AC MOTORAC MOTORINDUCTION MOTOR

Purpose of an “Electrical Purpose of an “Electrical Motor”Motor”

MechanicalEnergy

ElectricalEnergy

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

Type of electric motorsType of electric motors

AC motor

DC motorUnivers

al motor

Single phase

3 phase

Synchronous motor

Asynchronous motor

Squirrel cage

Wound motor

Separately excited

Self excited

series Shunt

Compound

AC INDUCTION MOTORAC INDUCTION MOTOR

Stator Construction

Stator Windings

Rotor Construction

AC MOTORAC MOTOR

Motor Fundamental 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 PolarityChanging Polarity

Induced Voltage Induced Voltage

3 Phase Induction Motor3 Phase Induction Motor

Stator WindingsStator Windings

Electromagnetic Coils(Motor Windings)

Core

Stator

B2

A1

C2

B1

A2

C1

3 Phase AC Power

Frequency - defines how many times a second AC changes direction: from positive to negative

Note: US Standard = 60 cycles per second (60 Hz)

Voltage - defines the amplitude of electric power

Each phase is displace 120°

Rotating Magnetic FieldRotating Magnetic Field

Synchronous SpeedSynchronous 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.

Induced voltage in rotorInduced voltage in rotor

Speed of RotorSpeed 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 Working Principle of 3 Phase Induction MotorInduction 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 Starting method of 3 phase Induction motorInduction motor

DOL(Direct on Line)RDOL (Reverse Direct on Line)Start Delta starterSoft starter(Drive)Auto transformer

To provide starting but to reduce heavy starting currents and provide overload and no-voltage protection.

DOL STARTERDOL STARTER

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

ADVANTAGE OF DOLADVANTAGE OF DOL

The place occupy is less. Can be used for high torque

motor.Trouble shooting is very simple. Less cost.

DISADVANTAGE OF DOLDISADVANTAGE 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.

STAR DELTA STARTERSTAR 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.

How reduce starting How reduce starting current current

ADVANTAGE OF STAR ADVANTAGE OF STAR DELTADELTA 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 DISADVANTAGE OF STAR DELTADELTA the disadvantage of using star delta starting is the

reduction in starting torque