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Induction Motor

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Poly Phase Induction motor

Two Important parts are

Stator

Rotor

Wound RotorSquirrel Cage Rotor

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Schematic View of Three phase Induction Motor

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Cut-way View of Induction Motor

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Stator & Yoke of Induction Motor

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Stator of Induction Motor

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Important points on Stator Stator consisted of stator frame, stator

core, poly phase (2 or 3 ) distributed winding, end covers ,bearing etc

Stator core is stack of cylindrical steel lamination slotted along inner periphery for housing 3 phase winding.

Stator core is screwed to stator frame . Stator frame is made of Cast Iron

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Rotor

1. Squirrel cage rotor :- Winding Consist of un-insulated conductor, in form of copper or aluminum bars embedded in semi- closed slots. These bars are short-circuited at the end by end-rings of same materials. The rotor bars form uniformly distributed winding along rotor slots

2. Wound Rotor

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Squirrel Cage Rotor

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Wound Rotor

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Important points of Wound Rotor motor The rotor slots accommodate an

insulated winding. The windings are uniformly distributed

and usually connected in star. Three leads from the star connection are

connected to slip rings or collector rings mounted on but insulated from shaft.

Carbon brushes pressed on slip ring allow external resistance to be inserted in series with rotor winding for speed and starting torque control

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Important Note. In both type of rotor, the rotor slots are

not parallel to the shaft but it is skewed for obtaining quieter and smoother operation

Wound rotor is used where driven load require speed control and high starting torque.

Squirrel cage rotor is more simple and economical than wound-rotor motor.

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Motor Magnetic Field

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Comparison between two types of Motor

Characteristic

Squirrel-cage

Slip-ring

Speed Almost Constant, Decrease slightly with increased load

Speed decreases more rapidly with load

Torque Starting torque is less, Running torque good

Starting torque 3 times full load torque, running torque is good

Current Starting current is 5-6 times full load current

Starting current is about 2 times full load current

Speed control

Done by changing pole

By changing external resistance of rotor circuit

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Comparison contd…

Characteristic

Squirrel cage

Slip-ring

Power factor Low (about 0.7 t0 0.8) About 0.8 to 0.9

Cost low Higher

Maintenance Cost

Very low High due to brushes, Extra Resistance

Application Lathes , drill, Printing Machine, blower etc.

Lift, cranes etc where high starting torque is necessary

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Difference Between Transformer and Induction motor Transformer primary and secondary are

concentrated winding , require no winding factor. In Induction motor stator and rotor both are distributed winding with different winding factor.

No load current in transformer is 2 to 6 % but in induction motor it is 30 to 50 % due to presence of air gap which create high reluctance between stator and rotor.

Transformer primary and secondary frequency are same but in induction motor the rotor frequency is slip times the stator frequency

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Concept of Rotating Magnetic field

øR=øM Sinwt

øY=øM Sin(wt-1200)

øB=øMsSin(wt-2400)

øT=øR+øY+øB

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Production of Rotating Magnetic field in3-phase winding

øY

øy=-0.866øm

øT

øB=0.866øm

600

øR = ømSinwt = 0øY= øm Sin(-120 )=-0.866øm

øB= øm Sin(-2400)=0.866øm

øT=øR+øY+øB

øT=2×0.866ømcos300=1.5øm

Case1 Ə=wt=00

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Production of Rotating Magnetic field in3-phase winding

øY Assumed positive Direction

øy=-0.866øm

øT

øR=0.866øm

øR = ømSin600 =0.866øm

øY= øm Sin(60-1200 )=-0.866øm

øB= øm Sin(60-2400)=0øT=2×0.866ømcos300=1.5øm

Case2: Ə=wt=600

600

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Production of Rotating Magnetic field in3-phase winding

øB Assumed positive Direction

øB=-0.866øm

øT

øR=0.866øm

øR = ømSin1200 =0.866øm

øY= øm Sin(120-1200 )=0øB= øm Sin(120-2400)=-0.866øm

øT=2×0.866ømcos300=1.5øm

Case3: Ə=wt=1200

600

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Production of Rotating Magnetic field in3-phase winding

600

øR = ømSinwt = 0øY= øm Sin(180-120 )=0.866øm

øB= øm Sin(180-2400)=-0.866øm

øT=2×0.866ømcos300=1.5øm

Cas4: Ə=wt=1800

øB = -0.0866øm

Øy=0.866øm

øT

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Gist Rotating magnetic field is produced

when three stationary stator winding are excited by balance three phase.

The speed of Rotating magnetic field is given by 120f/P , and called synchronous speed.

P = number of poles f = Supply frequency

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Principle of Operation

Three phase winding in stator when get supply produce rotating magnetic field [or flux] of Constant magnitude but rotating at synchronous speed

This rotating magnetic field induce emf in rotor circuit and current start flowing in rotor circuit as rotor is short Circuited.

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Cont.. The torque is produced on the rotor due

to interaction of two fields. As per Lenz’s law ,under the influence of

this toque the rotor start rotating in same direction to catch the rotating magnetic field. The relative speed between rotating magnetic field and the rotor is called slip speed.

Note:- The rotor rotate near synchronous speed but can not attend the synchronous speed. So induction motor is called Asynchronous motor.

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Slip of Induction Motor

[ ] [ ][ ]

[ ]

[ ] 100%

s

s

s

s

Synchronous speed N Rotor Speed NSlip s

Synchronous Speed N

N Ns

N

Slip is Always expressed as percentage

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Frequency of Stator current & Rotor current

Stator current frequency is same as supply frequency f

Rotor induced emf frequency or Rotor current frequency= sf

Where ‘s’ is the slip

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Thank You