Class 16 3-phase induction motor

Document Sample

```					AC Motor
An AC motor is an electric motor that is
driven by an alternating current

Example

3-phase induction motor
3-phase induction motor
• The 3-phase induction motor is a rotating
electric machine designed to operate from
a three-phase source of alternating
voltage.
Three phase AC induction motors rated 1 Hp (746 W) and 25 W with small motors
Applications

 Washing machines
 Compressors
 Air conditioning units
 Pumps
 Dishwasher
 Record Player
Construction

An induction motor has 2 main parts

a) a stator

b) a rotor
a) a stator

The outside stationary stator having coils is
supplied with AC current to produce a rotating
magnetic field
b) a rotor
The inside rotor attached to the output shaft
is given a torque by the rotating field

The rotor is wound for a definite number of
poles.
Rotor is of two types :

i) Squirrel cage

ii) Slip ring
Disassembled 250W motor from a washing machine. The 12 stator windings
are in the housing on the left. Next to it is the "squirrel cage" rotor on its shaft.
• In 3-phase induction motors, the rotor
receives electric power by induction in the
same way as the secondary of a 2-winding
transformer

• So, the name induction motor

• In fact, an induction motor can be treated
as a rotating transformer in which the
primary winding is stationary but the
secondary is free to rotate.
Production of rotating field

When stationary coils, wound for
3 phases, are supplied by 3 phase
A.C. supply, a uniformly rotating
magnetic flux of constant magnitude
is produced
φ1   m sin t
φ                               
φ 2   m sin  t  120 0            
φ 3   m sin  t  240                 0

Phase 1        Phase 2          Phase 3
φ1               φ2      φ 3

φm

0   1          2    3
t

φ3

1200

1200                           φ1
1200

φ2
The Resultant Flux                                                        300

i) At 0 when ωt = 0                                      φ3           φR
1  0                                                            3
m        300           3
2                           m

 2   m sin  120                              3                                       2
0
  m sin 120  
0
m                             1200
φ1
2                   1200
1200

3   m sin  240 0    m sin 240 0  
3
m
2
φ2
3                3
R        m cos30 0      m cos30 0
2                2
3                        3 3
2        m cos30  3  m
0
 m
2                        2     2
 1.5 m
The Resultant Flux                                       
3
m
φ3                      2
300
ii) At 1 when ωt = 600
1200        300    φR
3
1   m sin 60   0
m                  1200
2                                                           φ1
           
 2   m sin  60    m
0

2
3                              1200
2
3
m


3   m sin  180 0  0   
φ2
3                3
R       m cos30 0      m cos30 0
2                2
3                        3 3
2        m cos30  3  m
0
 m
2                        2     2
 1.5 m
The Resultant Flux
φ3
iii) At 2 when ωt = 1200
1200
3                                                      3
m
1   m sin 120 
0
m                      1200
2
2                                                300                 φ1
 2   m sin 0 0  0                                    1200

3
φR    300

3   m sin  120         
        m
0                 3                      2
m
2
φ2
3                3
R       m cos30 0      m cos30 0
2                2
3                        3 3
2        m cos30  3  m
0
 m
2                        2     2
 1.5 m
The Resultant Flux
φ3
iv) At 3 when ωt =    1800

1   m sin 180  0
0                                           1200

3                               1200
 2   m sin 60 
0
m                                                            φ1
2                               3
m
1200
300        3

3   m sin  60         
2                           m
0                 3                                   2
m
2                  φR
φ2
300
3                3
R       m cos30 0      m cos30 0
2                2
3                        3 3
2        m cos30  3  m
0
 m
2                        2     2
 1.5 m
3
The resultant flux is      m
2

It rotates around the stator at a synchronous speed

120 f
NS 
P
in the clockwise direction            Ns is in rpm

f – Frequency of the supply voltage
P - No. of poles
Through electromagnetic induction, the rotating
magnetic field induces a current in the
conductors of the rotor, which in turn sets up a
counterbalancing magnetic field that causes the
rotor to turn in the direction the field is rotating.
Synchronous Speed Ns

or Speed of the Stator flux Ns

120  f
Ns 
P

Ns is in rpm

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