3.18 AC Motors Flashcards
What are the two main categories of AC motors?
Synchronous motors and asynchronous motors (induction motors)
These categories differ primarily in their rotor speeds relative to the stator magnetic field.
Define synchronous motors.
Machines whose rotor speeds are equal to the speeds of the stator magnetic field
These motors do not experience slip.
Define asynchronous motors.
Machines where rotors rotate at speeds less than the synchronous speed
These are commonly referred to as induction motors.
What is the primary advantage of using AC motors?
They cost less than DC motors and require less maintenance
They also eliminate issues with brushes and commutators.
What is the purpose of damper windings in synchronous motors?
To provide starting torque and minimize hunting
They are typically made of heavy copper bars inserted in the rotor pole faces.
What is the Load Angle in synchronous motors?
The difference between the speed of the stator magnetic field and the rotor speed
It equates to the ‘Slip’.
What distinguishes salient pole machines from non-salient pole machines?
Salient pole machines have radially arranged pole shanks; non-salient pole machines have cylindrical rotors
Salient pole machines are suitable for low speeds, while non-salient ones are for high speeds.
True or False: Synchronous motors have starting torque.
False
Synchronous motors require external starting equipment to reach rated speed.
What is the primary construction of induction motors?
They consist of a stator and a rotor
The stator has three-phase windings and the rotor typically has a squirrel cage design.
How is torque produced in induction motors?
By the induced EMF in the rotor due to the stator’s rotating magnetic field
The rotor current opposes the rotating magnetic field, creating torque.
What is the formula for slip in induction motors?
Slip = Synchronous speed - Actual speed
Slip is essential for the operation of induction motors.
What is the typical phase arrangement in a three-phase induction motor?
Three coils placed 120° apart
This arrangement ensures smooth rotation of the magnetic field.
What is the primary characteristic of induction motors?
They are self-starting and robust in construction
They are also economical and easy to maintain.
Fill in the blank: Synchronous motors operate at a _______ speed.
[constant]
Their speed is determined by the number of poles and supply frequency.
Which type of induction motor has its stator made up of two windings placed at right angles?
Two-phase induction motor
These motors are not common due to impracticality in modern applications.
What effect does increasing load have on low-impedance induction motors?
It does not greatly reduce speed but increases torque
This characteristic helps maintain consistent speed despite load changes.
What is the synchronous speed for a two-pole machine at 60 Hz?
3600 RPM
This speed is determined by the frequency of the applied voltage.
What happens if the rotor speed equals synchronous speed in an induction motor?
No EMF is induced in the rotor, leading to no torque generated
This prevents the rotor from reaching synchronous speed.
What are the characteristics of synchronous motors compared to induction motors?
Synchronous motors are sensitive to load changes and have no starting torque
Induction motors, on the other hand, are less sensitive to load variations.
What is the relationship between rotor speed and the magnetic field in AC motors?
If the rotor reaches the same speed as the magnetic field, there is no induction and the motor torque drops.
What happens when the rotor lags behind the rotating field in an AC motor?
The rotor cuts the flux lines, producing torque.
What term is used to describe the difference in speed between the motor and the field?
‘Asynchronous’
What are the two viable methods for controlling the speed of an AC induction motor without rebuilding it?
- Change the frequency
- Use a wound rotor
Why is changing frequency not viable for aircraft AC induction motors?
AC induction motors are designed for a specific speed in terms of construction.
What is a wound rotor induction motor?
A special type of 3-phase AC induction motor designed to provide high starting torque and variation in speed.
What is the function of the slip rings in a wound rotor motor?
To connect external resistance in the rotor circuit.
How does varying the resistance in the rotor circuit of a wound rotor motor affect its speed?
Increasing resistance decreases speed; decreasing resistance increases speed.
What is the effect of increased rotor resistance on torque in a wound rotor motor?
More voltage is induced in the rotor circuit, producing necessary torque and increasing it.
How is the direction of rotation changed in a 3-phase induction motor?
By reversing any two of the three wires.
What controls the direction of rotation in a single-phase induction motor?
A starting capacitor.
Fill in the blank: The rotor follows the _______ phase in AC motors.
[leading]
True or False: The speed of a wound rotor motor is equal to the synchronous speed of the rotor.
False
What is a single-phase induction motor?
A motor that runs on single-phase AC and is used for small low-output applications
Examples include communication equipment, fans, refrigerators, and power tools.
What are the two kinds of single-phase motors?
- Induction motors
- Series motors
Describe the characteristics of a series AC motor.
- Varying-speed machine
- Low speeds for large loads
- High speeds for light loads
- Very high starting torque
What is the primary difference between a single-phase induction motor and a series AC motor?
The single-phase induction motor has only one stator winding that generates a non-rotating field.
What is the purpose of the capacitor in a capacitor start motor?
To create a phase difference between the primary and auxiliary windings for self-starting.
What is synchronous speed?
The speed at which the magnetic field rotates in a rotary machine, determined by frequency and number of poles.
What are the two types of induction motors?
- Single-phase induction motor
- Three-phase induction motor
How does the rotor in an induction motor generate torque?
The rotor flux lags behind the stator flux, creating a torque that causes rotation.
What is the role of the starting capacitor in a capacitor start induction motor?
Provides additional starting torque by being connected in parallel during startup.
Fill in the blank: A shaded pole motor uses a _______ wound in a small notch in the stator pole piece.
shading coil
What happens when the shading coil in a shaded pole motor is energized?
It produces a field opposite to that of the main field, inducing rotation in the rotor.
True or False: A single-phase induction motor is self-starting.
False
What is the function of the centrifugal switch in a capacitor motor?
Turns off the starting capacitor after the motor reaches 70-80% of its rated speed.
List the categories of induction motors.
- Single-phase induction motor
- Split-phase induction motor
- Capacitor start induction motor
- Capacitor start and capacitor run induction motor
- Shaded pole induction motor
- Three-phase induction motor
- Squirrel-cage induction motor
- Slip ring induction motor
What is the effect of connecting a capacitor to the auxiliary winding?
It creates a 90-degree phase difference necessary for starting the motor.
What is the primary difference between squirrel-cage and wound induction motors?
Squirrel-cage motors have a simple rotor design, while wound motors have windings on the rotor.
How can the direction of rotation in a three-phase motor be changed?
By altering the phase sequence of the three-phase supply.
What is the importance of slip in an induction motor?
It is necessary to induce rotor current and generate torque.
