What is the primary function of the commutator in a DC motor?
A Reverse current direction
B Increase torque
C Generate magnetic field
D Control speed
The commutator in a DC motor reverses the direction of current in the armature windings every half‐turn. This ensures that the torque generated in the motor remains in the same direction, enabling continuous rotation of the motor.
What does the armature in a DC machine do?
A Supply current
B Convert electrical power to mechanical power
C Produce magnetic field
D Control speed
The armature in a DC machine is responsible for converting electrical energy into mechanical energy. The current flowing through the armature interacts with the magnetic field created by the field windings, generating the torque necessary for the motor’s rotation.
In a synchronous motor, what is the function of the excitation system?
A To generate torque
B To produce heat
C To maintain synchronization
D To regulate speed
The excitation system in a synchronous motor provides the necessary field current to the rotor, ensuring that the rotor’s magnetic field remains synchronized with the rotating magnetic field of the stator. This synchronization is crucial for the motor to operate at constant speed.
What happens during commutation in a DC motor?
A Speed increases
B Efficiency decreases
C Current is reversed in the rotor
D Magnetic field is created
Commutation in a DC motor involves reversing the current in the armature windings to ensure that the torque direction remains consistent. This process is facilitated by the commutator, allowing continuous rotation of the motor’s rotor.
What does the field winding in a DC machine do?
A Controls current
B Generates the magnetic field
C Converts mechanical energy to electrical
D Reverses current direction
The field winding in a DC machine generates the magnetic field required for operation. This field interacts with the current flowing through the armature, creating the mechanical torque that powers the motor or generator.
How does the armature resistance affect the performance of a DC motor?
A Increases speed
B Increases efficiency
C Reduces current
D Reduces torque
The armature resistance in a DC motor causes losses in the form of heat. These losses reduce the effective torque output of the motor and reduce efficiency, especially at higher loads, leading to decreased overall performance.
What is the role of the excitation system in an alternator?
A To regulate output voltage
B To generate power
C To reverse current flow
D To increase speed
The excitation system in an alternator provides the required field current to the rotor, which in turn controls the output voltage of the machine. By adjusting the field current, the excitation system ensures a stable and controlled voltage output.
What is a major consequence of poor commutation in a DC motor?
A Faster speed
B Increased wear and tear on brushes
C Voltage fluctuations
D Increased torque
Poor commutation in a DC motor results in inefficient current reversal in the armature windings, leading to sparking and excessive wear on the brushes. This reduces the motor’s lifespan and increases maintenance costs.
What is the primary cause of field weakening in a DC motor?
A Reduced excitation
B Increased load
C Decreased field current
D Increased armature current
Field weakening occurs when the field current in a DC motor is reduced. This weakens the magnetic field, allowing the motor to run faster but reducing torque. It is often used to control speed in motors with constant voltage.
What is the function of a commutator in a DC machine?
A To supply power to the armature
B To generate torque
C To provide speed control
D To change direction of current
The commutator in a DC machine periodically reverses the direction of current in the armature windings, ensuring that the torque generated by the motor is always in the same direction, allowing continuous rotation.
In a synchronous motor, what effect does the excitation have on the motor’s power factor?
A Decreases speed
B Improves power factor
C Increases torque
D Reduces efficiency
In a synchronous motor, adjusting the excitation (field current) can improve the power factor. Under‐excitation causes the motor to absorb reactive power, while over‐excitation allows the motor to supply reactive power, improving the overall power factor of the system.
What is the primary function of the stator in a synchronous motor?
A To generate torque
B To supply current
C To control speed
D To produce a rotating magnetic field
The stator in a synchronous motor produces a rotating magnetic field, which interacts with the rotor’s magnetic field to induce motion. This interaction allows the rotor to lock with the stator’s field and operate at synchronous speed.
What is the relationship between armature current and torque in a DC motor?
A Directly proportional
B Exponentially related
C Inversely proportional
D No relationship
In a DC motor, the armature current is directly proportional to the torque. As the armature current increases, the torque produced by the motor increases as well. This relationship is fundamental to motor control and load handling.
What happens when the excitation in a synchronous generator is reduced?
A Speed increases
B Power factor improves
C Voltage decreases
D Voltage increases
In a synchronous generator, reducing the excitation (field current) reduces the magnetic field strength, which in turn lowers the output voltage. Proper excitation control is essential for maintaining the desired voltage level.
How does commutation affect the efficiency of a DC machine?
A Has no effect
B Increases efficiency
C Improves speed
D Reduces efficiency
Poor commutation in a DC machine can cause sparking at the brushes, leading to energy losses and reduced efficiency. Good commutation ensures smooth current reversal and minimizes losses, improving the overall performance of the motor or generator.