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Electric Motor Basic Principles

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  1. Electric Motor Basic Principles • Interaction between magnetic field and current carrying wire produces a force • Opposite of a generator Kelvin Peng

  2. Conventional (Brushed) DC Motors • Permanent magnets for outer stator • Rotating coils for inner rotor  • Commutation performed with metal contact brushes and contacts designed to reverse the polarity of the rotor as it reaches horizontal Kelvin Peng

  3. 2 pole brushed DC motor commutation Kelvin Peng

  4. DC Motor considerations • Back EMF - every motor is also a generator • More current = more torque; more voltage = more speed • Load, torque, speed characteristics • Shunt-wound, series-wound (aka universal motor), compound DC motors Kelvin Peng

  5. Conventional (Brushed) DC Motors • Common Applications: • Small/cheap devices such as toys, electric tooth brushes, small drills • Lab 3 • Pros: • Cheap, simple • Easy to control - speed is governed by the voltage and torque by the current through the armature • Cons: • Mechanical brushes - electrical noise, arcing, sparking, friction, wear, inefficient, shorting Kelvin Peng

  6. Brushless DC Motors • Essential difference - commutation is performed electronically with controller rather than mechanically with brushes Kelvin Peng

  7. Brushless DC Motor Commutation • Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit) • Similarity with stepper motor, but with less # poles • Needs rotor positional closed loop feedback: hall effect sensors, back EMF, photo transistors Kelvin Peng

  8. Brushless DC Motors • Applications • CPU cooling fans • CD/DVD Players • Electric automobiles • Pros (compared to brushed DC) • Higher efficiency • Longer lifespan, low maintenance • Clean, fast, no sparking/issues with brushed contacts • Cons • Higher cost • More complex circuitry and requires a controller Kelvin Peng

  9. AC Motors • Synchronous and Induction (Asynchronous) • Synchronous: rotor rotation frequency = AC current frequency Kelvin Peng

  10. AC Induction Motors (3 Phase) • Use poly-phase (usually 3) AC current to create a rotating magnetic field on the stator • This induces a magnetic field on the rotor, which tries to follow stator - slipping required to produce torque • Workhorses of the industry - high powered applications Kelvin Peng

  11. Stepper Motors Jiasheng He

  12. Stepper Motor Characteristics Brushless Incremental steps/changes Holding Torque at zero speed Speed increase -> torque decreases Usually open loop Jiasheng He

  13. Stepper Speed Characteristics • Torque varies inversely with speed • Current is proportional to torque • Torque →∞ means Current →∞, which leads to motor damage • Torque thus needs to be limited to rated value of motor Jiasheng He

  14. Types of Stepper Motors Permanent Magnet Variable Reluctance Hybrid Synchronous Jiasheng He

  15. Permanent Magnet Stepper Motor • Rotor has permanent magnets • The teeth on the rotor and stator are offset • Number of teeth determine step angle • Holding, Residual Torques Jiasheng He

  16. Unipolar • Two coils, each with a center tap • Center tap is connected to positive supply • Ends of each coil are alternately grounded • Low Torque Jiasheng He

  17. Bipolar • Two coils, no center taps • Able to reverse polarity of current across coils • Higher Torque than Unipolar Jiasheng He

  18. Bipolar • More complex control and drive circuit • Coils are connected to an H-Bridge circuit • Voltage applied across load in either direction • H-Bridge required for each coil Jiasheng He

  19. Variable Reluctance • No permanent magnet – soft iron cylinder • Less rotor teeth than stator pole pairs • Rotor teeth align with energized stator coils Jiasheng He

  20. Variable Reluctance • Magnetic flux seeks lowest reluctance path through magnetic circuit • Stator coils energized in groups called Phases Jiasheng He

  21. Hybrid Synchronous • Combines both permanent magnet and variable reluctance features • Smaller step angle than permanent magnet and variable reluctance Jiasheng He

  22. Applications • Printers • Floppy disk drives • Laser Cutting • Milling Machines • Typewriters • Assembly Lines Jiasheng He