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AC current reverses direction Two parts: stator and rotor Stator: stationary electrical component PowerPoint Presentation
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AC current reverses direction Two parts: stator and rotor Stator: stationary electrical component

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AC current reverses direction Two parts: stator and rotor Stator: stationary electrical component

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  1. AC Motors • AC current reverses direction • Two parts: stator and rotor • Stator: stationary electrical component • Rotor: rotates the motor shaft • Two types • Synchronous motor • Induction motor

  2. AC Motors

  3. AC – Induction motor • Most common motors in industry • Advantages: • Simple design • Inexpensive • High power to weight ratio • Easy to maintain • Direct connection to AC power source

  4. Electromagnetics Rotor Stator AC – Induction motor • How induction motors work • Electricity supplied to stator • Magnetic field generated that moves around rotor • Current induced in rotor • Rotor produces second magnetic field that opposes stator magnetic field • Rotor begins to rotate

  5. AC – Induction motor • Single-phase induction motor • One stator winding • Single-phase power supply • Squirrel cage rotor • Require device to start motor • Up to 3 to 4 HP • Household appliances: fans, washing machines, dryers

  6. The shaded pole delays the creation of the magnetic field in that portion of the stator poles. This produces a magnetic field in the shaded portion that is approximately 90° apart from the magnetic field produced in the main portion of the pole. Considered a nonreversible motor. Shaded-pole motor

  7. Capacitor Start (disconnects capacitor after motor speed picks up) Capacitor Run (Keeps the capacitor connected during the operation of the motor, in order to keep the electric power consumption low) Capacitor Start-Run (uses two capacitors, one for starting and one for running. This further improves Power Consumption) Three Types of Capacitor Start Motors

  8. Start circuit has: Centrifugal switch Start winding Start capacitor This produces higher starting torque. Run winding Reverse direction of rotation by interchanging run winding or start winding connections (preferred). Capacitor-start motor

  9. The capacitor shifts the phase on one of the windings so that the voltage across the winding is at 90° from the other winding Run capacitor produces higher running torque. Start winding stays as part of the circuit Run winding Reverse direction of rotation by interchanging run winding or start winding connections (preferred). Capacitor-run motor

  10. Start circuit: Start winding Centrifugal switch Start capacitor Larger value produces higher starting torque. Run winding Run capacitor Smaller value produces higher running torque. Reverse direction of rotation by interchanging run winding or start winding connections (preferred). Capacitor-start/capacitor-run motor

  11. Capacitor start-run reverse connections

  12. 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

  13. AC - Synchronous motor • Requires DC voltage for starting excitation • Has low starting torque • Suited for low load applications • Rotor of the synchronous motor travels at the same speed as the rotating magnetic field

  14. Parts Rotor with single winding Slip rings and brushes Three-phase stator windings Synchronous motorThree-Phase Motors

  15. AC - Synchronous motor • Constant speed fixed by system frequency • Used where there is a need to improve the power factor • Synchronous speed (Ns): F = frequency of the voltage source supplied P = number of poles

  16. Synchronous speed

  17. 4-Pole stator winding • Each AC phase has 4 stator windings • Each winding is in opposite direction from preceding winding, making a N-S-N-S field • Field strength rotates with AC current of each phase

  18. AC – Induction motor • Three-phase induction motor • Three-phase supply produces magnetic field • Squirrel cage or wound rotor • Self-starting • High power capabilities • Fractional to 100’s of HP • Applications: pumps, compressors, conveyor belts, grinders • 70% of motors in industry!

  19. AC – Induction motor • Aka: Asynchronous motor • The induction ac motor is a common form of an asynchronous motor • Is basically an AC transformer with a rotating secondary

  20. AC – Induction motor • Components • Rotor • Squirrel cage: conducting barsin parallel slots • Wound rotor: 3-phase, double-layer, distributed winding • Stator • Stampings with slots to carry 3-phase windings • Wound for definite number of poles

  21. 3-phase Induction Motor Operation • Arrows shows stator magnetic field vector • Stator field precedes the rotor’s induction field http://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gif

  22. Interchange any two of the three stator leads. The industry standard is to switch T1 and T3 . The wound-rotor induction motor is considered to be a variable-speed motor. Initial cost is higher and maintenance costs are higher than for a squirrel-cage induction motor. Reversing Direction of a 3 Phase Motor

  23. Reversing Direction of a 3 Phase Motor A B C

  24. Changing AC Motor Speed • Voltage – Hertz Ratio: • Operating motor in a range different from rated frequency and voltage affects both torque and current .

  25. Changing AC Motor Speed • Voltage – Hertz Ratio: • Maintaining the ratio gives a constant torque range For a synchronous motor rated for 3 phase, 460 volts, 60 Hz and 3600 rpm, what will be the operating frequency and voltage if the motor controller commands the motor to run at 2750 rpm? 7.67 2 poles 45.83 Hz 351.52 volts

  26. Parts: Rotor Stator The squirrel-cage induction motor is considered to be a fixed-speed motor. Squirrel-cage induction motor

  27. AC – Induction motor • Speed and slip • Motor never runs at synchronous speed but lower actual rotor speed • Difference is “slip” • Install slip ring to avoid this • Calculate % slip: Ns = synchronous speed in RPM NR= rotor speed in RPM

  28. The rotor contains windings. Slip rings and brushes provide an electrical connection to the rotor windings. The wound-rotor induction motor is considered to be a variable-speed motor. Initial cost is higher and maintenance costs are higher than for a squirrel-cage induction motor. Wound-rotor induction motor

  29. Relationship: load, speed and torque “Breakdown” torque: 75% speed and highest torque = 178.6 ft-#’S “Pull-up” torque: lower torque and increasing speed Starting Torque (aka LRT): high torque and low speed Full load torque: motor operates at rated voltage, frequency and load and stator current are zero 30 HP 1765 RPM

  30. Torque Curve • Calculate: • Speed at 100% full load current • % Slip

  31. AC Motor Data Plate

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

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

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

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

  36. 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

  37. Summary • DC motors are: • permanent magnet • series-wound, • shunt-wound, • compound-wound • AC single phase motors are: • the shaded-pole, • split-phase, • capacitor-start, • capacitor-run, • capacitor-start/ capacitor-run

  38. Questions • Name two motors that do not need brushes for their rotor windings. • Which motor supplies the highest output torque to weight ratio? • Why is it not recommended to use dc motors in artificial hearts? • In an environment that contains explosive gases, such as in mines, which motor do you recommend using: a) series dc motor b) shunt dc motor c) induction motor d) universal motor