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Electric Motors

MECH1200. Electric Motors. MECH1200. Fundamentals of DC Electric Machinery. History Introduction Principle of Operation of Electric Machinery Principle of Operation of AC Synchronous Machines Principle of Operation of DC Machines Types of Electric Machines Construction of DC Machines

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Electric Motors

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  1. MECH1200 Electric Motors

  2. MECH1200 Fundamentals of DC Electric Machinery • History • Introduction • Principle of Operation of Electric Machinery • Principle of Operation of AC Synchronous Machines • Principle of Operation of DC Machines • Types of Electric Machines • Construction of DC Machines • DC Machine Torque Characteristics • DC Machine Speed Control • Exercise Questions

  3. MECH1200 History • Very advanced mechanisms are developed in the middle ages. • Development of the steam engine leads to the industrial revolution in the mid 1700’s. • Michael Faraday runs the first electric motor experiment in 1821. Faraday and Daniell in the lab A mechanical clock made in 1510 James Watt Images are in the public domain. Source: Wikimedia Commons, Wikimedia Foundation

  4. MECH1200 Introduction What is an Electric Motor? • Electromechanical device that converts electrical energy to mechanical energy • Mechanical energy used to: • Rotate pump impeller, fan, blower • Drive compressors • Lift materials • Motors in industry: 70% of electrical load

  5. MECH1200 • Electric Machinery: - Electric Motors: convert electrical energy into mechanical energy. - Electric Generators: convert mechanical energy to electric energy. • The Electric machine is composed of: - Stator - Rotor

  6. MECH1200 The Stator is the stationary part of a motor. The Rotor is a moving component of an electromagnetic system in the electric motor

  7. How Does an Electric Motor Work? MECH1200

  8. Principle of Operation of AC Synchronous Machines MECH1200 N S N Alternators such as the one shown above are three phase synchronous machines. In synchronous motors, the magnetic field in the stator rotates, and the rotor tries to follow the stator’s magnetic field, i.e. it tries to synchronize itself with it, and thence the name synchronous motor. N S S N S S N

  9. Principle of Operation of DC Machines A Stationary magnetic field in the stator An alternating magnetic field in the rotor But how can we make the magnetic field alternate in the rotor? Once the rotor rotates half a cycle, it switches polarity, so that the magnets are again repelling A circuit that uses a commutation technique is used to supply an alternating current into the rotor. Every half cycle the current flowing in the rotor switches direction due to commutation, watch: When the rotor is energized, it rotates to the most stable position: N Commutators N S S Brushes (To maintain electrical contact) Commutation Circuit MECH1200

  10. Types of Electric Motors MECH1200 Electric Motors DC Motors AC Motors Special Purpose Motors Stepper motor Permanent Magnet DC (PMDC) Induction Motor Series Motor Brushless DC motor Hysteresis motor Shunt Motor Reluctance motor Synchronous Motor Compounded Motor Universal motor Separately Excited motor

  11. MECH1200 Electric Motors Alternating Current (AC) Motors Direct Current (DC) Motors Synchronous Induction Separately Excited Self Excited Single-Phase Three-Phase Series Compound Shunt Classification of Motors

  12. MECH1200 Three types of Motor Load

  13. DC Motors Parts of a DC motor: Armature with windings Brush assembly Field windings Commutator MECH1200

  14. MECH1200 Construction of DC Machines • Field Windings: In DC machines, this is the winding in the stator. Note: Permanent magnet dc motors do not have field winding but rather a permanent magnet. • Armature Windings: The windings in the rotor . • Commutator segments: the method to convert the dc current to an alternating current in the rotor of the dc motor is called commutation. They are connected to the rotor windings, and stay in contact with the brushes. • Brushes: pieces made of carbon or graphite and pushed against a spring to maintain electrical contact with the commutator segments. • Interpole windings: A third set of windings are mounted on the stator, and connected in series with the rotor to reduce the sparking between brushes and the commutator.

  15. MECH1200 • If the armature and field windings are connected in series, then the machine that results is called the DC series machine. • If the armature and field windings are connected in parallel, then the machine that results is called the DC shunt machine. • If the armature is connected with a field winding in series and another field winding in parallel, then the resulting machine is called the compounded machine. • Note that the interpole windings are always connected in series with the rotor.

  16. MECH1200 • Name the type of each of the motors A, B, and C shown in the figure below: A: Shunt motor B: Series motor C: Compound motor

  17. MECH1200 DC Motor Construction

  18. Interpole Windings MECH1200 • Due to shifting of the magnetic flux lines, arcing and short-circuiting can result • Interpole windings counteract this shift and reduces the arcing • Are in series with field coils

  19. MECH1200 Counter (or Back) EMF Recall from 1100: an induced voltage in a wire (or any conductor) results from relative motion of either the wire or the magnetic field. Likewise, as the armature cuts through the magnetic field of the field coils (stator), a voltage is induced. (Think how a generator works.)

  20. MECH1200 Counter (or Back) EMF Combined Armature and Field Lines of Force

  21. MECH1200 Counter (or Back) EMF This induced voltage causes current to flow in the opposite direction of the applied DC current, thus “countering” it, reducing the overall current and voltage. Armature current is “into page” (-z direction) Counter EMF current is “out of page” (+z direction)

  22. Permanent Magnet DC Motors Excellent starting torque Good speed regulation Limited to low HP loads Torque is limited to 150% of rated torque. Reverse direction of rotation by interchanging armature connections. MECH1200

  23. Series Wound - DC Motors MECH1200

  24. Series Wound - DC Motors Highest starting torque Not used with loads that are belt or chain coupled Loads should always be directly coupled. Reverse direction of rotation by interchanging armature connections. Cannotbe used without a load attached Examples: Locomotives, Cranes, Hoists.. MECH1200

  25. Series Wound – DC Motors MECH1200 Runaway motor at low/no loads High starting torque

  26. Shunt Wound DC Motors Best speed regulation Open shunt field winding will cause motor speed to increase to dangerous levels. Field loss relay should always be used. Allows simplified control for reversing the motor. http://www.youtube.com/watch?v=VGmoKK5Iqrs MECH1200

  27. Shunt Wound DC Motors Field winding are connected in parallel to the armature windings. Field and armature windings can be hooked to same or different sources. “Separately excited” – independent control of armature and field. MECH1200

  28. Shunt Wound DC Motors Self excited – When constant speed is desired. Field is independent of armature current. Doesn’t provide high starting torque Ex: Elevator, Centrifugal Pumps Separately excited – Can operate above its rated speed. Speed can be increased by weakening the field in the field windings. Results in lower torque Requires two power supplies Ex: Woodworking Machinery, Printing Press MECH1200

  29. Shunt Wound DC Motors MECH1200 • Properties: Speed constant independent of load up to certain torque • Field winding parallel with armature winding • Current = field current + armature current Speed control: insert resistance in armature or field current

  30. Shunt Wound DC Motors MECH1200 • Reverse direction by switching either armature or field winding polarities

  31. MECH1200 Shunt DC Motor Terminal Characteristics Question: At which speed should we operate the motor to achieve maximum power? Hint: The power corresponds to the area of the rectangle under the operating point on the speed-torque line. Which of the three rectangles has the largest area? Speed (RPM) No load speed 3000 Maximum power is at 50% of no load speed 2400 1800 Stall point 1200 600 Torque (ft-lb) 10 5

  32. MECH1200 Shunt DC Motor Speed Control • There are three methods to control the speed of a DC motor: 1. Changing the armature voltage. 2. Changing armature resistance (by adding a resistor in series with armature) 3. Change the field (stator) magnetic field Speed (ω) Speed (ω) As Va increases As Ra increases Torque Torque

  33. Compound Wound DC Motors Starting torque : Better than shunt-wound DC motor. Not as good as series-wound DC motor. Speed regulation Better than series-wound DC motor Not as good as with shunt-wound DC motor. Reverse direction of rotation by interchanging armature connections. MECH1200

  34. MECH1200 DC Motors DC compound motor Good torque and stable speed Suited for high starting torque if high % compounding: cranes, hoists Higher % compound in series = high starting torque Field winding in series and parallel with armature winding

  35. MECH1200 Series DC Motor Terminal Characteristics Speed (RPM) If the series motor is unloaded, speed increases significantly (this condition is called Runaway, and it must be avoided) Shunt motor Very high starting torque Series motor Torque (ft-lb)

  36. MECH1200 DC Motor Rating Plates

  37. MECH1200 DC STEPPER MOTORS

  38. MECH1200 DC STEPPER MOTORS • Voltage Rating • provides desired torque • Resistance-per-winding determines • the current draw of the motor • Maximum operating speed • Degrees per Step • Sets the number of degrees the shaft will rotate for each full step

  39. MECH1200 STEPPER MOTOR CONTROL • 4 signal wires fired in the correct sequence will turn the motor

  40. Stepper motor location MECH1200

  41. How does the stepper motor fit in MECH1200

  42. Stepper motor MECH1200 STEPS

  43. MECH1200 What are Poles in a Motor? • Winding(s) which produce the magnetic field(s) necessary to cause the rotor to turn. 3 Phase; 2 Pole Motor

  44. Brushless DC Motors MECH1200 • Rotor is permanent magnet • Stator is wired to provide rotating magnetic field • http://www.youtube.com/watch?v=ZAY5JInyHXY

  45. Types of Motor Enclosures ODP – Open Drip Proof Air flows through motor (fan blades help flow) Used in environments free from contaminants MECH1200

  46. Types of Motor Enclosures TENV – Totally Enclosed Non-Ventilating Protect motor from corrosive and harmful elements Frame fins help to dissipate heat MECH1200

  47. Types of Motor Enclosures TEFC – Totally enclosed Fan Cooled Similar to TENV except has external fan for cooling MECH1200

  48. Types of Motor Enclosures XP – Explosion Proof Similar to TEFC but enclosures are cast iron MECH1200

  49. MECH1200 Hazardous Locations Division I – Hazardous material present in the air as a norm Division II - Hazardous material present in the air as an abnormal event

  50. Hazardous Locations/ Classes MECH1200

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