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.
What is the primary function of the rotor in an induction motor?
A To generate current
B To produce torque through interaction with the stator’s magnetic field
C To supply voltage to the stator
D To reduce losses
The rotor in an induction motor interacts with the rotating magnetic field generated by the stator to produce mechanical torque, which causes the rotor to rotate.
What is the main purpose of field windings in a synchronous machine?
A To generate the magnetic field necessary for synchronous operation
B To carry load current
C To improve cooling
D To increase rotor speed
Field windings in a synchronous machine create the magnetic field necessary for the machine to synchronize with the rotating magnetic field in the stator.
What happens to the torque in a DC motor when the armature current increases?
A Torque decreases
B Torque increases proportionally
C Torque remains constant
D Torque becomes zero
In a DC motor, torque is directly proportional to the armature current (\(T \propto I_a\)). As the current increases, the torque increases accordingly.
Which type of transformer has the primary and secondary windings connected electrically?
A Isolation transformer
B Autotransformer
C Step-up transformer
D Step-down transformer
An autotransformer has the primary and secondary windings electrically connected through a common winding, making it more compact and efficient than a two-winding transformer.
What type of current is supplied to the field winding of a synchronous machine?
A Alternating current
B Direct current
C Pulsating current
D High-frequency current
The field winding of a synchronous machine is supplied with direct current (DC) to generate the magnetic field needed for synchronization with the stator’s rotating magnetic field.
What is the role of slip rings in an AC machine?
A To transfer current between stationary and rotating parts
B To step up voltage
C To generate magnetic flux
D To improve torque
Slip rings are used in AC machines to transfer current between the stationary and rotating parts, maintaining an electrical connection without interrupting the current.
Which motor is commonly used in ceiling fans?
A Single-phase induction motor
B DC motor
C Universal motor
D Stepper motor
Single-phase induction motors are commonly used in ceiling fans because of their simple design, reliability, and low cost for AC power applications.
What is “voltage regulation” in a transformer?
A The efficiency of the transformer
B The change in output voltage from no-load to full-load
C The total power loss
D The insulation resistance
Voltage regulation indicates how much the output voltage of the transformer changes as the load varies from no-load to full-load.
What is the purpose of a shading coil in a shaded pole motor?
A To create a phase shift for starting torque
B To reduce resistance
C To improve commutation
D To step down voltage
The shading coil in a shaded pole motor creates a phase shift in the magnetic field, which produces the necessary starting torque for the motor to begin rotation.
Why is the core of a transformer laminated?
A To reduce eddy current losses
B To increase magnetic flux
C To improve insulation
D To step up voltage
Laminating the core in transformers reduces eddy current losses by providing multiple layers of insulation, which limits the circulation of these currents.
What is the purpose of the core in a transformer?
A To provide a low-reluctance path for magnetic flux
B To carry electrical current
C To dissipate heat
D To increase voltage
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 type of loss is caused by friction in bearings and windage in an electrical machine?
A Mechanical loss
B Copper loss
C Core loss
D Eddy current loss
Mechanical losses in electrical machines arise due to friction in bearings and windage, which are caused by the movement of parts through air.
Which type of motor is used in electric clocks?
A Synchronous motor
B Induction motor
C Universal motor
D Stepper motor
Synchronous motors are used in electric clocks because they maintain accurate time by operating at a speed synchronized with the supply frequency.
What is the primary cause of eddy current losses in electrical machines?
A Induced currents in the core due to a changing magnetic field
B Friction between moving parts
C Resistance in the windings
D Harmonics in the supply current
Eddy current losses are caused by the circulating currents induced in the core due to the alternating magnetic field, leading to energy dissipation as heat.
Which component of an AC motor is responsible for the production of the rotating magnetic field?
A Stator winding
B Rotor
C Bearings
D Shaft
The stator winding in an AC motor is responsible for generating the rotating magnetic field that induces current in the rotor to produce torque.
What type of motor is commonly used in air conditioning units?
A Induction motor
B DC motor
C Universal motor
D Stepper motor
Induction motors are widely used in air conditioning units because of their durability, efficiency, and simple construction, making them ideal for continuous operation.
What is the main advantage of a brushless DC motor over a brushed DC motor?
A Less maintenance due to the absence of brushes
B Higher efficiency at low speeds
C Higher starting torque
D Simpler construction
Brushless DC motors eliminate the need for brushes and commutators, reducing wear and maintenance. They also offer better efficiency and longer lifespans compared to brushed DC motors.
What is the effect of increasing the supply voltage on the speed of a DC motor?
A Speed increases proportionally
B Speed increases until a certain limit
C Speed decreases with increasing voltage
D Speed remains constant
In a DC motor, increasing the supply voltage increases the speed until the back EMF reaches a level that limits further acceleration, at which point speed stabilizes.
What is the main reason for the use of permanent magnets in motors?
A To provide a constant magnetic field without the need for field windings
B To increase current through the motor
C To improve efficiency at high speeds
D To provide variable speed control
Permanent magnets are used in motors to create a constant magnetic field, eliminating the need for additional field windings and reducing motor size and complexity.
What happens to the speed of an induction motor when the load decreases?
A Speed increases significantly
B Speed increases slightly
C Speed decreases
D Speed remains constant
When the load on an induction motor decreases, the motor’s speed increases slightly due to the reduced slip, but it remains very close to synchronous speed.
What is the main purpose of a power system?
A Generate and transmit electricity
B Provide lighting for buildings
C Distribute water in cities
D Control telecommunications
The primary purpose of a power system is to generate electrical energy, transmit it over long distances, and distribute it to consumers efficiently and reliably. This involves power generation, transmission, and distribution.
What is the typical frequency of AC power supply in India?
A 50 Hz
B 60 Hz
C 45 Hz
D 55 Hz
The standard frequency for AC power in India is 50 Hz, while in countries like the USA, it is 60 Hz. Frequency is a critical parameter in maintaining synchronization and efficiency in power systems.
What is the function of a transformer in a power system?
A Convert AC to DC
B Step up or step down voltage levels
C Store electrical energy
D Protect the system from faults
A transformer is an electrical device that changes the voltage level in an AC power system. Step-up transformers increase voltage for transmission, while step-down transformers decrease voltage for safe distribution.
What does “reactive power” represent in a power system?
A Power used to perform useful work
B Power wasted due to losses
C Power required to maintain the magnetic field
D Power supplied by batteries
Reactive power is the component of power that does not perform any useful work but is necessary to maintain the electric and magnetic fields in inductive and capacitive components, such as transformers and motors.
Which of the following is NOT a type of power generation?
A Thermal power
B Hydro power
C Solar power
D Mechanical power
Mechanical power is not a type of power generation; it is a form of energy that is often converted into electrical power using turbines in thermal, hydro, or wind power plants.
What is the main advantage of using High Voltage Direct Current (HVDC) for long-distance transmission?
A Reduced power losses
B Increased power factor
C Lower cost of transformers
D Easier voltage regulation
HVDC systems have lower power losses compared to AC systems during long-distance transmission because there is no reactive power loss and the current remains constant throughout the transmission line.
What is the function of a circuit breaker in a power system?
A Step up the voltage
B Open and close circuits under load conditions
C Measure electrical parameters
D Store electrical energy
A circuit breaker is a protective device that interrupts current flow in a circuit under normal or fault conditions. It can handle load currents and isolate faults to protect equipment.
Which of the following is a renewable energy source?
A Coal
B Natural Gas
C Wind
D Nuclear
Wind energy is a renewable energy source as it is derived from natural wind currents, which are inexhaustible and environmentally friendly compared to fossil fuels like coal and natural gas.
What is the purpose of load flow analysis in power systems?
A Determine the economic cost of generation
B Calculate voltage, current, and power in the system
C Protect the system during faults
D Manage battery storage systems
Load flow analysis is used to determine the steady-state operation of a power system by calculating voltage levels, current flows, and power transfer across various components under normal operating conditions.
What is the main purpose of a relay in a protection system?
A Generate electricity
B Detect and isolate faults
C Control power generation
D Step down voltage
A relay is a protective device that detects abnormal conditions such as overcurrent or short circuits and sends signals to circuit breakers to isolate the faulty section of the power system.