AC MOTOR INDUCTION MOTOR Purpose of an Electrical

AC MOTOR INDUCTION MOTOR

Purpose of an “Electrical Motor” Electrical Energy Mechanical Energy The purpose of an electical Motor is to Convert Electrical Energy into Mechanical Energy

Type of electric motors AC motor Single phase Universal motor Separately excited 3 phase Squirrel cage Synchronous motor DC motor Asynchronous motor Wound motor series Shunt Self excited Compound

AC INDUCTION MOTOR

Stator Construction Stator Windings

Rotor Construction

AC MOTOR

Motor Fundamental When current flows through a conductor, it produces a magnetic field around the conductor. The strength of the magnetic field is proportional to the amount of current.

Thumb Rule

Changing Polarity

Induced Voltage

3 Phase Induction Motor

Stator Windings Electromagnetic Coils (Motor Windings) A 1 B 2 C 1 B 1 Core A 2 Stator

3 Phase AC Power Each phase is displace 120° Frequency - defines how many times a second AC changes direction: from positive to negative Voltage - defines the amplitude of electric power Note: US Standard = 60 cycles per second (60 Hz)

Rotating Magnetic Field

Synchronous Speed � The 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.

Induced voltage in rotor

Speed of Rotor �The 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%.

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.

Starting method of 3 phase Induction motor �DOL(Direct on Line) �RDOL (Reverse Direct on Line) �Start Delta starter �Soft starter(Drive) �Auto transformer To provide starting but to reduce heavy starting currents and provide overload and no-voltage protection.

DOL STARTER � The DOL starter switches the supply directly on to the contacts of the motor.

ADVANTAGE OF DOL �The place occupy is less. �Can be used for high torque motor. �Trouble shooting is very simple. �Less cost.

DISADVANTAGE OF DOL �Starting 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 5 k. W. �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.

STAR DELTA STARTER �Star 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.

How reduce starting current

ADVANTAGE OF STAR DELTA � using Star-Delta starting is the huge reduction in the starting current of the motor, which will result in a significant cost saving on cables, transformers and switch gear.

DISADVANTAGE OF STAR DELTA � the disadvantage of using star delta starting is the reduction in starting torque