Motor theory

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Motor theory

  1. 1. AC Motor Theory <ul><li>Over View </li></ul><ul><ul><li>History </li></ul></ul><ul><ul><li>AC Motors Convert Electrical Energy to Mechanical Energy </li></ul></ul><ul><ul><ul><li>Induction </li></ul></ul></ul><ul><ul><ul><li>Motor Components </li></ul></ul></ul><ul><ul><ul><li>The Magnetic Field </li></ul></ul></ul><ul><ul><ul><li>Speed </li></ul></ul></ul><ul><ul><ul><li>Slip </li></ul></ul></ul><ul><ul><ul><li>Slip Control </li></ul></ul></ul>
  2. 2. AC Motor Theory <ul><li>Overview (cont.) </li></ul><ul><ul><li>AC Motors Convert Electrical Energy to Mechanical Energy </li></ul></ul><ul><ul><ul><li>Torque </li></ul></ul></ul><ul><ul><li>Speed Control </li></ul></ul><ul><ul><li>Nameplate Information </li></ul></ul><ul><ul><li>Motor Winding Connections </li></ul></ul><ul><ul><li>Types of Induction Motors </li></ul></ul>
  3. 3. AC Motor Theory <ul><li>First Electric Motor Was DC in 1833 </li></ul><ul><ul><li>Simple to control speed </li></ul></ul><ul><li>First AC Motor in 1899 </li></ul><ul><ul><li>Simple and Robust </li></ul></ul><ul><ul><li>Fixed speed and torque characteristics </li></ul></ul>
  4. 4. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>When a conductor is moving across a magnetic field a voltage is induced </li></ul></ul><ul><ul><li>If the conductor is part of a closed circuit there will be a current induced </li></ul></ul>
  5. 5. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>In a motor, the induction principle is utilized in reverse </li></ul></ul><ul><ul><ul><li>A live conductor is placed in a magnetic field </li></ul></ul></ul><ul><ul><ul><li>The conductor is influenced by a force which tries to move it through the magnetic field </li></ul></ul></ul>
  6. 6. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>The AC motor is made up of two parts </li></ul></ul><ul><ul><ul><li>The Stator </li></ul></ul></ul><ul><ul><ul><ul><li>The stationary section that contain the windings </li></ul></ul></ul></ul><ul><ul><ul><li>The Rotor </li></ul></ul></ul><ul><ul><ul><ul><li>The rotating section that contains the conductors </li></ul></ul></ul></ul>
  7. 7. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>The Magnetic Field </li></ul></ul><ul><ul><ul><li>Rotates in the air gap between the stator and the rotor </li></ul></ul></ul><ul><ul><ul><li>Has a fixed location in the stator core but its direction varies </li></ul></ul></ul><ul><ul><ul><ul><li>Speed of direction change is determined by the frequency of the AC line </li></ul></ul></ul></ul><ul><ul><ul><ul><li>The field changes 60 times per second with 60 Hz power </li></ul></ul></ul></ul>
  8. 8. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>The Magnetic Field </li></ul></ul><ul><ul><ul><li>When three phases are introduced in the motor three magnetic fields are introduced </li></ul></ul></ul><ul><ul><ul><ul><li>Make up a symmetrical rotating </li></ul></ul></ul></ul><ul><ul><ul><ul><li>120 degrees apart </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Poles of opposite polarity are formed </li></ul></ul></ul></ul><ul><ul><li>Speed = (frequency x 120) / # of poles </li></ul></ul><ul><ul><ul><ul><li>(60 Hz x 120) / 4 poles = 1800 rpm </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Theoretical or Synchronous - need to factor </li></ul></ul></ul></ul><ul><ul><ul><ul><li>in “slip” </li></ul></ul></ul></ul>
  9. 9. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>Slip </li></ul></ul><ul><ul><ul><li>The rotor is not quite able to keep up with the magnetic field rotation so rotates slightly slower </li></ul></ul></ul><ul><ul><ul><li>Slip is typically 3-8% of synchronous speed </li></ul></ul></ul><ul><ul><ul><li>So, actual motor speed equals: </li></ul></ul></ul><ul><ul><ul><li>Speed = ((Frequency x 120) / # of poles) - Slip </li></ul></ul></ul><ul><ul><ul><li>((60Hz x 120) / 4 poles) - 50 rpm) = 1750 rpm </li></ul></ul></ul>
  10. 10. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>Slip Control </li></ul></ul><ul><ul><ul><li>Slip can be controlled through motor voltage </li></ul></ul></ul><ul><ul><ul><li>If voltage is reduced then slip will increase </li></ul></ul></ul><ul><ul><ul><ul><li>Soft Starts </li></ul></ul></ul></ul><ul><ul><ul><li>Since AC motors are designed for a certain voltage and frequency ratio, changing the voltage alone will cause improper magnetization and increase losses and motor heat </li></ul></ul></ul>
  11. 11. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>Speeds of Typical Motors @ 60 Hz </li></ul></ul><ul><ul><li>2 pole = 3600 rpm - slip </li></ul></ul><ul><ul><li>4 pole = 1800 rpm - slip </li></ul></ul><ul><ul><li>6 pole = 1200 rpm - slip </li></ul></ul><ul><ul><li>8 pole = 900 rpm - slip </li></ul></ul>
  12. 12. AC Motor Theory <ul><li>AC Motors Convert Electric Energy into Mechanical Energy </li></ul><ul><ul><li>Motor torque is affected by the voltage </li></ul></ul><ul><ul><li>As voltage increases in relation to frequency, torque increases </li></ul></ul>
  13. 13. AC Motor Theory <ul><li>Torque in AC Motors </li></ul><ul><ul><li>A one hp, four pole motor (1800 rpm sync.) has approximately 3 ft-lbs. of torque </li></ul></ul><ul><ul><li>If the V/Hz ratio remains constant then the torque will remain constant over the speed range </li></ul></ul>
  14. 14. AC Motor Theory <ul><li>Pop Quiz! </li></ul><ul><ul><ul><li>Motor A is a Four Pole, 1750 rpm, 100 hp motor </li></ul></ul></ul><ul><li>Motor B is a Two Pole, 3500 rpm, 200 hp motor </li></ul><ul><li>Which motor has more torque? </li></ul>
  15. 15. AC Motor Theory <ul><li>Answer: The torque will be the same . Remember that horsepower is a function of speed and torque. Although motor B has twice the horsepower it also has to go twice the speed. </li></ul><ul><li>So, how much torque will they have? </li></ul>
  16. 16. AC Motor Theory <ul><li>Answer: 300 ft.-lbs. </li></ul><ul><li>Since a 1 hp, 1750 RPM motor creates 3 ft.-lbs. then a 100 hp, 1750 rpm motor will create 300 ft.-lbs. </li></ul><ul><li>Remember: Horsepower is not the whole story! </li></ul>
  17. 17. AC Motor Theory <ul><li>Speed Control In AC Motors </li></ul><ul><ul><li>Proper Speed Variation </li></ul></ul><ul><ul><ul><li>Since the rotor follows the rotating magnetic field then the rotor will slow with a lower frequency </li></ul></ul></ul><ul><ul><ul><li>Since the motor is designed for a certain voltage to frequency ratio, if we lower voltage in proportion to the frequency the torque will remain constant </li></ul></ul></ul>
  18. 18. AC Motor Theory <ul><li>Typical Motor Nameplate Data </li></ul><ul><ul><li>Frame size </li></ul></ul><ul><ul><ul><li>NEMA standard sizes </li></ul></ul></ul><ul><ul><ul><li>Lower rpm motors will have larger frame sizes to help cool since the cooling fan is moving less air </li></ul></ul></ul><ul><ul><li>Horsepower </li></ul></ul><ul><ul><li>RPM </li></ul></ul><ul><ul><li>Voltage </li></ul></ul><ul><ul><li>Full Load Amps </li></ul></ul><ul><ul><ul><li>For each voltage (208-230/460 VAC) </li></ul></ul></ul>
  19. 19. AC Motor Theory <ul><li>Typical Motor Nameplate Data </li></ul><ul><ul><li>Power Factor (not on all motors) </li></ul></ul><ul><ul><ul><li>Ratio of how much current is active to reactive </li></ul></ul></ul><ul><ul><ul><ul><li>Active current goes to shaft output </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Reactive current builds the magnetic field </li></ul></ul></ul></ul><ul><ul><li>Insulation class </li></ul></ul><ul><ul><li>Enclosure </li></ul></ul><ul><ul><ul><li>ODP - Open, drip proof </li></ul></ul></ul><ul><ul><ul><li>TEFC - Totally enclosed, fan cooled </li></ul></ul></ul><ul><ul><ul><li>TEBC - Totally enclosed, blower cooled </li></ul></ul></ul>
  20. 20. AC Motor Theory <ul><li>Motor Winding Connections </li></ul><ul><ul><li>Typical motor has nine leads </li></ul></ul><ul><ul><li>Follow motor instructions for connections for your nominal voltage </li></ul></ul><ul><ul><ul><li>230 VAC diagrams will have the windings in parallel </li></ul></ul></ul><ul><ul><ul><li>460 VAC diagrams will have the windings in series </li></ul></ul></ul><ul><ul><li>On part winding start motors tie both the start and the run together </li></ul></ul><ul><ul><ul><li>Verify that both windings have the same </li></ul></ul></ul><ul><ul><ul><li>rotation </li></ul></ul></ul>
  21. 21. AC Motor Theory <ul><li>Types of AC Induction Motors </li></ul><ul><ul><li>Standard efficient </li></ul></ul><ul><ul><li>Energy efficient </li></ul></ul><ul><ul><ul><li>Federal government requiring all new motors to be energy efficient </li></ul></ul></ul><ul><ul><li>Inverter Duty </li></ul></ul><ul><ul><ul><li>Higher class insulation, phase paper, mounts for constant velocity fan </li></ul></ul></ul><ul><ul><li>Vector Duty </li></ul></ul><ul><ul><ul><li>Same as inverter duty but with a shaft encoder </li></ul></ul></ul>
  22. 22. AC Motor Theory <ul><li>Conclusion </li></ul><ul><ul><li>AC Motors operate through rotating magnetic fields </li></ul></ul><ul><ul><li>The speed of the motor is determined by the frequency of the power supply </li></ul></ul><ul><ul><li>The torque of the motor is determined by the voltage applied </li></ul></ul><ul><ul><li>AC motors are designed for a fixed voltage to frequency ratio </li></ul></ul>

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