DC Motor Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. The armature shaft of a DC motor must be able to resist

1. Twisting stains
2. A bending moment is produced due to the weight of the armature.
3. Twisting stains, bending moment, and unstable magnetic pull
4. Any unbalanced magnetic pull

Ans. The correct answer is c. Twisting stains, bending moment, and unstable magnetic pull

Explanation: The armature shaft must be able to resist any unstable magnetic pull on the armature core, winding stains from the torque gearbox, and bending moment from the weight of the armature in order for the motor to function well and for an extended period of time.

Ques. The armature of the DC motor draws current, which is proportional to the _____.

1. Speed
2. Torque
3. Voltage
4. None of the above

Ans. The correct answer is b. Torque

Explanation: A DC motor draws current that is directly proportional to the torque required by the machine, according to the equation of torque produced in a DC machine.

Ques. Which among the following parts can withstand the maximum temperature rise in a DC motor?

1. Commutator
2. Armature winding
3. Field winding
4. Carbon Brushes

Ans. The correct answer is c. Field winding

Explanation: The field windings can withstand a maximum temperature rise since they are not used in rotating parts.

Ques. If the back emf of a DC motor is wiped out suddenly, then what will happen?

1. The motor will continue to run
2. The motor will stop
3. Armature may burn
4. None of the above

Ans. The correct answer is c. Armature may burn

Explanation: The motor circuit will use extra current from the supply if the back emf abruptly disappears in order to maintain the back emf. As a result, an armature with too much current may get too hot and burn.

Ques. If the speed of a DC motor is increased, then

1. Line current increase and back emf fall
2. Line current falls but back emf increase
3. Both back emf and line current increase.
4. None of the above

Ans. The correct answer is b. Line current falls but back emf increase

Explanation: In the case of a DC motor, speed and back emf are inversely related. As a result, backward emf increases as speed does. As a result, the armature current drops until it equals the line current in the case of a series motor.

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Short Answers Questions [2 Marks Questions]

Ques. What is a DC motor?

Ans. A DC motor is an electrical machine that transforms electrical energy into mechanical energy. The input electrical energy of a DC motor is direct current, which is converted into mechanical rotation.

Ques. What is a universal motor?

Ans. The universal motor is a type of electric motor that can run on either AC or DC power and generates its magnetic field using an electromagnet as its stator. It is a commutated series-wound motor in which the field coils of the stator are coupled in series with the rotor windings through a commutator.

Ques. What will happen if the DC shunt motor is linked across the AC supply?

Ans. It will not rotate at all in the case of a parallel field connection but instead will begin to buzz and vibrate since the torques created by the positive and negative cycles will cancel each other out. The DC motor will become hot and might burn.

Ques. Can a DC motor run on AC?

Ans. Yes, the DC series motor can be operated by a single-phase alternating current source. This is due to the fact that the torque, which changes as a function of the armature and field current, is always positive. As a result, a positive average torque forces the motor to rotate.

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Long Answers Questions [3 Marks Questions]

Ques. What are the three types of losses in a DC motor?

Ans. A DC motor may encounter three different types of losses. These are:

• Mechanical loss: This loss is the friction caused by brushes, bearings, and other spinning components.
• Iron loss: Various causes contribute to this loss, including eddy currents and hysteresis.
• Copper loss: The armature and the field windings are where this loss mostly takes place.

Ques. What is Fleming’s left-hand rule?

Ans. According to Fleming's Left Hand Rule, if your left hand's index finger, middle finger, and thumb are extended mutually perpendicular to each other, and if the index finger represents the direction of the Magnetic Field, the middle finger represents the direction of the current, and the thumb represents the direction of force experienced by the Shaft of the DC motor.

Ques. What are the types of DC motors? 

Ans. The different types of DC motors are

• Permanent Magnet DC Motor: Permanent magnets in the form of poles are used in this type of DC motor. These magnets are typically positioned around the circumference of a cylindrical steel stator. This stator, in turn, aids in the reception of magnetic flux. 
• Series Motor: One of the various types of DC motors that employ the self-excited approach is the series motor. This is because the field winding is connected in series and internally to the armature winding. This DC motor consists of a stator, rotor, and commutator brush. 
• Shunt Motor: In this case, the field winding connects to the parallel armature windings of the motor. It results in yet another sort of self-excited motor. Both of these components are powered by a constant source of the same voltages.
• Compound DC Motor: This DC motor and type performs both series and shunt motor operations. These diverse types of field coils create the necessary magnetic flux. It also connects to the armature coil and generates torque sufficient for spinning at the appropriate speed.
• Short Shunt DC Motor: This motor makes use of the parallel connection between the shunt field and armature winding. The provided current interacts with the series field coil. Following that, it splits into the armature. 
• Long Shunt DC Motor: Long shunting DC along with the armature, the motor is connected in parallel with the series field coil. These, in turn, interact with one another in a chain. 

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Very Long Answers Questions [5 Marks Questions]

Ques. What are the differences between AC and DC motors?

Ans. The differences between AC and DC motors are

Ques. What are the applications of DC motors?

Ans. Following are the applications of different types of DC motor

Shunt DC motors

The following applications employ shunt DC motors because they have a moderate starting torque and a relatively consistent speed:

• Lathe machines
• Drilling and milling machines
• Centrifugal and reciprocating pumps
• Machine tools
• Blowers and fans 

Series DC motors

The following applications employ series DC motors due to their strong starting torque and variable speed:

• Conveyors
• Cranes
• Hoists, Elevators
• Electric Locomotives

Cumulative Compound DC Motors

Cumulative compound DC motors are utilized in the following applications because of their strong beginning torque:

• Shears
• Rolling mills
• Heavy Planers
• Elevators

Ques. How does a DC motor work?

Ans. Fleming's Left-Hand Rule, which states that when a current-carrying conductor is put in a magnetic field, it feels a torque and has a propensity to move, is the working principle of a DC motor. 

• A magnetic field is produced in the air gap when the field coil of the DC motor is energized.
• The direction of the magnetic field that is produced is the same as the armature's radii.
• The North pole of the field coil serves as the entry point for the magnetic field, while the South pole serves as the departure point.
• A force of equal intensity acting in the opposite direction is applied to the conductors situated on the other pole.
• The torque produced by these two forces causes the motor armature to revolve.

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