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The electric motor

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A presentation done for part of my A-level studies, and also for my personal interest too.

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The electric motor

  1. 1. The electric motor Electromagnetic effect By Kelvin Lam
  2. 2. Topics: • Motor introduction • Motor vs. engines • Magnetism • Fleming’s Left/Right Hand Rule • Linear motor • DC Motor (brushed) • AC Motor (3-phase) • Alternator • Eddy current brake
  3. 3. What is a motor? • ‘A device which converts electrical energy to mechanical torque.’ The asynchronous three-phase AC traction motor in the bogie of the Eurostar train.
  4. 4. Motor vs. engines • An IC engine and an electric motor both produces a mechanical torque. But engine converts chemical energy to mechanical torque via thermal energy; a motor converts electrical energy to torque. • The ‘Otto’ engine cycle as illustrated below uses a 4 stroke system: ‘intake, compression, combustion, exhaustion’.
  5. 5. Permanent Magnets • Ferrimagnetism: An electron has a ‘spin’ quantum mechanical property. It defines the rotatory (angular momentum) of the electron orbiting around an atom. • They are orientated randomly. • At specific temperature (Curie), the electrons which induces electromagnetic dipole aligns itself, causing a magnetic field as it is polarised. Thus the metal is magnetised.
  6. 6. Electromagnetism • Electromagnets exhibits property of magnetism only when current runs through it. • As electric current passes through a ferromagnetic element the particles become charged and begins to move in a path. Again due to the ‘dipole’ it creates a magnetic field. • The strength of this field depends on the cross section area of the conductor, current and the frequency ‘of the change of current’.
  7. 7. Fleming’s Left Hand Rule • We can use a mnemonic, ‘Fleming’s Left Hand Rule’, to understand the relationship of electric current and the ‘thrust of motion’ caused by it.
  8. 8. Right hand rule • Maxwell’s Corkscrew Rule • Thumb shows the direction of current; rest of hand shows direction of magnetic field. • As magnetic field is applied across the flow of electrons, it affect the spin of the electrons which affect the ‘atomic’ magnetic field, causing a repulsion.
  9. 9. Linear motor • Using the left-hand rule, having a current perpendicular to the magnetic field produces a linear motion. • Coils (in loop) produces an Eddy current field, producing magnetic field. • Used in high-speed transportation.
  10. 10. Linear motor (The Shanghai Maglev Train in China has a top speed of 431km/h, equivalent to 268mph)
  11. 11. Simple Direct current motor • The motor effect is observed when there is a changing magnetic field. • Right diagram shows a split-ring commutator DC motor: earliest and least efficient of all. • What if the commutator is short-circuited – stuck in middle of the two brushes?
  12. 12. • But in reality, DC motor has 3 coils. Each with 120 degrees angle. • There are 3 commutators and 2 brushes. • One of the 3 coils is inactive.
  13. 13. ‘DC is now obsolete!’ • Cheap, easy to operate. • Excellent for acceleration/speed control. • High precision. • Maintenance of mechanical rotating brushes. • Friction is enormous: not efficient for high speed performance, i.e. trains. • Sparks from brushes may initiate explosions. DC Motor are used in new state-of-art electric aircrafts.
  14. 14. AC Current • The electric current repeatedly changes its direction. • Single & Three Phase • Single phase: 360 degrees • Three phase: 120 degrees
  15. 15. Three-phase AC • It carries 3 alternating current of the same frequency. • Each current has a time-separation. • It gives a constant electrical power to turn the AC motor. • Due to the ‘superposition’ of current, it tends to cancel the p.d. each other so that it reduce the size of neutral wire.
  16. 16. Three-phase AC motor • Stator produces a varying magnetic field with AC. • This induces a secondary current in the rotor due to magnetic flux of different direction. • Lenz Law: the rotor then induces a magnetic field that oppose the stator.
  17. 17. Motor control Direct current • Chopper control • PWM (Pulse-width modulation) • Resistance (Cam shaft) • Thyristor • Bridge rectifier Single/3 Phase AC • VFD (Variable-frequency drive) • Inverter
  18. 18. • http://www.youtube.com/watch?v=LpvyrN8 YOBo
  19. 19. Regenerative braking • In railway (or hybrid) vehicles, the vehicle’s inertia drives the rotor, generating induction current. • As electricity is generated, this causes a ‘negative’ torque, slowing the vehicle down. • The produced electricity is either fed into resistor (dynamic braking) or fed back to the electric supply.
  20. 20. Eddy current brake • Conventional brake uses friction. • Rotating disc (which are exposed to electromagnetic field) induces an eddy current (opposing current) on the coils. • This produces a opposing braking force for the train. • Magnets are placed 7mm away from the rail to allow room for the rotating disc. • Braking strength controlled by strength of magnetic field. • Only usable in high speed situation. • No energy wasted, no heat, no odour. (Eddy current brake in Japanese Shinkansen 700)
  21. 21. Dynamo & Generator • Dynamo is an older term that describe something that makes direct current. • Early inventors discovered that electromagnetic effect could generate AC, but too complicated to control them. • It has a commutator. • The rotating magnets produces a varying magnetic field, thus generate a varying current.
  22. 22. Types of motor DC Motor • Brushed motor Stepper, coreless, pancake AC Motor • Induction motor • Universal motor • Synchronous (Selsyn) motor • Shaded-pole motor
  23. 23. Questions?

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