What is the main function of stator windings in an induction motor?
A Generate torque
B Supply excitation current
C Create rotating magnetic field
D Provide mechanical power
The stator windings in an induction motor create a rotating magnetic field when energized with alternating current. This field induces a current in the rotor, which produces torque to drive the motor.
In which motor is a wound rotor typically used?
A Induction motor
B Synchronous motor
C Universal motor
D Stepper motor
A wound rotor is typically used in an induction motor to provide high starting torque. The wound rotor allows external resistances to be added during startup to control the inrush current and improve the starting performance.
What happens to the slip of an induction motor at no load?
A It is high
B It is zero
C It is negative
D It is maximum
At no load, the rotor speed approaches synchronous speed, meaning the difference between synchronous speed and rotor speed is minimal. As a result, the slip of the induction motor becomes zero.
What is the effect of a high slip in an induction motor?
A Reduced efficiency
B Increased speed
C Increased torque
D Stable operation
High slip in an induction motor indicates that the rotor is rotating at a significantly slower speed than the synchronous speed. This condition allows the motor to produce more torque to handle increased load.
What does a synchronous motor require for operation?
A DC excitation
B External resistances
C Rotor speed control
D High slip
A synchronous motor requires DC excitation to the rotor winding, which creates a magnetic field. This magnetic field interacts with the rotating magnetic field of the stator to synchronize the rotor’s speed.
What is the role of rotor windings in a wound rotor induction motor?
A Control rotor speed
B Maintain voltage stability
C Offer high starting torque
D Provide external power
The rotor windings in a wound rotor induction motor allow external resistances to be connected during startup. These resistances provide high starting torque and limit the inrush current, ensuring a smooth startup.
Which of the following factors determines the synchronous speed of an induction motor?
A Stator resistance
B Frequency and poles
C Rotor resistance
D Slip
The synchronous speed of an induction motor depends on the supply frequency and the number of poles in the motor. It is calculated using the formula: Ns = (120f)/P, where f is frequency and P is the number of poles.
What is the primary difference between induction motors and synchronous motors?
A Induction motors run at synchronous speed
B Synchronous motors require slip rings
C Induction motors have no stator windings
D Synchronous motors require external excitation
The primary difference is that synchronous motors require external DC excitation to generate a magnetic field, while induction motors generate their own field through induction and operate slightly slower than synchronous speed.
What is the effect of high voltage on the stator windings of a synchronous motor?
A Reduced current
B Increased torque
C Overheating
D Higher speed
High voltage applied to the stator windings of a synchronous motor can cause excessive current, leading to overheating. This can damage the windings and decrease the motor’s lifespan.
What does the term “slip” refer to in an induction motor?
A Speed variation
B Rotor current
C Voltage drop
D Magnetic field intensity
Slip refers to the difference between the synchronous speed and the actual rotor speed in an induction motor. It is typically expressed as a percentage and increases with the load on the motor.
Which of the following is a disadvantage of squirrel cage rotors in induction motors?
A Requires slip rings
B Difficult to control speed
C High starting torque
D Low maintenance
Squirrel cage rotors are simple and require low maintenance, but controlling speed is difficult without additional equipment, as they cannot be easily adjusted once the motor is running.
What is the impact of increased load on an induction motor’s power factor?
A Power factor remains constant
B Power factor becomes unity
C Power factor increases
D Power factor decreases
As the load on an induction motor increases, the motor’s power factor typically decreases due to the higher reactive power required to magnetize the motor and the increase in slip.
What is the main purpose of slip rings in a wound rotor motor?
A Provide high torque
B Control rotor speed
C Transfer electrical energy
D Generate power
Slip rings are used to transfer electrical energy to the rotor windings in a wound rotor induction motor. This connection allows the addition of external resistances during startup, improving performance.
What causes core losses in an induction motor?
A Eddy currents and hysteresis
B Stator winding losses
C Rotor friction
D Magnetic leakage
Core losses in an induction motor are caused by eddy currents and hysteresis in the stator and rotor cores. These losses result in heat generation and reduce the efficiency of the motor.
What is the main advantage of using a wound rotor in induction motors?
A Simple design
B Low slip
C High starting torque
D Low cost
The main advantage of using a wound rotor in an induction motor is the ability to add external resistances, which increases the starting torque. This helps the motor start under heavy load conditions more efficiently.