15 3 Phase AC Power Each phase is displace 120° Frequency - defines how many times a second ACchanges direction: from positive to negativeVoltage - defines the amplitude of electric powerNote: US Standard = 60 cycles per second (60 Hz)
17 Synchronous SpeedThe speed of the rotating magnetic field is referred to as synchronous speed (NS)The induction motor can't run at the synchronous speed because at synchronous speed the induction motor can not develop any torque to move the rotor from its stationary position.
19 Speed of RotorThe difference between the synchronous speed ns and the speed n in rated operation is called slip s and is generally expressed in percent.Depending on the size of the machine, in rated operation it is roughly 10 to 3%.
20 Working Principle of 3 Phase Induction Motor When three phase supply is given to the three phase stator winding of the induction motor, a rotating magnetic field is developed around the stator which rotates at synchronous speed. This rotating magnetic field passes through the air gap and cuts the rotor conductors which were stationary. Due to the relative speed between the stationary rotor conductors and the rotating magnetic field, an emf is induced in the rotor conductors.As the rotor conductors are short circuited, current starts flowing through it.And as these current carrying rotor conductors are placed in the magnetic field produced by the stator, they experiences a mechanical force i.e. torque which moves the rotor in the same direction as that of the rotating magnetic field.
21 Starting method of 3 phase Induction motor DOL(Direct on Line)RDOL (Reverse Direct on Line)Start Delta starterSoft starter(Drive)Auto transformerTo provide starting but to reduce heavy starting currents and provide overload and no-voltage protection.
22 DOL STARTERThe DOL starter switches the supply directly on to the contacts of the motor.
23 ADVANTAGE OF DOL The place occupy is less. Can be used for high torque motor.Trouble shooting is very simple.Less cost.
24 DISADVANTAGE OF DOLStarting current of an induction motor can be 6-8 times the running current.the DOL starter is typically only used for motors with a rating of less than 5kW.The starting torque must be about 50 to 100% more than the expected load torque t ensure that the motor runs up in a reasonably short time.
25 STAR DELTA STARTERStar Delta starting is when the motor is connected (normally externally from the motor) in STAR during the starting sequence. When the motor has accelerated to close to the normal running speed, the motor is connected in DELTA.