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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
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.
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°
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.
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.
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.