59 Questions
What is a key advantage of permanent magnetic generators (PMGs) in comparison to conventional generators?
They require no brushes or slip rings.
How do brushless alternators differ from traditional alternators in terms of cooling?
Brushless alternators are air cooled.
What is the main function of an exciter field in a brushless alternator?
To induce current into the field coil.
Why are brushless alternators more efficient at high altitudes compared to traditional alternators?
Because brushless alternators have no brushes or slip rings that can arc at high altitudes.
In a brushless alternator, what produces the magnetic flux to start the generator?
Permanent magnets
What is the purpose of the main output field in a brushless alternator?
To generate electrical output.
What determines the direction of current movement in an electrical circuit?
Variations in voltage terminal polarities
What do variations in the negative alternation of a sine wave indicate?
Voltage terminal polarities have reversed
What does the amplitude of a sine wave represent?
Magnitude of voltage
In which waveform is the alternating current (AC) most commonly identified?
Sine wave
How are both current and voltage represented in a sine wave?
Two vertical axes
When is current considered to be flowing in a positive direction on a sine wave?
Above the time axis
What is the phase difference between the voltages generated in the three windings of a three-phase alternator?
120°
What is the relationship between the voltage at terminal C and the phase voltages A and B in a two-phase alternator?
The voltage at C is 1.414 times the phase voltages
Why is the two-phase alternator seldom seen in actual use?
It has fewer advantages over the single-phase alternator
What is the purpose of the small vector diagram mentioned in the text?
To show the phase relationship between the three windings
What is the main advantage of the three-phase alternator over the single-phase type?
It is more widely used in aircraft
Why is the schematic diagram of a three-phase stator complex and difficult to understand?
It shows all the individual coils of each phase
What is the primary function of the exciter field winding in a brushless alternator?
To induce voltage into the exciter output winding
How is the output of the main generator regulated in a brushless alternator?
By controlling the current to the exciter field winding
Which component(s) rotate as a unit in a brushless alternator?
Permanent magnet, exciter output winding, six diodes, and output field winding
Where are the main output stator windings located in a brushless alternator?
Wound in slots in the laminated frame of the alternator housing
What is the purpose of the six silicon diodes in a brushless alternator?
To rectify the output from the exciter output winding
Why is there no need to 'flash the field' in a brushless alternator?
Because the alternator utilizes a permanent magnet
What is the primary advantage of using a permanent magnet in a generator?
It eliminates the need for brushes to carry current to the rotating assembly.
What does a sinusoidal sine wave represent in the context of a generator?
The value of induced EMF at each instant during a 360° rotation of the coil.
What distinguishes alternating current (AC) from direct current (DC)?
AC flows first in one direction and then in the opposite direction, while DC flows in one direction only.
What is the most common waveform associated with alternating current (AC)?
Sine (or sinusoidal) wave
When does a conductor produce a greater potential difference (or induced EMF) in a generator?
When it is cutting lines of flux quickly.
What does each cycle of a sine wave represent in the context of a generator?
Two identically shaped variations in voltage, indicating current movement in opposite directions.
What is the main difference in speed calculation between a three-phase motor with six poles and a single-phase motor with two poles?
The calculated speed for the three-phase motor is three times too small.
What is the primary characteristic of synchronous motors in terms of speed?
They operate almost exactly at the synchronous speed.
What is the percentage difference between the full-load speed and the synchronous speed in asynchronous motors called?
Slip
What are the two practical methods mentioned in the text to change the rotation speed of AC induction motors?
Use a frequency converter or use a motor with separate windings for different speeds.
What type of device are induction motors considered to be?
Practically fixed-speed devices.
For applications requiring accurate speed control, what is recommended?
Using a frequency converter.
What is a disadvantage of a synchronous motor in terms of starting?
It cannot be started by applying three-phase AC power to the stator.
What is the purpose of adding a squirrel-cage winding to the rotor of a synchronous motor?
To help the rotor start by interacting with the stator's magnetic field.
What does the heavy copper bars shorted together by copper rings in a squirrel cage winding resemble?
A squirrel's wheel
Why does a relatively large current flow in the squirrel cage of a synchronous motor?
Due to the short circuit in the squirrel cage winding.
What occurs to the rotor in a synchronous motor when AC is applied to the stator?
The rotor is repelled first in one direction and then in the other.
What is the main function of a squirrel-cage winding in a synchronous motor?
To interact with the rotating field of the stator for starting.
What is the main reason an induction motor cannot run at synchronous speed?
There would be no induced voltage in the rotor if the speeds were the same.
What is the name given to the difference between the speed of the rotating stator field and the rotor speed in an induction motor?
Slip
What is the purpose of the small air gap between the rotor and stator in an induction motor?
To increase the interaction between the stator and rotor fields.
What is the main force that causes the rotor of an induction motor to move in the same direction as the rotating stator field?
The induced EMF in the rotor.
How does increasing the resistance in an induction motor affect its speed?
It decreases the speed of the motor.
What is the purpose of using a star-delta arrangement for starting large 3-phase induction motors?
To limit the starting current and achieve full voltage at speed
What is the relationship between the torque angle and the load on a synchronous motor?
The torque angle increases as the load increases
What is the maximum value of torque that a synchronous motor can develop without losing synchronism?
Pull-out torque
What happens to a synchronous motor if the mechanical load is increased beyond the pull-out torque?
The motor will stop
In a star-wound stator winding, what is the effective voltage across each winding compared to a delta-wound winding?
$\frac{1}{\sqrt{3}}$ times the voltage
What is the purpose of using a star-delta arrangement for starting large 3-phase induction motors?
To limit the starting current and achieve full voltage at speed
What is the primary reason for split-phase motors being limited to small sizes?
The auxiliary winding is only a light winding, limiting the starting torque.
What is the primary advantage of permanent-split capacitor motors?
Their running performance and speed can be tailored by capacitor selection.
How does a resistance-start motor differ from a capacitor-start motor?
The phase shift is obtained by making the winding impedances unequal.
What is a common application for permanent-split capacitor motors?
Fans and blowers with low-inertia loads.
Why are split-phase motors used in applications where high starting torque is not required?
They have desirable characteristics despite the low starting torque.
How can you identify a capacitor start motor?
By the bulbous protrusion on the frame where the starting capacitor is located.
Learn about permanent magnetic generators (PMGs) which do not require power to be supplied to the field, eliminating the need for brushes or slip rings. Discover how PMGs can be designed to operate in either voltage mode or current mode, and their importance in providing power to various components on gas turbine engines.
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