Eee ppt


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Eee ppt

  1. 1. ELECTRIC MOTOR An electric motor is an electromechanical device that converts electrical energy to mechanical energy. The mechanical energy can be used to perform work such as rotating a pump impeller, fan, blower, driving a compressor, lifting materials etc.
  3. 3. TYPES OF MOTOR LOADS Motor loads Description Examples Constant torque loads Output power varies but torque is constant Conveyors, rotary kilns, constant- displacement pumps Variable torque loads Torque varies with square of operation speed Centrifugal pumps, fans Constant power loads Torque changes inversely with speed Machine tools
  4. 4. CLASSIFICATION OF MOTORS Electric Motors Alternating Current (AC) Motors Direct Current (DC) Motors Synchronous Induction Three-PhaseSingle-Phase Self ExcitedSeparately Excited Series ShuntCompound
  5. 5. TYPES OF AC MOTORS * Electrical current reverses direction * Two parts: stator and rotor  Stator: stationary electrical component  Rotor: rotates the motor shaft * Speed difficult to control * Two types  Synchronous motor  Induction motor
  6. 6. AC MOTOR: 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
  7. 7. COMPONENTS OF INDUCTION MOTOR A 3-phase induction motor has two main parts: • A stator – consisting of a steel frame that supports a hollow, cylindrical core of stacked laminations. Slots on the internal circumference of the stator house the stator winding. • A rotor – also composed of punched laminations, with rotor slots for the rotor winding.
  8. 8. COMPONENTS OF INDUCTION MOTOR contd…  There are two-types of rotor windings: • Squirrel-cage windings, which produce a squirrel-cage induction motor (most common) • Conventional 3-phase windings made of insulated wire, which produce a wound-rotor induction motor (special characteristics)
  9. 9. Induction Motor: Operating Principle  Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor.  Consider a series of conductors (length L) whose extremities are shorted by bars A and B. A permanent magnet moves at a speed v, so that its magnetic field sweeps across the conductors.
  10. 10. Operating Principle Contd…  The following sequence of events takes place: 1. A voltage E = BLv is induced in each conductor while it is being cut by the flux (Faraday’s Law) 2. The induced voltage produces currents which circulate in a loop around the conductors (through the bars). 3. Since the current-carrying conductors lie in a magnetic field, they experience a mechanical force (Lorentz force). 4. The force always acts in a direction to drag the conductor along with the magnetic field.  Now close the ladder upon itself to form a squirrel cage, and place it in a rotating magnetic field – an induction motor is formed!
  11. 11. Induction Motor: Rotating Field  Consider a simple stator with 6 salient poles - windings AN, BN, CN.  The windings are mechanically spaced at 120° from each other.  The windings are connected to a 3-phase source.  AC currents Ia, Ib and Ic will flow in the windings, but will be displaced in time by 120°.  Each winding produces its own MMF,which creates a flux across the hollow interior of the stator.  The 3 fluxes combine to produce a magnetic field that rotates at the same frequency as the supply.
  12. 12. Induction Motor: Stator Winding  In practice, induction motors have internal diameters that are smooth, instead of having salient poles.  In this case, each pole covers 180° of the inner circumference of the rotor (pole pitch = 180°).  Also, instead of a single coil per pole, many coils are lodged in adjacent slots.  The staggered coils are connected in series to form a phase group.  Spreading the coil in this manner creates a sinusoidal flux distribution per pole, which improves performance and makes the motor less noisy.
  13. 13. INDUCTION MOTOR : SLIP  The difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slip. s = (Ns – N) Ns Where s = slip, Ns = synchronous speed (rpm), N = rotor speed (rpm) • At no-load, the slip is nearly zero (<0.1%). • At full load, the slip for large motors rarely exceeds 0.5%. For small motors at full load, it rarely exceeds 5%. • The slip is 100% for locked rotor.
  14. 14. Induction Motor: Frequency induced in the rotor  The frequency induced in the rotor depends on the slip: fR = s f fR = frequency of voltage and current in the rotor f = frequency of the supply and stator field s = slip
  15. 15. Induction Motor: Active Power Flow  Efficiency – by definition, is the ratio of output / input power: η = PL / Pe  Rotor copper losses: PJr = s Pr  Mechanical power: Pm = ( 1-s)Pr  Motor torque: Tm = 30Pr πNs Where: Pe = active power to stator Pr = active power supplied to rotor PL = Shaft Power
  16. 16. Power Losses
  17. 17. Induction Motor: Relationship between Load, Speed and Torque At full speed: torque and stator current are zero At start: high current and low “pull-up” torque At start: high current and low “pull-up” torque At 80% of full speed: highest “pull- out” torque and current drops
  19. 19. THE END