What is the primary purpose of using a star delta starter in induction motors?
A Increase torque
B Control speed
C Limit inrush current
D Reduce slip
The star delta starter is used to reduce the inrush current during startup. By initially connecting the motor in a star configuration, the voltage is reduced, limiting the current drawn by the motor.
What is the typical braking method used in an induction motor?
A Plugging
B Dynamic braking
C Regenerative braking
D Both B and C
Induction motors typically use dynamic braking (where the motor is connected to resistors to dissipate energy) and plugging (where the direction of rotation is reversed to stop the motor quickly).
How does the braking method “regenerative braking” work in an induction motor?
A By converting kinetic energy to electrical energy
B By reversing the motor direction
C By applying external resistances
D By using slip rings
In regenerative braking, the induction motor acts as a generator. The motor’s kinetic energy is converted back into electrical energy, which can be fed back into the power supply or dissipated through resistors.
What happens during the plugging of an induction motor?
A Speed increases significantly
B Direction of rotation is reversed
C Motor loses power
D Motor accelerates rapidly
Plugging occurs when the motor’s supply connections are reversed while the motor is running, causing the rotor to move in the opposite direction. This rapid reversal creates a braking effect, quickly stopping the motor.
What type of motor is typically used in applications requiring frequent starts and stops?
A Permanent magnet motor
B Synchronous motor
C Induction motor
D Universal motor
Induction motors are widely used for applications requiring frequent starts and stops because of their simple construction, reliability, and ability to withstand thermal stresses during startup and shutdown cycles.
What starting method is typically used for large induction motors to reduce starting current?
A Star delta start
B Capacitor start
C Direct on line start
D Rotor resistance start
The star delta start method is used for large induction motors to reduce the starting current. It initially connects the motor in a star configuration, lowering the voltage and reducing the current draw during startup.
What is the key difference between a wound rotor and a squirrel cage rotor in induction motors?
A Squirrel cage is more expensive
B Wound rotor has external resistors
C Wound rotor is simpler
D Squirrel cage requires external resistors
The wound rotor has windings on the rotor and allows the addition of external resistors during startup to control the inrush current and increase starting torque. The squirrel cage rotor does not require this feature.
Which method is commonly used to increase the torque output of an induction motor?
A Use an external resistor
B Decrease supply frequency
C Increase slip
D Add capacitors
Increasing the slip in an induction motor increases the relative speed between the rotor and the rotating magnetic field. This allows the motor to produce more torque, which is useful for handling higher loads.
What happens when the load on an induction motor is reduced?
A Torque decreases
B Slip increases
C Power factor increases
D Slip decreases
When the load is reduced, the rotor speeds up and the slip decreases. Slip is the difference between the synchronous speed and rotor speed, and it decreases as the load on the motor decreases.
What does the term “synchronous speed” refer to in a synchronous motor?
A Speed of the motor at maximum load
B Speed of the rotor
C Speed of the stator’s magnetic field
D Speed at no load
Synchronous speed is the speed at which the magnetic field in the stator of a synchronous motor rotates. The rotor must match this speed in order to operate synchronously with the stator’s magnetic field.
Which of the following is a disadvantage of a wound rotor motor compared to a squirrel cage motor?
A Higher cost
B Requires external resistors
C Lower starting torque
D Higher efficiency
Wound rotor motors are more expensive than squirrel cage motors because they require additional components like external resistors and slip rings. This makes their initial cost and maintenance higher.
What is the typical purpose of the capacitor in a capacitor start induction motor?
A Increase power factor
B Improve efficiency
C Reduce slip
D Create a rotating magnetic field
The capacitor in a capacitor start induction motor helps create a phase shift between the stator windings, producing a rotating magnetic field that starts the motor. Once the motor reaches a certain speed, the capacitor is disconnected.
What is the purpose of the excitation system in a synchronous motor?
A Regulate power factor
B Create the rotor magnetic field
C Control rotor speed
D Control starting torque
The excitation system in a synchronous motor provides DC current to the rotor windings, creating a magnetic field that interacts with the rotating magnetic field of the stator to maintain the motor’s synchronous speed.
What method is often used for braking high speed induction motors?
A Dynamic braking
B Plugging
C Both B and C
D Regenerative braking
High speed induction motors are often braked using dynamic braking, where the motor’s kinetic energy is dissipated through resistors, or by plugging, where the direction of rotation is reversed to rapidly stop the motor.
What happens if a synchronous motor is overloaded?
A It loses synchronization
B It accelerates
C It maintains speed
D It produces more torque
If a synchronous motor is overloaded, it cannot maintain synchronization with the stator’s rotating magnetic field. This causes the rotor to lose.