What is the primary function of the stator windings in an induction motor?
A To produce the rotor’s magnetic field
B To create a rotating magnetic field
C To provide excitation current to the rotor
D To regulate the motor’s speed
The stator windings in an induction motor create a rotating magnetic field when energized by a three-phase supply. This magnetic field induces current in the rotor, which causes the rotor to rotate and produce mechanical power.
Which of the following is true about the slip in an induction motor?
A Slip is zero when the motor is running at synchronous speed
B Slip increases as the load on the motor decreases
C Slip is inversely proportional to the load
D Slip is constant regardless of load
Slip refers to the difference between the synchronous speed and the actual rotor speed. When the motor reaches synchronous speed, slip is zero. Slip increases as the motor experiences load, which causes the rotor to slow down slightly.
What type of rotor is most commonly used in small induction motors?
A Wound rotor
B Squirrel-cage rotor
C Solid rotor
D Brushless rotor
The squirrel-cage rotor is widely used in small induction motors due to its simplicity and rugged construction. It consists of conductive bars that are short-circuited by end rings, providing high reliability and ease of maintenance.
What is the effect of increasing the load on an induction motor?
A The rotor speed decreases slightly
B The motor operates at synchronous speed
C The power factor increases
D The slip decreases
As the load increases, the rotor speed decreases slightly, and the slip increases. The increase in slip generates more torque to drive the increased load. This is a typical characteristic of induction motors.
What does the torque-speed characteristic of an induction motor indicate?
A The relationship between the motor’s speed and the input voltage
B The relationship between the rotor speed and the torque produced
C The power factor at various speeds
D The efficiency of the motor at different loads
The torque-speed characteristic of an induction motor shows how torque varies with rotor speed. Generally, the torque increases as the slip increases, peaking at a certain value before it starts to decrease as the rotor approaches synchronous speed.
In a synchronous motor, what does the rotor speed always match?
A The rotor’s mechanical speed
B The stator’s magnetic field speed
C The supply frequency speed
D The motor’s rated speed
In synchronous motors, the rotor speed always matches the speed of the stator’s rotating magnetic field. This speed is determined by the power supply frequency and the number of poles in the motor, and it remains constant during operation.
Which of the following is a characteristic of a wound rotor induction motor?
A It has a simple rotor design with no external connections
B It requires an external power supply for the rotor
C It has a rotor with short-circuited windings
D It operates at synchronous speed
A wound rotor induction motor uses windings in the rotor, which are connected to external resistors via slip rings. This allows the motor’s speed and torque to be controlled by adjusting the resistance in the rotor circuit.
What is the purpose of using a capacitor in a single-phase induction motor?
A To reduce core losses
B To create a phase difference between the current in the windings
C To increase the motor’s speed
D To improve cooling efficiency
In single-phase induction motors, capacitors are used to create a phase difference between the currents in the two windings, allowing the motor to produce the necessary starting torque.
In a wound rotor induction motor, what happens when external resistance is added to the rotor circuit?
A The motor runs at synchronous speed
B The starting torque increases
C The efficiency decreases
D The slip becomes zero
Adding external resistance to the rotor circuit of a wound rotor induction motor increases the starting torque. The resistance helps to limit the rotor current during startup, which helps to reduce the inrush current.
What is the typical cooling method used for large synchronous motors?
A Air cooling
B Oil cooling
C Water cooling
D Liquid nitrogen cooling
Large synchronous motors typically use water cooling systems, as water is more efficient than air in dissipating the heat generated during operation. Proper cooling helps maintain the motor’s temperature and extends its lifespan.
What is the primary advantage of using a squirrel-cage rotor over a wound rotor in induction motors?
A It provides higher starting torque
B It requires no external resistors or slip rings
C It offers better speed control
D It is more efficient at low speeds
The squirrel-cage rotor is simpler and more robust, requiring no external resistors or slip rings. This simplicity makes it ideal for most industrial applications, reducing the cost and maintenance requirements.
In which type of motor does the rotor always rotate at the synchronous speed?
A Induction motor
B Synchronous motor
C Permanent magnet motor
D Universal motor
In a synchronous motor, the rotor always rotates at the synchronous speed, which is determined by the power supply frequency and the number of poles in the motor. This speed does not change with load variations, unlike in induction motors.
What is the effect of increasing the number of poles in a synchronous motor?
A The motor speed increases
B The motor speed decreases
C The motor’s efficiency improves
D The motor generates more torque
Increasing the number of poles in a synchronous motor decreases the synchronous speed because synchronous speed is inversely proportional to the number of poles. This is useful for applications that require slower speeds.
What happens to the efficiency of an induction motor when it operates under no-load conditions?
A Efficiency increases
B Efficiency decreases
C Efficiency remains constant
D Efficiency becomes zero
When an induction motor operates under no-load conditions, it still consumes power to overcome core losses, friction, and windage losses. Since there is no useful mechanical work being done, the motor operates less efficiently than under load conditions.
What does the term “synchronous speed” refer to in the context of induction and synchronous motors?
A The actual speed of the rotor
B The speed at which the stator magnetic field rotates
C The speed of the rotor when it reaches maximum load
D The speed at which the motor is started
Synchronous speed is the speed at which the stator’s rotating magnetic field rotates in both induction and synchronous motors. This speed is determined by the supply frequency and the number of poles in the motor. The rotor of a synchronous motor always matches this speed.
In an induction motor, what is the effect of increasing the load on the motor?
A The rotor speed decreases
B The motor runs at synchronous speed
C The power factor increases
D The slip decreases
When the load on an induction motor increases, the rotor speed decreases slightly and the slip increases. The motor generates more torque to handle the increased load, and the rotor slows down slightly to accommodate this additional load.
What is the main disadvantage of a synchronous motor compared to an induction motor?
A It cannot be started without external equipment
B It has a higher starting torque
C It operates at variable speed
D It cannot be used for high-power applications
A synchronous motor requires external equipment for starting because it cannot start on its own. Unlike induction motors, which are self-starting, synchronous motors need an external mechanism like a damper winding to bring them up to synchronous speed before they begin operating.
What is the purpose of a damper winding in a synchronous motor?
A To increase the power factor
B To prevent the motor from losing synchronism during startup
C To cool the motor
D To reduce rotor losses
The damper winding in a synchronous motor helps the motor start and prevents it from losing synchronism during transient conditions. It provides the necessary torque for startup until the motor reaches synchronous speed.
What is the effect of increasing the air gap between the stator and rotor in an induction motor?
A Efficiency increases
B Efficiency decreases
C Torque increases
D Power factor improves
Increasing the air gap in an induction motor reduces the efficiency because it weakens the magnetic coupling between the stator and rotor. A larger air gap leads to higher losses and decreased torque production, making the motor less efficient.
What type of motor is typically used for precise speed control in applications such as clocks and watches?
A Synchronous motor
B Induction motor
C Permanent magnet motor
D Universal motor
Synchronous motors are ideal for applications requiring precise speed control, such as clocks and watches, because they maintain a constant speed regardless of load, making them suitable for applications that need accuracy.
What type of rotor construction does an induction motor typically use?
A Wound rotor
B Permanent magnet rotor
C Squirrel-cage rotor
D Brushless rotor
The squirrel-cage rotor is the most common rotor type in induction motors due to its simplicity, robustness, and low maintenance requirements. It consists of laminated sheets with conductive bars short-circuited by end rings, making it reliable and cost-effective.
What is the primary difference between an induction motor and a synchronous motor?
A Induction motors operate at synchronous speed, while synchronous motors do not.
B Synchronous motors have no slip, while induction motors always have some slip.
C Induction motors use external resistors for control, while synchronous motors do not.
D Synchronous motors are generally used for small loads, while induction motors are used for large loads.
The key difference between an induction motor and a synchronous motor is that synchronous motors run at a constant speed (synchronous speed) with zero slip, whereas induction motors always have some slip, meaning the rotor speed is slightly less than the synchronous speed.
In an induction motor, what is the effect of increasing slip?
A The motor operates at synchronous speed.
B The torque produced by the motor increases.
C The motor efficiency increases.
D The rotor speed decreases.
When the slip increases, the difference between the rotor speed and the stator’s rotating magnetic field increases. This results in more induced current in the rotor and increased torque production. However, too much slip can lead to inefficiency and excessive heating.
What does “synchronous speed” refer to in the context of synchronous motors?
A The speed at which the rotor accelerates
B The speed at which the stator’s magnetic field rotates
C The speed at which the rotor rotates under no-load conditions
D The speed of the electrical current in the windings
Synchronous speed refers to the speed at which the stator’s rotating magnetic field rotates. In a synchronous motor, the rotor must match this speed exactly to maintain synchronization with the stator field. The speed is determined by the supply frequency and the number of poles in the motor.
What is the role of slip rings in a wound-rotor induction motor?
A To allow external resistances to be added to the rotor circuit
B To cool the rotor
C To maintain constant motor speed
D To transfer electrical power to the stator
Slip rings in a wound-rotor induction motor provide a means to connect external resistances to the rotor circuit. These external resistances are used to control the motor’s starting torque and speed characteristics, making wound-rotor motors suitable for applications requiring variable speed and load adjustment.
Which of the following characteristics is true about a synchronous motor?
A It has a variable speed under varying loads.
B It operates at a constant speed independent of the load.
C It requires a mechanical starting device.
D It operates at a higher efficiency than induction motors at light loads.
Synchronous motors operate at a fixed speed, known as synchronous speed, which is determined by the supply frequency and the number of poles in the motor. The speed remains constant under varying load conditions, unlike induction motors, where speed decreases as the load increases.
In an induction motor, what happens when the load increases?
A The rotor speed increases.
B The slip increases.
C The motor operates at synchronous speed.
D The power factor improves.
When the load on an induction motor increases, the rotor slows down slightly, causing the slip to increase. This allows more torque to be produced to handle the increased load, but the motor will not reach synchronous speed because slip is always present.
What is the typical use of squirrel-cage rotors in industrial applications?
A High starting torque applications
B Low power factor applications
C Constant-speed applications
D Applications requiring high efficiency at light loads
Squirrel-cage rotors are commonly used in industrial applications where constant-speed operation is required. These rotors are durable, simple, and efficient, making them ideal for a wide range of applications in industries like pumping, fans, and compressors.
What is the effect of increasing the number of poles in a synchronous motor?
A The synchronous speed increases.
B The synchronous speed decreases.
C The torque produced by the motor increases.
D The motor becomes more efficient.
The synchronous speed of a motor is inversely proportional to the number of poles. Increasing the number of poles decreases the synchronous speed, making the motor suitable for applications requiring slower speeds, such as large fans or pumps.
How does the addition of a capacitor to a single-phase induction motor affect its performance?
A It improves the power factor and starting torque.
B It increases the efficiency at full load.
C It reduces the starting current.
D It improves the motor’s cooling.
In a single-phase induction motor, a capacitor is added to improve the power factor and provide the necessary phase difference between the currents in the two windings. This results in increased starting torque and better overall performance.
What is the main advantage of using a wound rotor instead of a squirrel-cage rotor?
A Higher efficiency at high speeds
B Ability to control the speed and torque
C More compact design
D Lower initial cost
A wound rotor allows for the addition of external resistances to control the motor’s starting torque and speed characteristics. This makes wound-rotor motors more flexible and suitable for applications requiring variable speed and load adjustment.
In which of the following applications would a synchronous motor be preferred over an induction motor?
A Low-speed, high-torque applications
B Constant-speed operations
C High-efficiency variable-speed applications
D High-slip operations
Synchronous motors are preferred for applications where constant-speed operation is crucial. These motors run at a fixed speed determined by the supply frequency and number of poles, making them ideal for precision applications such as timing devices and certain industrial processes.
What is the key feature of the torque-speed characteristic curve of an induction motor?
A The torque increases as speed decreases.
B The torque decreases as speed increases.
C The torque remains constant throughout the speed range.
D The torque is constant only at synchronous speed.
In an induction motor, the torque increases as the speed decreases, up to a point. This is due to the increased slip at lower speeds, which results in higher induced currents in the rotor, producing more torque. However, this trend stops when the motor approaches synchronous speed.
In a three-phase induction motor, what is the main cause of the motor’s starting current being higher than the rated current?
A The rotor windings require high voltage to begin rotating.
B The rotor initially requires a higher magnetic field to induce current.
C The stator windings need a higher current to create the magnetic field.
D There is a delay in the motor reaching full speed.
At startup, the rotor of an induction motor is stationary, and the slip is maximum. This requires a large current to be induced in the rotor to create the torque necessary for the motor to begin rotating. The inrush current is typically several times the motor’s rated current.
What is the main disadvantage of a synchronous motor compared to an induction motor?
A Synchronous motors are more difficult to maintain.
B Synchronous motors cannot be started easily without additional equipment.
C Synchronous motors are less efficient at high speeds.
D Synchronous motors require higher voltage for operation.
Synchronous motors require additional equipment, such as a damper winding or an external starting mechanism, because they cannot start by themselves. Unlike induction motors, which are self-starting, synchronous motors need to reach synchronous speed before they can operate normally.
What is the effect of the air gap between the stator and rotor in an induction motor?
A It reduces the motor’s power output.
B It reduces the efficiency of the motor.
C It allows the rotor to rotate smoothly.
D It increases the magnetic coupling between stator and rotor.
The air gap between the stator and rotor affects the magnetic coupling. A larger air gap reduces the coupling, leading to higher losses and decreased efficiency. Maintaining a small air gap improves motor performance by ensuring better magnetic field interaction.
What happens when the slip of an induction motor becomes negative?
A The motor will stop.
B The motor will operate in reverse direction.
C The motor will lose its torque.
D The motor will run at synchronous speed.
Negative slip occurs when the rotor speed exceeds the synchronous speed, which typically happens if the motor is driven mechanically. In this condition, the motor operates in reverse direction, as the rotor moves faster than the stator’s rotating magnetic field.
What is the function of a damper winding in a synchronous motor?
A To prevent overheating of the motor
B To help the motor start and synchronize
C To reduce vibrations in the motor
D To regulate the motor’s speed
A damper winding is used in synchronous motors to provide torque during startup. It helps the motor accelerate to synchronous speed and prevents it from losing synchronism during transient conditions, making the motor easier to start and maintain stable operation.
What is the effect of the air gap between the stator and rotor in an induction motor?
A Efficiency increases
B Efficiency decreases
C Magnetic flux is stronger
D Motor operates faster
A larger air gap between the stator and rotor reduces the magnetic coupling, resulting in lower efficiency. The optimal motor performance requires minimal air gap to ensure strong magnetic flux transfer and improved efficiency.
What type of motor is typically used for precise speed control in applications such as clocks and watches?
A Synchronous motor
B Induction motor
C Permanent magnet motor
D Universal motor
Synchronous motors are ideal for precise speed control applications like clocks and watches, as they maintain a constant speed regardless of the load. This makes them suitable for applications where accuracy is critical.
What is the key factor that determines the synchronous speed of a motor?
A The size of the motor
B The supply voltage
C The frequency of the power supply and the number of poles
D The type of rotor used
The synchronous speed is determined by the frequency of the supply and the number of poles in the motor. The formula for synchronous speed is \( N_s = \frac{120 \times f}{P} \), where \( N_s \) is the synchronous speed in RPM, \( f \) is the supply frequency, and \( P \) is the number of poles.
In an induction motor, what is the significance of the slip?
A It indicates the motor’s efficiency
B It shows the difference between the stator and rotor speed
C It determines the motor’s operating temperature
D It controls the motor’s torque
Slip in an induction motor is the difference between the synchronous speed and the actual rotor speed. It is necessary for torque generation in an induction motor. Without slip, there would be no induced current in the rotor, and no torque would be produced.
What is the primary difference between a squirrel-cage rotor and a wound rotor in an induction motor?
A Squirrel-cage rotors require external resistors, while wound rotors do not.
B Squirrel-cage rotors are simpler and more durable.
C Wound rotors have a fixed resistance, while squirrel-cage rotors have variable resistance.
D Squirrel-cage rotors are used only for high-power applications.
Squirrel-cage rotors are simpler, more durable, and require less maintenance compared to wound rotors. Wound rotors have windings connected to external resistors via slip rings, which allow for better control over the motor’s starting torque and speed characteristics.
What is the primary purpose of using a wound rotor in an induction motor?
A To provide higher efficiency
B To control the speed and torque characteristics
C To reduce the starting current
D To improve the power factor
The wound rotor design allows for external resistances to be added to the rotor circuit, providing better control over the motor’s starting torque and speed characteristics. This makes wound-rotor motors suitable for applications requiring variable speed and load adjustments.
What happens to the torque of an induction motor as the slip increases?
A Torque decreases
B Torque increases
C Torque remains constant
D Torque becomes zero
As the slip increases, the relative speed between the rotor and stator increases, resulting in more current being induced in the rotor. This leads to an increase in the torque produced by the motor, which is necessary for driving heavier loads.
What is the effect of under-excitation in a synchronous motor?
A It leads to a leading power factor
B It leads to a lagging power factor
C It increases the motor’s speed
D It decreases the motor’s efficiency
Under-excitation occurs when the field current is too low, which results in the motor consuming more reactive power and causing a lagging power factor. This behavior is typical when the motor is not supplying enough reactive power to the system.
In an induction motor, what is the relationship between the slip and the motor’s torque?
A Torque is inversely proportional to slip
B Torque is directly proportional to slip
C Torque remains constant throughout the speed range
D Torque is maximum when slip is zero
In an induction motor, torque increases as slip increases, up to a certain point. Higher slip means the rotor is rotating slower relative to the stator’s magnetic field, inducing more current and generating more torque. However, excessive slip can reduce efficiency and cause overheating.
What is the typical use of squirrel-cage rotors in industrial applications?
A High starting torque applications
B Constant-speed applications
C High-efficiency applications
D Low-power applications
Squirrel-cage rotors are widely used in industrial applications that require constant-speed operation. These rotors are simple, durable, and easy to maintain, making them ideal for applications like pumping, ventilation, and compressors.
What is the effect of increasing the number of poles in a synchronous motor?
A The synchronous speed increases
B The synchronous speed decreases
C The torque produced by the motor increases
D The motor becomes more efficient
The synchronous speed of a motor is inversely proportional to the number of poles. Increasing the number of poles decreases the synchronous speed, which is beneficial in applications that require slower speeds, such as large fans or pumps.
What is the role of slip rings in a wound-rotor induction motor?
A To allow external resistances to be added to the rotor circuit
B To maintain constant motor speed
C To cool the rotor
D To transfer electrical power to the stator
Slip rings in a wound-rotor induction motor provide a means to connect external resistances to the rotor circuit. These external resistances allow for better control of the motor’s starting torque and speed, making it suitable for applications requiring variable speed or load adjustment.