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
