Motor theory

AC Motor Theory Over View History AC Motors Convert Electrical Energy to Mechanical Energy Induction Motor Components The Magnetic Field Speed Slip Slip Control

AC Motor Theory Overview (cont.) AC Motors Convert Electrical Energy to Mechanical Energy Torque Speed Control Nameplate Information Motor Winding Connections Types of Induction Motors

AC Motor Theory First Electric Motor Was DC in 1833 Simple to control speed First AC Motor in 1899 Simple and Robust Fixed speed and torque characteristics

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy When a conductor is moving across a magnetic field a voltage is induced If the conductor is part of a closed circuit there will be a current induced

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy In a motor, the induction principle is utilized in reverse A live conductor is placed in a magnetic field The conductor is influenced by a force which tries to move it through the magnetic field

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy The AC motor is made up of two parts The Stator The stationary section that contain the windings The Rotor The rotating section that contains the conductors

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy The Magnetic Field Rotates in the air gap between the stator and the rotor Has a fixed location in the stator core but its direction varies Speed of direction change is determined by the frequency of the AC line The field changes 60 times per second with 60 Hz power

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy The Magnetic Field When three phases are introduced in the motor three magnetic fields are introduced Make up a symmetrical rotating 120 degrees apart Poles of opposite polarity are formed Speed = (frequency x 120) / # of poles (60 Hz x 120) / 4 poles = 1800 rpm Theoretical or Synchronous - need to factor in “slip”

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy Slip The rotor is not quite able to keep up with the magnetic field rotation so rotates slightly slower Slip is typically 3-8% of synchronous speed So, actual motor speed equals: Speed = ((Frequency x 120) / # of poles) - Slip ((60Hz x 120) / 4 poles) - 50 rpm) = 1750 rpm

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy Slip Control Slip can be controlled through motor voltage If voltage is reduced then slip will increase Soft Starts Since AC motors are designed for a certain voltage and frequency ratio, changing the voltage alone will cause improper magnetization and increase losses and motor heat

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy Speeds of Typical Motors @ 60 Hz 2 pole = 3600 rpm - slip 4 pole = 1800 rpm - slip 6 pole = 1200 rpm - slip 8 pole = 900 rpm - slip

AC Motor Theory AC Motors Convert Electric Energy into Mechanical Energy Motor torque is affected by the voltage As voltage increases in relation to frequency, torque increases

AC Motor Theory Torque in AC Motors A one hp, four pole motor (1800 rpm sync.) has approximately 3 ft-lbs. of torque If the V/Hz ratio remains constant then the torque will remain constant over the speed range

AC Motor Theory Pop Quiz! Motor A is a Four Pole, 1750 rpm, 100 hp motor Motor B is a Two Pole, 3500 rpm, 200 hp motor Which motor has more torque?

AC Motor Theory Answer: The torque will be the same . Remember that horsepower is a function of speed and torque. Although motor B has twice the horsepower it also has to go twice the speed. So, how much torque will they have?

AC Motor Theory Answer: 300 ft.-lbs. Since a 1 hp, 1750 RPM motor creates 3 ft.-lbs. then a 100 hp, 1750 rpm motor will create 300 ft.-lbs. Remember: Horsepower is not the whole story!

AC Motor Theory Speed Control In AC Motors Proper Speed Variation Since the rotor follows the rotating magnetic field then the rotor will slow with a lower frequency Since the motor is designed for a certain voltage to frequency ratio, if we lower voltage in proportion to the frequency the torque will remain constant

AC Motor Theory Typical Motor Nameplate Data Frame size NEMA standard sizes Lower rpm motors will have larger frame sizes to help cool since the cooling fan is moving less air Horsepower RPM Voltage Full Load Amps For each voltage (208-230/460 VAC)

AC Motor Theory Typical Motor Nameplate Data Power Factor (not on all motors) Ratio of how much current is active to reactive Active current goes to shaft output Reactive current builds the magnetic field Insulation class Enclosure ODP - Open, drip proof TEFC - Totally enclosed, fan cooled TEBC - Totally enclosed, blower cooled

AC Motor Theory Motor Winding Connections Typical motor has nine leads Follow motor instructions for connections for your nominal voltage 230 VAC diagrams will have the windings in parallel 460 VAC diagrams will have the windings in series On part winding start motors tie both the start and the run together Verify that both windings have the same rotation

AC Motor Theory Types of AC Induction Motors Standard efficient Energy efficient Federal government requiring all new motors to be energy efficient Inverter Duty Higher class insulation, phase paper, mounts for constant velocity fan Vector Duty Same as inverter duty but with a shaft encoder

AC Motor Theory Conclusion AC Motors operate through rotating magnetic fields The speed of the motor is determined by the frequency of the power supply The torque of the motor is determined by the voltage applied AC motors are designed for a fixed voltage to frequency ratio

Motor theory

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