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ELECTRIC DC MOTORS

ELECTRIC DC MOTORS. NY Robotics Group Meet-up April 25, 2013 Roger Mosciatti Foodinie. What does an electric more do?. Converts Electrical Energy into Rotating Mechanical Energy. Magnetic attraction and repulsion.

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ELECTRIC DC MOTORS

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  1. ELECTRIC DC MOTORS NY Robotics Group Meet-up April 25, 2013 Roger Mosciatti Foodinie

  2. What does an electric more do? Converts Electrical Energy into Rotating Mechanical Energy

  3. Magnetic attraction and repulsion N NN S F F F S S N Motion Motion How to reverse polarity? Make one an electromagnet!

  4. Electromagnet to switch poles

  5. How Does an Electric Motor Work? How does a DC motor work?

  6. DC Motor Operation This is an active graphic. Click on bold type for further illustration. How does a DC motor work?

  7. How does a DC motor work?

  8. How does a DC motor work?

  9. How does a DC motor work? By connecting the conductor through a lever arm to a point of rotation, a torque can be created. Torque = Radius x Force R F f

  10. Types of DC motors Brush Brushless Brushless/Frameless Stepper

  11. Brush DC Motor Configurations

  12. Brush DC Motor Structure Components: Armature/Rotor Commutator/Brushes Permanent Magnets Housing Bearings

  13. Coreless DC Brush motor

  14. Brush Motor vs. Brushless Motor

  15. Brushless Coreless DC Motor

  16. Coreless Brushless DC Motor Rotating Magnets Winding Backiron Magnetic Field

  17. Industrial Grade Brushless

  18. Lamination Stack (Stator)

  19. Brushless DC Motor Structure Components: Electronic Commutator Rotor/Magnets Stator/Windings Housing Bearings

  20. Different Configuration of Brushless DC motor Outer Rotor Low cogging torque Poor heat dissipation High Inertia Inner Rotor Good heat dissipation Higher performance Low Inertia

  21. http://educypedia.karadimov.info/library/4-pole_bldc_motor.swfhttp://educypedia.karadimov.info/library/4-pole_bldc_motor.swf How a brushless motor works

  22. Brushless frameless BENEFITS: Allows for direct integration with a mechanical transmission device, eliminating parts that add size, complexity, response and settling time. The design engineer is not constrained to the mounting interface and shaft dimensions of a typical framed motor.

  23. Brush vs. Brushless ADVANTAGES of BRUSH MOTOR Two wire control Replaceable brushes for extended life Low cost of construction Simple and inexpensive controls DISADVANTAGES Periodic maintenance is required for brush wear At higher speeds, brush friction increases, thus reducing useful torque Poor heat dissipation due to internal rotor construction Higher rotor inertia which limits the dynamic characteristics Lower speed range due to mechanical limitations on the brushes Brush Arcing will generate noise causing EMI

  24. Brush vs. Brushless ADVANTAGES of BRUSHLESS MOTOR Electronic commutation based on Hall position sensors Less required maintenance due to absence of brushes High efficiency, no voltage drop across brushes Higher speed range - no mechanical limitation imposed by brushes/commutator Essentially no (EMI) electric noise generation The windings in the stator laminations assembly are connected to the housing yielding excellent heat dissipation resulting in: Higher output power/frame size. Smaller size than brush motors for same power. DISADVANTAGES Higher cost of construction Control is complex and expensive

  25. Stepper DC Motor What is it? Is a brushless motor that divides a full rotation into a number of equal steps. The motor’s position can be then commanded to move and hold at one of these steps without feedback sensor, open loop, as long as the motor is properly sized for the application. How does it work? Brush or brushless motors rotate continuously when voltage is applied. Stepper motors have multiple “toothed” electromagnets arranged around a central shaped piece of iron or permanent magnet. To make run, first, one electromagnet is given power, which makes the gear's teeth magnetically attracted to the electromagnet’s teeth. When the gear’s teeth are aligned to the first electromagnet , they are slightly offset from the next electromagnet. Then the next electromagnet is turned on and the process starts again.

  26. How a Stepper DC motors works

  27. Stepper DC motors

  28. Stepper DC motors

  29. Stepper DC motors

  30. Stepper vs. Brush, Brushless motor Stepper smaller power to weight size ratio Stepper are usually 70% efficient where brush or brushless are 80% to 90% efficient Stepper usually have a low resolution .9 to 1.8 deg. That is 400 steps/ rev. Where the other can go to 4,096 steps/rev Stepper motors may stall and skip steps if the motor is not powerful enough by misapplying it. Stepper motors draw excess current regardless of load. The excess power is dissipated as heat. Stepper motors produce a slight hum due to the control process. Stepper motors vibrate slightly and have some resonance issues because of how the stepper motor operates.

  31. DC Motor constants Kt Torque/Current Torque = Force x R (Lever Arm) Torque is proportional to: Number of conductors R (Length of Lever Arm) Magnetic Field Strength Current Magnitude If the number of conductors is fixed and The length of the Lever Arm is fixed and The Magnetic Strength is Fixed THEN TORQUE IS PROPORTIONAL TO CURRENT (I) Kt = TORQUE /CURRENT Typically: Nm/Amp, oz.in./Amp, in.Lbs/Amp

  32. DC Motor constants Ke Velocity/Voltage Motion: Velocity = 2 ∏ x R (Lever Arm) x RPM Voltage is proportional to: Number of conductors R (Length of Lever Arm) Magnetic Field Strength Velocity If the number of conductors is fixed and The length of the Lever Arm is fixed and The Magnetic Strength is Fixed THEN VOLTAGE IS PROPORTIONAL TO THE VELOCITY Ke = VOLTAGE / VELOCITY Typically: Volts/1000 RPM

  33. Speed / torque Curve Continuous Operation Defines motor mounted on an aluminum plate of a certain size to dissipate the motor heat Intermittent Operation Defines motor operation for a pre-determined time based on motor design

  34. Feedback sensors Optical Encoder Up to 5,000 PPR Absolute Encoder Optical, Magnetic, Capacitive 16 to 20 bit resolution (1,048,576 PPR) Resolver 14 bit resolution (16,384 PPR)

  35. Torque multiplier Why use a gear box? A gearbox uses mechanical advantage to increase output torque and reduce RPM Improves the Inertia matching. Should the load Inertia significantly exceeds that of the motor it causes excessive overshoot and increase settling times. It greatly enhances system responsive time. Reduces system cost by the need of smaller motors and drives.

  36. Types of Gearboxes Spur Gear System Planetary Gear System Harmonic Drive System

  37. Small DC brush gearmotor with encoder

  38. Industrial DC Brushless Gearmotor

  39. PWM Motor Drive P = Pulse W = Width M = Modulation How do they work? Supplies the motor with high frequency power pulses (15 KHz to 35 KHz) whose width is proportional to the required power level as determined by the input command. Since the power transistors are either completely “ON” or “OFF”, they experience dissipation only while they are switching or they are “ON” with a very low voltage drop, resulting in an efficiency of 85% to 95%

  40. Closed Loop Servo System Parameter requirements for motor system selection: Load Weight Load Inertia Distance & Time from position A to B Velocity Profile Gearbox Ratio Inertia Matching Power Available

  41. Thank you Questions?

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