DC motors are DC electrical machines that convert electrical power to mechanical power. DC motors use direct current electricity to generate rotational motion, which makes them ideal for applications that require precise speed control and high torque output. DC machines are widely used in various applications due to their reliable performance and efficiency. They are commonly found in everyday appliances and power tools.
The basic principle of a DC motor involves the interaction between the magnetic field and the electric current. The motor consists of a stationary part called the stator and a rotating part called the rotor. The stator contains a series of electromagnets that create a magnetic field. In contrast, the rotor comprises a permanent magnet or an electromagnet that interacts with the stator’s magnetic field. Electrical Engineering Online shares Objective Electrical MCQ Questions on DC Motors.
The function of the commutator in a DC motor is to
The function of the commutator in a DC motor is to:
- Control the speed of the motor
- Reverse the direction of rotation
- Control the torque output
- Provide electrical connections to the armature windings
Correct answer: 4. Provide electrical connections to the armature windings
Explanation: The commutator in a DC motor is a segmented cylindrical structure connected to the armature windings. Its primary function is to provide electrical connections to the armature windings while ensuring that the current direction in the windings is maintained, allowing for continuous rotation.
In a DC motor, the armature winding is usually placed on the
In a DC motor, the armature winding is usually placed on the:
- Stator
- Rotor
- Commutator
- Brush assembly
Correct answer: 2. Rotor
Explanation: The armature winding in a DC motor is typically placed on the rotor. As the rotor rotates, the armature windings cut across the magnetic field produced by the field winding, generating the driving force. The armature windings are connected to the commutator segments.
The back EMF (electromotive force) in a DC motor is directly proportional to the following
The back EMF (electromotive force) in a DC motor is directly proportional to the following:
- Armature resistance
- Field winding resistance
- Speed of the motor
- Applied voltage
Correct answer: 3. Speed of the motor
Explanation: The back EMF in a DC motor is directly related to the speed of the motor. The back EMF increases as the motor speeds up, opposing the applied voltage and regulating the current flow through the armature windings.
Which type of DC motor provides high starting torque and is commonly used in applications such as electric vehicles and elevators
Which type of DC motor provides high starting torque and is commonly used in applications such as electric vehicles and elevators?
- Shunt motor
- Series motor
- Compound motor
- Permanent magnet motor
Correct answer: 2. Series motor
Explanation: A series motor provides high starting torque and is employed in applications that require high initial torque, such as electric vehicles and elevators. In a series motor, the field winding and the armature windings.
Which type of DC motor provides a nearly constant speed regardless of the load
Which type of DC motor provides a nearly constant speed regardless of the load?
- Shunt motor
- Series motor
- Compound motor
- Permanent magnet motor
Correct answer: 1. Shunt motor
Explanation: A shunt motor provides a nearly constant speed regardless of the load. The field winding in a shunt motor is connected in parallel with the armature windings, allowing it to maintain a relatively constant magnetic field strength, resulting in a consistent speed.
The term “cogging” in a DC motor refers to
The term “cogging” in a DC motor refers to:
- The noise produced during motor operation
- The jerky or uneven motion of the rotor
- The effect of back EMF on the armature windings
- The process of reversing the direction of rotation
Correct answer: 2. The jerky or uneven motion of the rotor
Explanation: Cogging in a DC motor refers to the jerky or uneven motion of the rotor when starting from rest. This happens because of the interaction between the magnetic poles of the field winding and the armature windings, resulting in intermittent or hesitant rotation.
The efficiency of a DC motor is calculated as the ratio of
The efficiency of a DC motor is calculated as the ratio of:
- Output power to input power
- Speed to torque
- Voltage to current
- Rotor to stator losses
Correct answer: 1. Output power to input power
Explanation: The efficiency of a DC motor is calculated by dividing the output power (mechanical power delivered by the motor) by the input power (electrical power supplied to the motor). It represents the effectiveness of the motor in converting electrical energy into mechanical energy.
In a regenerative braking system using a DC motor, the motor acts as a
In a regenerative braking system using a DC motor, the motor acts as a:
- Generator, that converts mechanical energy into electrical energy
- Transformer, stepping up the voltage during braking
- Rectifier, converting AC power into DC power
- Resistor, dissipating excess energy as heat
Correct answer: 1. Generator, converting mechanical energy into electrical energy
Explanation: The motor operates as a generator in a regenerative braking system using a DC motor. When the vehicle decelerates, the kinetic energy is converted into electrical energy, fed back into the power system, stored, or used to power other systems.
The function of the brushes in a DC motor is to
The function of the brushes in a DC motor is to:
- Supply power to the field winding
- Control the speed of the motor
- Reverse the direction of rotation
- Provide an electrical connection to the commutator
Correct answer: 4. Provide electrical connection to the commutator
Explanation: The brushes in a DC motor provide electrical link between the power source and the rotating commutator segments. They ensure continuous contact and transfer of current between the stationary power supply and the rotating armature windings.
The term “armature reaction” in a DC motor refers to the effect of
The term “armature reaction” in a DC motor refers to the effect of:
- Load fluctuations on the motor’s output
- Magnetic field on the brushes
- Rotor speed on the motor’s torque
- Armature current on the magnetic field
Correct answer: 4. Armature current on the magnetic field
Explanation: Armature reaction in a DC motor refers to distortion of the main magnetic field produced by the field winding due to the presence of the armature current. The armature current generates its magnetic field that interacts with the field winding’s magnetic field, causing a shift and distortion of the resultant field.
The type of DC motor that is commonly used in applications that require precise speed control, such as robotics and CNC machines, is
The type of DC motor that is commonly used in applications that require precise speed control, such as robotics and CNC machines, is:
- Shunt motor
- Series motor
- Compound motor
- DC Servo motor
Correct answer: 4. DC Servo Motor
Explanation: Brushless DC motors are commonly used in applications that require precise speed control. They offer advantages such as high efficiency, low maintenance, and accurate speed regulation, making them suitable for applications like robotics, CNC machines, and electric vehicles.