Upload
kaushik-patnaik
View
173
Download
4
Tags:
Embed Size (px)
Citation preview
04/08/23 L.N Tripathy 1
Induction Motor
2
Poly Phase Induction motor
Two Important parts are
Stator
Rotor
Wound RotorSquirrel Cage Rotor
04/08/23 L.N Tripathy 3
Schematic View of Three phase Induction Motor
4
Cut-way View of Induction Motor
04/08/23 L.N Tripathy 5
Stator & Yoke of Induction Motor
04/08/23 L.N Tripathy 6
Stator of Induction Motor
7
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
8
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
9
Squirrel Cage Rotor
04/08/23 L.N Tripathy 10
Wound Rotor
11
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
12
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.
04/08/23 L.N Tripathy 13
Motor Magnetic Field
14
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
15
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
16
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
17
Concept of Rotating Magnetic field
øR=øM Sinwt
øY=øM Sin(wt-1200)
øB=øMsSin(wt-2400)
øT=øR+øY+øB
18
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
19
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
20
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
21
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
22
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
23
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.
24
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.
25
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
26
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
27
Thank You