1. Topic 9.33.4.1 AC Motors

2. AC Motor Construction An AC motor is structurally identical to a DC motor exceptthat it has a slip-ring commutator rather than the split ring ofa DC motor. This is because the current in the coil changes directiondue to its AC nature and doesn't need to be mechanicallyflipped. This means that the rotation speed is harder to control asit is related to the frequency of the AC supply. It is common in AC motors to use electromagnets (field coils)to create the stator field. 3. AC Motor Construction 4. Brush-less AC Motors As AC electricity changes phase (direction) with aregular frequency this can be used to set up a rotatingmagnetic field around the rotor. If the rotor is replaced with a magnet, then a brush-lessmotor can be constructed. 5. Induction Motors An induction motor consists of astator and a rotor. The stator consists of a series ofwire coils wound on soft iron coresthat surround the rotor. These are connected to theexternal power supply in such away that they produce amagnetic field whose polarityrotates at constant speed in onedirection. The rotor consists of coils wound ona laminated iron armature mountedon an axle. 6. Induction Motors The rotor coils are notconnected to the external powersupply An induction motor has neithercommutator nor brushes. Instead eddy currents areinduced in the rotor coils by therotating magnetic field of thestator. 7. Induction Motors By Lenz's law the eddy currentsproduce magnetic fields whichtry to oppose the motion of therotor. However in this case this resultsin the rotor trying to keep upwith the stator field (so thatthere is no relative motion) andgenerating a torque. The Shading ring disrupts themagnetic field and allows themotor to start spinning. 8. Induction Motors The rotor coils are often simplifiedto single copper or aluminiumbars capable of carrying a largecurrent, embedded in the surfaceof the soft iron armature. The bars are connected at theends by a ring or disc of copperwhich allows current to flow in aloop between opposite bars. This physical arrangement isreferred to as a squirrel cage even though it looks more likea hamster wheel! 9. Energy Transfers in the Home andIndustry There are many many examples of energytransfers in the home. Remember that no energy transfer in the realworld is ever 100% efficient energy will always be wasted as thermalenergy (and light and sound) in each change. 10. Energy Transfers in the Home andIndustry Good examples to give couldinclude:Electrical Kinetic:Electrical Thermal:Electrical Light:Electrical Chemical:Electrical Sound: any device with a motor such as awashing machine. any device that heats up such akettle or toaster. anything that glows or gives offlight: lightbulb? Battery re-chargers are goodexamples. The best example is speakers(even though we know this goesvia kinetic energy)