What is the primary function of the rotor in an electric motor?
A To rotate and produce mechanical output
B To generate electrical power
C To create a magnetic field
D To regulate voltage
The rotor in an electric motor interacts with the magnetic field produced by the stator, and as a result, it rotates, providing mechanical output (torque) that can be used to drive loads.
What type of electrical machine is used to charge batteries?
A DC motor
B DC generator
C Induction motor
D Stepper motor
A DC generator is used to convert mechanical energy into DC electrical energy, which is typically used to charge batteries.
What is the relationship between slip and speed in an induction motor?
A Slip increases as speed increases
B Slip decreases as speed increases
C Slip remains constant regardless of speed
D Slip is independent of speed
Slip in an induction motor is the difference between the synchronous speed and rotor speed. As the rotor speed increases, the slip decreases.
What type of electrical machine is used in domestic water pumps?
A Universal motor
B Single-phase induction motor
C DC motor
D Stepper motor
Single-phase induction motors are widely used in domestic water pumps because of their simple design, reliability, and ability to operate with single-phase AC supply.
What type of loss is directly proportional to the square of the current?
A Copper loss
B Core loss
C Eddy current loss
D Mechanical loss
Copper losses in electrical machines are caused by the resistance of the windings and are proportional to the square of the current (\(I^2R\)).
What type of motor is commonly used in electric vehicles?
A Stepper motor
B Synchronous motor
C Brushless DC motor
D Induction motor
Brushless DC motors are preferred in electric vehicles for their high efficiency, torque, and low maintenance, making them ideal for battery-powered applications.
What is the primary purpose of field windings in electrical machines?
A To carry current
B To produce the magnetic field
C To step up voltage
D To reduce losses
Field windings in electrical machines generate the magnetic field necessary for the motor or generator to function.
What is the purpose of an autotransformer in electrical machines?
A To step up or step down voltage with a single winding
B To isolate circuits
C To improve power factor
D To step up current
An autotransformer uses a single winding for both primary and secondary, making it more compact and efficient for voltage transformation.
What is the main purpose of cooling oil in a transformer?
A To provide insulation and cool the transformer
B To step up voltage
C To increase power factor
D To increase current
Cooling oil in transformers provides both insulation and heat dissipation, preventing overheating and ensuring the transformer operates safely.
What type of rotor is used in a squirrel cage induction motor?
A A rotor with bars short-circuited by end rings
B A rotor with salient poles
C A rotor with permanent magnets
D A rotor with windings connected to slip rings
Squirrel cage rotors are simple and rugged, with rotor bars short-circuited by end rings, making them highly reliable and efficient in induction motors.
What is the main function of the core in a transformer?
A To provide a low-reluctance path for magnetic flux
B To insulate the windings
C To step up or step down voltage
D To reduce eddy current losses
The core of a transformer provides a low-reluctance path for the magnetic flux, ensuring efficient energy transfer between the primary and secondary windings.
What is the function of the centrifugal switch in a single-phase motor?
A To disconnect the starting winding after the motor reaches a certain speed
B To increase torque
C To improve power factor
D To reduce resistance
The centrifugal switch disconnects the starting winding once the motor reaches around 70-80% of its rated speed, preventing overheating.
What is the purpose of a yoke in a DC machine?
A To provide mechanical support and act as a protective cover
B To carry current
C To step up voltage
D To increase speed
The yoke is the outer frame that provides mechanical support and protection for the internal components, like the armature and field windings.
What is the primary purpose of excitation in a synchronous machine?
A To provide the magnetic field for operation
B To reduce copper losses
C To regulate speed
D To step up voltage
Excitation supplies the DC current to the field windings of synchronous machines, creating the magnetic field necessary for operation.
What type of rotor is used in synchronous machines for high-speed applications?
A Squirrel cage rotor
B Cylindrical rotor
C Salient pole rotor
D Hollow rotor
Cylindrical rotors are used in synchronous machines for high-speed applications because they are designed to withstand high centrifugal forces.
What is the main cause of mechanical losses in electrical machines?
A Eddy current losses
B Friction in bearings and windage
C Copper losses
D Core losses
Mechanical losses are mainly caused by friction in bearings and windage due to air resistance, which dissipates energy as heat.
What is the role of the stator in a three-phase induction motor?
A To produce a rotating magnetic field
B To generate mechanical output
C To carry current to the rotor
D To reduce losses
The stator in a three-phase induction motor generates a rotating magnetic field, which induces current in the rotor, causing it to turn.
What happens to the torque in an induction motor as the load increases?
A Torque increases
B Torque decreases
C Torque remains constant
D Torque becomes zero
In an induction motor, as the load increases, the torque increases to meet the increased demand, causing a corresponding decrease in the speed (slip increases).
What type of motor is most commonly used in electric clocks?
A Synchronous motor
B DC motor
C Universal motor
D Induction motor
Synchronous motors are used in electric clocks because they run at a constant speed synchronized with the supply frequency, ensuring accurate timekeeping.
What is the main advantage of a squirrel cage rotor?
A Simple, rugged, and reliable
B High starting torque
C Easily controlled speed
D Low maintenance cost
Synchronous motors are used in electric clocks because they operate at a constant speed synchronized with the supply frequency, ensuring accurate timekeeping.