Three-Phase Induction Motors. By Sintayehu Challa ECEg332:-Electrical Machine I

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Darcy Kelly
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1 Three-Phase Induction Motors 1

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4 Classification of AC Machines 1. According to the type of current Single Phase and Three phase 2. According to Speed Constant Speed, Variable Speed and Adjustable Speed 3. According to Principle of Operation Synchronous and Asynchronous Motors 4. According to Structural Features Open, Enclosed,Semi-Enclosed, Ventilated, Explosion proof,water proof, etc 4

5 Contd. Most Industrial electric motors are equipped by three phase Induction motor. Its characteristic features are- Simple and rugged construction Low cost and minimum maintenance High reliability and sufficiently high efficiency Needs no extra starting motor and need not be synchronized 5

The stator windings are connected directly to the three phase power source. The windings are geometrically spaced 120 degrees apart.

6 Principal Machine Components An induction motor has two main parts:-stator and Rotor a) stationary stator:- The Stator is made up of a number of stampings with slots to carry three phase windings. It is wound for a definite number of poles. The stator windings are connected directly to the three phase power source. The windings are geometrically spaced 120 degrees apart. The Stator consisting of a steel frame that supports a hollow, cylindrical core The core, constructed from stacked laminations, having a number of evenly spaced slots, providing the space for the stator winding 6

7 Contd. 7

8 Revolving Rotor A rotor composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding Two types of rotors are used in Induction motors 1) Wound rotor and 2)Squirrel-cage rotor 8

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10 Wound rotor The wound rotor has 3- phase windings made of insulated wire (wound- rotor). The three phase winding terminate in slip-rings mounted on the rotor shaft Brushes ride on the slip-rings during the start period of the motor they are connected to a three phase resistance bank 10

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aluminum rings, forming a squirrel- cage shaped circuit

14 Squirrel-cage Continued Squirrel-cage:-similar to the winding on the stator aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel- cage shaped circuit (squirrel- cage).almost 90% Induction motors are Squirrel-cage rotor 14

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16 Disadvantage Squirrel-cage 16

17 ENCLOSURE 17

18 ENCLOSURE The enclosure consists of 1. Frame (or yoke) and two end brackets (or bearing housings). 2. The stator is mounted inside the frame. 3. The rotor fits inside the stator with a slight air gap separating it from the stator. There is no direct physical connection between the rotor and the stator. 4. Bearings, mounted on the shaft, support the rotor and allow it to turn. 5. A fan, also mounted on the shaft, is used on the motor shown below for cooling. Note:- The enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates. 18

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STATOR COIL ARRANGEMENT 1. The coils on diametrically opposite sides are connected in series 2. The two coils are connected to produce mmf s that act in the same direction 3.

21 STATOR COIL ARRANGEMENT 1. The coils on diametrically opposite sides are connected in series 2. The two coils are connected to produce mmf s that act in the same direction 3. The Windings creates three identical sets of windings, labeled AN, BN, CN 4. The windings mechanically spaced at 120 degrees to each other 5. The three windings are wyeconnected, with a common neutral line- to- neutral impedances are equal, constituting a balanced load 21

The coils are wrapped around the soft iron core material of the stator. These coils are referred to as motor windings.

22 STATOR COIL ARRANGEMENT The following schematic illustrates the relationship of the coils. In this example six coils are used, two coils for each of the three phases. The coils operate in pairs. The coils are wrapped around the soft iron core material of the stator. These coils are referred to as motor windings. Each motor winding becomes a separate electromagnet. The coils are wound in such a way that when current flows in them one coil is a north pole and its pair is a south pole. For example, if A1 were a north pole then A2 would be a south pole. When current reverses direction the polarity of the poles would also reverse. 22

23 Contd. 6) The line currents are displaced in time by 120 degrees 7) sequence rotation: Positive phases sequence = ABC,Clockwise rotation Negative phases sequence = ACB,Counter Clockwise rotation 8) the magneto- motive force is in- phase with the line currents, the mmf s are displaced in time by 120 degrees and also the mmf s are displaced around the stator by 120 mechanical degrees 9) The total mmf within the stator hollow space is the sum of the three phase mmf s 10) The resulting mmf is a magnetic field that varies in time and space, the magnitude of the total mmf is constant 11) The direction of the mmf revolves around the center axis of the stator 23

24 POWER SUPPLY The stator is connected to a 3-phase AC power supply. In the following illustration phase A is connected to phase A of the power supply. Phase B and C would also be connected to phases B and C of the power supply respectively. 24

25 Contd. Phase windings (A, B, and C) are placed 120 apart. In this example, a second set of three-phase windings is installed. The number of poles is determined by how many times a phase winding appears. In this example, each phase winding appears two times. This is a two-pole stator. If each phase winding appeared four times it would be a four-pole stator. 25

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27 Production of Rotating magnetic Flux The magnetic flux produced in the stator winding φ a b c = φ m m m sin ( ωt) φ = φ sin( ωt 120) φ = φ sin ( ωt + 120) The resultant flux becomes φ = φ + φ + φ R a b Exercise Find the resultant flux a) ωt = 0 ( e) ωt = 180 ( b) ωt = ( 60 f ) ωt = 270 ( c) ωt = 90 ( g) ωt = 360 c ( d) ωt =

28 Production of Rotating Magnetic Field Suppose we have a balanced three phase current supplying the three phase winding of stator winding i = I sin( ωt) i i a b c = = I I m m m sin( ωt 120) sin( ωt + 120) The mmf distribution for the stator winding of phase a can be expressed as KN KN Fa ( θ, t) = ia sin θ = I m sin( ωt) sin θ P P = F sin( ωt) sin θ = m F 2 m cos( ωt θ ) F 2 m cos( ωt + θ ) 28

29 The Rotating Field 29

30 30 Contd. The three phase winding rotating magnetic field becomes ) sin( 2 3 ) cos( 2 ) cos( 2 ) 120 sin( ) 120 sin( ), ( ) sin( 2 3 ) cos( 2 ) cos( 2 ) 120 sin( ) 120 sin( ), ( ) cos( 2 ) cos( 2 sin ) sin( ), ( θ ω θ ω θ ω θ ω θ θ ω θ ω θ ω θ ω θ θ ω θ ω θ ω θ = + + = = = + = = t t F t F t F t F t t F t F t F t F t F t F t F t F m m m c m m m b m m m a The resultant mmf at the point in the air-gap becomes ) cos( 2 3 ), ( ), ( ), ( ), ( θ ω θ θ θ θ = + + = t F t F t F t F t F m c b a R

Direction of Rotation The positive crests of the currents in a positive ABC phase sequence Follow each other in the order A B C A This phase sequence produces a magnetic field that rotates

31 Direction of Rotation The positive crests of the currents in a positive ABC phase sequence Follow each other in the order A B C A This phase sequence produces a magnetic field that rotates clockwise for windings arranged ABC in a clockwise layout around the stator Changing either the phase sequence or the winding layout will cause the magnetic field to rotate in the opposite direction 31

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37 Principle of Action Contd. A) Synchronous speed The rotating magnetic field determined by the synchronous speed of stator given by Where N s Synchronous speed also called speed of rotating field f supply frequency p-no. of poles N s S ω = 120 = p 2 π 60 N S f rpm rps 37

38 Contd. Exercise Find the synchronous speed of a three phase induction motor for 60-Hz supply frequency in rpm and rps a) p=2 b)p=4 c) p=6 d)p=8 38

39 contd B) Slip The mode of operation of IM is determined by the relative speed between the rotating magnetic field and the rotor speed. These relative speed is called slip Slip is the difference between the synchronous speed and rotor speed and always expressed as in per unit. Where N is called Rotor speed S = N S N S N x100 39

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42 Continued c) Rotor Speed (N) From Slip equation the rotor speed expressed as N = N S ( 1 S) The rotor speed always less than the synchronous speed 1)When the rotor stationary is called at stand-still, N=0 NS N S = N NS 0 = =1 N S S The frequency of the rotor current is the same as the supply frequency 42

43 Continued 2) The synchronous speed equals the rotor speed (N S =N) S = N S N S No relative motion, this implies there is no induced voltage N The induction motor can not run at synchronous speed = N S N S N S = 0 43

44 Continued When the rotor starts revolving then the rotor frequency depends upon the relative speed (slip) To define slip speed Slip speed = Slip = N S N N 120 f = p 120 f = p p x 120 f S Where f r is called frequency of rotor N N S S N r 120 f = p r = f f r f r = Sf 44

45 Conclusion Conversion of electrical power into mechanical power takes place in the rotating part of an electric motor In Induction machines the rotor receive power by induction in exactly the same way as the secondary of the two winding transformer receives the power from primary. An Induction motor can be treated as a rotating transformer, i.e. one in which primary winding (the stator) is stationary but the secondary (Rotor) is free to rotate by induction. 45

46 Exercise 1. A three phase,4-pole,50 Hz induction motor is running at 1440 rpm. Determine the slipspeed and slip? 2. A three phase induction motor has 2 poles and is connected to 400V,50hz supply. Calculate the actual rotor speed and rotor frequency when the slip is 4% 3. A three phase alternator having 12-poles is driven at a speed of 500 rpm. It supplies power to an 8-pole, 3-phase induction motor, if the slip of the motor at full load is 4%. Find the full-load speed of the induction motor? 46

Placement Paper For Electrical

Placement Paper For Electrical Q.1 The two windings of a transformer is (A) conductively linked. (B) inductively linked. (C) not linked at all. (D) electrically linked. Ans : B Q.2 A salient pole synchronous