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.
What is the primary advantage of using an induction motor over a synchronous motor in industrial applications?
A Higher efficiency
B Easier to start
C Constant speed
D Lower operating cost
Induction motors are easier to start compared to synchronous motors. They are self-starting, which eliminates the need for external equipment or complex starting procedures, making them more practical for a wide range of industrial applications.
What is the primary disadvantage of using a synchronous motor?
A It is difficult to start without external equipment
B It has low efficiency at light loads
C It requires a high starting current
D It operates at variable speeds
Synchronous motors cannot start by themselves because they need to reach synchronous speed before they can operate normally. External equipment, such as a damper winding or an auxiliary motor, is required to bring the motor up to speed.
What is the primary advantage of using a squirrel-cage rotor over a wound rotor?
A It requires no external electrical connections
B It provides better speed control
C It allows for higher starting torque
D It is more efficient at low speeds
The main advantage of a squirrel-cage rotor is its simplicity and reliability. It does not require external electrical connections such as slip rings, making it more durable and easy to maintain compared to wound rotors.
What is the role of the rotor in an induction motor?
A To create a rotating magnetic field
B To interact with the stator’s magnetic field to produce mechanical power
C To provide excitation current
D To regulate the motor’s speed
The rotor interacts with the stator’s rotating magnetic field to induce current, which produces mechanical power. The torque generated by this interaction drives the rotor to rotate and provides useful work for the motor.
What is the typical use of squirrel-cage rotors in industrial applications?
A Constant-speed applications
B Variable-speed applications
C High-efficiency applications
D Low-power applications
Squirrel-cage rotors are widely used in applications that require constant speed. They are durable and easy to maintain, making them ideal for applications such as pumps, fans, and compressors where a stable speed is needed.
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 fluctuates
At no-load, an induction motor operates less efficiently because it consumes energy to overcome core losses and other non-productive losses, even though it is not performing useful work. This makes the motor less efficient under low-load conditions.
What is the typical application of a wound rotor induction motor?
A Applications requiring precise control of speed and torque
B General-purpose motor applications
C Low-power applications
D Applications requiring high starting torque
Wound rotor induction motors are used in applications where precise control over speed and torque is required. The ability to add external resistance to the rotor circuit allows for improved control of motor performance, making it ideal for variable-speed applications.
What is the effect of increasing the number of poles in an induction motor?
A It increases the motor’s speed
B It decreases the motor’s speed
C It increases the motor’s torque
D It decreases the motor’s efficiency
Increasing the number of poles in an induction motor decreases the synchronous speed. The synchronous speed is inversely proportional to the number of poles, making it beneficial in applications that require slower speeds and more torque, such as fans and pumps.
What is the effect of slip in an induction motor when operating under full-load conditions?
A Slip increases
B Slip decreases
C Slip remains constant
D Slip becomes zero
When an induction motor operates under full-load conditions, slip increases. As the load increases, the rotor speed decreases slightly, and the slip increases to generate the required torque for the load.
What is the purpose of using laminated cores in induction motors?
A To reduce eddy current losses
B To reduce mechanical friction
C To increase the motor’s power factor
D To improve the motor’s efficiency
Laminated cores in induction motors are used to reduce eddy current losses. By using thin sheets of magnetic material, the formation of circulating currents (eddy currents) is minimized, which helps to improve motor efficiency and reduce heat generation.