Three-Phase Induction Motors · PDF fileequipped by three phase Induction motor. ... Wound...

By Sintayehu Challa ECEg332:-Electrical Machine I

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Three-Phase Induction Motors


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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


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


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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 ofpoles. 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


7

Contd.


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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


9

Contd.


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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


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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


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Disadvantage Squirrel-cage


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ENCLOSURE


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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.


19

Contd.


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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. 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 wye-connected, with a common neutral line- to- neutral impedances are equal, constituting a balanced load


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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.


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Contd.

6) The line currents are displaced in time by 120 degrees

7) sequence rotation: Positive phases sequence = ABC,Clockwise rotationNegative 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


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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.


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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.


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Production of Rotating magnetic Flux

The magnetic flux produced in the stator winding

)120(sin

)120sin(

)(sin

+=

−=

=

t

t

t

mc

mb

ma

ωφφ

ωφφ

ωφφ

cbaR φφφφ ++=�The resultant flux becomes

Exercise Find the resultant flux

360)(270)(180)(

120)(90)(60)(0)

===

====

tgtfte

tdtctbta

ωωω

ωωωω


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Production of Rotating Magnetic Field

Suppose we have a balanced three phase current supplying the three phase winding of stator winding

)120sin(

)120sin(

)sin(

+=

−=

=

tIi

tIi

tIi

mc

mb

ma

ω

ω

ω

The mmf distribution for the stator winding of phase a can be expressed as

)cos(2

)cos(2

sin)sin(

sin)sin(sin),(

θωθω

θω

θωθθ

+−−=

=

==

tF

tF

tF

tIP

KNi

P

KNtF

mm

m

maa


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The Rotating Field


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Contd.

The three phase winding rotating magnetic field becomes

)sin(2

3)cos(

2)cos(

2

)120sin()120sin(),(

)sin(2

3)cos(

2)cos(

2

)120sin()120sin(),(

)cos(2

)cos(2

sin)sin(),(

θωθωθω

θωθ

θωθωθω

θωθ

θωθωθωθ

+−++−=

++=

++++−=

−−=

+−−==

ttF

tF

tFtF

ttF

tF

tFtF

tF

tF

tFtF

mm

mc

mm

mb

mmma

The resultant mmf at the point in the air-gap becomes

)cos(2

3

),(),(),(),(

θω

θθθθ

−=

++=

tF

tFtFtFtF

m

cbaR


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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


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By Sintayehu Challa


33By Sintayehu Challa


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Principle of Action Contd.

A) Synchronous speed

The rotating magnetic field

determined by the synchronous speed of stator given by

Where Ns –Synchronous speed also called speed of rotating field

f – supply frequency

p-No. of poles

rpsN

rpmp

fN

Ss

S

60

2

120

πω =

=


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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


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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

100xN

NNS

S

S −=


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Continued

c) Rotor Speed (N)

From Slip equation the rotor speed expressed as

The rotor speed always less than the synchronous speed

1)When the rotor stationary is called at stand-still, N=0

The frequency of the rotor current is the same as the supply frequency

)1( SNN S −=

10

=−

=−

=S

S

S

S

N

N

N

NNS


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Continued

2) The synchronous speed equals the rotor speed (NS=N)

� No relative motion , this implies there is no induced voltage

� The induction motor can not run at synchronous speed

0=−

=−

=S

SS

S

S

N

NN

N

NNS


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Continued

� When the rotor starts revolving then the rotor frequency depends upon the relative speed (slip)

� To define slip speed

� Where fr is called frequency of rotor

Sfff

f

f

px

p

f

N

NNSlip

p

fN

p

fNNspeedSlip

r

rr

S

S

S

r

S

=⇒==−

=

=

=−=−

120

120

120

120


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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.


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Exercise

1. A three phase,4-pole,50 Hz induction motor is running at 1440 rpm. Determine the slip-speed 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?

Three-Phase Induction Motors · PDF fileequipped by three phase Induction motor. ... Wound...

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