series motor should always be started on load.
  • When the motor is connected across the supply mains without load, it draws small current from the supply mains. This current will flow through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude.
  • The increase in back emf weakens the armature current and hence the field current. This cause again increases the speed so the back emf. Thus, the field continues to weaken and speed continues increase until the armature produced such centrifugal force that it is coming out from its shaft and gets damaged.
  • 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.
2. The direction of rotation of a D.C. series motor can be changed by
A. interchanging supply terminals
B. interchanging field terminals
C. either of A and B above
D. None of the above

Answer: B. interchanging field terminals

Explanation: 
DC series motor:
  • In the case of DC series motor, the field and armature winding are connected in series. If the polarity of the input voltage or supply terminals are interchanged, then it changed the direction of the field and armature current both get changed so the direction of speed will remain the same.
  • The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.
  • To reverse the motor, you need to change the polarity of the supply voltage to either the field winding or the armature winding, but not both.
Separately excited DC motor:
  • The direction of the torque and the speed can be reversed by changing the direction of either the field current or armature current by changing the polarity of the DC voltage. 
  • However, this method is used for separately excited DC motor where it is possible to change the polarity of the field or armature.

3. Which of the following application requires high starting torque ?
A. Lathe machine
B. Centrifugal Pump
C. Locomotive
D. Air blower

Answer: C. Locomotive

Explanation:
Railway locomotives need high starting torque to move from a stationary position due to their heavy load.
DC motors are well suited due to the locomotive's high torque. This means that the motor can drive a consistent level of power over longer periods of time, ideal for trains that operate over 12 hours per day.
  • DC series motor is used for an electric locomotive
  • It has high starting torque which is important in a traction system
  • It requires less power for high torque production because torque production is directly proportional to the square of the armature current
Key Points:

In a DC series motor,

T ∝ ϕIa and E­b ∝ Nϕ

From the above two relations, when speed is high, the torque is low and vice-versa. Therefore, the starting torque of dc series motor is high.

Applications of DC series motor:
  • As It has a very high starting torque, it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works etc.
  • Therefore, both statements I and statement II are individually true and statement II is the correct explanation of statement I.

Important points:
Applications of other DC motors is given below.
Shunt-wound motor: 
  • This is an almost constant speed motor and its torque varies nearly as the current. Hence, it is used for driving constant speed line shafts, lathes, constant speed head b, fans wood working machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines etc.

Cumulative compound motor: 
  • It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing load. Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators etc.

Differentially compound motor: 
  • It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

4. If a D.C. motor is to be selected for Conveyor which motor would be preferred 
A. Series  Motor
B. Shunt motor
C. Differentially compound motor
D. Cumulative compound motor 

Answer: A. Series  Motor

Explanation:  
Conveyors need high starting torque initially, and constant torque later. So that, the DC series motor is used in conveyors as it provides very high starting torque, which is practically 5 times the rated torque.


Important Notes:
Applications of different DC motors are given below.

DC Series wound motor: 
  • It has a very high starting torque.
  • Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works, etc.

DC Shunt-wound motor: 
  • This is an almost constant speed motor and its torque varies nearly as the current.
  • Hence, it is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans,  woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.
DC Cumulative compound motor:
  • It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
  • Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.
DC Differentially compound motor: 
  • It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.
5. Which D.C. motor will be preferred for machine tools ?
A. Series motor 
B. Shunt motor
C. Cumulative compound motor 
D. Differential compound motor 

Answer: B. Shunt motor

Explanation:
DC Shunt-wound motor: 
  • This is an almost constant speed motor and its torque varies nearly as the current.
  • Hence, it is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans,  woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.

6. Which D.C. motor will be preferred for constant speed line shafting ? 
A. Cumulative compound motor
B. Differentially compound motor 
C. Series Motor
D. Shunt Motor

Answer:  D. Shunt Motor

Explanation:
  • DC shunt motor is a constant speed motor.
  • It can be used where the speed is required to remain almost constant from no load to full load, where the load has to be driven at a number of speeds and any one of which is nearly constant.
  • Hence DC shunt motor is used to drive the constant speed line shafting, lathes, blower and fan.

7. Differentially compound D.C. motors can find applications requiring
A. high starting torque
B. low starting torque
C. variable speed
D. frequent on-off cycles

Answer:  B. low starting torque

Explanation:
DC Differentially compound motor has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

DC Cumulative compound motor:
  • It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
  • Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.

8. Which D.C. motor is preferred for elevators ?
A. Shunt motor
B. Series motor
C. Differential compound motor 
D. Cumulative compound motor

Answer: D. Cumulative compound motor

Explanation:
DC cumulative compound motor having high starting torque up to 450% depending upon the degree of compounding.
The speed regulation is varying up to 25 ~ 30%. That's why these motors are used in elevators.

Key Notes:
Applications of different DC motors are given below.

DC Series wound motor: 
  • It has a very high starting torque.
  • Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works, etc.
DC Shunt-wound motor: 
  • This is an almost constant speed motor and its torque varies nearly as the current.
  • Hence, it is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans, woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.
DC Cumulative compound motor:
  • It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
  • Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.
DC Differentially compound motor: 
  • It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.
Important Points:
  • In DC series motors, torque increases as the square of armature current. Hence these motors are used in high starting torque and variable speed applications.
  • In DC shunt motors, torque is directly proportional to armature current. It is a constant speed motor. It is used for constant speed applications.
  • The compound motors are used where higher starting torque and fairly constant speed is required.
9. According to flaming's left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of
A. current in the conductor
B. movement of the conductor  
C. resultant force on conductor
D. none of the above

Answer: A. current in the conductor

Explanation:
Fleming Left-hand rule gives the force experienced by a charged particle moving in a magnetic field or a current-carrying wire placed in a magnetic field.
  • This rule was originated by John Ambrose Fleming.
  • It is used in an electric motor.
  • It states that "stretch the thumb, the forefinger, and the central finger of the left hand so that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the central finger points in the direction of motion of charge, then the thumb points in the direction of force experienced by positively charged particles."
  • In Fleming's left rule, the middle finger represents the direction of current flowing through the conductor. So option 3 is correct.
  • The thumb represents the direction of the magnetic force.
  • The forefinger represents the direction of the magnetic field.
10. If the field of a DC shunt motor gets opened 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 run at constant speed

Answer: C.  the motor will attain dangerously high speed

Explanation:
  • For a constant back emf, flux is inversely proportional to the speed of the motor.
  • If field winding is disconnected accidentally, 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 the motor.
  • In the case of a series machine if field winding disconnected, then the motor circuit is open and no current will pass through the armature, for the operation of machine interaction of two fluxes is necessary thus the machine won't operate.







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