B To convert electrical energy into mechanical energy
C To regulate voltage
D To generate electricity
A motor’s primary function is to convert electrical energy into mechanical energy, which is used for performing mechanical work.
Which component of a transformer prevents flux leakage?
A Core
B Stator
C Rotor
D Windings
The core in a transformer helps confine the magnetic flux to prevent leakage and ensure efficient energy transfer between windings.
What does “rotor” refer to in an electrical machine?
A Stationary magnetic part
B Rotating part of the machine
C The winding for generating voltage
D The frame for supporting the machine
The rotor is the rotating part of an electrical machine that interacts with the stator to produce torque or generate power.
What is the synchronous speed of a machine dependent on?
A Resistance of windings
B Supply frequency and number of poles
C Rotor mass
D Core material
The synchronous speed of an electrical machine depends on the supply frequency and the number of poles as per the formula: \( N_s = \frac{120f}{P} \).
What is the purpose of a starter in a motor?
A To limit the initial current
B To increase speed
C To reduce torque
D To maintain constant temperature
A starter limits the initial current drawn by a motor during starting, preventing damage to the motor and the power supply system.
Which loss is caused by alternating magnetization of the core?
A Friction loss
B Copper loss
C Hysteresis loss
D Eddy current loss
Hysteresis loss occurs due to the repeated magnetization and demagnetization of the core material in an alternating magnetic field.
Which material is commonly used for the core of electrical machines?
A Aluminum
B Silicon steel
C Copper
D Carbon
Silicon steel is used for cores in electrical machines because of its high magnetic permeability and low hysteresis loss.
What does an increase in slip in an induction motor indicate?
A Increased speed
B Decreased rotor speed
C Improved efficiency
D Reduced torque
As slip increases, the rotor speed decreases relative to the synchronous speed, which can result in higher torque but lower efficiency.
What is the main advantage of a three-phase motor over a single-phase motor?
A Higher efficiency and smoother operation
B Easier starting without additional components
C Lower weight for the same power rating
D Less complex construction
Three-phase motors are more efficient and provide smoother operation due to the uniform power delivery from three-phase currents.
What is the main characteristic of a synchronous generator?
A Produces DC power
B Operates without excitation
C Operates at constant speed regardless of load
D Produces variable voltage
A synchronous generator operates at a constant speed (synchronous speed) irrespective of the load, as its speed is determined by the supply frequency and the number of poles.
What causes eddy current losses in electrical machines?
A Improper insulation
B Circulating currents in the core
C Friction between moving parts
D High-frequency operation
Eddy current losses occur due to circulating currents induced in the core by the changing magnetic field, resulting in heat generation.
What is the role of bearings in electrical machines?
A To generate power
B To control magnetic flux
C To support the rotating shaft
D To increase voltage
Bearings provide mechanical support to the rotating shaft, reducing friction and ensuring smooth operation.
What is the main advantage of brushless motors?
A Low torque output
B Low-speed operation
C Maintenance-free design
D Simple control mechanism
Brushless motors eliminate the need for brushes and commutators, reducing wear and maintenance requirements, making them more durable.
What is the purpose of a flywheel in electrical machines?
A To store rotational energy
B To reduce slip
C To step up voltage
D To improve insulation
A flywheel stores rotational energy and helps maintain smooth operation by reducing fluctuations in speed.
Which part of a DC motor experiences commutation?
A Stator
B Field winding
C Armature winding
D Core
Commutation occurs in the armature winding of a DC motor, where the direction of current in conductors is reversed to ensure unidirectional torque.
Why are permanent magnets used in some motors?
A To control speed
B To reduce resistance
C To provide a fixed magnetic field
D To improve cooling
Permanent magnets create a fixed magnetic field, eliminating the need for external field excitation and simplifying motor design.
What is the primary function of field winding in a synchronous machine?
A To produce the main magnetic field
B To reduce vibrations
C To increase speed
D To improve cooling efficiency
The field winding in a synchronous machine is responsible for producing the magnetic field required for operation.
What type of cooling is used in high-power electrical machines?
A Natural air cooling
B Forced air or liquid cooling
C Radiation cooling
D Magnetic cooling
High-power electrical machines use forced air or liquid cooling to efficiently dissipate heat and maintain operating temperatures.
Which of the following is an essential step in electrical machine maintenance?
A Increasing speed periodically
B Reducing power supply voltage
C Inspecting insulation and connections
D Operating the machine without load
Regular inspection of insulation, connections, and components is crucial to ensure the machine’s proper functioning and safety.
Which type of motor is commonly used in computer hard drives?
A Stepper motor
B Brushless DC motor
C Universal motor
D Induction motor
Brushless DC motors are used in computer hard drives due to their high precision, reliability, and maintenance-free design.