EET-325 Advanced Control Wiring Lecture 4 Induction Motors PDF

EET-325 Advanced Control Wiring School of Engineering Technology & Applied Science (SETAS) Lecture #4 Induction Motors  OBJECTIVES Operating principal of induction motor Motor slip Squirrel cage rotor Wound Rotor Startin...

EET-325 Advanced Control Wiring School of Engineering Technology & Applied Science (SETAS) Lecture #4 Induction Motors  OBJECTIVES Operating principal of induction motor Motor slip Squirrel cage rotor Wound Rotor Starting and speed control Direction control Motor name plate information 2 Lecture #4 - Induction Motors  SQUIRREL-CAGE MOTOR Squirrel-cage rotor Complete rotor wo/ laminations w/ the iron core 3 Lecture #4 - Induction Motors  WOUND-ROTOR MOTOR Rotor of wound-rotor motor Rotor connected to external resistors 4 Lecture #4 - Induction Motors  OPERATING PRINCIPAL - 1 The rotating magnetic field on the stator induces a current in the closed loops of the rotor windings The induced current creates a magnetic field that follows the rotating stator field As the rotor accelerates, the magnitude of the induced current and corresponding rotor field decreases 5 Lecture #4 - Induction Motors  OPERATING PRINCIPAL - 2 An induction motor needs to rotate at less than the synchronous speed to maintain a rotor field As motor load decreases the speed will increase While, in theory, the motor will turn at synchronous speed at no load, the inertial of the rotor, friction and windage losses always impose some load 6 Lecture #4 - Induction Motors  TORQUE SPEED CHARACTERISTIC 7 Lecture #4 - Induction Motors  SYNCHRONOUS SPEED Rotational speed of the magnetic field Determined by: - Number of stator poles (per phase) - Frequency of applied voltage S P 𝟏𝟐𝟎 𝒇 3600 2 𝑺= 1800 4 𝑷 1200 6 900 8 - S: Speed (RPM) Synchronous speed at 60 Hz for - f: Frequency in (Hz) different numbers of starter poles - P: Number of stator poles per phase 8 Lecture #4 - Induction Motors  SYNCHRONOUS SPEED - EXAMPLE What is the synchronous peed of a 4-pole motor connected to a 50 Hz power source? 𝟏𝟐𝟎 𝒇 𝑺= 𝑷 Solution: 𝟏𝟓𝟎 × 𝟓𝟎 𝑺= = 𝟏𝟓𝟎𝟎𝑹𝑷𝑴 𝟒 9 Lecture #4 - Induction Motors  PERCENT SLIP Percent slip: Induction motor speed performance Determined by subtracting rotor speed from synchronous speed (Synchronous speed – rotor speed) % Slip 𝑺𝒔𝒚𝒏𝒄 −𝑺𝑹 % 𝑺𝒍𝒊𝒑 = × 𝟏𝟎𝟎 𝑺𝒔𝒚𝒏𝒄 Rotor frequency 𝑷 × 𝑺𝑹 - SR: Rotor speed (RPM) 𝒇= - f: Frequency in (Hz) 𝟏𝟐𝟎 - P: Number of stator poles per phase 10 Lecture #4 - Induction Motors  PERCENT SLIP - EXAMPLE Calculate percent slip of a motor connected to a 60 Hz power source & turning at a speed of 1725 RPM at full load. 𝑺𝒔𝒚𝒏𝒄 − 𝑺𝑹 % 𝑺𝒍𝒊𝒑 = × 𝟏𝟎𝟎 𝑺𝒔𝒚𝒏𝒄 Solution: Since the Ssync is 1800 (Next higher sync speed), 𝟏𝟖𝟎𝟎 × 𝟏𝟕𝟐𝟓 % 𝑺𝒍𝒊𝒑 = × 𝟏𝟎𝟎 = 𝟒. 𝟏𝟔 % 𝟏𝟖𝟎𝟎 11 Lecture #4 - Induction Motors  SPEED REGULATION Speed regulation: Ability of a motor to maintain its speed under varying load conditions Motors are designed to operate at full load (theoretically) Overload: Operation above full load Partial load: Operation less than full load 12 Lecture #4 - Induction Motors  3Ø MOTOR DIRECTION CONTROL The direction of the rotating magnetic field can be reversed by interchanging any 2 phases This causes the rotor to follow the rotating magnetic field in the opposite direction 13 Lecture #4 - Induction Motors  NAME PLATE DATA - HORSE POWER (HP) The power rating of the motor The motor will be able to continuously deliver this power as long as the operating conditions are within tolerance 14 Lecture #4 - Induction Motors  NAME PLATE DATA - VOLTAGE RATING The voltage at which the motor is designed to operate Motors are typically designed for a ±10% voltage tolerance It is possible to have multiple voltages 15 Lecture #4 - Induction Motors  NAME PLATE DATA - FULL-LOAD AMPERAGE (FLA) The current drawn when the motor is fully loaded (at rated HP) For multi voltage motors, the various currents correspond to various voltages 16 Lecture #4 - Induction Motors  NAME PLATE DATA - SERVICE FACTOR (S.F.) Service factor gives an estimate of the motor’s tolerance for overload A 1.0 service factor motor can continuously handle its rated power A 1.15 service factor motor can be continuously overloaded by 15% S.F. only applies when the motor is running at full rated voltage The heat from running a motor above rated power decreases the motor’s life cycle 17 Lecture #4 - Induction Motors  NAME PLATE DATA - FREQUENCY AND PHASE Rated motor frequency Identifies 1Ø or 3Ø motors 18 Lecture #4 - Induction Motors  NAME PLATE DATA - % EFFICIENCY AND POWER FACTOR (PF) The efficiency (ratio of power out to power in) at full load Efficiency will be lower when the motor is partially load The power factor (P/S) at full load Power Factor will be lower when the motor is partially loaded 19 Lecture #4 - Induction Motors  NAME PLATE DATA - VERIFICATION EXAMPLE A 460V, 5 HP induction motor with an efficiency of 89.5% draws 6.7A at full load. Calculate the Power Factor PF, and prove that the given power factor (PF) is accurate. Solution: VA = 460 x 6.7 x 1.732 = 5,338 VA Watt = 5 HP x 746 W = 3,730 W Power Input = Watts / Efficiency = 3,730 W/ 0.895 = 4,168 W PF = Power Input / VA = 4,168 / 5,338 = 0.78 or 78% Check the PF of the previous Motor Nameplate PF = 0.78 20 Lecture #4 - Induction Motors