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  1. 1. Electric Motors <ul><li>Classification / types </li></ul><ul><ul><li>DC Motors </li></ul></ul><ul><ul><li>AC Motors </li></ul></ul><ul><ul><li>Stepper Motors </li></ul></ul><ul><ul><li>Linear motors </li></ul></ul><ul><li>Function </li></ul><ul><ul><li>Power conversion - electrical into mechanical </li></ul></ul><ul><ul><li>Positional actuation – electrical signal to position </li></ul></ul>
  2. 2. DC Motors <ul><ul><li>DC Motors </li></ul></ul><ul><ul><ul><li>Fundamental characteristics </li></ul></ul></ul><ul><ul><ul><ul><li>Basic function </li></ul></ul></ul></ul><ul><ul><ul><li>Types and applications </li></ul></ul></ul><ul><ul><ul><ul><li>Series </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Shunt </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Combination </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Torque characteristics </li></ul></ul></ul></ul><ul><ul><ul><li>Modelling </li></ul></ul></ul>
  3. 3. Fundamental characteristics of DC Motors End view Time 0 End view Time 0+ Shifting magnetic field in rotor causes rotor to be forced to turn
  4. 4. Nature of commutation <ul><li>Power is applied to armature windings </li></ul><ul><ul><li>From V+ </li></ul></ul><ul><ul><li>Through the +brush </li></ul></ul><ul><ul><li>Through the commutator contacts </li></ul></ul><ul><ul><li>Through the armature (rotor) winding </li></ul></ul><ul><ul><li>Through the – brush </li></ul></ul><ul><ul><li>To V- </li></ul></ul><ul><li>Rotation of the armature moves the commutator, switching the armature winding connections </li></ul><ul><li>Stator may be permanent or electromagnet </li></ul>
  5. 5. DC motor wiring topologies
  6. 6. Series Wound DC motors <ul><ul><ul><li>Armature and field connected in a series circuit. </li></ul></ul></ul><ul><ul><ul><li>Apply for high torque loads that do not require precise speed regulation. Useful for high breakaway torque loads. </li></ul></ul></ul><ul><ul><ul><ul><li>locomotives, hoists, cranes, automobile starters </li></ul></ul></ul></ul><ul><ul><ul><li>Starting torque </li></ul></ul></ul><ul><ul><ul><ul><li>300% to as high as 800% of full load torque. </li></ul></ul></ul></ul><ul><ul><ul><li>Load increase results in both armature and field current increase </li></ul></ul></ul><ul><ul><ul><ul><li>Therefore torque increases by the square of a current increase. </li></ul></ul></ul></ul><ul><ul><ul><li>Speed regulation </li></ul></ul></ul><ul><ul><ul><ul><li>Less precise than in shunt motors </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Diminished load reduces current in both armature and field resulting in a greater increase in speed than in shunt motors. </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>No load results in a very high speed which may destroy the motor. </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Small series motors usually have enough internal friction to prevent high-speed breakdown, but larger motors require external safety apparatus. </li></ul></ul></ul></ul></ul>
  7. 7. Shunt wound DC motors <ul><ul><ul><li>Field coil in parallel (shunt) with the armature. </li></ul></ul></ul><ul><ul><ul><ul><li>Current through field coil is independant of the armature. </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Result = excellent speed control. </li></ul></ul></ul></ul></ul><ul><ul><ul><li>Apply where starting loads are low </li></ul></ul></ul><ul><ul><ul><ul><li>fans, blowers, centrifugal pumps, machine tools </li></ul></ul></ul></ul><ul><ul><ul><li>Starting torque </li></ul></ul></ul><ul><ul><ul><ul><li>125% to 200% full load torque (300 for short periods). </li></ul></ul></ul></ul>
  8. 8. Compound wound DC motors <ul><ul><ul><li>Performance is roughly between series-wound and shunt-wound </li></ul></ul></ul><ul><ul><ul><li>Moderately high starting torque </li></ul></ul></ul><ul><ul><ul><li>Moderate speed control </li></ul></ul></ul><ul><ul><ul><li>Inherently controlled no-load speed </li></ul></ul></ul><ul><ul><ul><ul><li>safer than a series motor where load may be disconnected </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>e.g. cranes </li></ul></ul></ul></ul></ul>
  9. 9. Permanent magnet DC motors
  10. 10. Permanent Magnet DC Motors <ul><ul><li>Have permanent magnets rather than field windings but with conventional armatures. Power only to armature. </li></ul></ul><ul><ul><li>Short response time </li></ul></ul><ul><ul><li>Linear Torque/Speed characteristics similar to shunt wound motors. Field magnetic flux is constant </li></ul></ul><ul><ul><ul><li>Current varies linearly with torque. </li></ul></ul></ul><ul><ul><li>Self-braking upon disconnection of electrical power </li></ul></ul><ul><ul><ul><li>Need to short + to – supply, May need resistance to dissipate heat. </li></ul></ul></ul><ul><ul><li>Magnets lose strength over time and are sensitive to heating. </li></ul></ul><ul><ul><ul><li>Lower than rated torque. </li></ul></ul></ul><ul><ul><ul><li>Not suitable for continuous duty </li></ul></ul></ul><ul><ul><ul><li>May have windings built into field magnets to re-magnetize. </li></ul></ul></ul><ul><ul><li>Best applications for high torque at low speed intermittent duty. </li></ul></ul><ul><ul><ul><li>Servos, power seats, windows, and windshield wipers. </li></ul></ul></ul>
  11. 11. Modeling DC motors <ul><li>A linear speed/torque curve can be used to model DC motors. This works well for PM and compound designs and can be used for control models for narrow ranges for the other configurations </li></ul><ul><li>Model will assume! </li></ul><ul><ul><li>Linearity </li></ul></ul><ul><ul><li>Constant thermal characteristics </li></ul></ul><ul><ul><li>No armature inductance </li></ul></ul><ul><ul><li>No friction in motor </li></ul></ul>
  12. 12. DC Motor modeling Motor equations From the circuit Substituting the above: For stalled rotor torque And no-load speed In terms of no-load speed torque/speed equation is: Power is: Max power is: Units:
  13. 13. Application <ul><li>Use motor voltage and no-load speed to calculate K t </li></ul><ul><li>K t = K e in SI units </li></ul><ul><li>Use stalled rotor torque, V, and K e to find R </li></ul><ul><ul><li>Note, R varies with speed and cannot be measured at rest </li></ul></ul><ul><li>See web download for explanation of K t , K e : </li></ul><ul><li> </li></ul><ul><li>Development of Electromotive Force.pdf </li></ul>
  14. 14. DC motor control – H-bridge <ul><li>Switches control direction </li></ul><ul><ul><li>“A” switches closed for clockwise </li></ul></ul><ul><ul><li>“B” switches for counter-clockwise </li></ul></ul><ul><li>PWM for speed control </li></ul><ul><ul><li>“A’s” duty cycle for clockwise speed </li></ul></ul><ul><ul><li>“B’s” duty cycle for counter-clockwise speed </li></ul></ul><ul><li>Can be configured to brake </li></ul><ul><ul><li>Bottom “B” and “A” to brake </li></ul></ul>
  15. 15. H-Bridge implementation <ul><li>Elements in box are available as single IC </li></ul>
  16. 16. Brushless designs <ul><li>Commutation is done electronically </li></ul><ul><ul><li>Encoder activated switching </li></ul></ul><ul><ul><li>Hall effect activated switching </li></ul></ul><ul><ul><li>Back EMF driven switching </li></ul></ul><ul><li>PM armature </li></ul><ul><li>Wound/switched fields </li></ul><ul><li>Application </li></ul><ul><ul><li>Few wearing parts (bearings) </li></ul></ul><ul><ul><li>Capable of high speed </li></ul></ul><ul><ul><li>Fractional HP </li></ul></ul><ul><ul><ul><li>Servos </li></ul></ul></ul><ul><ul><ul><li>Low EMC </li></ul></ul></ul>
  17. 17. Stepper Motors <ul><li>Description </li></ul><ul><ul><li>Generally a two phase motor </li></ul></ul><ul><ul><li>permanent magnet rotor and wound fields </li></ul></ul><ul><ul><li>Rotor normally has many poles </li></ul></ul><ul><ul><ul><li>200 poles = 1.8 degrees per step </li></ul></ul></ul><ul><ul><li>Used primarily for position or velocity control </li></ul></ul><ul><ul><li>Typically no position feedback </li></ul></ul><ul><ul><ul><li>Torques are managed so that an intended step is always achieved </li></ul></ul></ul><ul><ul><ul><ul><li>Accelerations, decelerations and loads must be managed intelligently </li></ul></ul></ul></ul><ul><li>Two general types of windings </li></ul><ul><ul><li>Unipolar </li></ul></ul><ul><ul><li>Bi-polar </li></ul></ul>
  18. 18. Winding configurations <ul><li>Bi-polar design </li></ul><ul><ul><li>6 wire </li></ul></ul><ul><li>Unipolar design </li></ul><ul><ul><li>4 wire </li></ul></ul>
  19. 19. AC Motors <ul><li>AC Motors </li></ul><ul><ul><li>Fundamental characteristics </li></ul></ul><ul><ul><li>Types </li></ul></ul><ul><ul><ul><li>Fractional horsepower (single phase) </li></ul></ul></ul><ul><ul><ul><li>Integral </li></ul></ul></ul><ul><ul><ul><ul><li>Single phase (Cap start Induction run) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Three phase </li></ul></ul></ul></ul><ul><ul><li>NEMA Torque characteristics </li></ul></ul><ul><ul><li>Modelling </li></ul></ul>
  20. 20. Fractional horsepower designs <ul><ul><li>Shaded Pole (low starting torque, simple, cheap) </li></ul></ul><ul><ul><ul><li>uses a short circuited coil embedded in face of field to cause one side of field to be magnetized before the other </li></ul></ul></ul><ul><ul><li>Split phase (low starting torque) </li></ul></ul><ul><ul><ul><li>Two windings (2-phase), one with high resistance hence different RL and phase </li></ul></ul></ul><ul><ul><ul><li>Centrifugal switch on starting winding </li></ul></ul></ul><ul><ul><li>Capacitor Start Induction Run (medium starting torque) </li></ul></ul><ul><ul><ul><li>Two windings (2-phases) </li></ul></ul></ul><ul><ul><ul><li>Capacitor used on second winding to create leading phase </li></ul></ul></ul><ul><ul><ul><li>Centrifugal switch on starting winding </li></ul></ul></ul><ul><ul><li>Universal? (intermittent use, brushes!) </li></ul></ul><ul><ul><ul><li>DC motor with inductance managed to allow AC operation </li></ul></ul></ul><ul><ul><li>Synchronous (clocks, synchronization) </li></ul></ul><ul><ul><ul><li>Permanent magnet rotor always in phase with AC </li></ul></ul></ul>
  21. 21. AC motor model <ul><li>See Siemens AC motor info for modeling info. </li></ul>
  22. 22. AC Motors <ul><li>Relationship between number of poles and motor synchronous speed </li></ul><ul><li>Squirrel cage motors must operate with some slip .5 to 8% to allow the rotor to be magnetized. </li></ul><ul><ul><li>Actual speed is synchronous speed reduced by the slip. </li></ul></ul>1200 6 1800 4 3600 2 Synchronous Speed (RPM) Poles
  23. 23. Squirrel Cage Rotor Seimens AG, 2002
  24. 24. Inducing magnetism in the rotor <ul><li>Difference between angular velocity of rotor and angular velocity of the field magnetism causes squirrel cage bars to cut the field magnetic field inducing current into squirrel cage bars. </li></ul><ul><li>This current in turn magnetizes the rotor </li></ul>
  25. 25. Torque/speed curve
  26. 26. Typical starting current
  27. 27. Motor characteristics <ul><li>Enclosure / frame </li></ul><ul><li>Voltage / frequency </li></ul><ul><li>3 or 1 phase </li></ul><ul><li>Poles / speed </li></ul><ul><li>Service factor </li></ul><ul><ul><li>Fraction of rated HP that motor can be operated at </li></ul></ul><ul><li>Insulation class/ Temp rise </li></ul><ul><ul><li>(operating temperature compatible) </li></ul></ul><ul><li>NEMA Design A,B,C,D, etc. (Torque curve type) </li></ul><ul><ul><li>See next page </li></ul></ul><ul><li>Efficiency </li></ul>575 220/380 460 425 230 400 200 380 115 50 Hz 60 Hz
  28. 28. NEMA Torque characteristics summarized High Punch Press High ------- Low Very high D Loaded compressor Loaded conveyor Normal Low Normal High C Same as Design &quot;A&quot; Normal Normal Normal Normal B Mach. Tools, Fans Low High High Normal A TYPICAL APPLICATIONS FULL LOAD SLIP BREAK- DOWN TORQUE STARTING CURRENT STARTING TORQUE NEMA DESIGN
  29. 29. NEMA Motor Characteristics High Low Med Med-High Med-High Efficiency 0.5-3 800-1000 160-200 60-140 74-190 E 5-8 600-700 275 NA 275 D 1-5 600-700 190-225 140-195 200-285 C 0.5-5 600-700 175-300 65-190 70-275 B (most common) 0.5-5 NA 175-300 65-190 70-275 A Slip % Locked Rotor Current % FL Breakdown Torque % FL Pull-up Torque % FL Locked Rotor Torque % FL Design
  30. 30. PWM Variable Frequency Drives <ul><li>Variable frequency drives use AC to DC converter then a DC to AC converter (inverter) </li></ul><ul><ul><li>Inverter frequency and voltage output can be varied to allow motor speed to be varied. </li></ul></ul><ul><ul><li>Very efficient and cost effective variable speed for 1 HP and up </li></ul></ul>