AC and DC

• DC and AC Motors and Generators

  Generators - convert mechanical energy into electrical energy<|>Motors - convert electrical energy into mechanical energy<|>Both operate through the interaction between a magnetic field and a set of windings
• Forms of rotating Electrical Machines


  • Generators
  • Motors
• DC Motor


  • Converts electrical energy into mechanical energy
  • Depends on the force on a conductor carrying current in a magnetic field
• DC Machine Construction

  1. Armature

  2. Commutator

  3. Field windings
• Armature


  Soft iron drum mounted on the motor shaft, houses the armature conductors or coils and provides low reluctance path for flux
• Commutator


  Made of insulated copper segments, two brushes pressed to permit current flow, brushes made of carbon or graphite
• Types of DC Motors

  • Series wound
  • Shunt wound
  • Compound wound
• Series Wound Motor


  Field coils connected in series with the armature
• Examples of series wound motor use

  • Engine starter motors
  • Flap operating motors
  • Landing gear operating motors
• Series wound motors should never be allowed to operate without a mechanical load applied as they are liable to over-speed, possibly to destruction
• Shunt Wound Motor


  Field coils connected in parallel with the armature windings
• Typical shunt wound motor applications in aircraft

  • Fuel pumps
  • Fans
• Compound Wound Motor


  Has two sets of field windings, one in series with the armature and one in parallel
• Compound wound motor applications in aircraft


  • Hydraulic pumps
  • Starter/generator
• DC Generator/Dynamo

  Converts mechanical energy into electrical energy<|>Can use permanent magnets or electrically generated magnetic field
• Principle of Operation of DC Generator


  1. Conductor rotates in magnetic field

  2. Induces EMF in conductor
• AC Generators/Alternators do not require a commutator, allowing simpler construction
• AC Generator/Alternator


  Field coils rotate while armature windings are stationary<|>Armature windings produce the output

  
• Examples of AC Generator/Alternator

  • Example 1
  • Example 2
• Voltage Calculations for AC Generator

  1. E = BLv

  2. E = 4.44fNΦ
• Current Calculations for AC Generator


  I = E/Z
• Frequency Calculation for AC Generator


  f = P/2 * n/60
• Types of AC Motors

  • Synchronous motors
  • Induction motors

  
• High-power AC motors invariably operate from a three-phase supply, but single-phase versions are also widely used
• Synchronous Motor


  AC generators can be used as synchronous AC motors
• Induction Motor


  Current is induced in the rotor by transformer action, rather than using slip rings to pass current to the field coils
• The armature of a DC motor is laminated to reduce eddy current loss
• Around 25% of motors manufactured today are DC motors
• Causes of rapid brush wear in DC generators

  • Severe sparking
  • Rough commutator surface
  • Imperfect contact
• Alternating Current (AC)


  Large-scale generators produce AC, follows sine wave with n cycles per second, can be 1, 2 or 3-phase, has transforming ability
• Direct Current (DC)


  From batteries, photovoltaics, fuel cells, small DC generators, charge in one direction, has negative and positive terminals, easy conversion from AC to DC but not DC to AC
• Power generation is dominated by AC machines, ranging from automotive alternators to the synchronous generators used in power stations, with efficiency increasing with size up to 98%
• Both DC and AC motors are used, with high-power motors usually AC three-phase, and domestic applications often using single-phase induction motors, while DC motors are useful in control applications
• The rotation of a coil in a uniform magnetic field produces a sinusoidal EMF, which is the basis of an AC generator
• A commutator can be used to produce a DC generator
• The magnetic field in an electrical machine is normally produced electrically using field coils
• DC motors are often similar in form to DC generators
• Some forms of AC generator can also be used as motors
• The most widely used form of AC motor is the induction motor
• All AC supplies are sine waves, other wave shapes are encountered occasionally