Motors And Generators Flashcards
1.1 Outline the properties of permanent magnets
• Magnet that does not lose its magnetic force
Detail the handling precautions for permanent magnets
- don’t drop it
- No sharp blows
- Don’t influence with other magnets
- must be fitted with a keeper
2.1 Identify the factors that influence the magnitude of induced EMF in a DC generator
The strength of the induced voltage is dependant on the speed of the conductor and the strength of the magnetic field
2.2 Name the components of a dc generator
• Carcass, frame or yoke
• Field Coils
• Field pole
• Armature
• Commutators
2.3 Describe the operation of the components of a DC generator
• Carcass, frame or yoke is used to:
◦ Complete the magnetic circuit between the poles
◦ Mechanically support the other parts of the generator
• Field coils
◦ Produce the magnetic field by allowing current to flow through the coils
• Armature
◦ Cuts through the magnetic field
◦ Produces the electric current
• Commutators
◦ Converts the AC current into DC current
◦ Different sections of the commutator come into contact with the brushes and will alternate between each other
2.4 Identify the neutral plane of a DC generator
The point where the current in the armature produced is equal to zero
2.5 Explain armature reaction in DC generators
• The current flowing through the armature creates a seperate magnetic field which affects the magnetic flux of the generators poles, bending and disturbing them
• This distortion increases with an increase in load
• Geometric neutral plane is the position where the plan of two opposite armature coils is perpendicular to the undistorted magnetic field
2.6 Describe the methods of overcoming armature reaction in DC generators
• Brush positioning
◦ Moving the brushes so they align with the new disturbed field
• Compensating windings
• Interpoles
3.1 Determine the direction of resultant conductor movement through a magnetic field from an applied current
• Conductors will move in a way that they combine or subtract from each other, almost in a super positioning sense, when multiple coils are used in the armature
• Magnetic fields occur around the conductor, with areas of strong force and weak force
◦ This difference creates torque which spins a motor, converting electrical force into mechanical force
3.2 Explain the right hand rule
• Motion goes up with thumb
• Magnetic flux goes out with index finger
• Current points left from middle finger
3.3 Identify the construction features of DC motor components
• Armature assembly
◦ Laminated to reduce eddy currents
◦ Contains the commutator as well
• Field assembly
• Field poles and field coils
• Brushes
• End bells and bearings
3.5 Describe the construction features of common DC motor designs
• Series DC Motor
◦ High starting torque
◦ High starting current
• Shunt DC Motor
◦ Resistance is very high
◦ Current is constant through the field
◦ Suitable for when constant speed is required and high starting torque is not required
• Compound DC Motor
◦ Combination of the two previous motors
3.6 Identify typical aircraft applications for common DC motor designs
• Series motors are used in aircraft as engine starters, land gears, cowl flaps and wing flaps
• Shunt motors are ideal when constant speed is desired and when high starting torque isn’t
• Compound required when both characteristics are required
3.7 Explain the advantage of using DC starter-generators in aircraft
• Saves weight by eliminating a seperate starter that is only used during the start
3.8 Explain the effect of counter-EMF in DC motors
• Counter EMF opposes the current and the rotation of the armature
• The polarity of counter EMF is opposite the torque of the armature
• A build up of eddy currents that oppose the armatures magnetic field
3.9 Describe methods of speed control in DC motors
• Varying the current in the field windings
◦ A motor in which speed can be controlled is called a variable speed motor
3.10 Detail common DC motor maintenance checks
• Operation of motor
• Wiring, connection and terminations etc
• Clean and mounting bolts tight
• Check brushes for minimum length and spring tension
• Inspect commutator
4.1 State the common aircraft alternator design configurations
Rotating armature
Rotating field
4.2 Describe the operation of the components of an aircraft alternator
Slip rings
Brush block
Stator
Rotor
4.3 Identify physical characteristics of three phase aircraft alternators
Has three single phase windings spaced with 120 degrees between
4.4 List the common output connection methods for three phase aircraft alternators
Star/Wye
Delta
4.5 Describe line and phase voltage relationships of three phase aircraft alternators
Star connections have an increase in voltage
Delta connections have an increase in current
4.6 Describe the output phase sequence of three phase aircraft alternators
4.7 Identify the advantages of brushless aircraft alternators
More efficient and produces less wear as brushes, commutators are not required
Won’t arc at high altitudes with no brushes
4.8 Describe the operation of a brushless aircraft alternator
Uses multiple generators in the one alternator to induce and amplify a current
4.9 Explain aircraft alternator output frequency production
4.10 Detail common aircraft alternator maintenance checks
Secure attachment
Terminal board covers fitted and secured
Alternator clean from oil and grease
Electrical connections to the alternator are tight and correct phase sequence has been maintained
4.11 Explain the operation of an aircraft generator control unit
Regulates, senses and protect the AC generation system
Can regulate the generator output
4.12 Identify the purpose of constant speed drive units in aircraft
What are the two types of AC motors
Induction and synchronous
The three types of AC generators may be:
(hint: phase)
Single, two, three phase
What is three phase induction used for
Where large amounts of power are required
What is the purpose of single phase AC induction motors
Much smaller devices that require less power output
What is the characteristic and purpose of three phase synchronous AC motors
Operate at constant speed and are used for flux grate compasses and propeller synchroniser systems
What are the two sections of an inductive AC motor
Rotor and Stator
What is the plane of commutation
The resulting neutral plane that occurs after armature reaction
How do you reduce eddy currents
Laminate shit
What is the geometric neutral plane
position where the plane of the two opposite armature coils is perpendicular to the undistorted magnetic field in the generator.
What are generators rated with
Amps
What is the speed of frequency in AC motors called
Synchronous speed
What is a capacitive start motor
Uses two windings and a parallel circuit to create a split phase. A centrifugal switch will activate once split has been produced
What is the purpose of a reverse current relay
To prevent current leaving from a battery and feeding the generator
6.1 Identify typical aircraft applications for linear actuators
Trim systems, auxiliary door activation
6.2 Describe the design features of electrical linear and rotary actuators
6.3 Describe the construction features of electrical linear and rotary actuators
DC motor
Reduction Gear Train (Rotary gear box for rotary)
Ball in screw mechanism
Limit switches
6.4 Identify typical aircraft applications for electrical rotary actuators
Used to control fuel flow, oil flow, pneumatic systems and environmental control systems
What are three methods for starting a single phase induction motor
Split phase
Capacitor start
Shaded pole
Which electricity type can be operated by an AC series motor
AC AND DC
If you see a block diagram that looks dumb what is it
Regulated sine wave static inverter
Describe phase rotation
Swapping the phase of a an AC motor to change the direction of rotation
Induction AC Motor use a “” field in the stator
Rotating field
How does an induction motor spin
The stator induces a current into the rotor with a magnetic field, creating a magnetic field in the rotor, which creates relative motion in the motor.
How does a synchronous motor function
By applying DC current to a rotor winding, locking it into place with the multiphase rotating AC power source in the stator
What is slip in an AC motor
Difference in speed of rotation between the magnetic field of the stator and the rotor
What is the major advantage of a rotating field AC generator
Doesn’t require slip rings, which requires less maintenance
How should the direction of rotation be changed in an AC motor
By reversing two of the leads to the motor
What is the purpose of a constant speed drive
Regulates the frequency of AC power
What is the advantage of a series DC motor
Can draw large amounts of torque on starting due to low resistance
Memorise this
Describe the construction of a split field series motor
Uses two field windings wound in opposite directions on the same pole.
