The synchronous speed of a motor is determined by the supply frequency and the number of poles in the motor. It can be calculated using the formula: Ns = (120f)/P, where f is the frequency and P is the number of poles.
What does slip in an induction motor indicate?
A Magnetic field strength
B Speed difference
C Load applied
D Rotor resistance
Slip in an induction motor is the difference between the synchronous speed and the actual rotor speed. It is usually expressed as a percentage of synchronous speed and is necessary for the motor to produce torque.
What happens to the torque of an induction motor as the load increases?
A Torque increases
B Torque becomes unstable
C Torque remains constant
D Torque decreases
As the load on an induction motor increases, the slip increases, which allows more torque to be generated. The motor adjusts to meet the load by increasing the interaction between the rotor and stator magnetic fields.
What is the primary function of a synchronous motor?
A High starting torque
B High torque production
C Variable speed operation
D Constant speed operation
A synchronous motor operates at a constant speed, which is the same as the synchronous speed determined by the supply frequency and the number of poles. This makes it ideal for applications requiring precise speed control.
What is the effect of an increase in load on the slip of an induction motor?
A Slip remains constant
B Slip decreases
C Slip increases
D Slip becomes negative
As the load on an induction motor increases, the rotor slows down slightly, causing the slip to increase. This allows the motor to generate more torque to meet the increased load requirements.
Which part of the synchronous motor must be supplied with DC current?
A Shaft
B Rotor windings
C Bearings
D Stator windings
The rotor windings of a synchronous motor must be supplied with DC current to generate a magnetic field. This magnetic field interacts with the stator’s rotating magnetic field to synchronize the motor’s speed.
What is the primary cause of core losses in an induction motor?
A Eddy currents
B Stator windings
C Rotor resistance
D Slip rings
Core losses in an induction motor are primarily caused by eddy currents induced in the motor’s core due to the alternating magnetic field. These losses generate heat and reduce the efficiency of the motor.
How does the torque speed characteristic of an induction motor look?
A Horizontal
B Linear
C S shaped
D Parabolic
The torque speed characteristic of an induction motor typically follows an S shape. As the motor starts, the torque increases, then decreases as the motor approaches synchronous speed. At synchronous speed, the torque is almost zero.
In a synchronous motor, what happens when the excitation is too low?
A Motor operates at higher speed
B Motor accelerates
C Motor increases torque
D Motor loses synchronization
If the excitation in a synchronous motor is too low, the rotor cannot maintain synchronization with the stator’s magnetic field. This results in the motor losing its constant speed and performance being affected.
What is the primary purpose of using slip rings in a wound rotor motor?
A To provide external resistances
B To regulate voltage
C To generate power
D To control speed
In a wound rotor motor, slip rings provide a continuous connection to external resistances, which are used to control the starting torque. These resistances help reduce the inrush current during startup and improve motor performance.
What happens to the rotor current when the slip of an induction motor increases?
A Remains constant
B Increases
C Becomes zero
D Decreases
As the slip of an induction motor increases, the relative speed between the rotor and the rotating magnetic field increases. This results in higher induced currents in the rotor, which produce more torque to handle the increased load.
What is the characteristic of a squirrel cage rotor in an induction motor?
A Uses external resistances
B Requires slip rings
C Simple and maintenance free
D Provides high starting torque
Squirrel cage rotors are known for their simplicity and durability. They are maintenance free and do not require slip rings or external resistances, making them widely used in industrial applications due to their robust design.
What happens to an induction motor if the load is removed?
A Rotor speed increases
B Efficiency decreases
C Slip increases
D Rotor speed decreases
When the load is removed from an induction motor, the rotor speed increases as the motor approaches synchronous speed. The slip decreases, and the motor operates at a higher speed with less torque demand.
How does an increase in voltage affect the speed of an induction motor?
A Reduces efficiency
B Increases speed
C Decreases speed
D No effect on speed
In an induction motor, the speed is primarily determined by the supply frequency and the number of poles, not the voltage. Therefore, increasing the voltage does not affect the motor’s speed but can affect its torque and efficiency.
What is the role of the exciter in a synchronous motor?
A Control power factor
B Regulate rotor speed
C Provide DC current to the rotor
D Start the motor
The exciter in a synchronous motor provides DC current to the rotor to create a magnetic field. This field interacts with the rotating magnetic field of the stator, allowing the motor to operate at synchronous speed.